Management of phosphorus, potassium, and sulfur in intensive, irrigated lowland rice

Transcription

1 ELSEVIER Field Crops Research 56 (1998) Field Crops _ Research Management of phosphorus, potassium, and sulfur in intensive, irrigated lowland rice A. Dobermann a,*, K.G. Cassman b, C.P. Mamaril a, J.E. Sheehy a a International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines b Department of Agronomy, University of Nebraska-Lincoln, P.O. Box , Lincoln, NE , USA Abstract Management of soil phosphorus (P), potassium (K) and sulfur (S) resources in intensive, irrigated rice systems has received less attention than increasing cropping intensity and yields with new cultivars, irrigation, and fertilizer N. Crop requirements, input-output balance, and soil supplying capacity of P, K and S in irrigated lowland rice are reviewed. Based on projected rice production requirements, we estimate that the total annual nutrient demand for irrigated rice will be about 9 to 13 x l06 t N, 9 to 15 X 10 6 t K, 1.2 to 2.4 X 10 6 t P and 0.9 to 1.5 X 10 6 t S in 2025, amounts that represent an increase of 65 to 70% above 1990 requirements. At present, negative K balances are widespread and K deficiency has become a constraint to increasing yields, even on heavy-textured lowland soils with high inherent fertility. Because opportunities are limited for breeding cultivars that acquire more P, K or S from soil or have higher internal nutrient-use efficiencies, long-term management strategies must focus on maintaining adequate nutrient balances in the topsoil layer. Interactions among nutrients have a large influence on physiological and agronomic efficiency that result from nutrient applications. Strategies that only aim at increasing P or K application rates without considering the indigenous supply from soil reserves are inefficient; they may not sustain yield increases to meet rice demand. Little improvement in fertilizer use efficiency can be expected from the present system of providing blanket recommendations for a given production domain. Instead, site-specific nutrient-management approaches will be needed to accommodate the tremendous variability in indigenous nutrient supply found in the irrigated lowlands of Asia Elsevier Science B.V. Keywords: Long-term experiments; Nutrient balance; Nutrient uptake; Nutrient-use efficiency; Nutrient management; Phosphorus; Potassium; Rice; Sulfur 1. Importance of P, K, and S in lowland rice in Asia 1.1. Overview Before the introduction of modem high-yielding cultivars, net nutrient removal by rice crops was small and even poor soils had the capacity to supply * Corresponding author. sufficient nutrients to sustain the low yield levels of traditional cultivars. The introduction of modem rice production technologies based on early-maturing, N-responsive, semidwarf cultivars has increased both the cropping intensity (double- and triple-crop rice systems) and rice yields in the irrigated lowlands of Asia. Although Green Revolution had tremendous impact on N fertilizer use in Asia, it had much less impact on the use of P and K fertilizers. Most countries in South and Southeast Asia lack indige /98/$ Elsevier Science B.V. All rights reserved. PII S (97)00124-X

2 114 A. Dobermann et al. / Field Crops Research 56 (1998) nous sources of commercially exploitable P and K ores and must depend on imports (Belmehdi and Nyiri, 1990; Maene, 1995). The removal of nutrients such as P, K and S with harvested rice grain and straw has increased markedly with the greater yields of the new systems. At issue is whether current management strategies practiced by rice farmers are adequate to maintain yield levels of 4 to 6 t ha-1 over the long term and to support yields near 8 to 10 t ha -1 that are needed to meet growing demand. Research on management of P, K and S in irrigated rice has been reviewed frequently (Chang, 1976; Blair et al., 1980; Kemmler, 1980; De Datta and Mikkelsen, 1985; Uexkuell, 1985; Tandon, 1987; Tandon and Sekhon, 1988; Mohanty and Mandal, 1989; De Datta et al., 1990a,b; Palmer et al., 1990; Lefroy et al., 1992; Uexkuell and Beaton, 1992). This research review will focus on crop nutrient requirements, assessment of soil-nutrient supplying capacity, and fertilizer management strategies based on nutrient balance concepts. To discuss these issues in detail, we will use recent data of a standardized analysis of long-term fertility experiments (LTFE) conducted across a wide range of environmental conditions. The focus is mostly on P and K, but S is included where it appears to be most important Current and projected requirements and use Currently, the annual net-nutrient removal of harvested grain from irrigated ricefields in Asia is in a range of 0.9 to 1.2 Mt K, 0.5 to 0.9 Mt P and 0.2 to 0.5 Mt S (Table 1). Actual net nutrient removal from rice-based cropping systems is much greater because the amount of straw recycled is decreasing because of its use as forage or fuel, or its removal to facilitate manual threshing and rapid turnaround time between rice crops. To achieve the projected yield goals in irrigated rice systems of Asia, total annual crop demand will be about 9 to 15 Mt K, 1.2 to 2.4 Mt P and 0.9 to 1.5 Mt S in 2025 (Table 1). Reliable estimates of the regional fertilizer use on irrigated rice in South and Southeast Asia are not available. Current total K fertilizer use on all crops in the region is only 3.7 Mt K (Maene, 1995). Including Japan, total P use on all crops was 2.0 Mt P in 1987, but the fertilizer supply and crop demand deficit was 1.4 Mt P, and is projected to rise to 1.9 Mt P by 1993 (Belmehdi and Nyiri, 1990). Much of the P and K fertilizer is applied to crops other than rice, and most intensive irrigated rice systems have a clear K deficit (Dobermann et al., 1995c) Extent of deficiencies Because of a general lack of detailed soil fertility surveys, we can only provide examples of P, K and S deficiencies in various regions rather than exact figures. Phosphorus deficiency occurs widely in lowland soils that possess high native P-fixing capacity, such as acid soils (South China, Indonesia), acid sulfate soils (Mekong Delta), alkaline soils (Bangladesh, Pakistan, and India) and volcanic soils of the region (Kawaguchi and Kyuma, 1977; Boje-Klein, 1986; Islam, 1995). Average use of P fertilizer on all crops is particularly low (< 10 kg P ha -1 ) in Thailand, the Philippines, India, Bangladesh, Pakistan, and Sri Lanka (Belmehdi and Nyiri, 1990). Recent data from long-term experiments indicate increasing yield response to fertilizer P on soils that do not have high native P-fixing capacities (Dobermann et al., 1996c) and P deficiency is widespread in rice-wheat systems in India, Pakistan, Bangladesh, and Nepal. Potassium deficiency typically occurs in highly weathered tropical soils (Oxisols, Ultisols) and coarse-textured soils. Most soils of the great alluvial floodplains in Asia were generally regarded as high in extractable K, and it was thought that additional K supply from irrigation water would make K a rare limiting factor in irrigated rice systems (Kawaguchi and Kyuma, 1977; De Datta and Mikkelsen, 1985; Bajwa, 1994), but K deficiency is now more widespread even on heavy-textured soils. Examples include alluvial, illitic soils in India (Tiwari, 1985), lowland rice soils of Java (Sri Adiningsih et al., 1991), and vermiculitic clay soils of Central Luzon, Philippines (Dobermann and Oberthuer, 1997). In the latter study, a detailed survey of a ha, intensive, irrigated rice domain found 64%, 54%, and 0.5% of the area to be classified as low in available soil P, K, and S, respectively. Sulfur deficiency was reported in Bangladesh, India, Indonesia, Myanmar, Pakistan, Philippines, Sri Lanka and Thailand, and, more recently, also from rice fields in Yunnan province, China (Blair et al.,

