Mineral nutrition of cassava

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Mineral nutrition of cassava G. Byju*, G. Suja ICAR-Central Tuber Crops Research Institute, Thiruvananthapuram, Kerala, India *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 1.1 Cassava production 1.2 Cassava plant 2. Cassava productivity 2.1 Crop characteristics 2.2 Climate 3. Soil requirements 4. Nutrient requirements 5. Methods of fertilizer recommendations for cassava 5.1 Blanket fertilizer recommendations 5.2 Targeted yield approach 5.3 Leaf color chart and SPAD-502 meter based real time N management 5.4 Site-specific nutrient management 5.5 Method and timing of fertilizer application 5.6 Drip fertigation 6. Organic vs inorganic nutrition of cassava 7. Knowledge gaps and recommended research 8. Conclusions Acknowledgments References

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Abstract Cassava is an important tropical root crop with versatile uses in food, feed, green energy and industrial sectors. It is grown mostly for food in Sub-Saharan Africa, food and other uses in Latin America and feed and other uses in Asia. The current global level of productivity of about 12 t/ha is only 12% of its potential productivity. Major reasons for the wide yield gap are lack of scientific nutrient management and unbalanced crop nutrition. In this review, we update the gap in cassava productivity, soil requirements, nutrient requirements and evolution of fertilizer recommendations and its impact on bridging the yield gap. Research work on mineral nutrition of cassava, especially in Asia and Latin America is aplenty. Among the major nutrients, cassava removes large quantity of potassium along with the harvested produce compared to the other nutrients. The range of critical levels of P and K reported for cassava was lower than the

Advances in Agronomy ISSN 0065-2113 https://doi.org/10.1016/bs.agron.2019.08.005

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corresponding values reported for most of the crops, which shows its adaptability to grow and yield under poor soil fertility conditions. Studies on nutrient requirements of cassava showed that the N:P:K uptake ratio in total plant dry matter was 6.2:1:6.6. The total N, P and K uptake requirements for producing one ton of fresh cassava tuber ranged from 2.9 to 6.9 kg for N, 0.68 to 1.3 kg for P and 3.9 to 7.9 kg for K. Large variations in blanket NPK recommendations could be observed among different countries. An attempt to compare inorganic nutrient management with organic nutrition on growth, yield, quality and soil properties is also made. Presently, more knowledge-intensive and computer simulation model-based site-specific nutrient management (SSNM) using models such as quantitative evaluation of fertility of tropical soils (QUEFTS) has proved to be a better alternative in reducing the yield gap of cassava in India and some West African countries. More basic knowledge need to be emanated from further research for developing field level nutrient recommendations under changing climate, especially drought. Development of a cassava SSNM network (CSN), analysis and interpretation of already existing data of different national and international research institutes and generation of required additional field data will help to further improve the SSNM technology and its wider application for precision nutrient management, combating wide spread nutrient disorders and enabling better yields among cassava growing countries across the globe.

1. Introduction Cassava (Manihot esculenta Crantz), a starchy tropical tuber crop (approximately 85% starch on dry weight basis), cultivated by small farmers in the marginal lands of Sub-Saharan Africa, Asia and Latin America, provides subsistence as well as cash incomes (Sarma and Kunchai, 1991). It is also known as manioc in French, maniok in German, mandioc in Portuguese, cassave in Dutch, yuca in Spanish, ketela pohon in Indonesia, mihogo in Kenya, akpu in Nigeria, san in Vietnam and maracheeni or kappa in Malayalam language of Kerala state, India and is a perennial shrub belonging to a group of 100 species of the genus Manihot (Howeler et al., 2013). The name cassava is derived from the word Casavi or Cazabi, which in Arawak (the language of the first indigenous people who lived in the great Antilles and were in contact with Christopher Colombus) means bread (Parmar et al., 2017). Several molecular studies suggest that Manihot esculenta ssp. Flabellifolia and Manihot pruinosa are the likely progenitors of modern day cassava cultivars (Allem, 1999, 2002; Olsen, 2004; Olsen and Schaal, 1999). Ancient ancestors of the above two progenitors of cassava were endemic to Brazilian Cerrado Savannah and later spread to the Amazon basin in the form of other species (Allem, 2002; Howeler et al., 2013). Archeological findings suggest that

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domestication of cassava as an agricultural crop began 5000–10,000 years ago in the Amazonian rain-forest (Allem, 2002; Howeler et al., 2013; Shigaki, 2016) and hence is one of the oldest cultivated crops. It was then spread to tropical parts of Africa and Asia through Spanish and Portuguese explorers, where it established itself as early as 1800. Cassava is one of the energy dense crops, providing an energy of 250 kcal/ha/day compared to maize (200), sweet potato (180), rice (176), sorghum (114) or wheat (110) (El-Sharkawy, 2012). The high calorie yield per hectare, adaptability to highly infertile soil conditions, relative resistance to diseases, and flexibility of harvesting time are the major advantages of this crop compared to many other crops. It is the most important tuber crop and fourth most important source of food calories in the tropics and is a staple for more than 800 million people (Cock, 1985; Nasser et al., 2007). About 700 million people receive more than 500 cal/day of energy from cassava and more than 500 million people consume more than 100 cal/day in the form of cassava roots throughout the tropics. The tubers are mainly consumed as human food in various forms after cooking and processing to remove the inherent cyanogenic glucosides, used as animal feed, as well as for starch extractions besides various other industrial uses (Alves, 2002; Cock, 1985; El-Sharkawy, 2004; Lebot, 2009; Splittstoesser and Tunya, 2002).

1.1 Cassava production World cassava production more than doubled from 124 million tons in 1980 to 277 million tons in 2016 (FAOSTAT, 2018), which can be attributed to the ever-increasing demand for this crop in the food, feed, green energy and industrial sectors. As depicted in Fig. 1, global area under cassava showed a steady increasing trend of about 0.22 million ha per year over the past 56 years, but it started stagnating since 2012 at around 23 million ha. About 72% of world’s cassava area is in Africa, 18% in Asia and 10% in Americas. Global annual production of cassava also showed a similar trend as in the case of area under cassava. World cassava production showed a steady increasing trend of about 3.48 million tons per year. It is also interesting to note that production did not show stagnating trend after 2012 as noticed in the case of area, which was mainly due to increase in productivity in Asian countries (Fig. 1). About 57% of world’s cassava is produced in Africa, with Nigeria being the top producer at 21% of global production. Asian countries produce about 32% and Americas about 11% of global cassava production. The productivity of

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Fig. 1 Production, area harvested, and productivity of cassava in the world and different continents. Based on data from FAOSTAT, 2018. Production; Cassava; all Countries; 1961–2016 (Online). Food and Agriculture Organization of the United Nations Download data (http://www.fao.org/faostat/en/#data/QC). (accessed 01/12/2018).

cassava increased only by 4.4 t/ha during the past 56 years, which stood at 11.8 t/ha in 2016 and this world average is below the productivity of 46 of the 104 cassava producing countries (Fig. 2). Considering the facts that further expansion in area is not possible in most of the cassava growing countries due to limited land availability and wider variability that exists in average yields across cassava producing countries as low as 1.13 to as high as 32.68 t/ha, there is an urgent need to increase the productivity of cassava. In Africa, which has the major area under cassava, the crop is grown mainly on small holdings by low-income farmers who apply little or no external inputs (Howeler et al., 2013). It is usually grown with other crops, such as maize, rice, legumes, melons, banana and oil palm. About 90% of the produce is used for direct human consumption as fresh tubers or after processing into fermented flour products. In Asia, cassava is mostly produced to meet the demand for dried cassava chips and cassava starch for use in commercial livestock feed and for industrial processing (Howeler et al., 2013). About 26% of world’s cassava is produced from four Asian countries, namely, Thailand, Indonesia, Vietnam and Cambodia (which is 82% of Asia’s production of 89.2 million tons) (FAOSTAT, 2018). The major consumer of cassava in the region is China. China’s annual import of dried

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Fig. 2 Variation in productivity (t/ha) of cassava across growing countries. Map prepared by R. Shiny, Ph.D. Scholar, ICAR-CTCRI, India. Based on data from FAOSTAT, 2018. Production; Cassava; all Countries; 1961–2016 (Online). Food and Agriculture Organization of the United Nations Download data (http://www.fao.org/faostat/en/#data/QC). (accessed 01/12/2018).

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cassava in 2016 was 7.7 million tons, while import of cassava starch was 2.4 million tons. In Asia, cassava is also utilized as feed stock for the production of biofuel and one ton of dried cassava chips yields about 300 L of 96% pure ethanol (UNCTAD, 2013). Many companies in China, Japan and the Republic of Korea are supporting large scale cassava cultivation in Cambodia, Indonesia, Lao PDR and The Philippines as a source of dried chips for ethanol production. Cassava is also used for direct human consumption in some parts of Asia and in Kerala state, India, cassava is grown mainly for food as a secondary staple as well as a delicacy in star hotels and the farmers realized average yields of 34 t/ha (Farm Guide, 2018) thanks to development and popularization of high yielding varieties, better agronomy and soil fertility management. In Americas, cassava is usually grown in marginal, acid, infertile soils. About 70% of Americas’ production of 30.3 million tons is from Brazil which harvested 21.08 million tons in 2016 (FAOSTAT, 2018), followed by Paraguay (3.17 million tons), Colombia (2.11 million tons) and Peru (1.18 million tons). Cassava is an important staple food in Colombia and northeast Brazil and about half of the production in Americas is used as human food and remaining half as animal feed (Howeler et al., 2013). Under the changing climate scenario, cereal grain production in the tropics is predicted to decline (IPCC, 2006; Rosenzweig and Parry, 1994) and this will definitely reduce the dependence on crops like maize as main staple in regions like Sub-Saharan African countries. The most important alternative and suitable crop under such situations will be cassava which also has got varied uses in animal feed and many industries such as starch, alcohol and biofuel (Eke-Okoro et al., 2009; Jansson et al., 2009; Jarvis et al., 2012; Johnston et al., 2009; Sabitha et al., 2016). In this review, the scientific research regarding the mineral nutrition of cassava is synthesized. We bring together current knowledge on nutrient requirements of cassava, and yield response to fertilizer application in relation to factors such as climate, soil and crop management. We also provide the important knowledge gaps and recommendations for further research on the potential of fertilizer for closing the yield gap under different production situations across major growing environments. This review can definitely help to develop future research programs required to increase the sustainable productivity of cassava. It is believed that this review will be an encouragement to all the stakeholders in the cassava sector to intensify research efforts on the nutrient requirement of cassava under the ever-changing climate scenario.

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1.2 Cassava plant Cassava is a perennial shrub belonging to the family Euphorbiaceae cultivated mainly for its starchy roots. The plant is 1–4 m in height and there exists wide variations in morphological characteristics, which indicate that it has undergone a high degree of interspecific hybridization. Since cassava tuber is a true root anatomically, it cannot be used for vegetative propagation. The tuber has an outer bark (periderm), peel or cortex (sclerenchyma, cortical parenchyma and phloem) and the edible portion (parenchyma). The parenchyma consists of xylem vessels and a matrix of starch containing cells (Wheatley and Chuzel, 1993). The stem is woody and cylindrical, the branching is sympodial, and the main stem is divided di-, tri- or tetrachotomously and produce secondary branches which again produce other branches (Fig. 3). Cassava plant has simple leaves with leaf blade or lamina and a long petiole. The lamina is deeply lobed with palmated veins. The number of lobes will generally be odd, and it usually ranges from 3–9 and occasionally up to 11 (Rogers and Fleming, 1973). The leaves near the inflorescence are usually reduced in size with three lobes and the leaf closest to the inflorescence is generally simple and unilobed. The leaves have an alternate arrangement and have a phyllotaxy of 2/5, indicating that from any leaf (leaf 1) there are two rotations around the stem to reach the sixth (leaf 6) in the same

Fig. 3 View of a cassava field. Photo: G. Byju, ICAR-CTCRI, India.

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orthosticky as leaf 1. In these two rotations there are five successive intermediate leaves (not counting leaf 1) (Alves, 2002). The mature leaves are glabrous and are surrounded by two stipules (0.5–1.0 cm long) (CIAT, 1984). The length of the petiole ranges from 5 to 40 cm. The upper leaf surface is covered with a shiny, waxy epidermis. Most of the stomata are located on the lower leaf surface (abaxial) and a few stomata are present along the vein on the upper surface (adaxial) (Cerqueira, 1989). The number of stomata in leaf ranges from 278 to 700/mm2. Cassava is monoecious and male (pistillate) and female (staminate) flowers are produced on the same plant. The female flowers are present on the lower part of the inflorescence and are fewer in number compared to the male flowers. On the same inflorescence, the female flowers open 1–2 weeks before male flowers (protogyny) and the male and female flowers on different branches of the same plant can open at the same time. Cassava is generally cross pollinated by insects and hence is a highly heterozygous plant. No separate calyx and corolla are present in the flowers, but an indefinite structure known as perianth or perigonium is present, which is made up of five yellow, reddish or purple tepals. The pedicel of the male flower is thin, straight and very short, while that of the female flower is thick, curved and long. Inside the basal flower, there is a basal disk divided into 10 lobes. Ten stamens originate from them and are arranged in two circles and provide support to the anthers. The five stamens in the outer circle are separated and are longer than inner ones, which join together on the top to form a set of anthers. The pollen is generally yellow or orange, varying from 122 to 148 μm in size (Ghosh et al., 1988). The female flower also has a 10-lobed basal disk, which is less lobulated than the male flower. The ovary is tricarpellary with six ridges and is mounted on the basal disk. The three locules contain one ovule each. A very small style is located on top of the ovary, and a stigma with three undulated, fleshy lobes originates from the style. The cassava has trilocular capsule fruit, which is ovoid or globular in shape, 1–1.5 cm in diameter and with six straight, prominent longitudinal ridges or aristae. There is one carunculate seed in each locule. The fruit has a biocidal dehiscence, which is a combination of septicidal and loculicidal dehiscences, with openings along the parallel plane of the dissepiments and along the midveins of the carpels, respectively. This biocidal dehiscence helps the fruits to open into six valves with an explosion and eject the seeds at some distance (Rogers, 1965). The fruits mature in 75–90 days after

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pollination. The seed is ovoid-ellipsoidal, approximately 100 mm long, 6 mm wide and 4 mm thick (Ghosh et al., 1988).

