Tuber yield, water and fertilizer productivity in early potato as affected by a combination of irrigation and fertilization

Agricultural Water Management 101 (2011) 35–41

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Tuber yield, water and fertilizer productivity in early potato as affected by a combination of irrigation and fertilization Anita Ierna a,∗ , Gaetano Pandino b , Sara Lombardo b , Giovanni Mauromicale b a b

Istituto per i Sistemi Agricoli e Forestali del Mediterraneo, CNR, Sezione di Catania, Str.le V. Lancia, Zona Industriale, Blocco Palma, I-95121 Catania, Italy Dipartimento di Scienze delle Produzioni Agrarie e Alimentari, DISPA, Università degli Studi di Catania, Via Valdisavoia 5, 95123 Catania, Italy

a r t i c l e

i n f o

Article history: Received 29 January 2011 Accepted 26 August 2011 Available online 22 September 2011 Keywords: Solanum tuberosum Irrigation Fertilization Irrigation Water Productivity Fertilizer productivity Yield

a b s t r a c t Excessive amounts of irrigation water and fertilizers are often utilized for early potato cultivation in the Mediterranean basin. Given that water is expensive and limited in the semi-arid areas and that fertilizers above a threshold level often prove inefficacious for production purposes but still risk nitrate and phosphorous pollution of groundwater, it is crucial to provide an adequate irrigation and fertilization management. With the aim of achieving an appropriate combination of irrigation water and nutrient application in cultivation management of a potato crop in a Mediterranean environment, a 2-year experiment was conducted in Sicily (South Italy). The combined effects of 3 levels of irrigation (irrigation only at plant emergence, 50% and 100% of the maximum evapotranspiration – ETM) and 3 levels of mineral fertilization (low: 50, 25 and 75 kg ha−1 , medium: 100, 50 and 150 kg ha−1 and high: 300, 100 and 450 kg ha−1 of N, P2 O5 and K2 O) were studied on the tuber yield and yield components, on both water irrigation and fertilizer productivity and on the plant source/sink (canopy/tubers dry weight) ratio. The results show a marked interaction between level of irrigation and level of fertilization on tuber yield, on Irrigation Water Productivity and on fertilizer productivity of the potato crop. We found that the treatments based on 50% ETM and a medium level of fertilization represent a valid compromise in early potato cultivation management. Compared to the high combination levels of irrigation and fertilization, this treatment entails a negligible reduction in tuber yield to save 90 mm ha−1 year−1 of irrigation water and 200, 50 and 300 kg ha−1 year−1 of N, P2 O5 and K2 O, respectively, with notable economic savings for farmers compared to the spendings that are usually made. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Potato is a very important crop in the Mediterranean Basin, occupying an overall area of about one million ha and producing 18 million tonnes of tubers (FAO, 2008). In several countries such as Tunisia, Egypt, Cyprus, Israel, Lebanon, Turkey and in southern Italy, potatoes are not grown in the usual cycle (spring–summer) owing to the high temperatures and considerable demand for irrigation water, but are largely grown in a winter–spring cycle (planting from November to January and harvesting from March to early June) for early production. Early potatoes, defined as “potatoes harvested before they are completely mature, marketed immediately after harvesting and whose skin can easily be removed without peeling” (UNECE of Geneva, FFV-30/2001), are highly appreciated and are mainly exported to northern European countries, with considerable profit (Ierna, 2010). The substantial commercial value of the product and the intensive use of the

∗ Corresponding author. Tel.: +39 095 292871; fax: +39 095 292870. E-mail address: [email protected] (A. Ierna). 0378-3774/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2011.08.024

land prompt farmers to supplement the potato crop with water, nutrients and other management needs, which have undoubtedly been responsible for increased early potato yields in recent decades. In the Mediterranean area, irrigation plays a fundamental role in early potato cultivation. Indeed, the crop is planted during winter months when rainfall usually exceeds evaporation, but in the successive stages of growth of the aerial part and of tubers from the end of winter to the whole of spring, the rainfall decreases at the same time that evapotranspiration and temperatures increase thus causing substantial soil water deficits. In general, potato crops are affected by drought at all stages of growth, but during the periods of tuber initiation and bulking this has a drastic effect on yield (Jefferies and Mackerron, 1993; Van Loon, 1981). Therefore, early potato cultivation in this area usually resorts to irrigation throughout the spring, coinciding with the phase of tuber bulking and growth. The amount of water supplied, like the number of irrigations and irrigation intervals, is dependent on the rainfall distribution and may differ from one season to another; nonetheless, excessive water inputs are common due to inefficient irrigation methods (furrow, macro-sprinklers).