3 A. Dobermann et al. / Field Crops Research 56 (1998) I I o~ t-q o e~ I I I ~- r~ tr I I I "~ "Tt l ~ tt) t'q I.~.<.r. ~r I e. ~d r.,,, ~ e~ z

4 116 A. Dobermann et al. / Field Crops Research 56 (1998) ; Deng et al., 1989). In Bangladesh, 81% of soil samples collected in 1988 from different agro-ecological zones were classified as S-deficient (Islam, 1995) and 53% of the lowland rice soils in Java have low S status (Sri Adiningsih et al., 1991). Replacing S-containing fertilizers with non-s fertilizer (Yoshida, 1981) exacerbates S deficiency in rice, although volcanic activity and S deposition associated with rapid industrial development may offset this trend in parts of Asia Potential for solutions: improved cultivars and crop management Insufficient data are available to assess accurately the potential for breeding rice cultivars for increased K- and P-use efficiencies in irrigated systems. Most of the progress made appears to be related to genetic variation in external P-acquisition mechanisms, i.e., differences in root morphology or plant-induced P solubilization, that lead to higher P uptake (Kirk et al., 1998). Because P, K and S have important physiological functions, it is likely that plants have evolved with high internal nutrient-use efficiencies (grain yield increase per kilogram of nutrient taken up). Whether significant improvements are possible to extend this natural selection remains to be determined. Improvement of harvest index (HI) remains another option, but there are limits to how much more HI can be increased through breeding (Peng et al., 1994). Selection for lines that acquire more P, K or S from soil by developing an extensive root system may be a strategy for adaptation in harsh rainfed environments where both drought and nutrient-deficient soils limit yields. Most irrigated lowland soils possess relatively high native fertility compared to acid, highly weathered upland soils, and the scope for improving levels of nutrient uptake beyond those observed in current high-yielding cultivars is uncertain. Several investigators have suggested that genetic variation in the internal efficiency of P and K may exist in rice (Katyal et al., 1975; Fageria et al., 1988), but systematic studies are few. In Thailand, differences between two cultivars in P-use efficiency were mainly due to external P-acquisition mechanisms rather than different physiological require- ments, and no differences in yield were found when the supply of P was adequate (Koyama and Chamnek, 1971). A key issue is whether root-uptake mechanisms actually constrain P- and K-use efficiencies. Cultivar differences in K uptake were observed in solution culture and field experiments conducted in Arkansas (Teo et al., 1992, 1995), but only few studies of nutrient uptake by rice roots have been made. Rates of P transport to roots are more limiting than rates of uptake at the root surface (Kirk and Saleque, 1995), so there is little scope for increasing P uptake by manipulating the influx characteristics of the root itself (Kirk et al., 1998). Because more than 90% of the total root length is located within the topmost 0-20 cm soil (De Datta et al., 1988a 1988; Teo et al., 1995; Table 4 in Cassman et al., 1998), long-term strategies for P and K management in intensive, irrigated rice systems must focus on sustaining soil fertility in this layer to increase root uptake rates of P and K by creating adequate buffering power and nutrient concentrations in solution. Tolerance of genotypes to slight differences in soil fertility or nonhomogeneous fertilizer application may be an important adaptive trait to account for within-field variation in soil fertility or rice growth (Dobermann, 1994; Dobermann et al., 1995b). Whether there is significant variation among rice genotypes in their plasticity to smooth out small-scale spatial variation in nutrient supply needs further investigation in the field. 2. Nutrient requirements or rice 2.1. Internal P and K use efficiencies Definitions of nutrient-use efficiency vary greatly (Gourley et al., 1993). We restrict our discussion to aggregate physiological efficiency (PE), defined as the amount of grain yield produced per kilogram of nutrient present in aboveground plant biomass. Our subsequent discussions are based mostly on an analysis of crop uptake and nutrient-use efficiency of P and K in LTFE by irrigated rice at 10 sites in five Asian countries (Table 2). These sites represent a wide range of soil types and rice culti-

5 A. Dobermann et al. / Field Crops Research 56 (1998) Table 2 Potassium and P use efficiency in 10 long-term fertility experiments with irrigated rice in 1993 (Dobermann et al., 1996b,c) Site Cultivar Grainyielda(kgha -1) Yield increase PE b AE([Akggrain]/ due to (%): ([kg grain]/[kg uptake]) [kg applied]) + NPK + NP + NK P K P K pc K d PhilRice IR a 5464b 5203b IRRI IR a 7385a 7367a Qingpu Hanfeng 7792a 7804a 7920a BRIARC 1R a 7403a 5214b Pantnagar Pant Dhan a 5083b 3905c Coimbatore CR a 4840b 4506b Maros IR a 5422a 5351a Cuu Long IR a 5280a 3966b Shipai Guichaoxuan 4001a 3904a 3751a Jinxian ER a 3633a 3339b "Grain yields adjusted to 14% moisture content. Means with common letters are not significantly different within sites by LSD (5%). baverage _+ standard deviation of six treatments (control, + N, + NP, + NK, + PK, + NPK). ~Agronomic P efficiency = (GYNp K -- GYNK)/fertilizer P applied (all measured as kg ha- 1 ). Agronomic K efficiency = (GYNp K - GYNp)/fertilizer K applied (all measured as kg ha- 1 ). vars, including the best-performing modem cultivars. Continuous annual double-cropped rice-rice systems were used at eight sites, whereas two sites used an annual rice-wheat sequence (Dobermann et al., 1996d). The PE at these sites ranged from 220 to 900 kg grain kg-~ P (Fig. 1), compared with 410 to 625 kg grain kg-~ P as reported in a review by van Keulen (1986). On average, 70% of the total P in aboveground biomass was contained in the grain (Dober- Grain yield (kg ha "1) 9000 ~maximum dilution. q maximum dilution / ] 8ooo4Y=9 /x- ', / _*'-./ / '' -. -a,/ / ] /.~'U~A _~" / al ot e" -- " or 6000 xmumoum' / 4000 ~,.~z,~/,. y = 220 (x-0.1). ~2~0~/ y= 30 (x-3) / ~ /'/ LTFE at,l sites 1993DS / / A Farmers'fields CentralLuzon 1994DS / / / / Total P uptake (kg ha "1) Total K uptake (kg ha "1) Fig. 1. Relationship between total P and K in aboveground biomass at maturity and grain yield (14% moisture) in LTFEs with irrigated rice conducted at 10 sites in 5 countries and in 60 farmers' fields of Central Luzon, Philippines. Dashed lines show the envelope of maximum accumulation ( = minimum PE) and maximum dilution ( = maximum PE) of P and K in the plant (Dobermann et al., 1996c,d; Dobermann, A., unpublished data).

