A critical assessment of the system of rice intensification (SRI)

Agricultural Systems 79 (2004) 261–281 www.elsevier.com/locate/agsy

A critical assessment of the system of rice intensification (SRI) A. Dobermann* Department of Agronomy and Horticulture, University of Nebraska-Lincoln, PO Box 830915, Lincoln, NE 68583-0915, USA Received 28 September 2002; received in revised form 16 May 2003; accepted 6 June 2003

Abstract The system of rice intensification (SRI) has been proposed as an integrated and agroecologically sound approach to rice (Oryza sativa L.) cultivation. It was mainly developed through participatory on-farm research conducted in Madagascar, but its evaluation is ongoing in Asia as well. This paper critically discusses some of the assumptions underlying the SRI and its scope for improving rice production in Asia. A review of cropping practices at known highyield sites showed that techniques such as SRI are not necessary for growing rice near the yield potential. A move from permanent flooding to intermittent irrigation bears short- and long-term risks that are not well understood, but non-flooded conditions will generally favor rice growth on poor soils with potential for Fe toxicity. Deep root systems are associated with low input rice cropping and the intermittent water management practiced in SRI, but they are not a necessity for maximum rice performance, particularly in favorable environments with intensive cropping, short growth duration, and good water and nitrogen management. Approaches such as SRI may serve the important needs of resource-poor farmers in areas with poor soils, but are likely to have little potential for improving rice production in intensive irrigated systems on more favorable soils, where high yields can be achieved through implementation of more cost-efficient management practices. # 2003 Elsevier Ltd. All rights reserved. Keywords: Rice; System of rice intensification; Yield potential; Root system; Water management

Abbreviations: PAR; photosynthetically active radiation; RUE; radiation use efficiency; SRI; system of rice intensification * Tel.: +1-402-472-2811; fax: +1-402-472-7904. E-mail address: [email protected] (A. Dobermann). 0308-521X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0308-521X(03)00087-8

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1. Introduction Intensification of irrigated double- and triple-crop rice systems in Asia since the mid 1960s involved an increase in the number of crops grown per year and greater yield per crop cycle. Higher yields resulted from the combination of increased yield potential of modern varieties, improved crop nutrition made possible by fertilizer application, and improved host-plant resistance and pest management (Cassman and Pingali, 1995). Growth rates of yield and total irrigated rice production have, however, slowed down in recent years due to lower rice prices and a slowdown in demand growth, but concern was also raised about resource degradation (Dawe and Dobermann, 2001). In many large irrigated rice production domains, where farmers were early adopters of modern technologies, yields have stagnated since the mid1980s (Cassman and Dobermann, 2001). At issue is how rice yield growth in Asia can potentially be re-energized. In several recent publications, Uphoff (2001), Stoop et al. (2002), and Uphoff et al. (2002) described the system of rice intensification (SRI), which mainly evolved through participatory on-farm experimentation conducted in Madagascar during the 1980s and 1990s. They suggested that SRI represents an integrated and agroecologically sound approach to irrigated rice (Oryza sativa L.) cultivation, which may offer new opportunities for location-specific production systems of small farmers. They also proposed that such approaches could unlock currently untapped production potentials of rice, allowing farmers to realize yields of up to 15 Mg ha 1 or more with reduced irrigation and mineral fertilizer inputs (Stoop et al., 2002). Many development-oriented organizations have therefore begun to evaluate the SRI system or some of its components in other regions, including major rice-growing areas of south and southeast Asia (Fernandes and Uphoff, 2002). However, although most published and unpublished reports on SRI tend to be optimistic, they are incomplete in their coverage of the existing scientific literature, and there is a general lack of detailed field research following high scientific standards. Both Uphoff (2001) and Stoop et al. (2002), for example, do not report research data that would allow a thorough examination. Could SRI have advantages over current input-intensive, irrigated rice production in Asia and elsewhere, or is it mainly a system geared towards rice cropping practiced by resource-poor farmers on poorer soils? The objectives of this paper are to critically discuss some of the stated SRI principles and assess its scope for improving irrigated rice production. Due to the lack of detailed SRI research, emphasis will be given to current scientific understanding of selected biophysical characteristics of irrigated rice cropping and implications for different management approaches. Key questions that need to be raised include: are changes in crop management as those proposed in SRI required to reach yields close to current ceilings and to re-accelerate yield growth in Asia? If so, what are the biophysical processes involved and do they really represent new knowledge? Is SRI more ecologically sound and resourceefficient than a well-managed ‘‘conventional’’ rice system? Under what conditions is SRI likely to have little advantage over less labor-intensive crop management practices?