2. Cassava productivity Cassava is considered as a subsistence crop grown traditionally by poor and marginal farmers, but is becoming increasingly important as a commercial crop in most of the growing areas across the world. The crop has a unique capability to produce harvestable yield even under hostile environmental conditions, where other crops fail (Duque and Setter, 2013; Howeler, 2002). A number of mechanisms are attributed to its ability to tolerate adverse weather and soil conditions. It includes control over stomata under moisture stress (Connor and Palta, 1981), reduction in canopy growth and leaf size (Ike and Thurtell, 1981), a deep, fibrous root system (Connor et al., 1981), extension of leaf life (El-Sharkawy et al., 1992) and potential hydraulic lift capacity (El-Sharkawy, 2006). The optimum leaf photosynthetic rate of cassava under field conditions is between 25 and 30 μmol CO2/m2/s (El-Sharkawy et al., 1990). The maximum leaf photosynthetic rate at an optimum leaf temperature of 30–35 °C is reported to be between 40 and 50 μmol CO2/m2/s (El-Sharkawy, 2004). Cassava follows an intermediate photosynthetic pathway between the C3 and C4, and has been proposed to be an adaptive step toward the more “efficient” C4 pathway of higher plants (El-Sharkawy and Cock, 1987). The phosphoenolpyruvate carboxylase enzyme (C4 plants) is active in cassava plant, and was found to be comparable to several other C3-C4 intermediate species (El-Sharkawy, 2006). The leaf Krantz anatomy is not seen in cassava, but the leaves have distinct green bundle sheath cells, spatially separated below the palisade cell, which not only performs C3 photosynthesis, but also transports photosynthates in the leaf, thus enhancing the photosynthetic capacity of the leaves (Allem, 2002). Under favorable environmental conditions, cassava can yield 80 t/ha of fresh tuber in experimental farms and up to 40 t/ha of fresh tuber in commercial fields with improved cultivars. But its productivity in seasonally dry and semi-arid environments without fertilization is much less (El-Sharkawy, 2010; Ramanujam, 1990). A large yield gap exists between yield potential and farmers yields due to lack of adoption of improved varieties, better agronomy and soil fertility management (El-Sharkawy, 1993; El-Sharkawy et al., 1990; Fermont, 2009). Because of its inherent tolerance to prolonged drought and infertile soils (El-Sharkawy, 2006), cassava production is

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expanding into more marginal lands and drier environments for subsistence (Fermont, 2009; Romanoff and Lynam, 1992). The differences between potential and actual yields, known as yield gaps, are large for cassava in most of the growing countries. Average yield of fresh tuber of cassava varies from 1.13 to 32.68 t/ha among 104 cassava growing countries with a global average yield of 11.80 t/ha (FAOSTAT, 2018), which is far below the potential yield of 80–100 t/ha (Byju et al., 2012). One of the plausible reasons for this wide yield gap is the spatial and temporal variability in soil and plant properties and managing this variability will be one of the important solutions to bridge the yield gap of cassava.

2.1 Crop characteristics The potential yield of a crop is decided by physiological and phenological traits which are influenced by genotype, environment and management. Cassava varieties vary widely in their physiological and phenological traits and being a perennial shrub, the growth of cassava alternates between vegetative growth, storage root development and dormancy. A number of factors related to cultivar, environmental conditions such as temperature and moisture and existing farming practices influence occurrence and duration of these phases (Allem, 2002). According to Cock et al. (1979), cassava plant develops shoots and a fine root system during the first 9 weeks after planting (WAP) or 63 days after planting (DAP). A number of researchers have used leaf area index (LAI) as a parameter for the evaluation of potential yield since they could observe a positive correlation between LAI and tuber yield (Cock et al., 1979). The LAI of a particular cultivar is affected by branching pattern, leaf size and duration of leaf retention. Cock et al. (1979) also reported that if the crop can attain and maintain an LAI value of 3–3.5 early in the season, there will be optimal tuber bulking rate. Bulking of roots into tubers starts at about 9–17 WAP, when supply of photoassimilates in the shoots exceeds demand. A computer-based simulation model to determine the ideal plant type of cassava for maximum yield under favorable growth conditions, both edaphic and climatic, was developed by Cock et al. (1979). According to them, an ideal plant type should have the following characteristics: late branching at 6–9 months after planting, maximum leaf size of about 500 cm2 per leaf blade at 4 months after planting, long duration of leaf retention of about 100 days, LAI between 2.5 and 3.5 during most of the growth cycle, a harvest index (HI) greater than 0.5, nine or more tubers per plant at a population density of

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10,000 plants per ha, and each plant having two vegetative shoots originating from the original cuttings. Such an ideal cassava plant was predicted to give an yield of about 90 t/ha of fresh tuber (about 30 t/ha dry matter), provided that the growing environment is optimum with no stress condition.

2.2 Climate The distribution of cassava is related to climatic conditions and most of the cassava producing regions are between Tropic of Cancer and Tropic of Capricorn and elevations up to 2300 m above mean sea level (Allem, 2002). For optimum growth and production, cassava requires an annual rainfall of more than 1000 mm, annual temperature above 18 °C and a daily mean solar radiation higher than 16 MJ/m2 (Ekanayake et al., 1998). The cassava plant is very sensitive to low temperature especially frost and it cannot be grown successfully in areas where the temperature is below 15 °C (Cock, 1985). In areas with marked seasonal temperature changes, cassava is grown only when the annual mean temperature is greater than 20 °C. The minimum, optimum and maximum temperature for sprouting are 12–17, 28–30 and 36–40 °C, respectively. Photosynthetic rates declined rapidly above 40 °C and stopped completely at 50 °C. Sprouting and establishment from stem cuttings are most rapid when soil temperature is between 28 and 30 °C. Sprouting ceases when the soil temperature decreases below 17 °C or increases above 37 °C (Keating and Evenson, 1979). Certain cassava cultivars grow and produce tuber in cool climate and in such cases the optimum temperature range is 25–27°C and the growth will cease below 10 °C and above 35 °C. For cassava cultivars that come up well in hot-climate, the optimum temperature range is 25–40 °C and the maximum range is 30–35°C (El-Sharkawy, 2006). Cassava can survive in areas with large variation in precipitation between 500 and 5000 mm (Tan and Bertrand, 1972) or <600 mm in semiarid tropics to >1600 mm in subhumid/humid tropics (Allem, 2002). In order to calibrate the Ecocrop model for climate suitability studies of cassava, Ceballos et al. (2011) used minimum killing temperature of 0 °C (Tkill), minimum temperature of 15 °C (Tmin), minimum optimum temperature of 22 °C Topmin), maximum optimum temperature of 32 °C (Topmax), and maximum temperature of 45 °C (Tmax). The minimum and maximum optimum rainfalls were set to 800 (Ropmin) and 2200 mm (Ropmax), respectively, and the minimum and maximum rainfalls were set to 300 (Rmin) and 2800 mm (Rmax), respectively. Sabitha et al. (2016) calibrated the same model for cassava under Indian conditions and the climatic parameters developed are given in Table 1.

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Table 1 Parameter set developed for calibrating Ecocrop model for cassava in India. Parameter Calibrated value

Tkill

5 oC

Tmin

18 oC

Topmin

20 oC

Topmax

30 oC

Tmax

48 oC

Rmin

400 mm

Ropmin

600 mm

Ropmax

2000 mm

Rmax

3300 mm

Source: From Sabitha Soman, Byju, G., Sreekumar, J., 2016. Projected changes in mean temperature and total precipitation and climate suitability of cassava (Manihot esculenta Crantz) in major growing environments of India. Indian J. Agric. Sci. 86(5), 647–653.

Since cassava is generally cultivated as a rainfed crop in dry environments, it is important to plant the crop with the onset of monsoon. Once it sprouts and establishes itself with available soil moisture from rainfall, cassava is capable of tolerating droughts due to physiological mechanisms and it will be able to survive without moisture for up to 12–24 weeks (El-Sharkawy, 2006). Water stress can be divided into three stages: early water stress (8–24 weeks after planting), mid-season stress (16–32 weeks after planting) and terminal stress (24–48 weeks after planting). El-Sharkawy and Cadavid (2002) reported that avoiding early water stress was most important for biomass growth, tuber bulking and yield compared to mid-season and terminal water-stress. This shows the importance of planting rainfed cassava with the onset of monsoon to avoid water stress during early growth. Large reserves of carbohydrate allow rapid regeneration when sufficient moisture becomes available once more (Rogers and Appan, 1972). The crop growth rate was reduced during periods of stress by reduction in leaf area and stomatal closure (Connor et al., 1981). In India, cassava is grown in areas with contrasting climatic conditions. It is grown in areas with rainfall as high as 3000 mm (Kerala) to as low as 600 mm (Tamil Nadu), and at elevations from below sea level to 2200 m above mean sea level. In Idukki district of Kerala and the Kolli hills of Tamil Nadu, India which are

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located at altitudes of 2200 and 1200 m above mean sea level, respectively, cassava performs well (Byju, 2006). It has been gaining importance as an industrial crop in the western, eastern and north-eastern states of India and grows well in a wide range of soils and climates (Nedunchezhiyan et al., 2008; Nedunchezhiyan and Mohanty, 2005). Fig. 4 shows the variation in monthly minimum and maximum temperature and total rainfall in the three major cassava growing states in India (Byju et al., 2010). In the tropics where cassava is grown, day length usually varies between 10 and 12 h a day. In these tropical regions, photoperiod will have little effect on cassava growth patterns. Bolhuis (1966) reported that cassava cultivars showed slight differences in critical photoperiod, but generally it required a 12-h photoperiod for maximum growth. Veltkamp (1985) observed that when the day length was more, shoot growth got increased, while tuber development decreased. Reverse trend was noticed during shorter photoperiods with decreased shoot growth and increased tuber development without any change in total dry weight. It was also observed that while tuber development was enhanced under shorter photoperiods, shorter days were not required for tuber initiation. Due to this fact, long photoperiods will result in reduced carbohydrates available for tuber development (Keating et al., 1982; Veltkamp, 1985). El-Sharkawy et al. (1992) reported that cassava required a high solar radiance to express its full photosynthetic potential with an average daily solar radiation of 18 MJ/m2. Cassava grown under shade resulted in etiolation or increase in plant height through stem elongation and increased internodal length. The leaf area also showed increasing trend in cassava grown under shade (Fukai et al., 1984) and length of leaf retention was reduced under severe shaded conditions of 95–100% (Cock et al., 1979).

3. Soil requirements Bulking of tubers and early harvesting of cassava is facilitated in a soil, which allows the development of an adequate rooting volume. Even though cassava is able to grow in varied soil types, the most preferred soil types that allow tuber development and ease of harvesting are sandy, sandy loam and clay loam soils (Asher et al., 1980; Rubatzky and Yamaguchi, 1997). These textural classes allow good drainage of water and soil aeration without restricting root growth. On the other hand, poorly aerated soil will increase susceptibility to root diseases and compaction. Compared to sandy textured

Ja nu Fe ar br y ua r M y ar ch A pr il M ay Ju ne Ju A ly Se u g pt ust em O b er ct N ob ov e e r D mb ec e em r be r

Rainfall Ja n Fe uar br y ua M ry ar ch Ap ril M ay Ju ne Ju A ly Se ug pt u s em t O ber N c to ov b e e r D mb ec e em r be r

Min Temp Ja nu Fe ary br ua r M y ar ch Ap ril M ay Ju ne Ju A ly Se u g p t us t em O b er ct N obe ov e r D mb ec e em r be r

Max Temp

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41

39

37

35

33

31

29

27

25

30

25

20

15

10

5

500

450

400

350

300

Kerala

Tamil Nadu

Andhra Pradesh

250

200

150

100

50

0

Fig. 4 Average variation in monthly minimum and maximum temperature (oC) and total rainfall (mm) in the three major cassava growing states in India.