6 118 A. Dobermann et al / Field Crops Research 56 (1998) PE (kg grain kg P uptake -1) O o, g 0 i i Control N PE (kg grain kg K uptake 1) o 8 o 8, O Outside 10th and 90th percentile 25th to 75th percentile Median o o o o 8 8 o i i i i l NK NP PK NPK O i i Control N NK NP PK NPK Fig. 2. Physiological efficiency (PE) of P and K in 6 fertilizer treatments of LTFE at 10 sites in Boxplots are based on three replicates of each treatment at all sites. mann et al., 1996c). Distribution of plant mass between grain and straw had a large impact on P partitioning. Dry matter HI and the P harvest index were positively correlated (r = 0.74 * ). The PE was 610 +_ 173 (mean of sites _+ SD) kg grain kg -1 P in + NK treatments, compared to 359 _+ 108 kg grain kg-] P in + NPK treatments (Fig. 2). At Maros, the PE was lowest and did not vary much among fertilizer treatments (282 _+ 9 kg grain kg-1 p, Table 2). This result could be attributed to the very high soil P content at this site, where Olsen-P ranged from 37 to 47 mg kg ~ in all six fertilizer treatments (Dobermann et al., 1996c). The slope of the relationship between grain yield and K uptake varied from 30 to 110 kg grain kg -~ K o o absorbed (Fig. 1). van Keulen (1986) reported a range of 55 to 80 kg grain kg- 1 K for rice, based on a small number of published reports. To some extent the scatter in Fig. 1 may be caused by variations in HI, but the relationship between K harvest index and HI was much poorer than for N or P (r = 0.49). Losses of K due to leaching from leaves or leaf senescence at the end of the grain-filling period, and partial replacement of K in the plant by other cations such as Na (van Keulen, 1986), are other important factors that may affect the PE of K. Large differences were found among sites and treatments in PE (Table 2) and, within each site, significant differences were evident among fertilizer treatments. At sites with high soil K-supplying capacity, such as Lanrang and IRRI, data points were located close to the line of maximum K accumulation. Treatments with balanced nutrition (+NPK) had significantly lower PE (53 +_ 15 kg grain kg -1 K uptake) than + NP treatments (68 _+ 26 kg grain kg-1 K uptake, Fig. 2). Results of these studies confirm strong nutrient interactions as determinants of PE (Janssen et al., 1990). With a limited nutrient supply, there is maximum dilution of the nutrient in the plant, and uptake is not influenced by growth but only by supply (source limitation). Conversely, when the supply of a nutrient is large and growth is not limited by uptake, the internal nutrient concentration is high, and there is maximum accumulation (sink limitation). In this situation, growth is limited by other factors (Janssen et al., 1990). In irrigated rice, water is generally not a major growth-limiting factor and the PE for a given nutrient depends on the factors that determine yield potential (e.g., light flux, temperature), availability of other nutrients, HI, nutrient losses through leaching from plant foliage during the reproductive phase, pest damage, and genotypic variation in internal utilization efficiency. In most environments, nutrient interactions are the major determinants of PE and must be considered in estimating crop nutrient demand Estimating current and projected nutrient uptake requirements Irrigated rice takes up approximately equal amounts of N and K on a mass basis, and three times