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2. Key elements of SRI Stoop et al. (2002) and Uphoff et al. (2002) provide a detailed overview of the rationale and key components of SRI and they also discuss its scope for adoption. Therefore, only a brief summary will be given here. SRI is understood as a set of principles and a set of mostly biophysical mechanisms. It originated in the humid highlands of Madagascar with rainfall mostly ranging from 1000 to > 2000 mm (Oldeman, 1990), mostly on poor soils with low pH, low CEC, low available P, and high concentrations of soluble Fe and Al. Major SRI principles include (1) raising seedlings in carefully managed nurseries, (2) careful transplanting of single, young (8–15 days old) seedlings at wide plant spacing (starting at 2525 cm, but going up to 5050 cm), (3) intermittent irrigation to avoid permanent flooding during the vegetative growth phase, (4) addition of nutrients to the soil, preferably in organic forms such as compost instead of chemical fertilizer, and (5) intensive manual or mechanical weed control without herbicide use. It should be noted, however, that SRI is not a ‘‘standard package’’ of specific practices, but rather represents empirical practices that may vary to reflect local conditions (Uphoff, 2002). Variants of SRI have also been tested in which only some of the basic components were practiced. The key physiological principle behind the principal SRI measures is to provide optimal growing conditions to individual rice plants so that tillering is maximized and phyllochrons are shortened, which is believed to accelerate growth rates (Nemoto et al., 1995). It was also observed that tiller mortality is reduced. Furthermore, intermittent irrigation is believed to improve oxygen supply to rice roots, thereby decreasing aerenchyma formation and causing a stronger, healthier root system with potential advantages for nutrient uptake (Stoop et al., 2002).

3. Climate and yield potential Some SRI publications claim rice yields that appear to exceed known physiological yield limits. For example, Rafaralahy (2002) lists nine examples of grain yields ranging from 15.0 to 23.4 Mg ha 1 in farmers’ fields in the highland regions of Madagascar, but he provides no further details about the methods used for measuring yield and whether it was adjusted to a standard moisture content. Maximum SRI yields in the range of about 8–12 Mg ha 1 appear to be more common in other studies, including those conducted outside Madagascar (Fernandes and Uphoff, 2002; Stoop et al., 2002). Are yields of 515 Mg ha 1 possible in these environments or are they merely unverified claims? Based on the present scientific understanding of photosynthesis, the maximum achievable radiation use (or conversion) efficiency (RUE) of C3 rice is about 2.7 g CH2O MJ 1 photosynthetically active radiation (PAR), which suggests ceiling grain yields of about 12.5 Mg ha 1 in the tropics (at 110 day growth duration) or 18 Mg ha 1 in cooler environments with long growth duration of 150–160 days (Sheehy, 2000). More typical for rice-grown under nonlimiting conditions in the field is a RUE of 2.2 g CH2O MJ 1, which suggests a yield

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potential of 10 and 15 Mg ha 1, respectively. These yield ceilings are less than some of the yield claims made for SRI in on-farm studies in Madagascar (Rafaralahy, 2002), but they are comparable with maximum yields achieved in several research experiments conducted elsewhere. Grain yields of 10–12 Mg ha 1 have been achieved in tropical dry season crops, whereas maximum yields in the 13–15 Mg ha 1 range have been measured at sites with subtropical or temperate climate in Australia and China (Horie et al., 1997; Kropff et al., 1994; Sheehy, 2000; Ying et al., 1998).1 The latter are typically sites with high solar radiation and/or cooler night temperatures, often associated with higher elevation, and a growth duration that is typically 30–50 days longer than in the tropics (Horie et al., 1997; Ying et al., 1998). Most of the SRI sites in Madgascar for which super yields have been reported are found on higher elevation (500 to > 1500 m). Climatically, many of these environments are characterized by cool temperature and solar radiation regimes that increase the duration of each growth stage of rice, which causes problems for adaptation of normal crop management to these extreme conditions (Chabanne and Razakamiaramanana, 1997). At higher elevation sites in Madagascar average temperature rarely exceeds 20  C and night temperatures drop to about 10  C or less during rice growth. Therefore, climatically, very high yields of a single, long-duration rice crop appear to be possible, although without more accurate measurements it remains unclear whether the cumulative amount of intercepted PAR during a typical SRI growing season would be sufficient for yields in the 20 Mg ha 1 range. November/December to April is the typical period in which rice is grown at the original SRI sites, which also coincides with most of the rainfall. Rainfall from November to April is typically about 1000–1300 mm (at some sites up to 2000 mm), that is it rains on about 15–20 days per month during the main rice season and cloudy weather during November to February significantly reduces the interceptable solar radiation (Oldeman, 1990). More detailed climate data and plant phenology measurements are needed to clarify these uncertainties. Radiation use efficiency and PAR must be measured and attempts should be made to simulate the yield potential for these environments, similar to the studies done for Asia (Kropff et al., 1994; Matthews et al., 1995). Obviously, the yield potential is lower at tropical lowland sites, particularly in double- or triple-crop system with short growth duration of rice (Kropff et al., 1994), but also at sea level in Madagascar where the climate is tropical. Climatic differences in yield potential must be emphasized when trying to promote the adoption of SRI elsewhere. A related question is whether yields close to the climatic-genetic potential can only be achieved with cultivation practices such as those proposed in SRI. Studies in the Philippines (yields of 9.5–11 Mg ha 1) were typically conducted with rice transplanted at 2020 cm (25 hills m 2, 3–4 seedlings per hill, 14-day-old seedlings), permanent flooding at 5–10 cm water depth throughout the whole growth period, and use of mineral fertilizer as the sole nutrient source (Dobermann et al., 2000; 1 Yields reported here only refer to published, replicated field experiments in which rice was grown under nearly non-limiting conditions. It should be noted, however, that a maximum yield of almost 18 Mg ha 1 has recently been reported for a hybrid testing plot in China (Yuan, 2002).