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soils, sandy loams to clay loams would also provide better soil nutrient retention and reduced nutrient losses through leaching. The optimum pH for cassava is 5.5, but the crop is able to withstand soil acidity and high levels of Al (Chew et al., 1981; Islam et al., 1980). In acidic soils, aluminum (Al) and/or manganese (Mn) toxicity, and N, P, K, calcium (Ca) and magnesium (Mg) deficiency tend to become more prominent. The tolerance of cassava to soil acidity was reported by Chew (1971), who observed adequate growth in acidic peat soils of pH 3.7, when it was limed to pH 4.7 in Malaysia. The response of 138 cassava cultivars to soil acidity tolerance was studied in a highly acidic oxisol (pH 4.3) and concluded that acidity tolerance differed among cultivars (Spain et al., 1975). Edwards and Kang (1978) studied the performance of two cassava cultivars, cv. Ojukaniye and cv. Apuwuru in an acidic ultisol with a soil pH of 4 and observed that both these cultivars were adapted to high soil acidity. Cassava is renowned as the species that will still produce a harvestable yield (5–6 t/ha) in tropical soils of low fertility, where other crops will fail (Cock and Howeler, 1978). Edwards et al. (1977) concluded from solution culture experiments with cassava, maize, soybean and sunflower that cassava was able to tolerate low N, K and Ca in the root environment better than any of the other species tested. On the other hand, cassava has been reported not to grow favorably in alkaline soils, but was more susceptible to Zn deficiency (Howeler, 1985). In contrast to this, cassava is very successfully cultivated in alkaline soils (Vertisols) of pH 8.5 in Tamil Nadu state, India, where the farmers harvest an average yield of 35–40 t/ha (Byju et al., 2010). The optimal soil physical requirements for cassava cultivation are summarized in Table 2 (Byju, 2006). Extensive studies to develop the critical levels for satisfactory growth and yield of cassava were carried out in all major cassava growing areas of SubSaharan Africa, Asia and Latin America. The soil chemical characteristics in relation to cassava nutrient requirement are summarized in Table 3. There are various compounding factors that affect plant nutrient availability such as soil pH, soil moisture, soil texture and soil structure. However, soil tests that take into consideration compounding factors provide a closer correlation to plant responses such as yield or biomass. It is evident that soil chemical testing can only provide empirical approximations of plant root activity, and are often specific to the nutrient, crop, soil and region (Perverill et al., 1999). A lot of studies are available to find out the critical levels of nutrients in soils for cassava, which is the concentration of the nutrient in the soil above which there is no further significant response to application of the nutrient

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Table 2 Indicative optimal soil physical requirements for cassava. Parameter Unit Value

Rooting depth

cm

90

Soil slope

%

<3.0

Electrical conductivity

dS/m

0.7

Exchangeable sodium percent

%

<5

Texture

Loam, sandy loam

Infiltration

cm/h

1–2

Available water capacity

mm/m

120–150

Hydraulic conductivity

cm/h

3–5

Source: Based on Byju, G. 2006. Soil health in relation to tuber crops. In: Byju, G., (Ed.), Quality Planting Material Production in Tropical Tuber Crops. CTCRI, Thiruvananthapuram, India. pp. 138–151.

Table 3 Soil chemical characteristics in relation to cassava nutrient requirement. Soil parameter Unit Very low Low Medium High Very high

pH

<3.5

3.5–4.5

4.5–7.0

7.0–8.0

>8.0

Organic matter %

<1.0

1.0–2.0

2.0–4.0

>4.0

—

Salinity

mS/cm

—

<0.5

0.5–1.0

>1.0

—

Na saturation

%

—

<2.0

2–10

>10.0

—

P

mg/kg

<2.0

2.0–4.0

4–15

>15

—

K

cmolc/kg <0.1

0.1–0.15 0.15–0.25 >0.25

Ca

cmolc/kg <0.25

0.25–1.0 1–5

>5

—

Mg

cmolc/kg <0.2

0.2–0.4

0.4–1.0

>1.0

—

S

mg/kg

<20

20–40

40–70

>70

—

Fe

mg/kg

<1

1–10

10–100

>100

—

Mn

mg/kg

<5

5–10

10–100

100–250 >250

Zn

mg/kg

<0.5

0.5–1.0

1–5

5–50

>50

Cu

mg/kg

<0.1

0.1–0.3

0.3–1.0

1–5

>5

B

mg/kg

<0.2

0.2–0.5

0.5–1.0

1–2

>2.0

—

pH—H2O 1:5, OM—Walkley and Black, P—Bray 2, K/Ca/Mg—1N NH4 acetate, S—CaSO4, B—Hot water, Cu/Fe/Mn/Zn—0.05 N HCl + 0.025 N H2SO4 Source: Based on Howeler, R.H., 1996. Mineral nutrition of cassava. In: Craswell, E.T., Asher, C.J., O’Sullivan, J.N., (Eds.), Mineral Nutrient Disorders of Root Crops in the Pacific. Proc. Workshop, Nuku alofa, Kingdom of Tonga, 17–20 April 1995, ACIAR Proceedings No. 5, Canberra, Australia, pp. 110–116.

ARTICLE IN PRESS 17

Mineral nutrition of cassava

Table 4 Range of critical levels of organic matter and nutrients in soils for cassava. Range of Soil parameter Unit critical level Source

Organic matter

%

1.3–3.1

Gomes (1998) and Howeler (1998)

Phosphorus

mg/kg

3.1–8.23 (Bray I)

Howeler (2002) and Susan John et al. (2004)

Phosphorus

mg/kg

4.0–10.0 (Bray II)

Howeler (1985) and Hagens and Sittibusaya (1990)

Potassium

cmolc/kg

0.08–0.19

Hagens and Sittibusaya (1990) and Wargiono et al. (1998)

Calcium

cmolc/kg

0.25

CIAT (1979)

Magnesium

cmolc/kg

<0.20

Kang (1984)

(usually defined as 95% of maximum yield) (Howeler, 2002). According to him, critical levels are 3.1% for organic matter, 7 mg/kg for P (Bray II) and 0.14 cmolc/kg for exchangeable K. The range of critical levels of organic matter and nutrients for cassava reported in literature is summarized in Table 4. The range of critical levels of P given in Table 4 are much lower than the values reported for most other crops (10–18 mg/kg), which clearly shows that cassava will grow well and yield reasonably on soils that are low in P, where P deficiency symptoms would appear in most other crops (Howeler, 2002). The main reason for this is the AM fungal association with cassava roots (Potty, 1990). Similar observation can be seen in the case of exchangeable K, where the critical levels for cassava reported in literature ranged from 0.08 to 0.19 cmolc/kg, which was lower than the values reported for most other crops, which ranged from 0.16 to 0.50 cmolc/kg. This is a clear indication that cassava can grow well and yield reasonably on soils with medium levels of K, even though its K requirement is high (Howeler, 2002). In India, cassava is cultivated in a wide range of agroclimatic and soil environments. In Kerala state, where the crop was introduced into India more than 300 years ago, it is cultivated mostly in laterite soils (Ultisols) (Byju and Varghese, 1999a, 2001a,b). In Tamil Nadu state, where it is cultivated mainly for industrial uses, cassava is grown in black soils (Vertisols) and red soils (Alfisols). In Andhra Pradesh state, where it is also cultivated mainly for industrial uses, cassava is grown in sandy loam and coastal alluvial soils (Inceptisols). Tables 5–8 summarize the physico-chemical

Fe

cmolc/kg

kg/ha

Mn

Zn

Cu

Sand

mg/kg

Silt

Clay

%

Depth (cm)

pH

OC %

0–28

5.05

1.96

182.24

29.34

103.34

0.41

0.07

6.55

34.12

10.18

1.25

1.17

64.81

5.59

29.60

28–63

5.00

1.47

58.34

11.38

52.39

0.29

0.05

3.21

23.18

5.52

0.65

0.87

58.30

7.51

34.20

63–87

5.20

0.52

42.14

6.59

34.11

0.21

0.03

2.34

13.45

3.21

0.32

0.52

50.72

8.54

40.75

87–200

4.95

0.33

38.81

4.32

22.87

0.17

0.02

0.96

8.91

1.08

0.27

0.19

57.04

7.47

35.50

Source: Based on Byju, G., Nedunchezhiyan, M., Ramanandam, G., 2010. Soil fertility research for cassava in India. In: CIAT (Ed.), A New Future for Cassava in Asia: Its Use as Food, Feed and Fuel to Benefit the Poor. Proc. 8th Regional Workshop, Vientiane, LaoPDR, 20–24 October, 2008. CIAT, Cali, Colombia. pp. 275–297.

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Table 5 Properties of Ultisols where cassava is cultivated in Kerala, India. N P K Ca Mg S

Table 6 Properties of Vertisols and Alfisols where cassava is cultivated in Tamil Nadu, India. N P K Ca Mg S Depth (cm)

Fe

cmolc/kg

kg/ha

Mn

Zn

Cu

B

mg/kg

pH

OC %

0–20

7.8

0.72

109.20

30.66

500.0

9.88

2.76

17.0

1.40

3.9

4.61

3.17

0.83

20–40

8.1

0.34

87.36

10.51

198.0

10.00

2.75

5.0

0.64

1.4

0.73

2.93

0.61

40–60

8.3

0.34

80.08

10.51

176.0

9.63

2.74

5.0

0.57

1.0

0.82

2.19

0.52

60–80

8.3

0.36

50.96

7.01

144.0

9.75

2.66

5.5

0.52

1.2

0.67

1.34

0.41

80–100

8.5

0.39

43.68

10.51

76.0

7.63

2.20

3.1

1.47

0.3

0.52

0.77

0.20

100–120

8.5

0.28

43.68

7.01

84.0

7.63

2.34

1.8

0.89

0.8

0.80

0.81

0.13

0–20

7.02

0.36

133.74

34.64

184.93

1.02

0.95

12.2

22.65

2.8

0.95

1.03

0.30

20–40

7.13

0.15

75.28

21.62

82.15

0.73

0.63

6.3

12.15

0.9

0.32

0.69

0.15

40–60

7.15

0.15

68.71

14.15

65.68

0.51

0.51

2.5

10.63

0.5

0.30

0.51

0.13

60–80

7.16

0.11

47.13

9.68

35.12

0.32

0.42

1.6

8.15

0.4

0.25

0.42

0.08

80–100

7.11

0.09

34.15

8.53

28.83

0.25

0.33

0.9

5.33

0.3

0.21

0.38

0.05

100–120

7.13

0.06

30.68

5.15

21.61

0.22

0.28

0.6

3.15

0.3

0.18

0.35

0.04

Vertisols

Source: Based on Byju, G., Nedunchezhiyan, M., Ramanandam, G., 2010. Soil fertility research for cassava in India. In: CIAT (Ed.), A New Future for Cassava in Asia: Its Use as Food, Feed and Fuel to Benefit the Poor. Proc. 8th Regional Workshop, Vientiane, LaoPDR, 20–24 October, 2008. CIAT, Cali, Colombia. pp. 275–297.

ARTICLE IN PRESS

Alfisols

cmolc/kg

kg/ha

Zn

Cu

B

Sand

mg/kg

Silt

Clay

%

Depth (cm)

pH

OC %

0–20

6.8

0.18

98.64

46.43

104.87

0.23

0.21

6.25

2.75

1.23

1.06

2.38

0.28

94.2

3.9

1.9

20–50

6.9

0.09

41.25

10.34

30.45

0.17

0.14

4.12

1.31

1.03

0.59

1.04

0.15

95.3

2.2

2.5

50–100

7.0

0.05

37.53

6.75

16.75

0.11

0.10

2.18

1.15

0.82

0.21

0.58

0.07

94.6

2.7

2.7

100–150

7.0

0.03

35.64

4.83

10.49

0.06

0.05

1.27

1.06

0.51

0.15

0.30

0.04

95.2

2.6

2.2

Source: Based on Byju, G., Nedunchezhiyan, M., Ramanandam, G., 2010. Soil fertility research for cassava in India. In: CIAT (Ed.), A New Future for Cassava in Asia: Its Use as Food, Feed and Fuel to Benefit the Poor. Proc. 8th Regional Workshop, Vientiane, LaoPDR, 20–24 October, 2008. CIAT, Cali, Colombia. pp. 275–297.

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Table 7 Properties of Inceptisols where cassava is cultivated in Andhra Pradesh, India. N P K Ca Mg S Fe Mn

Table 8 Range and mean values of physico-chemical characteristics of soils where cassava is cultivated in India. Kerala Tamil Nadu Ultisols

Alfisols

Inceptisols

Mean

Range

Mean

Range

Mean

Range

Mean

pH

4.1–5.8

5.0

7.2–8.5

8.0

5.7–8.5

6.8

4.90–6.20

5.5

Organic carbon %

0.31–2.70

1.08

0.48–1.11

0.70

0.16–0.71

0.38

0.15–0.53

0.35

N

kg/ha

165.20–300.15 174.23 87.36–189.30 123.80 107.25–268.13 168.70 94.52–128.89 106.55

P

kg/ha

15.61–39.62

28.94

16.64–92.70

41.35

1.57–82.77

19.82

16.78–74.98

54.60

K

kg/ha

60.4–174.2

98.3

60.5–1050.0

439.3

78.4–705.0

298.1

82.6–96.7

86.2

Ca

cmolc/kg 0.22–0.63

0.39

3.13–13.43

8.44

2.63–11.50

5.70

0.15–0.48

0.31

Mg

cmolc/kg 0.02–0.08

0.06

1.38–3.50

2.42

0.14–1.13

0.78

0.02–0.05

0.03

S

mg/kg

3.47–9.55

6.51

–

–

–

–

–

–

Fe

mg/kg

5.98–82.90

19.43

1.82–33.40

9.84

0.60–3.83

1.80

0.69–2.57

1.89

Mn

mg/kg

2.48–84.76

19.90

1.09–20.43

6.85

0.50–5.60

1.50

0.41–1.75

0.93

Zn

mg/kg

0.26–1.96

0.78

0.37–2.71

0.83

0.50–4.60

1.10

0.88–2.43

1.65

Cu

mg/kg

0.24–10.20

1.24

0.95–4.53

2.68

1.69–4.66

2.80

0.59–1.29

0.86

B

mg/kg

0.02–0.40

0.27

0.61–0.95

0.82

0.16–0.29

0.22

0.26–0.35

0.31

CEC

cmolc/kg 1.39–7.70

3.04

–

–

–

–

–

–

Bulk density

Mg/m3

1.30–1.60

1.40

1.18–1.27

1.23

1.03–1.22

1.18

1.24–1.39

1.34

Sand

%

32.10–65.80

53.70

24.75–38.50

33.69

22.61–36.48

31.21

90.50–95.90

93.80

Silt

%

3.10–8.40

5.20

21.30–32.30

28.09

23.65–33.31

29.27

1.70–4.10

3.70

Clay

%

26.30–62.50

42.00

29.25–54.00

38.96

28.72–47.61

39.53

1.50–3.00

2.50

Source: Based on Byju, G., Nedunchezhiyan, M., Ramanandam, G., 2010. Soil fertility research for cassava in India. In: CIAT (Ed.), A New Future for Cassava in Asia: Its Use as Food, Feed and Fuel to Benefit the Poor. Proc. 8th Regional Workshop, Vientiane, LaoPDR, 20–24 October, 2008. CIAT, Cali, Colombia. pp. 275–297.