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A. Ierna et al. / Agricultural Water Management 101 (2011) 35–41

Three macronutrient elements, namely nitrogen, potassium and phosphorus, are the predominant fertilizers applied. All have been shown to improve yield and quality of potato tubers where native soil supplies are limited (Westermann, 2005). In early potato crops, farmers may use as much as 600 kg ha−1 of N, 300 of P2 O5 and 400 of K2 O (Bianco, personal communication). These are far greater than the usual crop uptake, which for an aerial production of 20 t ha−1 are equal to 102, 27 and 197 kg ha−1 of N, P2 O5 and K2 O, respectively (Mauromicale et al., 2000). The excess of N fertilizers may increase nitrate concentration in the groundwater table (Darwish et al., 2003); P is thought to have very low solubility in soil systems and when applied excessively it can disperse through runoff and erosion and potentially affect the quality of surface waters. In addition, high amounts of nitrogen augment the nitrates in tubers to levels above the threshold allowed by the major European distributors which is set at 200 mg/kg f.w. (Ierna, 2009). Considering that irrigation water is expensive and limited in the semi-arid areas of the Mediterranean basin, and that fertilizers above a threshold level often prove ineffective for production purposes while eventually damaging the environment, it is crucial to provide crop-specific irrigation and fertilization management. This would improve farmers’ incomes by saving water and reducing fertilizer costs as well as minimising nutrient loss. Since water plays an important role in the fate and transport of nutrients and their absorption by crops, appropriate fertilizers and water application should be considered together in a comprehensive approach. Extensive research has been conducted in the Mediterranean to determine yield response of potato crop to water (Fabeiro et al., 2001; Foti et al., 1995; Ierna and Mauromicale, 2006; Karafyllidis et al., 1996; Onder et al., 2005), and yield response to simultaneous application of water and nitrogen (Darwish et al., 2003; ˇ Navarro Mohammad et al., 1999; Papadoupolus, 1988; Pedreno et al., 1996). However, up to date literature is lacking on the effects of complete fertilization programmes (N, P and K) and on the interaction between water supply and fertilizer applications on yield performances. In addition, the understanding of the productive crop response to these resources is of crucial importance to reduce these inputs in cultivation management without sacrificing yield. Improvements in irrigation management are a way of increasing agricultural production and reducing the demand for water (Perry et al., 2009). Environmental protection is one of the priorities of the new objectives of European agricultural policy (European Union, 2000); a compromise between the need to maximize yield and profit and an adequate use of irrigation water and N fertilizer is therefore required to reduce the impact of crop production on the environment. The aim of the present work was to evaluate the effects of different combinations of amounts of irrigation water and nutrient application on (a) tuber yield and yield components; (b) Irrigation Water Productivity; (c) fertilizer productivity and (d) source/sink (canopy/tubers) ratio to achieve their appropriate combination in the cultivation management of a potato crop.

the potato crop season for early production (from January to May), the mean maximum day temperatures and the mean minimum night temperatures of the 30-year period 1977–2006 were, respectively, 15.4 and 7.1 ◦ C in January, 16.2 and 7.6 ◦ C in February, 17.7 and 8.8 ◦ C in March, 20.2 and 10.9 ◦ C in April, 24.3 and 14.4 ◦ C in May. Rainfall over the same period averages about 180 mm. In the 2 years of the experiment, we used two adjoining plots in the same field. A layer, 0.25 m thick (from −0.05 to −0.30 m), where more or less 90% of active roots were located, was considered for the soil analysis. The soil type is Calcixerollic Xerochrepts (USDA, Soil Taxonomy). Analysis made before the start of the trials indicated the following characteristics: clay 30%, silt 25%, sand 45%, organic matter 2.0%, pH 8.4, total nitrogen 1.8‰, assimilable P2 O5 78 kg ha−1 , exchangeable K2 O 337 kg ha−1 . The field capacity at −0.03 MPa was 0.29 g g−1 dry weight, the wilting point at −1.5 MPa was 0.11 g g−1 dry weight and bulk density was 1.2 g cm−3 . All analyses were performed according to procedures approved by Italian Society of Soil Science (Violante, 2000). 2.2. Experimental design, plant material and management practices In both 2007 and 2008, the experiment was conducted on potato (Solanum tuberosum L.) cv. Spunta using a randomized split-plot design with three replications, including 3 levels of irrigation [I1 (unirrigated control), I2 (50% of the maximum evapotranspiration – ETM) and I3 (100% of ETM) as main plots; and 3 levels of mineral fertilization (low F1: 50, 25 and 75 kg ha−1 of N, P2 O5 and K2 O, medium F2: 100, 50 and 150 kg ha−1 of N, P2 O5 and K2 O and high F3: 300, 100 and 450 kg ha−1 of N, P2 O5 and K2 O)] as sub-plots. Subplot size was 4.2 m × 4.2 m, with 84 plants. The width of borders between irrigation treatments was 2 m. The level F2 coincides with the crop uptake determined in a previous study (Mauromicale et al., 2000) in which it was found that an early potato crop in the same environment had an average uptake of 102, 27 and 197 kg ha−1 of N, P2 O5 and K2 O, respectively; levels F1 and F3 represent half and twice the nutrients uptake, respectively. Spunta is a well-adapted and widespread cultivar in the Mediterranean region; it is an early potato, with long, regular and very large tubers; plants produce a moderate number of tall erect and vigorous stems (Mauromicale et al., 2003). In both years, virus-free seed-tubers of Spunta imported from North European countries were utilized for planting. In 2007, whole tubers with a mean weight of roughly 100 g were planted on January 12, whereas in 2008 half tubers with a weight of about 60–70 g were planted on February 5. The half tubers were always cut lengthwise to ensure an equal number of buds per tuber-seed unit. Both whole and half tubers were planted at 0.3 intervals, in rows 0.7 m apart (equivalent to a planting density of 4.76 plants m−2 ). All plants emerged 40 days after planting (DAP) in the first year and 30 DAP in the second one. ETM was calculated using the following formula: ETM =