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Kropff et al., 1994; Sheehy, 2000; Ying et al., 1998). Management at the Chinese high-yield site in Yunnan (yields of 13–15 Mg ha 1) typically involves planting at 1016.5 cm spacing with just one old (30–40 days) seedling per hill, permanent flooding at 5–10 cm water depth throughout the whole growth period, and use of farmyard manure (22 Mg ha 1) plus mineral fertilizer as nutrient sources (Ying et al., 1998). Practices at Yanco, Australia (13–14 Mg ha 1) include high plant density due to dry seeding at a rate of 120 kg seed ha 1, shallow flooding (5 cm) until panicle initiation followed by temporary deep flooding (20 cm) and gradual decrease of the water depth thereafter, and use of mineral fertilizer only (Horie et al., 1997). In summary, techniques such as those proposed in SRI are not a requirement for growing rice near climatic-genetic yield potential ceilings.

4. Reported yield increases over conventional management of irrigated rice Most publications on SRI report large yield increases compared to ‘‘conventional’’ or ‘‘traditional’’ management of irrigated rice (Stoop et al., 2002). Fernandes and Uphoff (2002) have recently summarized SRI reports from 17 countries. Unweighted average grain yields in all these studies were 6.8 Mg ha 1 for SRI as compared to just 3.9 Mg ha 1 for control treatments that represented the recommended conventional irrigated rice management at these sites. The extraordinary yields reported from Madagascar have, however, not been achieved elsewhere. Although yield advantages were claimed for the majority of reports, there were also examples for no yield increases over the control in Bangladesh, China, India, Myanmar, Nepal, and Thailand. Numerous uncertainties are associated with these studies. Most of the papers published in Uphoff et al. (2002) do not include sufficient descriptions of site characteristics (e.g., soil, climate), experimental protocols, field management, sampling methods, and statistical data analysis to properly assess the validity of the results reported. Many reports represented on-farm experimentation or even unreplicated demonstration plots with limited data collection and statistical analysis. Little information was given on how the ‘‘conventional’’ plots were managed in comparison with SRI, and also with regard to known best practices for water, nutrient, and pest management. Many comparisons of SRI with conventional management practices also tended to be confounded because total nutrient inputs in SRI and conventional treatments differed, and because the conventional management treatments did not represent average or advanced levels of modern rice production. For example, the average grain yield reported for conventional irrigated rice management (3.9 Mg ha 1) in the SRI studies (Fernandes and Uphoff, 2002) was significantly less than the current global average irrigated rice yield of about 5.3 Mg ha 1 (Dobermann, 2000). To resolve such issues, SRI must focus on carefully conducted, replicated field experiments in which known best-management practices are fully implemented at yield levels that approach location-specific ceilings. The studies by Wang et al. (2002) are among few published examples in which SRI was compared to a

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well-managed conventional treatment at high yield levels (8.5–11.5 Mg ha 1). Despite differences in various physiological characteristics measured during the growing season, final numbers of tillers, grain yields, and components of yield of Indica varieties or hybrids were either the same or even higher with conventional management than under SRI.

5. Continuous flooding or water-saving irrigation? Based on the assumption that aerenchyma formation due to submergence may adversely affect plant performance, the SRI challenges the common notion that rice performs best under flooded conditions. Instead, intermittent irrigation is practiced in SRI during vegetative growth to keep the soil just saturated or moist enough to avoid drought stress, which typically results in water savings as compared to continuous flooding. Saving water has become one of the priorities of rice research (Barker et al., 2000) and many water-saving irrigation techniques are being studied. Instead of keeping fields flooded, the soil can be kept near saturation, or alternate wetting and drying regimes are imposed through intermittent irrigation (Bouman and Tuong, 2001; Tabbal et al., 2002). However, it is necessary to differentiate between environments in which growing rice under flooded conditions is most sustainable and those where periodical aeration or oxygenation can be the single-most important management factor for increasing yields. The former appears to apply to more fertile lowland rice environments, whereas the latter is mainly the case on marginal soils with need for aeration to improve oxygen supply to roots, and to avoid accumulation of toxic concentrations of reduced substances such as ferrous iron (Fe2+) or hydrogen sulfide (H2S). Why has rice adopted itself to waterlogged conditions? Like many native wetland plants, rice has found wetlands to be its niche in the evolutionary sequence because those environments provided many advantages. Oryza glaberrima, for example, evolved as a wild species in seasonally flooded portions of west African river valleys and mostly flourished as deep-water rice. Soils were better, water was abundant, nutrients were available, and there was less competition by other species. To take advantage of this, it had to develop an aerenchyma and, due to the abundance of water and nutrients, it could easily thrive on a relatively shallow root system. Root porosity or aerenchyma formation (Armstrong, 1967) is directly related to redox potential, but plants that are adapted to wetland conditions appear to perform best under slightly reducing conditions as compared to strongly reduced or strongly oxidized soil (Kludze and Delaune, 1995; Kludze and Delaune, 1996). A floodwater layer has unique benefits. It acts as a buffer zone that stabilizes many soil processes and has a rich biological life, as summarized by Roger (1996). Estimates vary, but biological activity in the floodwater is a major component of the long-term sustainability of rice systems, mainly due to the large C and N inputs associated with it. Primary productivity of the floodwater community ranges from