ARTICLE IN PRESS

Range

Parameter

Unit

Vertisols

Andhra Pradesh

ARTICLE IN PRESS 22

G. Byju and G. Suja

characteristics of the major soil orders where cassava is cultivated in India (Byju, 2000; Byju and Varghese, 1998, 1999b; Byju et al., 2010; Sheeja, 1994; Susan John and Venugopal, 2004). The impact of cassava cultivation on soil physico-chemical characteristics, laterization process and soil acidity parameters was studied by Byju and Varghese (2001a,b, 2002). The data presented in Tables 5–8 show three contrasting soil conditions where cassava is produced in India. In Kerala, India, where cassava is consumed as a food, it is grown mostly in highly leached, tropical, laterite soils, where the soil is acidic with an average pH of 5.0. About 50% of cassava cultivation of India is concentrated in Tamil Nadu, India, where the crop is raised in neutral to alkaline soils with mean pH value of 6.8 in red soils and 8.0 in black soils. In Andhra Pradesh, cassava production is concentrated in sandy and sandy loam soils with a mean soil pH value of 5.5. Byju et al. (2016) reported average cassava yields of 41.25, 36.32 and 20.00 t/ha, respectively, in these three peninsular Indian states of Tamil Nadu, Kerala and Andhra Pradesh.

4. Nutrient requirements Soil test values mentioned in the previous section will be of no use if it is not correlated with plant growth response and most commonly the relative yield is used for such correlation (Dahnke and Olson, 1990). There are two approaches to calibrate a soil test in the field. In the first approach, nutrient omission plots are used in combination with plots receiving the particular nutrient and plant yield is measured. In the second approach, the plant yield is measured against various concentrations of the nutrient studied. A least squares curve fitting is performed followed by the implementation of a mathematical model on the basis of goodness-of-fit (Perverill et al., 1999). The Mitscherlich model is the most common mathematical model used in the soil test calibration. The quadratic models are generally found suitable for estimating plant response to increasing rates of nutrient application, still the nutrient recommendations based on this model is reported to be excessive (Black, 1993). The critical concentration ranges for deficiency, sufficiency and toxicity for that particular crop and soil test are estimated by developing statistical relationships between field data and the model. This estimate from the curve is usually taken at the point of marginal stress, or 95% relative yield and the critical level is deduced as the concentration of

ARTICLE IN PRESS 23

Mineral nutrition of cassava

the nutrient in the soil or plant tissue above which, there is no significant response to nutrient additions (Asher et al., 2002; Howeler, 1996). Other methods to assess nutrient availability include plant digestion/ analysis and glasshouse bioassays. While soil tests or glasshouse bioassays are proactive measures in correcting nutrient deficiencies, plant testing and analysis can be considered as a reactive measure (Asher et al., 2002). In plant analysis, index tissues or representative plant parts (youngest fully expanded leaf (YFEL) blade for cassava) are analyzed and the concentrations of different essential elements can be quantified using colorimetric, flame photometric and atomic absorption spectrophotometric methods. Critical nutrient concentrations of different elements thus developed for cassava is given in Table 9. Howeler (2002) reported a comprehensive review of the work done to find out the critical nutrient concentrations (CNC) for deficiencies and toxicities in youngest fully expanded leaf (YFEL) blade which is identified as the “indicator” or “index” tissue. The YFEL will be normally the fourth or fifth Table 9 Nutrient concentrations in youngest fully expanded leaf (YFEL) blade of cassava at 3–4 months after planting. Very Sufficient High Toxic Nutrient Unit deficienta Deficient Low

N

%

<4.0

4.1–4.8

P

%

<0.25

0.25–0.36 0.36–0.38 0.38–0.50 >0.50

K

%

<0.85

0.85–1.26 1.26–1.42 1.42–1.88 1.88–2.44 >2.40

Ca

%

<0.25

0.25–0.41 0.41–0.50 0.50–0.72 0.72–0.88 >0.88

Mg

%

<0.15

0.15–0.22 0.22–0.24 0.24–0.29 >0.29

–

S

%

<0.20

0.20–0.27 0.27–0.30 0.30–0.36 >0.36

–

B

mg/kg <7

7–15

15–18

18–28

28–64

>64

Cu

mg/kg <1.5

1.5–4.8

4.8–6

6–10

10–15

>15

Fe

mg/kg <100

100–110

110–120

120–140

140–200

>200

Mn

mg/kg <30

30–40

40–50

50–150

150–250

>250

Zn

mg/kg <25

25–32

32–35

35–57

57–120

>120

4.8–5.1

5.1–5.8

>5.8

– –

a Very deficient, <40% maximum yield; Deficient, 40–80%; Low, 80–90%; Sufficient, 90–100%; High, 100–90%; Toxic, <90%. Source: Based on Howeler, R.H., 1996. Mineral nutrition of cassava. In: Craswell, E.T., Asher, C.J., O’Sullivan, J.N., (Eds.), Mineral Nutrient Disorders of Root Crops in the Pacific. Proc. Workshop, Nuku alofa, Kingdom of Tonga, 17–20 April 1995, ACIAR Proceedings No. 5, Canberra, Australia, pp. 110–116.

ARTICLE IN PRESS 24

G. Byju and G. Suja

Table 10 Range of critical nutrient concentrations (CNC) of deficiencies and toxicities for different elements developed for cassava. Element Unit Value

A. CNCs for deficiencies N

%

4.6–5.7

P

%

0.33–0.41

K

%

1.1–1.90

Ca

%

0.46–0.64

Mg

%

0.29–0.33

S

%

0.27–0.33

Zn

mg/kg

33–51

B

mg/kg

35

Cu

mg/kg

6

Mn

mg/kg

50

Zn

mg/kg

120

B

mg/kg

100

Mn

mg/kg

250

B. CNCs for toxicities

Based on data compiled from Howeler, R.H., 2002. Cassava mineral nutrition and fertilization, In: Hillocks, R.J., Thresh, J.M., Bellotti, A.C., (Eds.), Cassava: Biology, Production and Utilization, CABI, New York, USA, pp. 115–147.

leaf from the top and should be collected at 3–4 months after planting (MAP) cassava. The range of CNCs developed by different workers is summarized in Table 10. Several nutritional disorders of cassava diagnosed by nutrient deficiency symptoms, soil testing and plant analysis were reported in different growing areas of the world and is well documented (Alvarez et al., 2012; Asher et al., 1980; Howeler, 1981, 1996, 2012, 2014; Howeler and Aye, 2014; Howeler and Fernandez, 1985; Lozano et al., 1981; Susan John et al., 2006). The leaf, stem and tuber of cassava showed very wide variations in their biochemical, mineral and proximate composition and a large body of evidence exists with variations among different varieties and regions. We have compiled the different values to arrive at the ranges of their composition and provided in Table 11.

ARTICLE IN PRESS 25

Mineral nutrition of cassava

Table 11 Biochemical and mineral composition of different plant parts of cassava. Parameter Tuber Stem Leaf

Biochemical composition (fresh weight) Moisture %

60.0–66.2

–

74.8–81.0

Energy (kJ/100g)

528–611

–

209–251

Protein (%)

0.54–2.00

–

5.1–6.9

Starch (%)

28.0–33.2

–

–

Sugar (%)

0.34–1.14

–

–

Carbohydrate (%)

32.4–35.0

–

7

Dietary fiber (%)

1.43–1.57

–

0.5–10

Crude fiber (%)

0.9–2.0

–

2.1–5.1

Fat (%)

0.1–0.3

–

1.0–2.0

Ash (%)

0.5–1.7

–

2.7

N, %

0.4–0.8

1–1.6

3.5–5.0

P, ppm

1000–1400

1800–2300

2500–3800

K, ppm

7000–10000

8700–20000

12000–16000

Ca, ppm

1300–1600

14000–21000

6700–10800

Mg, ppm

600

3000–4700

2500–2800

S, ppm

500

1100–1600

2500–3000

Fe, ppm

127

74–130

200–450

Mn, ppm

10–15

100–140

130–210

Zn, ppm

15–16

36–46

45–66

Cu, ppm

3–3.9

8.9–10.8

8.7–10.6

B, ppm

4

13–14

26–37

Al, ppm

1.06–2.60

–

–

Na, ppm

6.2–8.1

–

–

Vitamin A, mg/100g

5–35

–

8.3–11.8

Thiamine, mg/100g

0.03–0.28

–

0.06–0.31

N & minerals (dry weight)

Vitamins (fresh weight)

Continued

ARTICLE IN PRESS 26

G. Byju and G. Suja

Table 11 Biochemical and mineral composition of different plant parts of cassava.—cont’d Parameter Tuber Stem Leaf

Riboflavin, mg/100g

0.03–0.06

–

0.21–0.74

Nicotinic acid, mg/100g

0.6

–

1.5

Ascorbic acid, mg/100g

15–50

–

60–370

Vitamin C, mg/100g

14.9–36.0

–

80.0–200.0

Source: Based on Bradbury, J.H., Holloway, W.D., 1988. Chemistry of Tropical Root Crops: Significance for Nutrition and Agriculture in the Pacific. ACIAR Monograph No. 6, ACIAR, Canberra, Australia; Howeler, R.H., 2002. Cassava mineral nutrition and fertilization, In: Hillocks, R.J., Thresh, J.M., Bellotti, A.C., (Eds.), Cassava: Biology, Production and Utilization, CABI, New York, USA, pp. 115–147; Charles, A.L., Sriroth, K., Huang, T., 2005. Proximate composition, mineral contents, hydrogen cyanide and phytic acid of 5 cassava genotypes. Food Chem. 92, 615–620; Ceballos, H., Sanchez, T., Chavez, A.L., Iglesias, C., Debouck, D., Mafla, G., Tohme, J., 2006. Variation in crude protein content in cassava (Manihot esculenta Crantz) roots. J. Food Compos. Anal. 19, 589–593; Montagnac, J.A., Davis, C.R., Tanumihardjo, S.A., 2009. Nutritional value of cassava for use as a staple food and recent advances for improvement. Compr. Rev. Food Sci. Food Saf. 8, 181–194; Geethu Mohan, 2018. Biochemical, Mineral and Proximate Composition of Cassava Varieties. M.Sc. Thesis, Kerala University, India.