7 A. Dobermann et al. / Field Crops Research 56 (1998) more N on a molar basis. Nevertheless, the equivalence of N- and K-uptake requirements is not recognized in current fertilizer practices. In our analysis of 192 plots of 6 different fertilizer treatments at 10 sites, grain yield ranged from 2.0 to 8.3 t ha- 1. Total uptake in grain and straw was 20 to 200 kg N ha-1 (10 to 31 kg N t -I grain yield), 23 to 255 kg K ha -1 (8 to 35 kg K t-1 grain yield) and 4 to 34 kg P ha-l (1 to 5 kg P t -1 grain yield). Mean values were 88 kg N ha -1, 95 kg K ha-l and 12.6 kg P ha-i at an average grain yield level of 4.9 t ha -1. At yield levels of 4 to 8 t ha- i, K and P uptake requirements varied from 17 to 30 kg K t -I and 2 to 4 kg P t -1 of grain yield and increased with increasing yield level (Dobermann et al., 1996b,c). Sulfur removal is expected to range from 1 to 3 kg S t-1 grain yield (Yoshida, 1981; De Datta and Mikkelsen, 1985; Mohapatra et al., 1993; Mamaril, C.P., unpublished data). For yields greater than 8 t ha 1, total K uptake usually exceeds 200 kg ha -1, but it is difficult to make reliable extrapolations with the limited number of observations at that yield level. Uptake of 31 to 46 kg P ha-1 and 258 to 309 kg K ha-1 were reported for rice crops in the yield range of 9 to 10 t ha -1 in the Philippines (Yoshida, 1981; De Datta and Mikkelsen, 1985). The amount of K uptake required to produce 10 t grain ha -1 at least equals the estimated N uptake of 200 kg ha- l (Setter et al., 1994) and a minimum of 30 to 40 kg P ha- 1 and 15 to 20 kg S ha- 1 must be accumulated by the crop. On the soils in the 10 LTFEs, and in most lowland rice soils in Asia, uptake requirements for yields of 8 to 10 t ha- 1 exceed the nutrient supply from indigenous soil resources and current levels of fertilizer inputs by a large margin, particularly for K Nutrient uptake rates and root traits Patterns of P, K and S uptake follow dry matter production and N uptake. Between 75 and 90% of the total K uptake occurs between tillering and early grain filling, a period as short as 60 days (De Datta and Mikkelsen, 1985; Shi et al., 1990). In a field study with 200 kg N ha-1 applied to IR72 at IRRI, 30% of the total K and P accumulation at maturity was taken up between 32 to 45 days after transplanting (DAT; Dobermann, A., unpublished data). Peak uptake rates of 3.7 to 5.5 kg K ha- 1 day- 1 and 0.60 to 0.70 kg P ha-1 day-1 occurred between tillering and flowering. Yield levels were 7 to 8 t ha -1. Assuming a total uptake of 200 to 300 kg K ha-1 and 30 to 40 kg P ha- 1, these rates of uptake could sustain yields of up to 10 t ha -1. Cation uptake (NH~-, K +) by rice may be limited by impeded cation mobility, which results from rhizosphere acidification (Kirk et al., 1993). The rice plant seems to compensate by developing a dense, fibrous root system with root length densities that exceed values of other cereal crops (Ladha et al., 1998). Recent data from IRRI show, however, a significant response to K fertilizer at yield levels of more than 8 t ha- ~ in a long-term experiment (Table 3). In this LTFE, no response to K was observed between 1964 and 1991 and during this period yield levels decreased to 6 to 7 t ha -1. In 1992, fertilizer rates were increased from 140 to 200 kg N ha -~, 13 to 25 kg P ha -1, and 25 to 40 kg K ha -1, resulting in dry season (DS) yields of 8 to 9 t ha 1 and a significant response to K. Extractable soil K content (expressed as centimole charge kg -1) was high in soils of all treatments (> 0.5 cmol c kg-l). The higher grain yield in the + NPK treatment compared to + NP seems to result from small differences in K activity caused by fertilizer application (Table 3), but limitations in root K uptake due to rhizosphere processes may have occurred as well. We generally need a better understanding of interactions between redox status and K activity in flooded soils. Table 3 Grain yield (kg ha -1) of IR in the long-termfertility experiment at IRRI and PhilRice sites in the 1995 dry season (Descalsota, J., Cassman, K., unpublished data) a Fertilizer treatment b IRRI PhilRice b Unfertilized control 4150d 3050b + N 7550c 2640b + NP 8300b 3950b + NK 7730bc 3670b + NPK 9000a 9630a + NPK with compost 9600a - aaverage of four replications at IRRI and three replications at PhilRice. In a column within each site, means followed by a common letter are not significantly different at the 5% level by DMRT. byhe treatment with straw compost does not exist at PhilRice.

8 120 A. Dobermann et al. // Field Crops Research 56 (1998) In contrast to K, rhizosphere acidification has a positive effect on P uptake and, in most soils, rice plants seem to depend upon root-induced solubilization for the bulk of their P (Kirk and Saleque, 1995; Saleque and Kirk, 1995; Kirk et al., 1998) Critical plant tissue concentrations In early growth stages, nutrient contents in the uppermost fully expanded leaf (Y-leaf) are suitable indicators of the plant P, K and S status, and critical levels for occurrence of deficiency are well established (Table 4). During vegetative growth before flowering, concentrations of 1.8 to 2.6% K and 0.2 to 0.4% P in the Y-leaf and > 0.15% S in the whole shoot indicate sufficiency and that a response to the applied nutrient is unlikely (Blair et al., 1980; Bergmann, 1988). Potassium concentrations of grain from modern rice cultivars are typically in the range of 0.25 to 0.33%. Long-term use of K fertilizer in LTFEs did not result in higher K concentrations in grain compared to -K plots, whereas K fertilization did increase K concentration of straw (Dobermann et al., 1996d). Critical levels of K in the straw at harvest range from 1.0 to 1.8% (Table 4). Differences in soil and fertilizer P supply are reflected in both the P concentration of grain and straw, which may range from 0.09 to 0.40% and 0.02 to 0.21%, respectively (Dobermann et al., 1996c). In our analysis of these data, yields of greater than 7 t ha- l required > 1.2% K and >0.05% P in the straw at harvest and > 1.2% K and > 0.17% P in the flagleaf at flowering (Cassman, K., Sta. Cruz, P., Dobermann, A., unpublished data), which is consistent with the values reported by other researchers (Table 4). Sulfur deficiency symptoms need to be detected early if the problem is to be corrected before it affects yield. Because foliar symptoms are similar to those of N deficiency, S deficiency is often not diagnosed and plant tissue or soil analysis is important for its identification. Critical levels for S deficiency have been established (Table 4). Between tillering and flowering, either a concentration < 0.1% S in the shoot or a N:S ratio of > 15 indicate S deficiency (Blair et al., 1980) Transport of mineral elements to the grain Amino acids and mineral elements are transported from the roots in the xylem to sink tissues (Hayakawa et al., 1994). The xylem may be a major source of mineral elements for the developing grain (Biddulph et al., 1958), and transpiration is significant in regulating the thermal balance of the developing panicle and the supply of mineral elements. Table 4 Ranges of critical contents for occurrence of K, P and S deficiency in rice plants Growth stage Plant part Critical level (%) References Potassium Tillering Y leaf a Flowering top 2-3 leaves Maturity straw Phosphorus Tillering Y leaf 0.10 Flowering flag leaf 0.18 Maturity straw 0.06 Sulfur Tillering Y leaf 0.16 Tillering shoot Flowering shoot Maturity straw ay leaf = uppermost fully expanded leaf. (Yoshida, 1981; De Datta and Mikkelsen, 1985; Rao and Sekhon, 1988) (Reddy et al., 1982; Rao and Sekhon, 1988) (Kanareugsa, 1980; Yoshida, 1981; De Datta and Mikkelsen, 1985; Rao and Sekhon, 1988; Sutar et al., 1992) (De Datta, 1981; Yoshida, 1981) (Chang, 1978) (Kanareugsa, 1980) (Yoshida and Chaundhry, 1979) (Suzuki, 1978; Yoshida and Chaundhry, 1979) (Yoshida and Chaundhry, 1979) (Wang, 1976; Suzuki, 1978; Yoshida and Chaundhry, 1979)