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about 0.2 to 2.0 g C m 2 per day or about 10–15% of that of the rice crop, resulting in large C inputs that are responsible for maintaining soil organic matter content, even without any addition of organic amendments (Bronson et al., 1997a; Cassman et al., 1995). In Asia’s most intensive triple-crop long-term experiment, rice yields without fertilizer or manure input and with complete removal of all straw have remained nearly unchanged since the mid 1970s, at 3 to 3.5 Mg ha 1 per crop or 9 to 10 Mg ha 1 per year (Dobermann et al., 2000). Plant N accumulation in the unfertilized treatment of this long-term experiment has remained relatively constant at about 50 kg N ha 1 per crop in recent years, a level which probably represents a new equilibrium maintained by N input from biological N2 fixation, most of it taking place in the floodwater or the soil-floodwater interface. Similar observations were made in a neighboring experiment at the same site with different inorganic and organic N sources (Ladha et al., 2000). There is no comparable intensive agricultural system in which such zero-input productivity would be possible over such a long time, indicating a remarkable sustainability of nutrient-supplying capacity and maintenance of soil C (Buresh et al., 2001). Therefore, unless there is water scarcity, giving up the long-term benefits of continuous flooding during rice growth is a risky management strategy, particularly on more fertile, clayey soils found in those areas of Asia where, at present, water is not in short supply. Any system in which the period under flooding is reduced significantly or in which intermittent irrigation causes repeated agitation of biological turnover processes bears the risk of being less sustainable over the long term. Field studies suggest large differences in the C and N turnover and balances between monoculture systems with long periods of flooding and rice grown in a system with long aerobic phases. The latter is usually associated with more rapid and complete mineralization of crop residue and soil organic matter (Bronson et al., 1997a; Witt et al., 2000). This must be kept in mind for evaluating systems such as SRI because management techniques that stimulate soil organic matter mineralization and crop nutrient extraction from the soil with insufficient external replenishment may lead to a decrease in soil fertility. In the case of SRI, deeper root systems are promoted by increased soil aeration and the lack of mineral fertilizer use, which also leads to greater exploitation of available indigenous soil resources. No research has been conducted to understand the long-term consequences of such soil exploitation for the sustainability of SRI production systems. The socioeconomic and agronomic issues involved in water-saving irrigation are complex. Such techniques are relatively difficult to implement, because they require excellent land preparation, timely availability of irrigation water during critical periods of growth, good irrigation infrastructure, and efficient methods of weed control, particularly in areas with larger field sizes. If land leveling and water management are poor, the risk for yield reduction due to temporary drought stress, weeds, or nutrient losses increases. Fig. 1 illustrates this for a relatively large irrigated ricefield (3.6 ha) in southern Russia. Although the quality of land leveling at this site was considered to be good, numerous small areas with higher micro-elevation occurred within this field, at which soil aeration or even drought stress occurred occasionally during the growing season. As a result, nutrient availability decreased and weed

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populations increased at these locations as compared to continuously flooded areas within the same field, resulting in significantly smaller panicles (Fig. 1) and grain yield (Dobermann, 1994). This significantly increases the labor or herbicide demand for weed control and is likely to be even larger so in systems such as SRI because of the wide plant spacing and prolonged period until canopy closure. Other potential uncertainties include increases in soil greenhouse gas emissions (N2O) in systems with alternate wet-dry conditions (Bronson et al., 1997b). Research on rice yield performance under different water management regimes generally suggests that most water saving practices are associated with a slight yield decrease, the level of which depends on the groundwater table depth, the evaporative demand and the drying period in between irrigation events (Bouman and Tuong, 2001). However, relative water savings tend to be larger than relative yield decreases, often resulting in increased water productivity. In their review of published data, Bouman and Tuong (2001) found that average water savings under saturated soil conditions were 23% (  14%), whereas yield reductions were 6% (  6%). In a detailed case study of two villages in China, Moya et al. (2001) reported that farmers did not practice a pure form of either alternate wetting and drying or continuous flooding, but they did save water at the farm level without adversely affecting yields or farm profitability.

Fig. 1. Relationship between rice grain weight per panicle and relative microrelief position in a 3.6 ha direct-seeded irrigated ricefield in south Russia. Data shown are pooled measurements conducted at 45 sampling locations in 1988 and at 81 locations in 1989. Water management included a drainage period during early tillering and shallow irrigation during reproductive growth. At each location, microrelief position relative to the field mean was measured as floodwater depth at late tillering. Modified from Dobermann (1994).