2. Materials and methods 2.1. Site, climate and soil Experiments were conducted during 2007 and 2008 at the experimental station of the Catania section of I.S.A.Fo.M. – CNR (National Research Council of Italy) on the coastal plain, south of Siracusa (37◦ 03 N, 15◦ 18 E, 15 m a.s.l.). This is a typical area for early potato cultivation in Sicily. The climate is semi-arid Mediterranean, with mild winters, and commonly rainless springs. Frost occurrence is virtually unknown (two events in 30 years). During

n 

E Kc Kp

0

where n = the number of days since the last watering; E = daily evaporation from an unscreened class A pan placed about 100 m from the crop and mesh covered to prevent animals drinking the water; Kc = crop coefficient, which varied from 0.45 to 1.15 in relation to the phase of the crop’s biological cycle (Doorembos and Kassam, 1979); Kp = pan coefficient, which was taken to be 0.8 in our conditions using the criteria set out by Doorembos and Kassam (1979) in Table 17. Water was applied by drip irrigation when the accumulated daily evaporation corrected for rain reached about 30 mm, which corresponded to 50–60% of available soil water content at

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Table 1 Average monthly maximum and minimum temperatures, total rainfall and daily average evaporation for 2007, 2008 and 1977–2006. Meteorological variable

Maximum air temperature (◦ C)

Minimum air temperature (◦ C)

Rainfall (mm)

Class A pan evaporation (mm d−1 )

Year

2007 2008 1977–2006 2007 2008 1977–2006 2007 2008 1977–2006 2007 2008

Month January

February

March

April

May

June

17.3 16.2 15.4 5.7 7.9 7.1 36 66 65 1.1 1.2

15.2 17.3 16.2 6.0 8.0 7.6 20 35 38 1.7 2.1

16.8 18.0 17.7 8.8 8.3 8.8 112 5 25 2.6 3.0

20.1 20.2 20.2 11.0 11.6 10.9 1 72 31 3.0 3.2

26.1 24.3 24.3 16.5 15.8 14.4 0 17 20 4.8 4.6

31.2 28.6 29.7 20.7 19.1 18.9 0 1 5 5.8 5.4

0.30 m depth in the plots irrigated with 100% ETM. Drip lines were collocated along rows with pressure compensating drippers spaced at 0.30 m (0.70 × 0.30). Only rainfall exceeding 5 mm within 25 h was considered effective and included in the calculation. Analogical water meters were used to measure the volume of irrigation water applied in each treatment. Irrigation started on 3 March and on 11 March respectively in 2007 and 2008 and finished on 10 May and 29 May respectively in 2007 and 2008. Following the above formula, 8 irrigations were done in 2007 and 9 in 2008. The interval between one irrigation and the next in both years ranged between 7 and 12 days. The unirrigated control (I1) received 25 mm to aid the rooting of the crop each year in one input when 70% of plant emerged. The total quantity of irrigation water supplied to plots receiving 100% of ETM (I3) was 174 and 192 mm in the first and in the second year, respectively. These amounts of water represent about 50% of maximum seasonal evapotranspiration. The downward flux below the root zone and run off were assumed to be negligible because of the rainfall distribution during the seasons and equally because the irrigation management using controlled amounts by drip irrigation did not lead to leaching and surface run off. Consequently, different treatment responses were due only to the differing amounts of water supplied by irrigation. Two weeks after plant emergence until 2 weeks before harvest, the soil water tension was monitored twice a week in I3 plots by using tensiometers (model Jet Fill, Soil Moisture Equipment Corporation, Santa Barbara, CA) and in I2 and I1 plots by using gypsum blocks (Soil Moisture Equipment Corporation, Santa Barbara, CA). Both tensiometers and gypsum blocks were installed at a depth of −0.3 m in the centre of each plot. The soil water potential ranged between −0.12 and −0.71 MPa in I1 plots, between −0.06 and −0.55 MPa in I2 plots and between −0.04 and −0.09 MPa in I3 plots. In both trials, phosphorus (as triple superphosphate) and potassium (as potassium sulphate) were applied at planting, whereas 50% of nitrogen (as ammonium nitrate) was supplied at complete emergence and the remaining 50% 3 weeks after as top dressing. Chlorpyrifos (30 kg ha−1 ) was applied before planting; standard crop management was applied, involving post-emergence weeding with linuron and pest control when needed.