9 A. Dobermann et al. / Field Crops Research 56 (1998) Table 5 The average composition of glutelin, the average composition of an amino acid in the phloem sap and endosperm sap of rice (Sheehy, 1996) Elements Molecular weight (g) Carbon (g) Hydrogen (g) Nitrogen (g) Oxygen (g) Glutelin Phloem sap Endosperm sap Sulfur (g) Sulfur is taken up by the roots as the sulfate ion (SO42-) and transported from the roots in the xylem. Reduction can take place in the roots, but largely takes place in chloroplasts. The reduced S is incorporated into cystine, cysteine, and methionine and it is important for the formation of chlorophyll. Reduced forms of S move to the panicle during grain filling in the phloem. Hayashi and Chino (1990) reported concentrations of sucrose, amino acids, potassium and ATP of 574 mm, 125 mm, 40 mm and 1.8 mm, respectively, in the phloem of the uppermost internode. Glutelin is the primary storage protein in rice. Based on recent data, however, there appears to be insufficient S in the phloem sap to meet the synthetic demands of grain to produce the amounts of glutelin that are observed (Table 5). The additional S must be moved to the grain via the xylem, but it is generally unclear the extent to which the xylem is a major source of S and other mineral elements during grain filling, and how that supply affects grain yield and quality. Pot experiments at IRRI have revealed that S deficiency as indicated by N:S ratios of 16 and 25 may also significantly affect grain quality due to reduced cysteine and methionine contents (Juliano et al., 1987) Agronomic efficiency and yield response to P, K, and S Modern rice cultivars respond to N application, with the agronomic efficiency (AE; kg grain yield increase kg-1 N applied) usually ranging from 15 to 25 kg grain yield kg -1 N applied (Yoshida, 1981). In the early years of the Green Revolution, at most sites, yield responses to fertilizer P and K were small (De Datta, 1983; De Datta and Mikkelsen, 1985). Soils were still rich in these nutrients, and flooding was thought to increase their supply. Phosphorus became the first deficient nutrient revealed in longterm experiments after a few years of intensive cropping whereas it usually took longer until significant responses to K application were found (Shiga, 1982; De Datta, 1983; De Datta et al., 1988b). In 1993, in the LTFEs at 10 sites, AEs varied from 0 to 114 kg grain yield kg -1 P (mean = 49 kg grain yield kg-~ P) and 0 to 26 kg grain yield kg-1 K applied (Table 2). Most sites responded to P (average yield increase of 22%) whereas only three sites had significant K response. A significant K response was not observed at three sites that also had low yields and low N and P uptake (Cuu Long, Shipai, Jinxian, Table 2). Unlike K, substantial P additions from sources other than fertilizer are not likely to occur, so the yield response to applied P mainly depends on the effective P supply from the soil. What are the reasons for the small K response at many sites? One hypothesis is that relatively large amounts of K are added from sources other than fertilizer. At IRRI, for example, large K input with irrigation water has resulted in a buildup of the extractable soil K content, even in -K treatments (De Datta et al., 1988b). None of the other nine LTFE sites had such large K inputs from sources other than fertilizer. A second hypothesis is that release of slowly available K may have caused little or no K response at sites such as BRIARC, Lanrang, Maros and Qingpu (Dobermann et al., 1996b). A third hypothesis is that the NPK rates used did not provide sufficient nutrient supply to maximize crop growth and K demand, and that other constraints or nutrient interactions may have limited K uptake and/or K response. Applied S inputs come mostly from use of ammonium sulfate (24% S) or single superphosphate (12% S). In countries where S is recommended, such as Indonesia and Bangladesh, application of 20 to 40 kg

10 122 A. Dobermann et al. / Field Crops Research 56 (1998) S ha-1 is adequate for highest yields but, depending on the degree of S deficiency, yield level, nutrient interactions and the rate applied, AE of applied S is quite variable. An average yield increase due to S of 19% was reported from 28 sites in South Sulawesi (Blair et al., 1979). In multilocation trials in the same region, AE ranged from 65 to 157 kg grain kg-~ S applied, whereas in rainfed rice areas in Indonesia AE was only 25 to 33 kg grain kg -1 S applied (Mamaril, C., unpublished data). Under some conditions, response to S is not consistent. At Jakenan, Indonesia, response to S has been observed consistently during the dry season, but not in the higher-yielding wet-season crop grown in the same field (Mamaril, C., unpublished data). I 0 C o L 0 LTFE Sites: 15 site-seasons Dry season ~ ' o Wet season ~, q " " Y = X ~,k",; r 2 = 0.84"* n = 99 IRRI 94DS (N) o / 94DS (NK) c~ IRRI 93WS (N) o,'~,/ ~ ~, ol(~,, o--prri 93es (N) /It / --~PRRI 95DS(NK) / /,-o. ~PRRt94DS(NK) %/ /0 ~'pret 94ws <NP) ~0 7 'PRRI 94WS (N) o, q~'prri 95DS (N] X,* C~--PRRI 93WS (N) "~" 1:1 / line Grain yield of IR72 (kg ha -1) Fig. 3. Relationship between dry-season grain yield of an elite breeding line with higher yield potential (IR ) and yield of IR72 in LTFE conducted at three sites in the Philippines (IRRI, PhilRice, BRIARC, see Table 2). The values of three dry seasons ( ) represent various combinations of N, P and K fertilizers, including an unfertilized control. Each observation represents the mean yield in replicated treatment plots using a RCB design. Ten observations are identified by site, season, and fertilizer treatment (Descalsota, J., Cassman, K., Khush, G., unpublished data). Imbalanced nutrition may prevent significant response to added nutrients, even if soil levels of one or more nutrients are high. Interactions among nutrients limit or enhance the observed response to a single nutrient and are a key AE factor of added fertilizer nutrients in irrigated rice. At PhilRice, for example, both the climatic yield potential and the fertilizer rates in the LTFE were high (200 kg N ha -1, 25 kg P ha -1, 100 kg Kha 1). In the 1993 dry season, grain yield of IR72 was 8 t ha-1 in the + NPK treatment, but only 5.5 and 5.2 t ha-1 in the + NP and + NK treatments, respectively (Table 2). An even much greater interactive response to P and K was measured in the 1995 dry season for an improved breeding line (IR ), which yielded almost 10 t ha -1 (Table 3). The significant response to K of the same new line at IRRI (Table 3) appears to be partly caused by P K interactions because soil K levels were very high in all treatments. Imbalanced nutrition may also prevent new cultivars from expressing their full yield potential. In LTFE at three Philippine sites, average grain yields of IR were 500 to 600 kg ha higher than those of IR72, but yields lower than for IR72 were found in treatments that had not received fertilizer P or P and K for a long time (Fig. 3). Simply adding more of one nutrient without knowledge of the soil nutrient supply of other nutrients is an inadequate strategy, but it is current practice in many blanket recommendations for fertilizer use on rice. 3. Soil nutrient-supplying capacity 3.1. Soil nutrient pools and implications for soil testing Flooding a dry soil causes an initial increase in the P concentration in soil solution because of reduction of Fe(III) compounds that liberate sorbed and coprecipitated P. It was generally thought that the availability of soil P was higher under flooded conditions than under upland conditions on the same soil (De Datta, 1983; Diamond, 1985). The initial P flush, however, is followed subsequently by a decrease from resorption or precipitation of Fe(II)-P