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Thus, the present knowledge indicates that water saving irrigation can, at best, maintain yields at present levels, but it appears difficult to produce more rice with less water on the same field without complex and potentially costly changes in other management practices. Based on small-plot (2.52.5 m) trials in Madagascar, Randriamiharisoa and Uphoff (2002) suggested that rice yield can be increased significantly with less water through a high degree of synergy among all SRI practices. Without more detailed measurements conducted in larger plots and in other environments these results must, however, be treated with caution. Tabbal et al. (2002) showed that there are alternatives for saving water that may be more promising for Asian rice systems. In their study, with continuous flooding, wet-seeded rice yielded higher than transplanted rice (3–17%), but required 19% less water, increasing water productivity by 25–48%. Keeping the soil at saturation saved 35% water, but at the cost of small (5%) yield loss. Intermittent irrigation further saved water, but at the expense of relatively large yield losses.

6. Soil chemistry and root health A key justification for promoting intermittent irrigation as part of SRI is the stated assumption that rice is not an aquatic plant, and that under continuous submergence most of the rice plant’s roots remain in the top few cm of soil and degenerate by the reproductive phase (Stoop et al., 2002). At issue is how common this is and under what conditions providing greater soil aeration is likely to be a key management factor for improving rice growth. Kar et al. (1974) is cited by Stoop et al. (2002) and in other SRI papers as evidence for ‘‘serious (78%) root deterioration’’ at flowering under flooded conditions, which ‘‘is likely to negatively affect the efficiency of nutrient uptake and consequently yield’’. However, in the Kar et al. (1974) study, total root number, root dry weight, and shoot dry weight of rice grown under flooded condition were much larger than under unsaturated condition (at which the relative root deterioration was less). For example, at flowering, root dry weight was 35 g with flooding vs. just 8 g without flooding. Moreover, in the flooded soil, root degeneration only started to become significant about 50 days after germination, first affecting rootlets. This is evidence for a relatively slow initial soil reduction because the experiment was conducted on a lateritic sandy loam soil of very poor soil fertility (pH 5.4, CEC 6.5 cmol kg 1, 0.5% organic matter). Based on the common kinetics of soil redox processes (Ponnamperuma, 1972), it is likely that the low soil organic matter level was responsible for a slow decline in redox potential during the first few weeks, but toxic levels of ferrous iron in soil solution were probably reached after few weeks of flooding. This is by no means representative for the majority of rice soils, particularly those found in more favorable lowland regions. Thus, Kar et al. (1974) was limited in scope and should not be cited without full explanation and a broader literature analysis. Reddy and Kuladaivelu (1992), for example, observed that root volume and root-dry weight were higher under continuous submergence or at irrigation to submergence after reaching the soil-

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saturation point than under drier upland moisture regimes. Zhang et al. (1990) studied nutrient uptake in lowland and upland rice cultivars under hypoxia. They found that the cortex was more developed in the lowland cultivar, which was related to adaptation to waterlogging. Hypoxia decreased root growth, Ca and P uptake but had no effect on N uptake in either NO3 or NH+ 4 form. The decreases in root growth and P and Ca uptake were greater in the upland cultivars, which was related to their different capacities for oxygen transport to the roots. Differences among soils exist that also determine the need for different forms of water management. In the early 1990s, Kirk and co-workers conducted a series of studies on chemical processes in the rhizosphere of flooded rice grown in an acid Ultisol from Ilo Ilo in the Philippines (Begg et al., 1994; Kirk and Solivas, 1994), a soil with poor fertility and characteristics similar to those at most SRI sites in Madagascar and also the lateritic soil used by Kar et al. (1974). A key feature of this soil was its low pH and the ability to produce large amounts of soluble ferrous iron after flooding, which greatly increased root-induced Fe oxidation and acidification of the rhizosphere. However, subsequent research showed that on less acid and particularly on neutral rice soils less ferrous iron is produced, there is less need for oxygen excretion by roots, less rhizosphere acidification, faster diffusion of acid away from the root, and roots will likely remain healthier (Kirk and Bajita, 1995; Kirk and Saleque, 1995). As an example, Fig. 2 illustrates major differences among lowland rice soils in the kinetics of redox potential (Eh), pH, soluble and exchangeable Fe after submergence. The Ilo Ilo soil in this example is the same acid Ultisol used by Kirk and others (Begg et al., 1994; Kirk and Solivas, 1994; Saleque and Kirk, 1995). After submergence, the redox potential in the two fertile soils (Maahas and Maligaya) declined rapidly, with an associated fast increase in soil pH, reaching near-neutral levels within about one week. Increases in soluble Fe2+ were, however, small in both soils because most of the ferrous iron produced by reduction processes was adsorbed on clay particles (=exchangeable Fe2+). In contrast, Eh and pH changes in the Ilo Ilo soil proceeded more slowly due to acid initial pH, lower organic matter content, and large amounts of Fe oxides and hydroxides in the clay fraction. Because the initial pH was very low and soil pH increased to neutral levels only after about four weeks, and due to generally less buffering (low cation exchange capacity) submergence resulted in soluble Fe2+concentrations that were 3- to 4-fold larger than in the Mahaas and Maligaya soils and potentially harmful to root health under strongly reducing conditions. Although little research on the chemistry of soils at SRI sites has been published, it is apparent that most of the experimentation was done on soils with characteristics similar to those of the Ilo Ilo soil in Fig. 2 or the soil used by Kar et al. (1974). Wetland soils in Madagascar are mostly Oxisols, Ultisols, Entisols, and Inceptisols (IRRI, 1997; Soil Survey Staff, 1999) and about 60% of all soils in Madagascar belong to soil groups with ferralitic properties and low soil fertility (Oldeman, 1990). Most of these soils have acid pH, low CEC, low exchangeable cations, predominantly 1:1 layer clay minerals, large amounts of Fe- and Al-oxides, and significant potential for accumulation of soluble ferrous iron and manganese after