Nebraska, USA). All plant parts were weighed. Samples of about 50 g of above-ground biomass, tubers and roots were oven-dried at 70 ◦ C until constant weight and weighed to determine their dry matter content. When about 80% of leaves were dry (130 and 123 DAP in the 2007 and 2008, respectively), plants of each sub-plot were collected; the tubers were hand harvested, counted and weighed to determine total yield, number and average tuber weight. Deformed and diseased tubers were also counted and weighed and were considered discarded tubers. Discarded production was determined as % weight of deformed and diseased tubers of the total tuber yield. For both years, the soil water content in the 0.2–0.3 m depth interval was determined in each plot when the tubers were harvested using the gravimetric method. The following were calculated: source/sink as the ratio of dry canopy weight measured when maximum LAI was achieved (86 DAP in 2007 and 90 DAP in 2008) to the tuber dry weight measured at the end of the cycle (130 and 123 DAP in the 2007 and 2008, respectively); IWP (Irrigation Water Productivity) defined as total tuber yield per unit of applied water to the crop and expressed as kg ha−1 mm−1 (Van Cleemput, 2000); PFP (Partial Factor Productivity) defined as total tuber yield per amount of the 3 fertilizers applied to the crop and expressed as kg ha−1 kg−1 (Cassman et al., 1996). Meteorological data were recorded with a CR 21 data logger (Campbell Scientific, Inc., Utah, U.S.A.) located at the experimental station. Measurements were made every 30 min.

2.3. Data collection

2.5. Temperature and rainfall

Plant growth was determined by destructive samplings of plants within the experimental plots that were located near the sides of the plots at 86, 111, 121 and 139 DAP in 2007 and at 76, 87, 94, 107, 114, 123 and 133 DAP in 2008. Plants were harvested individually by hand and separated into above-ground biomass (stems + leaves), roots + stolons and tubers. Roots, stolons and tubers were washed in gently running water. The leaf area was measured with a LI-3100C area meter (Li-COR Inc., Lincoln,

The average monthly minimum and maximum temperatures during the trials were similar in both years, also with respect to the 1977/2006 30-year average (Table 1). The volume of rainfall during the 2007 test was equal to 170 mm, of which roughly 70% occurred in March and did not substantially vary from that of 2008 (196 mm) and that of the 30-year mean (184 mm). The mean daily evaporation, from January to June fluctuated between 1.1 and 5.8 mm d−1 in 2007 and from 1.2 to 5.4 mm d−1 in 2008.

2.4. Data analysis Bartlett’s test was applied to establish homogeneity of variance, following which the data were subjected to analysis of variance (ANOVA). Since different tuber units (whole or sliced tubers) were used in the 2 years, the ANOVA was performed separately for each year. In fact all data were subjected to 2-way ANOVA (irrigation water level x fertilization level). The percentage values were transformed, before the variance analysis into angu√ lar values by the Bliss formula arc sin % (Snedecor and Cochran, 1989).

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Table 2 Summary of statistical significance from analysis of variance for all studied variables. Variable

Source of variation

dfa

2007

2008

Soil water content (%)

Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F) Irrigation level (I) Fertilization level (F) (I) × (F)

2 2 7 2 2 7 2 2 7 2 2 7 2 2 7 2 2 7 2 2 7 2 2 7 2 2 7

***

***

NS NS

NS NS

***

***

***

***

***

***

NS NS NS

NS NS NS

***

***

***

NS

*

*

***

NS NS

NS NS NS

***

***

*

*

Tuber yield (t ha−1 )

N tubers plant−1

Average tuber weight (g)

Discarded tubers (% of total yield)

IWPb (kg ha−1 mm−1 )

PFPc (kg ha−1 kg−1 )

Source/sink

Tuber dry matter (%)

**

*

***

***

***

***

***

***

***

***

***

***

***

***

***

***

NS NS

NS NS

NS = not significant. a Degree of freedom. b Irrigation Water Productivity. c Partial Factor Productivity. * Significant at P ≤ 0.05. ** Significant at P ≤ 0.01. *** Significant at P ≤ 0.001.