Studies to estimate the nutrient uptake by cassava in certain growing areas of the world give contrasting results. The total nutrient uptake values reported by different workers are summarized in Table 12. The nutrient uptake data showed wide variation mainly due to differences in yield. The N uptake ranged from 93 kg/ha for an yield of 15 t/ha to 202 kg/ha for an yield of 45 t/ha. The P uptake ranged from 12 to 46 kg/ha and K uptake from 81 to 485 kg/ha. Nutrient management of cassava in India has been extensively reviewed by several workers (Byju and Suchitra, 2011; Byju et al., 2010; George et al., 2001; Mohankumar et al., 1998; Susan John et al., 2016). Total removal of N, P and K by cassava plant for producing one ton of tuber is provided in Table 13. When the data are averaged, it can be observed that the total N, P and K uptake requirements for producing one ton of fresh cassava tuber are 5.27, 0.84 and 5.60 kg per ton. The N: P:K uptake ratio in total plant dry matter is calculated as 6.2:1:6.6. The total N, P and K uptake requirements for producing one ton of fresh cassava tuber range from 2.9 to 6.9 for N, 0.68 to 1.3 for P and 3.9 to 7.9 for K. In the case of cassava, most of the absorbed K was exported by the tubers, while in cereals usually less than 20% of absorbed K was removed in the grain (Howeler, 1991). Howeler (1985) reported that about 34% of the total amount of absorbed N, 60% of P and 60% of K was found in the cassava

B

Source

–

124

46

485

–

–

–

–

–

–

–

–

Prevot and Ollagnier (1958)

45.0

202

32

286

–

–

–

–

–

–

–

–

Amarasiri and Perera (1975)

30.0

164

31

200

80

31

-

3.6

1.35

1.35

0.14

0.45

Asher et al. (1980)

30.0

180

22

160

66

27

1.3

2.8

0.46

0.81

0.14

0.07

CTCRI (1983)

25.0

172

26

97

–

–

–

–

–

–

–

–

Nayar et al. (1986)

50.0

188

34

233

–

–

–

–

–

–

–

–

Mohankumar and Nair (1996)

30.0

173

22

153

–

–

–

–

–

–

–

–

Byju et al. (2012)

29.0

180

23

156

82

26

–

–

–

–

–

–

Howeler (2014)

15.0

93

12

81

42

13

–

–

–

–

–

–

Howeler (2017a)

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Table 12 Yield and total nutrient uptake by cassava as reported by different workers. Nutrient uptake (kg/ha) Fresh tuber yield (t/ha) N P K Ca Mg S Fe Mn Zn Cu

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G. Byju and G. Suja

Table 13 Yield and nutrient removal by cassava plant (values are expressed on fresh weight basis). Nutrient removal (kg/ton tuber produced) N

P

K

Source

4.5

0.71

6.4

Amarasiri and Perera (1975)

5.5

1.03

6.7

Asher et al. (1980)

6.0

0.73

5.3

CTCRI (1983)

6.9

1.04

3.9

Nayar et al. (1986)

4.5

0.83

6.6

Howeler (1991)

3.8

0.68

4.7

Mohankumar and Nair (1996)

5.8

0.73

5.1

Byju et al. (2012)

6.2

0.79

5.4

Howeler (2014)

6.2

0.80

5.4

Howeler (2017a)

2.9–3.6

0.8–1.3

5.3–7.9

Polthanee and Wongpichet (2017)

tuber; thus most of the absorbed N was returned to the soil as fallen leaves or with the incorporation of plant tops after harvest, while most of the absorbed K is removed with harvested tuber. Howeler (2017a) reported that 75% of N, 92% of Ca and 76% of Mg were found in the plant tops (stem and standing leaves) and fallen leaves, and only 25%, 8% and 24%, respectively, in the tuber. In case of P, about equal parts were found in tuber and tops, while for K about 60% was found in the tuber and only 40% in tops and fallen leaves. Thus, if only tubers are removed, the ratio of N, P and K removed (expressed in terms of N, P2O5 and K2O) was 1.8:1.0:3.8 or about 2:1:4, while if all plant parts are removed, this will be 3.3:1.0:2.9 or about 3:1:3.

5. Methods of fertilizer recommendations for cassava The nutrient uptake requirements of cassava, especially for higher tuber yield, were substantial both in tuber and above ground biomass. The quantities of nutrients removed (especially N, P and Mg and except K) per unit of harvested produce were comparable or even smaller than the values reported for cereals and grain legumes (Howeler and Cadavid, 1983, 1990; Pellet and El-Sharkawy, 1997; Ravi and Mohankumar, 2004). When cassava

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29

is grown in hill production systems on steep, infertile areas prone to soil erosion, it resulted in significant loss of organic matter and nutrients in the absence of scientific agrotechnologies (Ghosh et al., 1989; Lal, 1997; Ramesh et al., 2018; Ruppenthal et al., 1997; Z€ obisch et al., 1995). Literature scan about the evolution of scientific research on arriving at fertilizer recommendations for cassava shows three distinct phases, (i) blanket fertilizer recommendations, (ii) targeted yield approach and (iii) site-specific nutrient management (SSNM) based on more knowledge-intensive computer simulation models.

5.1 Blanket fertilizer recommendations One of the plausible reasons for the large yield gap of cassava is nutrient limitations due to large spatial variability of soil fertility (Ezui et al., 2016). In general, fertilizer use in cassava in Sub-Saharan Africa is negligible, whereas in Asian countries it is somewhat marginal. Howeler (2002) reported that insufficient use of external nutrients leads to depletion of soil nutrients. Application of external nutrients was inevitable to replace the quantities of nutrients that were removed through harvested produce and crop residues (Ezui et al., 2016). The fertilizer recommendations for cassava production in most of the countries have been mostly blanket recommendations, which do not take into account the varietal and climatic yield potential as well as the spatial and temporal variabilities in soil and plant characteristics. In the absence of a better fertilizer recommendation, those blanket recommendations had definitely helped in improving cassava productivity from 5.67 to 9.24 t/ha in Africa, 8.08 to 21.34 t/ha in Asia and 11.90 to 13.37 t/ha in Americas (FAOSTAT, 2018). A synthesis of the work done worldwide on blanket fertilizer recommendations have been provided by various workers (Asher et al., 1980; Howeler, 1991, 2014, 2017a). Low soil fertility is one of the major reasons for low productivity of cassava in farmers’ fields (Carsky and Toukourou, 2005; Ezui et al., 2017; Fernandes et al., 2017; Kintche et al., 2017; Senkoro et al., 2018) and removal of soil related constraints could increase the yield of cassava by about 43%, 29% and 22% in Asia, Africa and Latin America, respectively (Henry and Gottret, 1996). The FAO conducted thousands of field experiments throughout the world between 1961 and 1977 to study the response of cassava to fertilizer applications and the results clearly showed that the crop is highly responsive to fertilization and fertilizer application is economical to farmers (FAO, 1980). Followed by these experiences and based on research work

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G. Byju and G. Suja

undertaken at international and national research institutes such as Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia, National Center for Research on Cassava and Tropical Fruit Culture (CNPMF), Brazil, ICAR-Central Tuber Crops Research Institute (CTCRI), India, Rayong Field Crop Research Center, Thailand, National Root Crops Research Institute, Nigeria and a number of institutes in other countries, blanket fertilizer recommendations for cassava have been developed by all major cassava growing countries and Table 14 gives some of the blanket recommendations followed in different countries. Cassava responds well to manures and fertilizers. Mohankumar et al. (1976) reported a fresh tuber yield of 27.1 t/ha, when cassava was grown with manures and fertilizers, compared to 15.8 t/ha in the control treatment without manures and fertilizers, in India. In Asia alone, at least 19 long-term NPK experiments have been conducted with a duration ranging from 4 to over 30 years of continuous cropping on the same plots with annual applications of the same treatments (Howeler, 2017b). The ICAR-Central Tuber Crops Research Institute (ICAR-CTCRI) in India has been conducting long-term experiments to study the influence of manures and fertilizers on soil fertility as well as tuber yield of cassava since 1977 (ICAR-CTCRI, 2018; Kabeerathumma and Mohankumar, 1990a,b; Table 14 Blanket NPK recommendations for cassava. Nutrient recommendation (kg/ha) Country

N

P2O5

India

100 100

100 Mandal et al. (1973) and Pillai et al. (1985)

India

100 50

100 Kabeerathumma and Ravindran (1996) (currently followed)

Thailand 100 50 50 25

100 Paisancharoen et al., (2010) (low fertility and high 50 fertility soils)

Indonesia 90

25

60

Wargiono et al. (1998)

Vietnam

80

40

80

Hy et al. (2010)

China

100 25

100 Jie et al. (2010)

Ghana

68

46

68

Boateng (2015)

Togo

76

30

30

Ezui et al. (2016)

K 2O

Source

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31

Kabeerathumma et al., 1990, 1993; Pillai et al., 1985; Susan John et al., 2005). The long-term experiments have been conducted in three different phases (1977–90, 1990–2003 and 2004-continuing) by modifying the treatments based on the results of the previous phase. The results of the first phase of the experiment, that spans for a period of 13 years, from 1977 to 1990, showed that continuous application of FYM at 12.5 t/ha and N, P2O5 and K2O @ 100:100:100 kg/ha respectively, produced a mean tuber yield of 29.0 t/ha for the cassava variety Sree Visakham (H-1687), whereas in the control treatment, without manures and fertilizers, the mean tuber yield was only 1.03 t/ha. A second phase of the long-term experiment was conducted during 1990–2003 with another set of treatments, which confirmed the significance of balanced fertilization in improving the organic carbon, available nutrient contents of soil and starch content of tubers. The starch content of cassava tuber was significantly enhanced under balanced NPK and Zn application. Continuous cropping of cassava with only chemical fertilizers decreased the levels of Ca, Mg, Zn and Cu in the soil, and lowered the soil pH. The results clearly indicate the need for organic manure application along with addition of chemical fertilizers. The long-term experiments also showed that the rate of application of farmyard manure (FYM) can be reduced to 50% (6.25 t/ha) by incorporating the residues of the previous cassava crop and by practicing green manuring in situ with cowpea as suggested earlier by different workers (Mohankumar and Nair, 1990; Nayar et al., 1993; Prabhakar and Nair, 1987; Sasidhar and Sadanandan, 1976). The third phase of the long-term experiment, which started during 2004 indicate that soil test-based application of FYM and NPK produced significantly higher tuber yield of 22.81 t/ha (ICAR-CTCRI, 2018). The blanket fertilizer recommendation for cassava in India is 100:50:100 kg/ha of N, P2O5 and K2O without soil testing and these recommendations are adjusted to 75%, 100%, or 125% of blanket recommendations depending on soil test results. In Kerala, India, the criteria proposed by Aiyer and Nair (1985) is used for the adjustment of blanket recommendation based on soil test data. Differences in soils, plant uptake, yield and site-specific data are not considered in their approach. Extensive field experimentation over 14 years at ICAR-Central Tuber Crops Research Institute, Thiruvananthapuram, Kerala, India, indicated that the short-duration cassava varieties (6–7 months), Sree Vijaya, Sree Jaya, Vellayani Hraswa and the early-maturing triploid, Sree Athulya, hold promise for better utilization of resources, diversification of food basket and income (Suja et al., 2010a) and can be taken as a component crop with rice,

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G. Byju and G. Suja

banana or vegetables. Nutrient management based on soil test data that resulted in saving of the entire quantity of P, 10% N and 15% K (FYM @ 12.5 t/ha and NPK @ 90:0:85 kg/ha) produced higher tuber yield (24.18 t/ha), dry matter and starch content of tubers and positive soil nutrient balance (Suja et al., 2010b, 2011, 2015). Further, sequential cropping of vegetable cowpea with short-duration cassava was feasible as it was profitable (₹ 97,398/ha) and saved the entire P for cassava (Suja and Sreekumar, 2015; Suja et al., 2015). In the subsequent experiments it was found that when pulses were a component of the system viz., rice-pulse-short duration cassava and rice-short-duration cassava + pulses, there was greater nutrient saving to the extent of 50% FYM and N and 100% P based on soil test-based fertilizer recommendation, besides being productive, profitable and energy efficient (Suja et al., 2018).

5.2 Targeted yield approach Ramamoorthy et al. (1967) established the theoretical basis and experimental proof for the fact that Liebig’s Law of Minimum operates equally well for N, P and K. This forms the basis for fertilizer application for targeted yields, first advocated by Truog (1960) who showed that a “prescription method” of fertilizer use helps attaining high yields of maize using empirical values of nutrient availability from soil and fertilizer. This method not only indicates soil test crop response (STCR) correlation-based fertilizer doze but also the level of yield the farmer can hope to achieve if good agronomic practices are followed in raising the crop (Subba Rao and Srivasthava, 2001). The essential basic data required for formulating fertilizer recommendations for targeted yield are (i) nutrient requirement (NR) in kg/ton, (ii) the percent contribution from the soil available nutrients (CS) and (iii) the percent contribution from the applied fertilizer nutrients (CF). This methodology was extended to different crops in different soils (Ramamoorthy et al., 1975; Randhawa and Velayutham, 1982). These were transformed into workable fertilizer adjustment equations for prescribing fertilizer dose for any yield target (T) based on soil test data as given below. Without FYM CS Fertilizer dose (kg/ha) ¼ NR CF  100  T  CF  STV . With FYM CS COM Fertilizer dose (kg/ha) ¼ NR CF  T  CF  STV  CF  OM. The STV and OM are the soil test values and nutrient supplied through organic manure, respectively, in kg/ha. Recommended rates of fertilizers for

ARTICLE IN PRESS 33

Mineral nutrition of cassava

Table 15 Derived STCR parameters for cassava in laterite soils of Kerala, India. Nutrient NR (kg/ha) CS (%) CF (%) COM (%)

N

6.58

40.17

54.38

78.24

P2 O 5

2.37

41.33

47.00

57.33

K2O

6.28

48.60

52.65

69.66

targeted yields of cassava in laterite soils of Kerala state, India were developed by Swadija and Sreedharan (1998) and for soils of Tamil Nadu, India by Selvakumari et al. (2001). The values developed by Swadija and Sreedharan (1998) for cassava in laterite soils of Kerala, India are given in Table 15. The fertilizer adjustment equations developed for cassava according to the above values are Without FYM FN ¼ 12.10 T  0.74 N FP2O5 ¼ 5.04 T  2.02 SP FK2O ¼ 11.93 T  1.10 SK Where FN, FP2O5 and FK2O are N, P2O5 and K2O in kg/ha, T is the target of tuber yield in t/ha and SN, SP and SK are soil available N, P and K in kg/ha, respectively. With FYM FN ¼ 12.10 T  0.74 N  1.44 ON FP2O5 ¼ 5.04 T  2.02 SP  2.79 OP FK2O ¼ 11.93 T  1.10 SK  1.58 OK Where ON, OP and OK are quantities of N, P and K supplied through organic manures in kg/ha. In a laterite-red loam soil association of Kerala, India, application of 100:300:300 kg/ha of N, P2O5 and K2O along with FYM @ 12.5 t/ha calculated following a systematic approach gave a tuber yield of 43 t/ha (Susan John et al., 2007). The optimum quantity of N, P and K calculated based on quadratic polynomial response function was 111–115 kg/ha and 95–116 kg/ha N, 228–408 kg/ha and 209–345 kg/ha P and 317–445 kg/ha and 289–392 kg/ha K without and with FYM, respectively. For red soils of Salem district, Tamil Nadu, India, Selvakumari et al. (2001) developed the following fertilizer adjustment equations. FN ¼ 5.60 T  0.61 SN  0.81 ON FP2O5 ¼ 3.53 T  1.80 SP  0.53 OP FK2O ¼ 9.42 T  0.67 SK  0.70 OK