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Fig. 2. Changes in redox potential, pH, solution iron concentration, and exchangeable iron in three Philippine rice soils during submergence (laboratory incubations, A. Dobermann, unpublished data). The three soils represent different geographical regions with a wide range in soil texture, clay mineralogy, and organic matter content. Maahas clay: Haplaquoll, 65% clay, 18.3 g organic C kg 1, CEC 31.1 cmcolc kg 1; Maligaya silt loam: Epiaquert, 29% clay, 11.2 g organic C kg 1, CEC 16.8 cmcolc kg 1; Ilo Ilo sandy loam: Epiaquult, 25% clay, 9.2 g organic C kg 1, CEC 7.4 cmcolc kg 1.

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flooding, which are toxic for rice plants under submerged conditions (Alaily et al., 1998; Vizier et al., 1990; Vizier, 1990). Under continuous submergence, rice yields tend to be very low on soils with such unfavorable physico-chemical characteristics, particularly due to negative effects on growth during the vegetative phase of rice (Vizier et al., 1990). Soils at some other tropical sites where SRI has been tested recently (e.g., Indonesia, Sierra Leone, Myanmar, Philippines) probably also fall into this category. Although acid soils with low fertility and potential Fe-toxicity are locally important for resource-poor farmers in Asia and elsewhere (Boje-Klein, 1986), their relative importance for irrigated rice production must be viewed unbiased for understanding where SRI has potential as opposed to other forms of management. Both general overviews (Moormann and van Breemen, 1978) as well as extensive soil surveys (Kawaguchi and Kyuma, 1977; Miura et al., 1995) suggest that most paddy soils in intensive irrigated areas of Asia are younger alluvial soils with relatively high fertility status, mostly Entisols, Inceptisols, Alfisols, Vertisols, and Mollisols. There are vast areas with Ultisols too, but they are mostly used for rainfed lowland and upland rice. Few larger irrigated rice areas with Aquults are found in Indonesia, southern Philippines, India, or Sri Lanka, but their overall share of irrigated rice is small. In Asia alone, the physical land area of irrigated double- and triple-crop rice monoculture systems is about 24 M ha (Huke and Huke, 1997). These systems alone account for about 40% of the global rice production (Dobermann and Cassman, 2002) and soils are mostly of better quality, that is they can be very productive with common nutrient and crop management because there are less inherent soil constraints to crop growth. Only few percent of the lowland rice soils in Madagascar belong to soil groups with similar fertile characteristics (Oldeman, 1990).

7. Root systems and nutrient uptake Observations for the SRI system suggest that deeper and stronger root systems are developed due to intermittent irrigation practiced on soils without physical barriers to root growth, planting of young, single seedlings at wide spacing, and application of slowly-releasing nutrient sources such as compost (Stoop et al., 2002). Although it remains unclear which of these SRI practices contributes most to such changes in root morphology, the key question is where a more rapidly developing shallow root system is more suitable to achieve the yield potential, and where a deeper, more aerated root system may be of greater advantage. Much of the original SRI work was done on soils with poor chemical fertility characteristics, but with physical properties that allow better water percolation and also favor deeper root growth. A deep root system is therefore desirable under the following conditions: (1) soils with no permanent floodwater layer and potential for water stress, (2) systems with low external inputs of rapidly released nutrients, and (3) systems with longer growth duration that allows enough time for root system development (single-crop rice). The Japanese literature has shown all this for single-crop systems with temperate climate and longer growth duration, not unlike the conditions found in the high-