3. Results

3.2. Tuber yield

3.1. Soil water content at harvest

Tuber yield was significantly higher in 2007 than in 2008 (32.1 t ha−1 vs. 26.0 t ha−1 ). In both years, yield was significantly (P ≤ 0.001) affected by irrigation level, fertilization level and their interaction (Table 2). In 2007, the highest tuber yield, equal to roughly 42 t ha−1 , was recorded for the three combination treatments: I3F2, I3F3 and I2F3, without significant differences among them (Table 3). The other combination treatments produced considerably less tuber yield. The lowest yield, equal to about 20 t ha−1 , was recorded for the level of irrigation I1, regardless of fertilization level (Table 3). The tuber yield was unaffected by the fertilization level in the plots treated with I1, whereas it declined significantly and proportionally as the fertilization level decreased in the irrigation treatments I2 and I3 (Table 3). In 2008, the highest tuber yield was confirmed only in the combination I3F3 (39.5 t ha−1 ). A significantly lower yield (33.2 t ha−1 ) was obtained in I3F2; while yields equal to roughly 30 t ha−1 and without statistical differences among them were obtained in the combinations I3F1, I2F3 and I2F2 (Table 4). The lowest yield, equal to roughly 14 t ha−1 was recorded in I1F3. The tuber yield dropped significantly as the fertilization level decreased in plots irrigated with maximum water level (I3) and intermediate water level (I2), whereas it increased as the level of fertilization decreased in plots irrigated only at plant emergence (Table 4).

Soil water content at harvest was significantly (P ≤ 0.001) influenced only by the irrigation level in both years (Table 2). Irrespective of the level of fertilization, soil water content in 2007 was 17.6% in the plots irrigated only at the plant emergence (I1) and increased significantly in I2 (20.5%) and I3 (23.1%); in 2008 soil water content was equal to 15.3% in I1 and increased significantly and comparably in I2 and I3 (19.0 and 20.3%) (Fig. 1). 30

Soil water content (%)

year 2007 year 2008

25 A B

a

20

a C b

15

10 I1

I2

I3

Level of irrigation Fig. 1. Effects of level of irrigation on soil water content at tuber harvest. Different letters within each year indicate significant differences at P ≤ 0.05 according to Duncan test (I1 = irrigation only at plant emergence; I2 = irrigation with 50% of ETM; I3 = irrigation with 100% of ETM).

3.3. Tuber yield components and discarded production In both years, the tuber yield differences between the interaction treatments “irrigation level × fertilization level” were due exclusively to the average tuber weight, because the differences in number of tuber per plant were not significant (Table 2). In both years, the highest average tuber weight was recorded in

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Table 3 Effects of interaction between “irrigation level × fertilization level” in 2007 on tuber yield, average tuber weight, discarded tubers, Irrigation Water Productivity (IWP), Partial Factor Productivity (PFP) and source/sink ratio. Treatment

Tuber yield (t ha−1 )

Average tuber weight (g)

Discarded tubers (% on total yield)

IWP (kg ha−1 mm−1 )

PFP (kg ha−1 kg−1 )

Source/sink

I1 F1 I1 F2 I1 F3 I2 F1 I2 F2 I2 F3 I3 F1 I3 F2 I3 F3

20.9 d 20.1 d 20.1 d 29.3 c 35.3 b 41.9 a 35.4 b 43.2 a 42.8 a

71 c 73 c 79 c 108 b 125 b 150 a 124 b 168 a 151 a

6.2 4.9 6.0 1.8 2.0 1.2 1.6 1.0 0.7

836 a 806 a 805 a 261 cd 315 c 374 b 178 de 217 de 215 e

139 c 67 e 24 g 195 b 118 d 49 f 236 a 144 c 50 f

0.24 b 0.19 c 0.28 a 0.19 c 0.13 d 0.12 d 0.14 d 0.12 d 0.14 d

Mean

32.1

117

2.8

501

114

0.18

I1 = irrigation only at plant emergence; I2 = irrigation with 50% of ETM; I3 = irrigation with 100% of ETM; F1 = low level (50–25–75 N–P2 O5 –K2 O; F2 = medium level (100–50–150 N–P2 O5 –K2 O); F3 = high level (300–100–450 N–P2 O5 –K2 O). Mean values followed by different letters within columns differ significantly at P ≤ 0.05 according to Duncan test. Table 4 Effects of interaction between “irrigation level × fertilization level” in 2008 on tuber yield, average tuber weight, discarded tubers, Irrigation Water Productivity (IWP), Partial Factor Productivity (PFP) and source/sink ratio. Treatment

Tuber yield (t ha−1 )

Average tuber weight (g)

Discarded tubers (% on total yield)

IWP (kg ha−1 mm−1 )

PFP (kg ha−1 kg−1 )