11 A. Dobermann et al. / Field Crops Research 56 (1998) compounds (Ponnamperuma, 1972; Chang, 1976; Willett, 1986; Kirk et al., 1990). Much of the reimmobilized P becomes acid-soluble (Kirk and Saleque, 1995), and a large proportion of P taken up by rice seems to be drawn from P pools that are soluble under alkaline, aerobic conditions [Fe(III)-P and A1-P], but which are transformed into acid-soluble P forms as a result of submergence and reduction (Jianguo and Shuman, 199 l; S aleque and Kirk, 1995). Therefore, suitable soil tests for rice should provide an index of this pool. Kirk et al. (1998) give a detailed review of P chemistry and its implications for root uptake. Effects of flooding on the chemistry of soil K have not been studied adequately (De Datta and Mikkelsen, 1985), but K availability is often said to increase in flooded soil owing to K displacement by Fe 2+ and NH~- from exchange sites (Ponnamperuma, 1972; De Datta and Mikkelsen, 1985). The result would be a short-term increase in the intensity factor (solution K+), and water saturation would also increase K diffusion rates. This effect probably occurs in soils with low K-fixation potential, because good correlations exist between extractable K and K uptake by irrigated rice on soils with predominantly l:l-layer minerals in their clay fraction (Dobermann et al., 1996b). Chemical extraction of these readily available K pools would provide a reasonable index of K availability in such soils. Many soils found in deltas, floodplains and inland valleys in Asia, however, contain vermiculite, illite or other K-fixing minerals. The high root density, relatively high maximum influx and low minimum solution concentrations for K uptake (Teo et al., 1992) indicate that rice depends on nonexchangeable K reserves for much of its K supply on such soils (Cassman et al., 1995). Recent studies with vermiculitic soil also indicate that due to increased K-fixation, plant-available K would decrease after flooding of dry soil (Olk et al., 1995). Particularly on alkaline soils, an excessive (Ca + Mg)/K ratio or reduced K activity in soil solution from preferential K adsorption may contribute to low K uptake by rice, even when ample K is available (Pasricha, 1983; Dobermann et al., 1996b). Similarly, excessive accumulation of Fe 2+ on soil exchange sites may lead to slow, long-term changes of K activity in paddy soils, but such mechanisms are poorly studied. Therefore, dynamic mea- sures integrating K release from different pools and K transport to the root surface are required to obtain reasonable estimates of the potential K supply across soils with different clay mineralogy (Dobermann et al., 1996b). Sulfur in soils occurs in four major forms, namely C-bonded S, ester sulfate, adsorbed SO42- and SO 2- in soil solution. Plants acquire S in the form of SO42- from the soil solution. Reduction of SO 2- to elemental S- and formation of sulfides follows iron reduction in flooded soil (Ponnamperuma, 1972). Oxidation of added or mineralized elemental S is rapid in aerated soil zones (He et al., 1994a). A significant fraction of soil S present as organic S is mineralized to sulfate or sulfide before it is available to plants (Lefroy et al., 1992). Soil tests for rice should measure inorganic S as well as some of the mineralizable organic S fraction Static soil tests Except in India, routine soil testing for specifying fertilizer recommendations is not widely used in Asia. The standard approach emphasizes the identification of deficient soils or plant response using rapid chemical tests with defined critical thresholds or threshold ranges (Dahnke and Olson, 1990; Peck, 1990). A large number of field experiments is required to establish relationships between a given soil test value and the probability of a response to applied nutrient. Mostly, such calibrations are relatively specific to a given soil type. The question arises as to what extent static soil tests are adequate to provide reasonable recommendations for P, K and S input requirements in intensive, irrigated rice systems in the tropics. Extractable soil K or K saturation of the CEC are still the most widespread measures of available K in rice soils. Their suitability as a measure of plantavailable K remains controversial and depends on the clay mineral composition and clay content (Dobermann et al., 1996b). Although 0.17 to 0.21 cmol c K kg ~ (1 N NH4OAc-extractable K) seems to be a generally accepted critical level for occurrence of K deficiency in rice soils, reported values range from 0.08 to 0.41 cmol c K kg 1 (Dobermann et al., 1996b). The 1 M HNO 3 method is widely used by Indian and Chinese researchers to provide a