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lands of Madagascar where SRI was developed. Morita and Yamazaki (1993) summarized the Japanese work on root morphology in response to the management factors that also form the basis of SRI. They suggested that the following practices help develop deep roots and achieve high yields: (1) good drainage, (2) deep plow layers, (3) compost application, (4) moderate, infrequent N application, (5) moderate water percolation, and (6) planting at moderate density. These are all components of SRI too, but Morita and Yamazaki (1993) also note that further studies are needed to understand relationships between root systems and yield. Figure 2-56 in their paper shows an example of an inconsistent relationship between the ratio of deep to shallow roots and rice yield. In general, the Japanese (and also Chinese) studies suggest that intermittent irrigation results in a greater number of superficial roots, whereas the effect on rooting depth appears to be less clear. Effects of mineral N fertilizer on root systems are clearer, generally suggesting a shallower root system with increasing amount of topdressed applied N (Figure 2-48 in Morita and Yamazaki, 1993). They also show a series of photos on the effect of compost application on root systems, suggesting an increase in fine roots that is probably caused by the need to capture slowly releasing nutrients from the compost. In contrast, high-yielding irrigated rice systems on favorable land in subtropical and tropical Asia are geared for maximum performance over a short time (100–110 days). This includes maximum capacity for uptake of surface-applied nutrients because there is less opportunity to incorporate large amounts of organic materials into the soil and growth time is limited. Soils are often heavy in texture and of predominantly 2:1 layer clays. Only relatively shallow tillage can be performed and soil physical properties often limit rooting depth. Root systems are very fibrous and about 80 to 90% of the root biomass is typically found in the top 20 cm of soil (Cassman et al., 1998), but there is little evidence for poorly functioning roots, except on marginal soils that are prone to toxicities under anoxic conditions. On fertile rice soils, the formation of superficial roots is a key performance factor. Some of the highest short-term nitrogen uptake rates ever measured for cereals were those for rice during the period from panicle initiation to flowering on a fertile Vertisol, after about 6 to 9 weeks of continuous flooding (Peng and Cassman, 1998). In their study, peak N uptake rates reached 9–12 kg N ha 1 day 1 over a 4-day period following topdressed urea application at panicle initiation. Kirk and Solivas (1997) concluded that ‘‘rates of [ion] diffusion will generally not limit [N] uptake in well-puddled soils, but they may greatly limit uptake in puddled soils that have been drained and re-flooded and in unpuddled flooded soils. Uptake of fertilizer-N broadcast into rice field floodwater and absorbed by roots in the floodwater or soil near the floodwater is not likely to be limited by root uptake properties or transport’’. Recent work also suggests that rice is efficient in absorbing and utilizing both nitrate- and ammonium-N and that a mixed nitrate/ammonium nutrition may have synergistic effects on growth (Kirk, 2001; Kirk and Kronzucker, 2000; Kronzucker et al., 1998; Kronzucker et al., 2000; Yang et al., 1999). Significant amounts of nitrate may be formed by nitrification of ammonium in the rhizosphere (Briones et al., 2003), even under submerged conditions. Superficial roots, not deep roots,

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appear to play a key role in this under warmer conditions, but genotypic variation must also be considered. Hybrid rice, for example, has an increased capacity to form a superficial root mat, which in turn appears to be efficient in absorbing N, much of it perhaps as nitrate-N (Yang et al., 1999; Yang and Sun, 1991). In summary, on an area basis, rice plants with deep root systems are unlikely to yield more than rice grown with a shallow root system, provided that nutrients are supplied in optimal congruence with crop demand and nutrient uptake is not constrained by other disorders. Tao et al. (2002) recently evaluated effects of SRI methods on physiology and growth of two rice hybrids at a site in southeast China in comparison with a conventional management (with flood irrigation). Although SRI plants had a much deeper root system and larger root and total plant dry matter per hill than plants grown under conventional management, total aboveground dry matter production expressed in kg ha 1 was not significantly different in both systems from tillering to maturity.

8. Organic amendments or mineral fertilizers? Mineral fertilizers were used in early SRI experimentation in Madagascar, but the addition of nutrients is not considered a requirement with SRI because it is assumed that higher yields can be achieved by using SRI practices without amendments, at least for several years (Uphoff, 2002). It is also assumed that chemical fertilizers do not contribute much over time to soil quality, so that the use of organic amendments, primarily as large amounts of compost, is encouraged. This is also the common practice at sites for which the largest SRI yields have been reported (Uphoff, 2002). Considerable uncertainties exist because most SRI evaluation studies conducted so far were confounded by unknown or unequal amounts of nutrients applied in SRI and conventional treatments. General differences in managing organic matter in irrigated lowland systems as compared with more aerobic systems are well known (Olk et al., 2000), but thorough field experiments on long-term effects of SRI practices on crop productivity, soil and environmental quality in comparison with wellmanaged mineral nutrient inputs do not exist. Moreover, the availability of organic materials and high labor cost for producing, transporting, and applying them are major constraints in large irrigated rice areas. Uncertain is also whether large applications of organic materials in SRI may cause significant increases in greenhouse gas emissions such as methane (CH4), which has been observed for irrigated rice systems in Asia (Wang et al., 2000; Wassmann et al., 1996). Large amounts of organic materials repeatedly applied on a soil with lower buffering capacity and high reducible Fe content may also accelerate soil reduction and thereby the potential for Fe toxicity in rice (Ponnamperuma, 1972). Myths and science often tend to get mixed up when benefits of organic amendments are discussed in comparison with a ‘‘recommended’’ (mineral) fertilizer treatment. Effects of organic amendments and recommended use of mineral fertilizer on absolute yield levels and long-term yield trends over time were recently reviewed for

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25 long-term experiments in Asia (Dawe et al., 2003). Three main conclusions emerged from this analysis. First, application of either manure or straw did not improve grain yield trends in rice-rice and rice-wheat cropping systems. Second, depending on socio-economic conditions, use of organic materials in these cropping systems may often be profitable, provided the organic materials are used as a complement to a recommended and balanced amount of mineral fertilizers (that is organic materials should not be used as the primary nutrient source). Third, experimental designs to assess the suitability of organic amendments need to be improved, particularly with regard to equal nutrient inputs, in order to allow a better comparison of the relative advantages of inorganic and organic nutrient sources. Although organic nutrient sources are important components of the nutrient cycle in agroecosystems and should be utilized where they are cost-efficient and available, cereal production at the global scale will likely depend on mineral nutrients to meet current and future food demand (Cassman et al., 2003). What has been found for rice systems with flood irrigation in Asia may not necessarily fully apply to a system such as SRI, but there is no credible evidence yet to support such an assumption. Unlike mineral fertilizers, organic fertilizers in smallholder agriculture do not add nutrients to a cropping system as whole, but rather are a means of nutrient transfer. For SRI technologies that include compost or manure, where will those nutrients come from and how sustainable is such a practice over the longer term?