Source/sink

I1 F1 I1 F2 I1 F3 I2 F1 I2 F2 I2 F3 I3 F1 I3 F2 I3 F3

16.2 ef 18.1 e 14.2 f 23.2 d 29.6 c 29.6 c 30.6 bc 33.2 b 39.5 a

79 d 91 d 73 d 127 c 128 c 117 c 142 bc 165 ab 169 a

0.7 1.4 0.4 2.9 0 0.7 2.9 1.9 0

648 b 724 a 568 c 192 de 245 d 245 d 141 e 153 e 182 de

108 cd 60 e 17 h 155 b 99 d 35 g 204 a 111 c 46 f

0.14 bc 0.14 bc 0.36 a 0.12 c 0.16 bc 0.23 b 0.16 bc 0.19 bc 0.13 bc

Mean

26.0

121

1.2

339

93

0.18

I1 = irrigation only at plant emergence; I2 = irrigation with 50% of ETM; I3 = irrigation with 100% of ETM; F1 = low level (50–25–75 N–P2 O5 –K2 O); F2 = medium level (100–50–150 N–P2 O5 –K2 O); F3 = high level (300–100–450 N–P2 O5 –K2 O). Mean values followed by different letters within columns differ significantly at P ≤ 0.05 according to Duncan test.

3.4. Irrigation Water Productivity (IWP) In both years, IWP was affected by irrigation level (P ≤ 0.001), fertilization level (P ≤ 0.05) and the interaction “irrigation level × fertilization level” (P ≤ 0.01) (Table 2). In both years, the highest IWP values were recorded in the plots irrigated only at the plant emergence (I1), whereas the lowest IWP values were found in the plots irrigated with 100% of ETM (I3). Intermediate IWP values were detected for irrigation treatment I2 (50% of ETM) (Tables 3 and 4). The effect of fertilization level was not significant in plots irrigated with 100% of ETM in both years, in plots irrigated with 50% ETM in 2008, and in plots irrigated only at plant emergence in 2007. By contrast, the medium level of fertilization improved the IWP values in comparison with the high and low fertilization level in plots irrigated with 50% ETM in 2007 and in plots irrigated only at plant emergence in 2008 (Tables 3 and 4).

3.5. Partial Factor Productivity (PFP) In both years, PFP was significantly (P ≤ 0.001) affected by irrigation level, fertilization level and their interaction (Table 2). The highest PFP values, 236 and 204 kg ha−1 kg−1 of fertilizers in 2007 and 2008, respectively were recorded in the plots irrigated with 100% ETM and fertilized with the lowest level of fertilization (I3F1). By contrast, the lowest PFP values, 24 kg ha−1 kg−1 in 2007 and 17 kg ha−1 kg−1 in 2008, were found in the plots irrigated only at the plant emergence and fertilized with the highest level of fertilization (I1F3) (Tables 3 and 4).

Discarded tubers (% of total yield)

the combinations I3F3 (151 g in 2007 and 169 g in 2008) and I3F2 (168 g in 2007 and 165 g in 2008). The lowest average tuber weight (from 71 to 91 g) was recorded in irrigation level I1 without significant differences in the fertilization levels (Tables 3 and 4). In 2007, the average tuber weight significantly decreased as the fertilization level decreased from F3 to F2 or F1 in the irrigation treatments I2 and as fertilization level decreased from F2 and F3 to F1 in the irrigation treatment I3 (Table 3). In 2008, the average tuber weight was unaffected by the fertilization level, both in the plots treated with I1 and I2, whereas it declined significantly as the fertilization level decreased from F2 and F3 to F1 in the irrigation treatments I3 (Table 4). The discarded tubers, as percentage of total yield, was significantly affected by irrigation level in 2007, when it decreased as irrigation level decreased (Fig. 2).

6

a

4

2

b b

0 I1

I2

I3

Level of irrigation Fig. 2. Effects of level of irrigation on discarded tubers in 2007. Different letters indicate significant differences at P ≤ 0.05 according to Duncan test (I1 = irrigation only at plant emergence; I2 = irrigation with 50% of ETM; I3 = irrigation with 100% of ETM).

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A. Ierna et al. / Agricultural Water Management 101 (2011) 35–41

Tuber dry matter content (%)

23 22

a

21 20 b

b

I2

I3

19 18 17 I1

Level of irrigation Fig. 3. Effects of level of irrigation on tuber dry matter content regardless of year. Different letters indicate significant differences at P ≤ 0.05 according to Duncan test (I1 = irrigation only at plant emergence; I2 = irrigation with 50% of ETM; I3 = irrigation with 100% of ETM).