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G. Byju and G. Suja

5.3 Leaf color chart and SPAD-502 meter based real time N management Haripriya Anand and Byju (2008) observed that leaf color chart (LCC) developed by IRRI, The Philippines and chlorophyll meter (SPAD-502 of Minolta Color Company, Japan) could act as proxies for chlorophyll content and leaf color of two cassava cultivars in India, Sree Vijaya and M-4. Leaf position, growth stage, variety and N fertilizer rate influenced LCC score and SPAD-502 values. The following equations were developed to describe the relationships between chlorophyll content and LCC score as well as between SPAD-502 value and LCC score of youngest fully expanded leaf (YFEL) blade of cassava which was found to be the best leaf tissue to describe the above relationships. Chlorophyll (a + b) ¼ 0.507 LCC score + 0.948 (r2 ¼ 0.85) SPAD value ¼ 10.981 LCC score  3.510 (r2 ¼ 0.85) Haripriya Anand and Byju (2008) also observed that a single relationship should not be used for the relation of LCC score and SPAD-502 value between cultivars as well as among different growth stages of cassava. Byju and Haripriya Anand (2009) further explored the relationship of LCC score and SPAD-502 values with leaf N content for real time N management (RTNM) of cassava. Results of their studies indicated the existence of strong relationships of LCC score and SPAD-502 values with leaf position as well as leaf N content of two cassava cultivars, Sree Vijaya and M-4. The YFEL blade exhibited significant, positive correlation of tuber yield with LCC score and SPAD-502 values and leaf N concentration. The following regression equations were developed to express the relationships between LCC score and leaf N concentration as well as between SPAD-502 value and LCC score. LCC score ¼ 0.358 (Leaf N content) + 0.78 (r2 ¼ 0.81) SPAD-502 value ¼ 10.981 (LCC score)  3.51 (r2 ¼ 0.82) The results of the study also indicated that a single regression equation cannot be used to describe the relationship of SPAD-502 value and leaf N concentration. The above study also developed the threshold LCC score and SPAD-502 value as 2.65 and 25, respectively, and can be used to decide the optimum time for N top dressing for cassava. Byju and Haripriya Anand (2011) validated the above results by conducting an on-station experiment at ICAR-CTCRI, Thiruvananthapuram, India. The LCC score and SPAD502 value guided N management resulted in significantly higher tuber yield and nutrient use efficiency as measured by agronomic, physiological and

ARTICLE IN PRESS Mineral nutrition of cassava

35

recovery efficiencies. Crop need based N management approaches using simple decision tools such as LCC and SPAD-502 could significantly increase crop yield, reduce loss of N to environment and reduce susceptibility of the crop to pests and diseases (Fairhurst et al., 2007).

5.4 Site-specific nutrient management The blanket fertilizer recommendations or their soil test-based adjustments described in the previous sections do not take into account the climatic yield potential of different regions, cultivar characteristics, yield targets and nutrient interactions. The fertilizer adjustment equations based on targeted yield approach (STCR equations) cannot be extrapolated to another soil condition. Moreover, the nutrient interactions according to Liebig’s Law of Minimum are not considered in both the above methodologies. Since cassava is cultivated in very diverse soil types in all major growing countries across the globe, the blanket or soil test-based adjusted blanket recommendations would lead to unbalanced crop nutrition and thereby increased nutrient losses (Adjei-Nsiah et al., 2007; Cassman et al., 2002; Kintche et al., 2017; Senkoro et al., 2018; Suchitra, 2015) which can adversely affect the productivity and profitability of the farm. In order to reduce the yield gap, the only alternative is to develop more knowledge-intensive and computer simulation model-based site-specific nutrient management (SSNM) technologies that take into account both the spatial and temporal variabilities in soil and plant properties. Another important milestone in fertilizer recommendation to cassava is the calibration, validation and modification of simulation models such as decision support system for agrotechnology transfer (DSSAT) and simulation model of cassava (SIMCAS) (Kaweewong et al., 2013; Matthews and Hunt, 1994; Mithra et al., 2013, 2018; Singh et al., 1998) and quantitative evaluation of fertility of tropical soils (QUEFTS) model (Byju et al., 2012, 2016; Ezui et al., 2016, 2017; Janssen et al., 1990; Sattari et al., 2014). Kaweewong et al. (2013) parameterized the DSSAT-CSM-CSCRPCassava model version 4.5 for cassava variety Kasetsart 50 in Thailand and the model could satisfactorily predict tuber yield in the range of 0–250 kg/ha N. This is a dynamic, process oriented model that simulates the growth and development of cassava restricted to potential, water and N-limited yields. Mithra et al. (2013) developed another process model, “simulation model of cassava” (SIMCAS) in which the crop phenology is simulated as a function of growing degree days (GDD). This model can

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G. Byju and G. Suja

simulate the growth and development of cassava restricted to potential, water, N and K-limited yields. Therefore, both the above models are not suited to those cassava producing soils which are highly impoverished of all the three major nutrients, N, P and K. The QUEFTS model addresses the above issues since it simulates N, P and K requirements of crops. It is a relatively simple and static model that calculates yields of crops based on the interactions of N, P and K, the physiological efficiency, the potential yield and yield potential of a particular location as boundary conditions ( Janssen et al., 1990). The model QUEFTS was originally developed to assess soil fertility and nutrient requirements of maize. Later on it was adapted for crops such as rice (Sattari et al., 2014; Witt et al., 1999), wheat (Pathak et al., 2003), grain legumes (Franke et al., 2014), sweet potato (Prince Kumar et al., 2016), potato (Prince Kumar et al., 2018) and taro ( Jinimol and Byju, 2019). Byju et al. (2012) developed site-specific nutrient management (SSNM) for cassava by calibrating the QUEFTS model. The calibrated QUEFTS model not only increases the yield of cassava but also maximizes the nutrient use efficiency of N, P and K and prevent the excessive application of some nutrients that may cause pollution of the environment. The SSNM technology was developed based on the observation that large spatial and temporal variability of soil fertility existed in the cassava growing areas and that the use of blanket fertilizer recommendations often resulted in excessive application of some nutrients and inadequate application of others, leading to low yields and high fertilizer costs. Thus, different soils may require a different balance of N, P and K as well as a different amount of each nutrient. Byju et al. (2012) used data from many fertilizer experiments that had been conducted between 1975 and 2002 in different cassava-growing areas of India, to validate the parameters in the QUEFTS model for cassava and adjust them to the specific relationship between yield and nutrient uptake in different environments. Minimum and maximum internal efficiencies of N, P, and K were estimated as 35 and 80 for N, 250 and 750 for P, and 32 and 102 for K (kg dry tuber yield per kg nutrient removed). The results of the study showed that the mean total nutrient removal by cassava per ton of tuber (dry weight basis) was 13.4 kg N, 2.3 kg P and 14.5 kg K. With the use of the calibrated QUEFTS model, the optimum rate of N, P and K application to reach a certain yield goal can be calculated. The NPK uptake at different yield potentials showed that the relation between yield and nutrient (N, P and K) uptake is linear at lower yield targets, indicating

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37

that plant growth was mainly limited by NPK uptake. At greater yield targets that are closer to the yield potential, there was a great reduction in the internal nutrient recovery efficiency. Thus, it is clear that maximizing the nutrient recovery efficiency by balanced NPK application will give more profit to farmers than aiming for greater yields. The calibrated QUEFTS model for cassava showed a linear increase in tuber yield with N, P and K uptakes of 17.6, 2.2 and 15.6 kg of applied N, P and K, respectively, per ton of dry tuber yield. The ratio of N, P and K in total plant dry matter was found to be 7.6:1.0:6.9. Byju et al. (2016) validated the model by conducting on-farm experiments at 30 locations of three cassava growing states in southern India, Kerala, Tamil Nadu and Andhra Pradesh, where the average cassava fresh root yields were 36.3, 41.2 and 20.0 t/ha, respectively. The fresh tuber yield under SSNM, averaged over all 30 sites and 2 years was 37.6 t/ha, while that under farmer fertilizer practice (FFP) was 30.6 t/ha. The SSNM treatment reduced the cost of fertilizers on average by $10/ha as compared to the FFP treatment, while the SSNM treatment increased the gross return above fertilizer cost by $254/ha. The results of the study showed the superiority of SSNM over FFP in increasing tuber yield and nutrient use efficiency, but it is not practical to make site-specific recommendations for each cassava field, as soil and plant tissue testing facilities are not always available and the service is too expensive for farmers. The short-term aim is to produce zonal NPK recommendation maps of the main cassava-growing areas showing zones with the same SSNM fertilizer recommendations based mainly on existing soil test data, yield data and general crop management practices (rainfed or irrigated), and for a certain target yield (Byju et al., 2016). Zone-specific SSNM-based fertilizer recommendations are now available for each zone and example of a map developed for Kerala state is shown in Fig. 5. An alternative approach is to produce SSNM fertilizer recommendation chart that shows recommendations for the application rates of N, P and K for cassava based on soil test results, for particular cassava-growing areas and different management practices and yield targets, as shown in Tables 16–21 (Byju et al., 2015). Byju et al. (2016) also developed custom-made nutrient formulations, as the basis of specific SSNM recommendations which is available for each zone. Besides the recommended N, P and K balance in these fertilizers, they will also be fortified with required secondary and micronutrients according

75°E 13°N

76°E

77°E

75°E 13°N 13°N

Zone NPK recommendation map for cassava (Kerala state, India, Ytarget = 30Mg ha–1)

76°E

77°E 13°N

Zone NPK recommendation map for cassava (Kerala state, India, Ytarget = 40Mg ha–1)

N

N

12°N 12°N

12°N

11°N

11°N 11°N

11°N

Zones

NPK level MN MP MK 10°N

MN HP HK HN M&HP MK HN HP HK LN MP HK

MN MP MK

Zone 2 10°N 10°N

Zone 3 Zone 4 Zone 5

LN MP HK

Zone 1 Zone 2 Zone 3

10°N

Zone 4 Zone 5

SSNM Recommendations Ytarget - 40t/ha

100:20:100 100:20:50 50:20:100 50:20:50 200:20:50

75°E

MN HP HK HN M&HP MK HN HP HK

SSNM Recommendations Ytarget - 30t/ha 9°N

Zones

NPK level

Zone 1

9°N

76°E

77°E

9°N

160:30:160 160:30:80 80:30:160 80:30:80 240:30:80

75°E

9°N

76°E

77°E

Fig. 5 Zone NPK recommendation map for cassava in Kerala state, India for yield target of 30 t/ha. Source: Reprinted with permission from Byju G., Nedunchezhiyan, M., Hridya, A.C., Sabitha Soman, 2016. Site-specific nutrient management for cassava in southern India. Agron. J. 108, 830–840.

ARTICLE IN PRESS

12°N

20

30

O.C (%)

N rate (kg/ha)

Below 0.4

100

200

0.4–0.8

50

0.8–1.2 Above 1.2

40

20

30

Yield target (t/ha)

40

Available P (kg/ha)

P2O5 rate (kg/ha)

–

Below 10

20

30

100

200

10–20

10

25

50

100

Above 20

10

15

30

50

20

30

40

Exchangeable K (kg/ha)

K2O rate (kg/ha)

–

Below 100

100

200

20

30

100–200

50

100

200

20

30

200–300

25

50

100

Above 300

15

30

50

Source: Based on Byju G., Nedunchezhiyan, M., Haripriya Anand, M., Suchitra, C.S., Hridya, A.C., Sabitha Soman, 2015. Handbook of Site Specific Nutrient Management of Cassava (CASSNUM). Technical Bulletin Series 62, ICAR-CTCRI, Thiruvananthapuram, Kerala, India.

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Table 16 Fertilizer calculation chart for cassava in Kerala State, India (Rainfed). Yield target (t/ha) Yield target (t/ha)

Table 17 Fertilizer calculation chart for cassava in Kerala state, India (Irrigated). Yield target (t/ha) Yield target (t/ha) 30

40

O.C (%)

N rate (kg/ha)

Below 0.4

80

160

240

0.4–0.8

40

80

0.8–1.2

20

Above 1.2

10

50

20

30

40

Available P (kg/ha)

P2O5 rate (kg/ha)

–

Below 10

15

30

45

160

240

10–20

10

15

40

80

160

Above 20

10

20

20

40

80

50

20

30

40

50

Exchangeable K (kg/ha)

K2O rate (kg/ha)

–

Below 100

80

160

240

–

30

45

100–200

40

80

160

240

25

30

200–300

20

40

80

160

Above 300

10

20

40

80

Source: Based on Byju G., Nedunchezhiyan, M., Haripriya Anand, M., Suchitra, C.S., Hridya, A.C., Sabitha Soman, 2015. Handbook of Site Specific Nutrient Management of Cassava (CASSNUM). Technical Bulletin Series 62, ICAR-CTCRI, Thiruvananthapuram, Kerala, India.