9. Different concepts for different needs Different approaches exist for improving the management of a cropping system: (1) strategies in which it is attempted to change various components simultaneously through a complex learning process and (2) those in which one tries to change individual components in a more stepwise manner, taking into account that socioeconomic changes are key determinants of adoptable (improved) technologies. Both may play a role in enhancing agricultural production and reducing negative impacts on the environments. Both require a solid scientific understanding of the processes involved before firm conclusions can be drawn and extrapolation and adoption should be attempted. The SRI is an example for the first approach, which may make it more suitable for niches such as the management of previously poor systems on mostly marginal land, provided that sufficient cheap labor is available. Its development has occurred locally through a long process of trial and error experimentation, leading to adaptation to the environments in which it was developed. Consequently, SRI is likely to work in such niches and may result in large gains as compared to management practices that do not fit these biophysical environments. However, recent studies in Madagascar have shown slow adoption of SRI and high disadoption rates (40%), even in areas where it was developed, mainly because the method requires additional knowledge and labor input at times of labor shortage or greater other opportunities for investment (Moser and Barrett, 2003a). This study also raised questions about the cost of diffusion because disadoption rates were high in the absence of extension, and because it may take farmers several years to become comfortable with SRI

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without further assistance. It is also questionable to what extent appreciable learning-by-doing effects occur. Commune surveys conducted in 2001 suggested that, after nearly 20 years of on-farm experimentation and NGO-led promotion efforts, SRI is practiced by more than 25% of farmers in less than 3% of all communes in Madagascar (Moser and Barrett, 2003b). A key limitation for potential adoption of SRI in Asia is that certain components of the technology conflict with significant changes in cropping technologies that have already occurred and continue to occur, mainly in response to socioeconomic constraints such as low rice prices, high labor cost, or decreasing labor availability. Direct seeding, use of herbicides and insecticides, and the use of mineral fertilizers are Asian farmers’ responses to such external pressures and they also determine future forms of more knowledge-intensive resource management (Pingali et al., 1998). The site-specific nutrient management developed by IRRI and its collaborators is an example for a technology tailored to such changes in crop management (Dobermann et al., 2002; Witt et al., 2002; Witt and Dobermann, 2002). It works under the premise that the weakest links are gradually improved on a broader basis, but allowing location-specific adoption. By using a uniform on-farm research approach quantitative knowledge can be gathered more quickly. It can be generalized and only manageable system components are changed at a time, not the whole system. For example, crop-based measures of indigenous nutrient supply and plant nutrient status were used as opposed to soil tests or some blanket fertilizer recommendation, mainly based on the assumption that those crop measures also better reflect the interactions between soil nutrient supply, root system functions, and crop nutrient demand (Dobermann et al., 2003). Work continues to make this part of a more integrated crop management approach in which other location-specific determinants of crop growth and response to nutrients are modified. That includes issues such as plant density (e.g. in Indonesia, India, China), water-saving intermittent irrigation (China), or integrated use of mineral fertilizer and manure (North Vietnam).

10. Conclusions The SRI has stimulated a substantial amount of debate about the principles of irrigated rice cropping. Unfortunately, little research following common scientific standards has been conducted to allow a thorough evaluation of SRI, which is a necessity before reliable conclusions about its potential impact can be drawn. Uncertainties persist about SRI performance in different environments, long-term effects on productivity, soil quality and the environment, and its overall adoption potential. Some components of SRI such as water-saving, intermittent irrigation require strict control of crop management and water availability, which increases the risk for failure. The agroecological conditions in Madagascar and similar environments in which SRI has been evaluated are not representative for most intensive rice environments in Asia. Two key hypotheses emerge from this review. First, based on the present knowledge of soil chemistry and crop physiology, it appears that SRI is mainly a suitable

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technology for increasing rice yields in environments with acid, Fe-rich soils, high labor availability, and a generally low level of crop intensification. Under such conditions, conventional management practices such as permanent submergence may fail to increase rice yield because rice growth and the response to mineral fertilizer applications is limited by toxicities caused by anoxic conditions. Second, benefits of SRI over conventional rice management are likely to be small on fertile rice soils with no constraints such as potential Fe-toxicity, provided that management follows known best practices. Although SRI may play a role for improving the livelihood in less favorable environments, most intensive lowland rice areas in Asia do not appear to fit into the categories of biophysical and socioeconomic conditions that would favor adoption of the SRI, so that its overall impact on the global rice supply is likely to remain small. To make progress in obtaining a more scientific understanding, future SRI research must focus on experimental approaches and measurements that allow systematic and rigorous quantification of the key processes involved across a range of different environments, and in comparison with the best currently available conventional rice management technologies.

Acknowledgements Contribution of the Nebraska Agric. Exp. Stn. Scientific J. Series Paper no. 14080.

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