In both years and in each irrigation level, the PFP significantly decreased as the fertilization level increased from F1 to F2, to F3. On the contrary, in each fertilization level, the PFP increased with the increased of irrigation level (Tables 3 and 4). 3.6. Source/sink In both years, source/sink was significantly (P ≤ 0.001) affected by irrigation level, fertilization level and their interaction (Table 2). The highest source/sink values, 0.28 and 0.36 in 2007 and 2008, respectively, were recorded in the plots irrigated only at emergence and fertilized with the highest N, P2 O5 and K2 O levels (I1F3) (Tables 3 and 4). In both years, the effect of fertilization level was not significant in plots irrigated with 100% of ETM (I3), whereas in the plots irrigated only at emergence (I1) the source/sink ratio dropped as the fertilization level decreased from F3 to F1 or to F2 (Tables 3 and 4). By contrast, in plots irrigated with 50% ETM, the source/sink increased from F2 and F3 to F1 in 2007, but decreased from F3 to F1 in 2008 (Tables 3 and 4). 3.7. Tuber dry matter content Tuber dry matter content was significantly (P ≤ 0.001) influenced only by the irrigation level in both years (Table 2). Irrespective of the year and level of fertilization, tuber dry matter content equal to 21.9% in the plots irrigated only at the plant emergence (I1), was reduced to 19.1% in the plots I2 and I3 (Fig. 3). 4. Discussion Under the specific conditions of these experiments, the study clearly demonstrates that there was a strong and significant interaction between the level of irrigation and level of fertilization on potato tuber yield, Irrigation Water Productivity, fertilizer productivity and plant source/sink ratio. The combined treatments based on 100% of ETM + medium or high level of N, P2 O5 and K2 O fertilization (I3F3 and I3F2) proved to be the best combination to improve the potato tuber yield. Although the nutrient uptake and water intake are two independent processes in nature, water status in soil greatly affects nutrient uptake and efficiency. Water management can affect the following: nutrient availability; transformation

of nutrients in soil or from fertilizers; nutrient uptake by plants and nutrient use efficiency; plant nutrient composition (Li et al., 2009). The high level of fertilization also improved the tuber yield in the plants irrigated with 50% of ETM (I2), whereas in plots irrigated only at emergence (I1) nutrient supply did not show any influence or only a very slight influence on tuber yield, in agreement with findings by Ojala et al. (1990). Water deficit causes water stress in plants, inhibits plant root growth, reduces absorbing areas and capacities of plant roots, and thereby decreases nutrient uptake, transfer and efficiency (Li et al., 2009). The reduction in the level of fertilization from F3 to F2 in the plots irrigated with 100% (I3) and 50% ETM (I2), led to a negligible reduction in yield, while a further reduction in the fertilization to F1 led to a reduction by 17% in I3 and by 23% in I2. These results concur with the findings of Ferreira and Carr (2002), who in a trial in a Mediterranean environment found the highest tuber yield with the highest levels of N studied (240 kg ha−1 of N) in plots with the full water satisfaction. This may be considered similar to our fertilization level F3, though they did not find significant differences in yield with lower levels of nitrogen. Instead, our results do not concur with the findings of Mohammad et al. (1999) in Jordan, that in a trial using the cultivar Spunta in a Mediterranean environment, found that fresh tubers were not significantly influenced by applying nitrogen from 49 to 98 kg ha−1 . The reduction in the input of irrigation water from 100% of ETM (I3) to 50% ETM (I2) led to a 13% reduction in the yield in the plots fertilized with a high level (F3) and of 15% in the plots fertilized with medium level (F2). Similar reductions in yield were also found in previous research by Proietti et al. (2005) and Ierna and Mauromicale (2006). Plots irrigated only at emergence (I1) with respect to 100% ETM led instead to a drastic reduction in yield equal to 55% in F3 and to 46% in F2. Similar yield reductions passing from 100% ETM to 0% ETM were found in comparable environments (Fabeiro et al., 2001; Ferreira and Gonc¸alves, 2007; Iqbal et al., 1999; Onder et al., 2005; Smith et al., 2002). This confirms the difficulty of obtaining satisfactory tuber yield in these semi-arid areas without water applications, as irrigation is essential in the Mediterranean environment. The fertilizer productivity expressed by the Partial Factor Productivity (PFP) increased with the increase of irrigation level in each level of fertilization. These results agree with those found in corn in a Mediterranean environment (Di Paola and Rinaldi, 2008), in which the productive efficacy of the nitrogen fertilizer increased with the increase of irrigation level. The increase of fertilizer productivity by increasing the water supplied confirms how the availability of water boosts the crop’s capacity for nutrient uptake and efficient use of nutrients in the soil. This work has also shown that the fertilizer productivity decreases with the increasing level of fertilization, in agreement with the observations of Darwish et al. (2006), Fontes et al. (2010), Kumar et al. (2007) and Love et al. (2005). The fertilizer productivity both in I3 and I2 decreased, however, to a greater degree passing from F1 to F2 rather than passing from F2 to F3. Our results point to the convenience of using the medium level of fertilization corresponding to the crop uptake, thus allowing savings with respect to the high levels of 200, 50 and 300 kg ha−1 N, P2 O5 and K2 O per hectare. The Irrigation Water Productivity decreased significantly with the increase of water supplied, reaching the minimum value in plots irrigated with 100% ETM. These results concur with several authors that have reported higher values of Irrigation Water Productivity under water deficit than under adequate water supply (Darwish et al., 2006; Fabeiro et al., 2001; Islam et al., 1990; Kashyap and Panda, 2003; Onder et al., 2005; Yuan et al., 2003). A high Irrigation Water Productivity value in itself is of little interest if not associated with high yields, whereas for growers