ARTICLE IN PRESS

20

Yield target (t/ha)

20

30

40

O.C (%)

N rate (kg/ha)

Below 0.3

100

200

300

0.3–0.6

50

100

0.6–1.0

25

Above 1.0

15

50

20

30

40

Available P (kg/ha)

P2O5 rate (kg/ha)

–

Below 15

25

50

75

200

300

15–22

15

25

50

100

200

Above 22

15

20

30

50

100

Yield target (t/ha)

50

20

30

40

50

Exchangeable K (kg/ha)

K2O rate (kg/ha)

–

Below 200

100

200

300

–

50

75

200–400

50

100

200

300

25

50

400–600

25

50

100

200

Above 600

15

25

50

100

Source: Based on Byju G., Nedunchezhiyan, M., Haripriya Anand, M., Suchitra, C.S., Hridya, A.C., Sabitha Soman, 2015. Handbook of Site Specific Nutrient Management of Cassava (CASSNUM). Technical Bulletin Series 62, ICAR-CTCRI, Thiruvananthapuram, Kerala, India.

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Table 18 Fertilizer calculation chart for cassava in Tamil Nadu state, India (Rainfed). Yield target (t/ha) Yield target (t/ha)

Table 19 Fertilizer calculation chart for cassava in Tamil Nadu state, India (Irrigated). Yield target (t/ha) Yield target (t/ha) 40

50

O.C (%)

N rate (kg/ha)

Below 0.3

160

240

–

0.3–0.6

80

160

0.6–1.0

40

Above 1.0

20

60

30

40

50

Available P (kg/ha)

P2O5 rate (kg/ha)

–

Below 15

40

60

80

240

–

15–22

20

40

80

160

240

Above 22

15

20

40

80

160

60

30

40

50

60

Exchangeable K (kg/ha)

K2O rate (kg/ha)

–

Below 200

160

240

–

–

60

80

200–400

80

160

240

–

40

60

400–600

40

80

160

240

Above 600

20

40

80

160

Source: Based on Byju G., Nedunchezhiyan, M., Haripriya Anand, M., Suchitra, C.S., Hridya, A.C., Sabitha Soman, 2015. Handbook of Site Specific Nutrient Management of Cassava (CASSNUM). Technical Bulletin Series 62, ICAR-CTCRI, Thiruvananthapuram, Kerala, India.

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30

Yield target (t/ha)

20

30

40

O.C (%)

N rate (kg/ha)

Below 0.2

100

200

–

0.2–0.7

50

100

0.7–1.2

25

Above 1.2

15

50

20

30

40

Available P (kg/ha)

P2O5 rate (kg/ha)

–

Below 15

25

50

–

200

–

15–30

15

25

50

100

200

Above 30

15

20

25

50

100

50

Yield target (t/ha) 20

30

40

50

Exchangeable K (kg/ha)

K2O rate (kg/ha)

–

Below 100

100

200

–

–

50

–

100–175

50

100

200

–

25

50

175–250

25

50

100

200

Above 250

15

25

50

100

Source: Based on Byju G., Nedunchezhiyan, M., Haripriya Anand, M., Suchitra, C.S., Hridya, A.C., Sabitha Soman, 2015. Handbook of Site Specific Nutrient Management of Cassava (CASSNUM). Technical Bulletin Series 62, ICAR-CTCRI, Thiruvananthapuram, Kerala, India.

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Table 20 Fertilizer calculation chart for cassava in Andhra Pradesh state, India (Rainfed). Yield target (t/ha) Yield target (t/ha)

Table 21 Fertilizer calculation chart for cassava Maharashtra state, India (Irrigated). Yield target (t/ha) Yield target (t/ha) 40

50

O.C (%)

N rate (kg/ha)

Below 0.3

160

240

–

0.3–0.6

80

160

0.6–1.0

40

Above 1.0

20

60

30

40

50

Available P (kg/ha)

P2O5 rate (kg/ha)

–

Below 15

40

60

80

240

–

15–22

20

40

80

160

240

Above 22

15

20

40

80

160

60

30

40

50

60

Exchangeable K (kg/ha)

K2O rate (kg/ha)

–

Below 200

160

240

–

–

60

80

200–400

80

160

240

–

40

60

400–600

40

80

160

240

Above 600

20

40

80

160

Source: Based on Byju G., Nedunchezhiyan, M., Haripriya Anand, M., Suchitra, C.S., Hridya, A.C., Sabitha Soman, 2015. Handbook of Site Specific Nutrient Management of Cassava (CASSNUM). Technical Bulletin Series 62, ICAR-CTCRI, Thiruvananthapuram, Kerala, India.

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30

Yield target (t/ha)

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45

to the soil fertility conditions in each zone. Fig. 6 gives the compositions of such formulations developed for agro-ecological units (AEUs) of Kerala state for yield target of 30 t/ha. These customized formulations were tested across the Kerala state in India during 2016–17 in 35 on-farm locations spread across five agro-ecological unit (AEU) zonations of Kerala (Malappuram, Palakkad, Idukki, Alappuzha and Pathanamthitta). The fresh tuber yield under customized formulation treatment, averaged over all 35 sites was 46.7 t/ha, while that under farmer fertilizer practice (FFP) was 38.3 t/ha. On average, the customized formulation treatment resulted in about 22% yield increase over farmer fertilizer practice (FFP) across the agro-ecological unit zonations studied (ICAR-CTCRI, 2018). In order to popularize the SSNM technology for cassava among farmers, the ICAR-Central Tuber Crops Research Institute (CTCRI), India has developed certain decision support tools. The CASSNUM (CAssava SiteSpecific NUtrient Management) version 1.0 is a nutrient decision support system (NuDSS) available in the website of ICAR-CTCRI which will help the farmers and extension personnel to calculate SSNM recommendations besides providing solutions to nutrient related issues of cassava cultivation (www.ctcri.in). A newer version of the NuDSS, CASSNUM version 1.1 is available in a CD and is distributed among farmers and extension personnel in India (Fig. 7). Considering the rapid spread of internet and mobile technology among Indian farmers, the ICAR-CTCRI recently launched a mobile app, Sree Poshini which is available for free download at Google Playstore (Fig. 8). Sree Poshini is a very simple mobile app which helps the tuber crops farmers to calculate the fertilizer requirements of cassava and other tuber crops based on SSNM technology. Ezui et al. (2016) validated the QUEFTS model for cassava in West Africa by calculating the N, P and K application rates for a balanced nutrition. They have used the concept of crop nutrient equivalent (CNE) under the assumption that balanced nutrition is achieved when the CNE become equal to each other. Results of their study showed nutrient use efficiencies of 20.5 and 31.7 kg dry tuber yield per kg CNE at balanced nutrition at harvest index values of 0.50 and 0.65, respectively. The N, P and K uptake requirements per ton of dry tuber yield were 16.2, 2.7 and 11.5 kg at HI of 0.50 and 10.5, 1.9 and 8.4 at HI of 0.65, respectively. The corresponding values for NPK uptake ratios are 6.0:1.0:4.2 and 5.3:1:4.2. The benefit:cost (B:C) ratios obtained for balanced fertilizer rate based on calibrated QUEFTS model was 3.8  1.1, whereas the corresponding value for blanket fertilizer rate was only 2.4  0.9. Ezui et al. (2017) further evaluated the calibrated

75°E 13°N

76°E

75°E

77°E 13°N

Customized fertilizer blends for cassava (Kerala state, India, Ytarget = 30 Mg ha–1)

76°E

13°N

77°E

N

N

12°N

12°N

11°N

11°N

11°N

12°N

11°N

Nutrient formula N:P2O5:K2O:Mg:Zn:B

Nutrient formula N:P2O5:K2O:Mg:Zn:B

18:3:18:2:0.3:0.1 21:3.5:10:3:0.3:0.1

16:3:16:3:0.4:0.2 19:3:9:4:0.5:0.2 10°N

10:3:20:4:0.5:0.2 12:5:12:4:0.6:0.2

10°N

11:4:22:3:0.3:0.1 13:5:13:3:0.4:0.2

10°N

25:3:9:2:0.3:0.1

26:3:7:2.5:0.3:0.1 No. of bags (50 kg) No. of bags (50 kg)

12 10 10 8 15

9°N

75°E

9°N

76°E

77°E

9°N

18 15 14 12 19

75°E

9°N

76°E

77°E

Fig. 6 Composition of secondary-and micronutrient-inclusive customized nutrient formulations developed for cassava in Kerala state, India. Source: Reprinted with permission from Byju G., Nedunchezhiyan, M., Hridya, A.C., Sabitha Soman, 2016. Site-specific nutrient management for cassava in southern India. Agron. J. 108, 830–840.

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12°N

10°N

13°N

Customized fertilizer blends for cassava (Kerala state, India, Ytarget = 40 Mg ha–1)

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Fig. 7 CASSNUM version 1.1, a NuDSS developed for SSNM of cassava in India.

Fig. 8 Sree Poshini, a mobile app developed for SSNM of tuber crops in India.

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G. Byju and G. Suja

QUEFTS model to assess yield response of cassava to soil and fertilizer nutrients in West Africa. The indigenous nutrient supply of N, P and K ranged from 86 to 177, 18 to 24 and 70 to 104 kg/ha, respectively. Minimum and maximum internal efficiencies at HI of 0.50 of N, P, and K were estimated as 41 and 96 for N, 232 and 589 for P, and 34 and 160 for K (kg dry tuber yield/kg nutrient removed). As developed by Byju et al. (2016) for India, fertilizer recommendations based on soil types or agro-ecological zones were proposed by Ezui et al. (2016) as more practical than recommendations for individual farms.

5.5 Method and timing of fertilizer application Normally, chemical fertilizers to supply N, P and K are applied as basal and top dressing. In India, it is generally recommended to apply full amount of P and half quantities of N and K as basal at the time of final land preparation. The remaining quantities of N and K are recommended to be applied 45–60 days after planting cassava (Nair et al., 2004). The beneficial influence of application of the secondary nutrients, namely, calcium, magnesium and sulfur has been studied by many researchers in India (Korah et al., 1989; Mohankumar and Nair, 1983, 1985; Pushpadas and Aiyer, 1976). Liming increased the tuber yield and starch content and decreased the HCN content (Nair and Varghese, 1970). Another study by Mohankumar and Nair (1985) with different rates of lime, from 0 to 2000 kg/ha CaO in addition to recommended dose of N, P2O5 and K2O (100:100:100 kg/ha) enhanced tuber yield by 35.6% by the application of 2000 kg/ha CaO compared to control. It improved the quality of the tuber also by increasing the starch content and reducing the HCN content. Magnesium deficiency was observed in cassava cultivated in Oxisols, Ultisols, Inceptisols and Entisols (Howeler, 2014). CIAT (1985) recommended to apply 60 kg/ha Mg as sulphomag (at intermediate rates) or band application of MgSO47H2O and broadcast application of MgO (at higher rates) while Susan John et al. (2005) recommended to apply 20 kg/ha of MgSO4 (about 2–4 kg/ha Mg). Application of S @ 50 kg/ha resulted in a significantly higher tuber yield and starch content and a lower HCN content. Sulfur addition also increased the total protein and methionine contents (Mohankumar and Nair, 1985). Cassava responds significantly to application of micronutrients. Micronutrient deficiencies of Fe, Mn, Zn, Cu and B are commonly reported in calcareous soils with alkaline pH, but Zn and B deficiencies are observed

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in acid soils also (Howeler, 2014). He recommended incorporation of ZnO or band application of ZnSO47H2O at the rate of 10–20 kg/ha of Zn. Foliar applications of 1–2% ZnSO47H2O or stake treatments in 2–4% ZnSO47H2O solution for 15 min before planting were also effective. Another study in India recommends soil application of 12.5 kg of ZnSO47H2O (about 2.6 kg/ha Zn) (Nair and Mohankumar, 1980). CIAT (1978) reported that in high-pH soils, soil application of Zn was not effective, and foliar application or stake treatment was recommended. Copper deficiency was reported in peat soils of Malaysia and a basal application of 2.5 kg/ha Cu as CuSO45H2O was recommended (Chew et al., 1978). Iron deficiency was reported to be a serious problem in calcareous soils of India, Mexico, Colombia, Thailand, Java and Indonesia (Howeler, 2014). He recommended stake treatment with 2–4% FeSO47H2O or its foliar application as a remedial measure. Manganese deficiency was also reported from certain calcareous soils such as in Tamil Nadu, India and stake treatment or foliar sprays of MnSO44H2O was recommended to alleviate the deficiency. In the case of B deficiency, in both acid and alkaline soils, band application of 1–2 kg/ha B as borax at the time of planting corrected the deficiency (Howeler, 2014). Nair and Mohankumar (1980) recommended soil application of 10 kg/ha borax to correct B deficiency. CTCRI (2007) developed a micronutrient product for cassava based on the nutritional requirements of cassava as well as by considering the average soil fertility status in major cassava growing areas. Further modifications were made in due course based on more data collected and two liquid foliar micronutrient formulations for cassava (in acid and neutral-alkaline soils) were commercialized in India through Agrinnovate, Indian Council of Agricultural Research (ICAR) and one private company, M/S Linga Chemicals, Madurai has brought the commercial formulations into Indian markets (ICAT-CTCRI, 2018).