12 124 A. Dobermann et al. / Field Crops Research 56 (1998) measure of nonexchangeable K, but the results were not always correlated to grain yield and total K uptake (Nath and Purkaystha, 1988; Panda and Panda, 1993). Other methods, such as quantity/intensity (Q/l) relationships (Pasricha, 1983; Basilio and San Valentin, 1990) and electro-ultrafiltration (Wanasuria et al., 1981; Ramanathan and Nemeth, 1982), are laborious or expensive and not used in routine analyses of rice soils. Olsen-P (0.5 M NaHCO 3 at ph 8.5) or, to a lesser extent, Bray-l-P (0.03 M NH4F M HC1), are mostly used as indices of available soil-p in flooded rice soils (Chang, 1976). Critical levels of Olsen-P reported for rice range widely (from 4 to 29 mg P kg-~), depending both on soil chemical properties that affect the storage of P in different inorganic pools and on how a critical level for P is defined (Dobermann et al., 1996c). Among the S soil tests used for rice, 0.01 M Ca(H2PO4) 2 is probably most common, and critical levels typically range from 7 to 11 mg kg-~ (Wang, 1976; Chang, 1978; Suzuki, 1978; Islam and Ponnamperuma, 1982). Depending on the analytical method used for determination of S in the extract, some organic S may be included in this measure (Anderson et al., 1992), and differences may occur in the critical levels established. Because it includes some of the ester sulfates, a 0.25 M KC1 extract heated at 40 C for 3 h may provide a better soil test for S (Blair et al., 1993). All rapid soil tests used for P, K and S for rice production have theoretical limitations, including that (1) nutrient availability in irrigated rice systems is extremely dynamic and tests on air-dried soil may not fully reflect nutrient status after submergence; (2) differences in clay mineralogy and physical properties have a strong impact on desorption characteristics and plant availability (He et al., 1994b; Dobermann et al., 1996b); (3) unextracted nutrient pools may also contribute to plant uptake; (4) diffusion is a key process of P and K transport to the root surface (Turner and Gilliam, 1976b; Teo et al., 1995); (5) external mechanisms such as root-induced solubilization of P by acidification contribute significantly to uptake by rice roots (Jianguo and Shuman, 1991; Kirk and Saleque, 1995); and (6) kinetics of nutrient release are not measured. Selecting appropriate extractants such as Olsen-P can reduce the first of these limitations, and the second and third limitations can be partly dealt with by using data on clay mineralogy or other soil properties to interpret static test thresholds. But none of the static soil tests currently in use accounts for--across a wide range of soils--diffusion, rhizosphere effects (see Kirk et al., 1998) or the kinetics of nutrient release Dynamic soil tests Ion-exchange resins can be used to estimate bioavailable nutrients dynamically because they maintain low ion concentrations in solution, thereby stimulating further release from soil solids. The state of the art has been reviewed by Skogley and Dobermann (1996) and several applications to rice soils have been reported (Turner and Gillam, 1976a; Xie and Du, 1988; Yang et al., 1992; Boruah et al., 1993). Resin-batch techniques--methods in which a soil suspension is shaken with the resin--are comparable to static soil tests and used mostly to extract the most labile nutrient fractions (Hedley et al., 1982; Saleque and Kirk, 1995). Resin-incubation techniques assess the nutrient supply to a strong resin sink over a longer time period and, in most situations, they account for diffusion as one process controlling the amount of nutrient adsorbed on the resin (Skogley and Dobermann, 1996). Between 1992 and 1995 we evaluated the Phytoavailability Soil Test (Yang et al., 1991) for use in irrigated rice. Capsules containing a mixture of cation and anion resin (H+/OH - form) are placed in a soil paste made from freshly sampled paddy soil from flooded ricefields. The adsorption kinetics of anions and cations are then measured in a subsequent 2-week anaerobic incubation at 30 C (Dobermann et al., 1994b). The capsule provides a strong and infinite nutrient sink over long incubation periods (Yang et al., 1991) and mimics some of the key processes occurring in the rooted zone of a paddy soil, including root nutrient uptake and rhizosphere acidification (Dobermann et al., 1995a). The dominating anion in flooded soils is HCO 3 (Kirk et al., 1993), which is not efficiently exchanged on OH--saturated anion resin. Therefore, net cation uptake in the resin capsule-soil system is larger than anion uptake. The capsule releases H + in exchange for cations taken up, causing acidification of the soil around it that is

13 A. Dobermann et al. // Field Crops Research 56 (1998) PhilRice IRRI Qingpu BRIARC Pantnagar Coimbatore Maros Cuu Long ~ (0.036) Shipai Jinxian [] (0.536) (0.481) (0.367) (o.318) RAQ at ld (= ap). (0.356) ~ RAQ ld to 14d (0.465) (0.338) (0,670) Resin P adsorption (RAQ, pmol cm "2) Fig. 4. Variation in resin adsorption quantities (RAQ) of P in the +NPK treatment of LTFE at 10 sites in the 1993 dry season. Soil samples were collected from the 0-20 cm layer between 20 and 30 days after transplanting and analyzed at IRRI within 3 to 4 days after sampling. The value of the adsorption rate coefficient b e is given in parenthesis (Dobermann et al., 1996c). similar to acidification measured in the rice rhizosphere (Begg et al., 1994; Dobermann et al., 1994a; Kirk and Saleque, 1995; Skogley and Dobermann, 1996). A simple two-constant rate model (RAQ = at b) can be used to describe the resin adsorption quantities (RAQ) of many cations and anions as a function of time (t) in a wide range of soils (Dobermann et al., 1994b). The method allows one to assess the variation in nutrient release between lowland rice soils in terms of kinetic parameters like a and b (Fig. 4). Rapid adsorption by the resin capsule during 1 day (initial adsorption rate constant a) mainly represents P and K fractions that are either in solution or in dynamic equilibrium with the soil solution (e.g., 1 N NH4Oac extractable K). The adsorption RAQ (pmol P cm -2) 2.0 +NP TRTM a b r 2 RAQ = a time b ~"" Control ~.. - "~ PK +N ~"~- ~" +NP "~''-~" ~" ~ +NPK +NK ~'" ~'~ - ~ 1.5 +PK ~''~-~ +NPK " i "'. 9' i ~ 6 i ~ ~ ' ~ ~ i ' _ i ' i ~ ' " i i ' ". ~ ii +NC ntr l 1.0 ~':" Time (d) Fig. 5. Phosphorus release kinetics measured with the resin capsule in six treatments of the LTFE at Qingpu, China. In this experiment, a rice-wheat cropping system has been practiced since In the 1993 dry season, soil samples (0-20 cm layer) were collected from replicate plots of six different combinations of N, P and K fertilizer 30 days after transplanting. Phosphorus adsorption kinetics on the resin capsule were measured during 2-week anaerobic incubation at 30 C (Pampolino, M.F., Adviento, M.A.A., Dobermann, A., unpublished data).