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it would be very important to increase this to have more available water without sacrificing yield. In plots fertilized with high and medium levels, passing from 100% to 50% of ETM, the yield decreased by about 14% whereas Irrigation Water Productivity increased by about 55%. It would therefore be convenient to supply water corresponding to 50% of ETM, saving roughly 90 and 100 mm ha−1 in 2007 and 2008, respectively. The dry matter content of the tubers, which is recognised as an important quality index, was higher in the plots irrigated solely at emergence stage (I1). This concurs with findings in the same environment by previous researchers (Foti et al., 1995) and in other environments (Darwish et al., 2006; Porter et al., 1999; Yuan et al., 2003), but does not concur with the results reported by Shock et al. (1998) and Waddell et al. (1999), in which the specific gravity – closely correlated with dry matter content (Schippers, 1976) – tended to diminish as water applied decreased. Our results showed that also in the less favourable combinations the content in tuber dry matter was always higher than 18%. 5. Conclusions Our research confirms the difficulty of obtaining high tuber yields in semiarid areas without adequate water supply. Nevertheless, we demonstrated that high yield levels of potatoes and both high water and fertilizer productivity, can be reached by a valid compromise based on the supply of 50% ETM and the application of medium levels of fertilization (100, 50 and 150 kg ha−1 of N, P2 O5 and K2 O), which more or less corresponds to the crop uptake measured in the same environment. This means that compared to the higher inputs of fertilizer and water, it is possible to make savings in one season alone of roughly 90 mm of water and 200, 50 and 300 kg ha−1 N, P2 O5 and K2 O per hectare. If one then considers that farmers regularly apply more water and above all much more fertilizer, the savings that can be achieved are much greater. Our results have also shown that the quantity of supplied fertilizer may further be reduced with far fewer irrigations. Other options to investigate in potatoes in order to optimize the efficiency of irrigation water and chemical fertilizers are through the adoption of new genotypes characterized by more efficient response to irrigation and fertilization or improved for earlier tuberization, higher harvest index and better sink/source balance. The water and fertilizer saved may be used more profitably to irrigate and fertilise supplemental lands in such a way to achieve a more efficient and rational land use both from an economical and environmental viewpoint. References Cassman, K.G., Gines, G.C., Dizon, M.A., Samson, M.I., Alcantara, J.M., 1996. Nitrogen use efficiency in tropical low land rice systems: contributions from indigenous and applied nitrogen. Field Crops Res. 47, 1–12. Darwish, T., Atallah, T., Hajhasan, S., Chranek, A., 2003. Management of nitrogen by fertigation of potato in Lebanon. Nutr. Cycl. Agroecosyst. 67, 1–11. Darwish, T.M., Atallah, T.W., Hajhasan, S., Haidar, A., 2006. Nitrogen and water use efficiency of fertigated processing potato. Agric. Water Manage. 85, 95–104. Di Paola, E., Rinaldi, M., 2008. Yield response of corn to irrigation and nitrogen fertilization in a Mediterranean environment. Field Crops Res. 105, 202–210. Doorembos, J., Kassam, A.H., 1979. Yield response to water. Irrigation and Drainage Paper 33, FAO, Rome. European Union, 2000. Special Report No. 14/2000 on Greening the Community Agricultural Policy together with the Commission’s replies. Official Journal C353/2000, August 30, 2001, pp. 0001–0056 (on-line), http://europa.eu.int/eurlex/en/lif/dat/2000/en300Y120801.html. Fabeiro, C., Martin de santa Olalla, F., De Juan, J.A., 2001. Yield and size of deficit irrigated potatoes. Agric. Water Manage. 48, 255–266. FAO, 2008. FAO. FAOSTAT. Agriculture. Rome (http://faostat.fao.org/). Ferreira, T.C., Carr, M.K.V., 2002. Responses of potatoes (Solanum tuberosum L.) to irrigation and nitrogen in a hot, dry climate. I. Water use. Field Crops Res. 78, 51–64. Ferreira, T.C., Gonc¸alves, D.A., 2007. Crop-yield/water-use production functions of potatoes (Solanum tuberosum L.) grown under differential nitrogen and irrigation treatments in a hot, dry climate. Agric. Water Manage. 90, 45–55.

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