5.6 Drip fertigation The role of cassava has been transforming from a subsistence to a commercial crop in many countries, especially in Asia, where the productivity is increasing at a faster rate than in other production domains. In Tamil Nadu, India where cassava is grown for industrial uses, majority of the farmers are interested in drip fertigation (Fig. 9). Based on the demand of the stakeholders, ICAR-CTCRI, India developed drip fertigation technology for cassava cultivation by conducting on-station experiments, since 2009. During

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G. Byju and G. Suja

Fig. 9 Drip fertigation, a very common practice by cassava farmers of Tamil Nadu, India.

2009–12, an experiment was conducted to study the response of cassava to drip fertigation. The 3-year long field experimentation showed that cassava responded well to drip fertigation. Drip irrigation at 100% pan evaporation along with application of 50% N and K fertilizers during the first 40 days, 30% during 40–80 days and remaining 20% during 80–120 days after planting (P applied as basal dose) resulted in threefold yield increase (Sunitha et al., 2013). The water productivity of cassava was worked out to be 2.6 kg/m3 under surface irrigation, whereas it was 8.2 kg/m3 under drip irrigation at the rate of 100% cumulative pan evaporation (Sunitha et al., 2016). During 2014–17, ICAR-CTCRI, India conducted another study with three different combinations of N and K rates through drip fertigation. The drip irrigation was given uniformly and a combination of N and K at the rate of 75:125 through drip fertigation was the best, yielding 48.68 t/ha (Sunitha et al., 2018).

6. Organic vs inorganic nutrition of cassava There is a growing concern for ecologically-grown foods due to combined awareness of food safety and security, environmental protection and biodiversity as well as human well-being (De Schutter, 2010, 2011; FAO, 2010; SCAR, 2011). Given the importance of soil for crop production as

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well as for providing wider ecosystem services for local and global societies, maintaining soil, in good biological condition is of vital importance (Bai et al., 2018), especially in crops like cassava that export large quantities of nutrients from soil. A case in point to highlight the significance of organic cassava in the present changing scenario is the proposal of Cerilles (2015) that Gahung-gahung organic cassava farming system of the Philippines was feasible and viable for adoption by smallholders as well as contributory for mitigating the effects of climate change. There is little understanding to date about the influence of organic nutrient management on the growth, yield, tuber quality and soil properties in cassava. The bio-organic fertilizer treatment promoted the leaf and stem growth, increased the chlorophyll content, photosynthesis of leaves, improved the physiological metabolism of cassava, transfer of photosynthates to storage roots and increased yield and starch content in the storage roots of cassava (Zhongyong et al., 2006). Similarly, tallest plants, greater leaf production, largest length and diameter of branches, highest total tuber yield and best physical and chemical characters were noticed in cassava plants fertilized with compost and inoculated with mixed biofertilizers (Shafeek et al., 2012). Under humid tropical conditions of south India, organic nutrient management promoted growth (plant height, leaf retention and total leaf production), biomass production and its partitioning to various plant parts (especially tubers), leaf area index, steady crop growth rate, tuber bulking rate (TBR) and mean TBR (Seena Radhakrishnan, 2017). Nutrient uptake being a function of dry matter production and nutrient contents, organic nutrient management also resulted in greater N and K uptake over chemical system, certainly due to higher total biomass and tuber biomass production (Fig. 10). All these were ultimately reflected in tuber yield under organic management (27.26 t/ha), though insignificant (+2.40%) over conventional system (26.62 t/ha) in this 3-year long experimentation. She further confirmed that industrial and domestic varieties of cassava responded similarly to organic and inorganic nutrient management. Contrary to this, in another study on conventional and organic cropping systems of farming in Osun state of Nigeria, LAI monthly average values were consistently higher by 39.89% for conventional chemical system than organic farming in cassava. The total yield was 118% higher for the chemically weeded conventional plot than the manually weeded organic plot (Agunbiade, 2013). Organic and inorganic nutrient sources had significant effect on quality of cassava tubers (Omar et al., 2012). Their assessment indicated that nutritional quality was improved by organic fertilization with

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Nutrient uptake (kg ha -1)

300 238.93 250

223.40 185.83

200

134.80

150 100 28.63 29.20

50 0 N uptake

P uptake Conventional

K uptake

Organic

Fig. 10 Nutrient uptake: Organic vs conventional system (Source). Source: From Seena Radhakrishnan A.R., 2017. Evaluation of Agronomic, Nutritional and Socio-Economic Impacts of Organic Production of Cassava (Manihot esculenta Crantz). Ph.D. Thesis submitted to University of Kerala, Thiruvananthapuram, Kerala, India.

significant effects on antioxidant activity and phenolic metabolites in cassava. Further, the phenolic and flavonoid content were significantly higher in the vermicompost treatment compared to mineral fertilizer and empty fruit bunch compost. The total flavonoids and phenolics content of vermicompost treated plants were 39% and 38% higher, respectively, than those chemically fertilized. Cyanogenic glucoside levels decreased with the application of organic fertilizers. Between the two types of compost, vermicompost resulted in higher nutritional value of cassava tubers. Of the two varieties tested, Medan variety showed the most promising nutritional quality with the application of vermicompost. Amanullah et al. (2007) pointed out that organic nutrient management especially through composted poultry manure positively influenced the quality characters such as peeled tuber yield, total and industrial starch content and sugar content of cassava as compared to inorganically nourished cassava. Later, Omar et al. (2013) also observed that application of organic fertilizers like vermicompost was favorable for enhancement of antioxidants and total phenolic acids in cassava leaves that contributed to nutritional value of leaves used as vegetable. The application of inorganic fertilizer, on the other hand, increased the level of cyanogenic glucoside and decreased the phytochemical contents in both the leaves and tubers. Varieties also varied in quality under organic vs inorganic fertilizers. Medan had higher

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Mineral nutrition of cassava

antioxidant activities as well as total content of phenolics and flavonoid in comparison with Sri Pontian. In yet another study in India, biochemical constituents were improved with lower cyanogenic glucoside content (19.60%), higher dry matter (+2.73%), starch (+8.60%), crude protein (+7.72%), sugar (+54.62%), ash (+38.42%) and fiber (+20.56%) contents under organic nutrition (Seena Radhakrishnan and Suja, 2017) (Fig. 11). Besides, the mineral contents like P, K, Ca, Mg, Fe, Mn and Zn status of the tubers were enhanced by 1.25%, 10.13%, 12.89%, 40.79%, 59.03%, 6.92% and 27.40%, respectively, over conventional nutrient management that advocated chemical fertilizers in the above research (Fig. 12). Thus, it could be inferred that organic nutrient management in cassava produced quality and safe tubers. In a typical acid Ultisol in south India, in-depth exploration after three cycles of cassava on organic vs inorganic nutrition revealed that soil physicochemical and biological properties were improved under organic system with higher aggregate stability, porosity, microbial counts of bacteria, fungi and actinomycetes, activity of soil enzymes (urease, acid phosphatase and dehydrogenase), significantly higher pH, CEC, organic C and exchangeable Ca and slightly higher available P, K, Mg, Fe and Zn status over conventional system. Thus, the organic system scored significantly higher soil quality index (SQI) (0.98) over conventional system (0.48) (Seena Radhakrishnan, 2017). 60

54.62

50 38.42

% increase/decrease

40 30 20.56

20 10

8.6

7.72

Starch

Crude protein

2.72

0

–10 –20

Dry matter

Cyanogenic Total sugars glucoside

Fibre

Ash

–19.6

–30

Fig. 11 Percent increase or decrease in biochemical composition of tuber under organic over conventional. Source: From Seena Radhakrishnan, A.R., Suja, G., 2017. How safe is organic cassava?. J. Root Crops. 43(2), 3–9.

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G. Byju and G. Suja

70 59.03

60

% increase

50 40.79

40 27.4

30 20 10.13

12.89 6.92

10 1.25

0

P

K

Ca

Mg

Fe

Mn

Zn

Fig. 12 Percent increase in mineral composition of tuber under organic over conventional in cassava. Source: From Seena Radhakrishnan, A.R., Suja, G., 2017. How safe is organic cassava?. J. Root Crops. 43(2), 3–9.

Nutrient dynamics of various organic and inorganic resources, like farmyard manure, green manure, wood ash, crop residue of cassava, chemical fertilizers and biofertilizers, commonly used in cassava production, was monitored at monthly intervals up to 6 months under humid tropical conditions (Seena Radhakrishnan and Suja, 2019). The release of almost all nutrients and activity of soil enzymes were higher at the mid stage of incubation (3 or 4 months). The pH showed an increasing trend and electrical conductivity, organic C and Fe content declined from initial. Averaging over stages, organic nutrient management favored the activity of soil enzymes and release of almost all nutrients over conventional system significantly, except N. Effectiveness of natural and commercial organic fertilizers on cassava was studied by Linn and Myint (2018) and found that the natural organic fertilizers increased the pH, moisture content, available N, and K, exchangeable K, Ca and Mg content of soil relative to control. The study revealed that both these organic fertilizers enhanced tuber yield by 79% over control, improved nutrient contents in tuber with least post-harvest soil nutrient depletion.

7. Knowledge gaps and recommended research The detailed review of literature given in the above sections clearly showed that yield gaps of cassava are wider in most of the growing countries

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across the globe. Sub-Saharan African countries have very low average productivities followed by Latin America. The highest productivities are noticed in Asian countries. The current global average productivity is only about one-tenth of its potential productivity. Fertilizer trials conducted across the world indicated that absence of sound nutrient management programs and unbalanced mineral nutrition were the major reasons for the poor yield. The effect of fertilizer application on yield depends on climate, soil and cultivar. Certain questions that need to be addressed in future to bridge the yield gap of cassava are given below. 1. What is the effect of QUEFTS model-based recommendations on cassava yield in major cassava growing countries? Creation of a cassava SSNM network (CSN) will help in sharing and testing the SSNM technology at global level. A huge volume of unpublished data available at national institutes (Annual Report, Technical Bulletin, etc.) can be thus utilized for initial testing the model before beginning field validation trails. 2. What are the interactions of macro- and micronutrients on nutrient partitioning in different plant parts of cassava? 3. What is the relationship between soil test data and actual indigenous supplying capacity in different soil types of major growing countries? Data already available with national/international institutes can be used for such studies. 4. What is the interaction of cassava nutrition with different cassava mosaic viruses across major growing areas? 5. How the QUEFTS model can be improved to increase its performance under drought? Studies so far indicate that the model overestimated yields under drought conditions and predicted and observed yields were closer only in years with normal rainfall. 6. How models could be used to calculate field specific fertilizer recommendations needed to maximize economic returns based on the current prices of fresh roots and the cost of fertilizers? 7. How to develop crop-tailored, customized plant nutrient formulations for SSNM of cassava across major growing areas considering the climate, soil and variety and QUEFTS calculations? 8. How artificial intelligence and IoT devices using cheaper, sensor-based technologies can be developed and used for field specific nutrient recommendations? 9. How to evolve sustainable nutrient management programs in cropping systems involving cassava in the major growing areas across the globe?

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10. How to develop low-cost integrated nutrient and water saving techniques to bridge the yield gap?

8. Conclusions In spite of the ever-increasing significance of cassava in food, feed, green energy and industry in 104 countries mostly located in Sub-Saharan Africa, Latin America and Asia, the use of mineral fertilizers is highly unbalanced or lacking. Evolution of fertilizer recommendations for cassava from blanket recommendations developed about 50 years ago to the current computer simulation model-based SSNM recommendations has shown the importance of balanced crop nutrition in reducing the yield gap of cassava substantially. Still, there exists knowledge gaps that need urgent attention by researchers. Under the changing climate, cassava has been dubbed as a future, smart crop and hence addressing issues related to balanced crop nutrition and enhancing nutrient use efficiencies under drought conditions gain more practical relevance. We look forward with great enthusiasm that this review will catch the attention of cassava crop nutrition researchers so that the knowledge gaps can be addressed by forming a network of cassava growing countries attracting national and international funding so as to meet the ever-increasing global demand for cassava.

Acknowledgments We are thankful to the Director General, Indian Council of Agricultural Research (ICAR), Director, ICAR-Central Tuber Crops Research Institute (CTCRI), India and external funding agencies such as Department of Science and Technology (DST), Government of India, Ministry of Environment and Forests (MOEF), Government of India and Kerala State Council for Science, Technology and Environment (KSCSTE), Government of Kerala, India for the facilities and funding to carry out research on mineral nutrition of cassava for the past 26 years at ICAR-CTCRI, Thiruvananthapuram, Kerala, India. We are also thankful to the Ph.D. Scholars, Haripriya Anand, M., Suchitra, C.S., Sabitha Soman and Seena Radhakrishnan, A.R. for their sincere efforts in conducting research studies related to mineral nutrition of cassava.

References Adjei-Nsiah, S., Kuyper, T., Leeuwis, C., Abekoe, M., Giller, K., 2007. Evaluating sustainable and profitable cropping sequences with cassava and four legume crops: effects on soil fertility and maize yields in the forest/savannah transitional agro-ecological zone of Ghana. Field Crop Res. 103, 87–97. Agunbiade, L.W., 2013. Conventional and organic cropping systems of farming in Osun State of Nigeria. Agric. Biol. J. N. Am. 4 (2), 103–109.

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