14 126 A. Dobermann et al. / Field Crops Research 56 (1998) rate coefficient b, on the other hand, characterizes the capability of a soil to maintain a nutrient flux to a strong sink, e.g., a resin or a plant root. At 10 sites, the resin capsule sensitively measured the buildup or depletion of soil P or K as a result of fertilizer history (Dobermann et al., 1996b,c). In the rice-wheat LTFE at Qingpu (China), for example, treatments that had received P for a long time (+ PK, + NP, + NPK) had a much greater P release than those in which P was not applied (Control, +N, + NK), with the greatest depletion of soil P found in the + NK treatment (Fig. 5). At this site, the treatment differences were mostly reflected by differences in the b coefficient describing slow P release over time. The b coefficient was highly correlated with Olsen-P and dithionite-citrate extractable Fe. Both methods seem to provide an index of the Feand A1-P pool that is alkali-soluble under aerobic conditions but acid soluble under anaerobic conditions (Dobermann et al., 1996c), and which contributes most to P uptake by rice roots (Saleque and Kirk, 1995). For K, the rate coefficient b was highly correlated with the (Ca + Mg)/K ratio and the CEC of the clay fraction (Dobermann et al., 1996b), indicating that it integrates measures of K exchange equilibrium, K mobility, and K release from fixed forms (Martin and Sparks, 1983; Havlin and Westfall, 1985), as well as some release of structural K from soil minerals due to acidification (Feigenbaum et al., 1981). Kinetic parameters of nutrient release such as a and b were suitable predictors of the effective soil K- and P-supplying capacity estimated by P and K uptake in LTFE conducted on very different soils (Table 6). Both Olsen-P and the resin capsule were similarly correlated with P uptake, but the resin capsule was a much better predictor of K uptake than a rapid soil test such as extractable K. A whole suite of static soil tests was needed to explain the variation in K uptake across soils with different texture and clay mineralogy (Table 6). This approach for estimating the indigenous soil nutrient supply assumes that no others factors (pests, solar radiation, other nutrients) have limited actual uptake (Janssen et al., 1990). Crop uptake would then be linearly related to soil supply over an agronomically reasonable range and the resin capsule would provide an estimate of Table 6 Regression equations describing the relationships between soil properties and P or K uptake of irrigated rice Regression equations a N r 2 Static soil tests P uptake = Olsen-P P uptake = Olsen-P fertilizer-n P uptake = Olsen-P CECclay/V-value fertilizer-n K uptake = K K uptake = K CECday/KS(Ca + Mg)/K 1on-exchange resin capsule P uptake = ap bp P uptake = ap bp fertilizer-n Kuptake= a K+227b K * * * * * * * * * * * * * * * * " * * * * * The equations for static soil tests are based on stepwise forward regression of three replicates of the + NK and + NPK treatments (for P) or +NP and +NPK treatments (for K) at 10 sites sampled in 1993 using a tolerance value of T= 0.1. The equations containing kinetic constants of resin adsorption are based on bulked samples for each treatment at 10 sites (Dobermann et al., 1996b,c). ap uptake = total P uptake with grain and straw at harvest (kg ha- l); Olsen-P (mg kg 1 ); CECclay = CEC of clay fraction (cmol kg l ); V-value = base saturation (%); a e = initial P adsorption rate constant; bp = P adsorption rate coefficient. K uptake = total K uptake with grain and straw at harvest (kg ha- 1 ); K = exchangeable K (cmol kg- 1 ); KS = K saturation of CEC (%); (Ca + Mg)/K = (Ca + Mg)/K ratio, exchangeable forms (cmol kg-1); a K = initial K adsorption rate constant; b K = K adsorption rate coefficient; fertilizer N = kg N ha -1.

15 A. Dobermann et al. / Field Crops Research 56 (1998) the potential soil nutrient supply as one basis for specifying fertilizer recommendations. More calibration data are needed to verify this hypothesis. Dynamic soil tests overcome many of the theoretical limitations associated with rapid chemical extractions. The resin capsule, for example, integrates intensity, quantity and delivery rate measures of P and K supply to the rice root in flooded paddy soils, it simulates rhizosphere acidification and its effects on nutrient availability, and it provides parameters that help to assess both short- and long-term nutrient-supplying power in a dynamic manner. Perhaps the major advantage of this method is that it can be used as an universal soil test for simultaneous measurement of N, P, K, S, Ca, Mg, Fe, Mn, Zn, Cu, and other nutrients across very different soil types. The resin capsule offers the potential for multi-element in situ soil nutrient extraction in flooded rice soils without the need for collecting and processing soil samples. Depending on the precision required and the nutrients of interest, spatial variability of ion supply to resin capsules placed in situ may complicate the task of obtaining representative RAQ values for a ricefield. Sampling of 8 to 10 soil cores, mixing the soil in a plastic bag and resin capsule incubation of the composite soil sample buried on-site is an alternative procedure for obtaining representative estimates of RAQ for a whole field (Dobermann et al., 1997). In any case, the resin capsule procedure avoids transport of soil to a laboratory and sample preparation such as drying, grinding and sieving. 4. Nutrient balances and management practices Estimating the complete P or K balance in an irrigated ricefield would require measurements of nutrient outputs from crop removal, leaching, runoff and seepage, and of nutrient inputs from fertilizer, recycled straw, irrigation water, rainfall, seepage, sedimentation and capillary rise. Many of these components are site-specific. Runoff is usually of minor importance and we may assume that inputs and outputs from seepage are equal. We do not have enough data about sedimentation as a source of nutrients, but it is mostly of importance in large deltas like the Mekong Delta. Rainfall, irrigation and leaching seem to be of more general importance for irrigated rice systems and we discuss them in more detail now Nutrient inputs from rainwater and irrigation water and leaching losses At an average concentration of 1 mg L -l, 1000 mm ha -1 of water adds 10 kg nutrient ha -1 to a ricefield. The relevant questions to answer are: (1) to what extent is this nutrient supply a net gain; and (2) how much of the crop nutrient demand is satisfied by such inputs? Rainwater is usually low in P and K and only occasionally will annual inputs exceed 10 kg K ha- (Handa, 1988; Abedin Mian et al., 1991). Sulfur concentrations in rainwater vary widely, and generally decline with increasing distance from the coast or from industrialized areas (Lefroy et al., 1992). Data from various rice-growing countries in Asia indicate an annual S deposition from rainfall of 4 to 29 kg S ha -1 (Amarasiri and Latheef, 1982; Lefroy et al., 1992). Preliminary measurements at IRRI have indicated high S content in rainwater (3 mg S L-l), which indicate a S deposition of approximately 60 kg S ha -j year-l (Sheehy, J., unpublished data). Assuming an annual rainfall of 1000 to 2000 mm, most sites of irrigated rice cultivation in Asia probably have annual inputs from rainwater in ranges of 0.2 to2kgpha -j,3to 10kgKha -l, and5to20s kg ha- 1. Phosphorus concentrations in most surface and deepwell waters used for irrigation are low (Kawaguchi and Kyuma, 1977), and annual P input from irrigation was less than 1 kg P ha- 1 in two studies in Bangladesh (Abedin Mian et al., 1991) and at IRRI (Dobermann, A., unpublished data). Only where irrigation water is taken from eutrophic rivers, may the P contribution be large (Minzoni and Moroni, 1987). At many sites in Asia, typical K concentrations in river or canal water range from 1 to 5 mg K L- (Kawaguchi and Kyuma, 1977; Handa, 1988). Using an estimated net irrigation water supply of 700 to 1200 mm, the K input per rice crop would be in the range of 7 to 60 kg ha-~ if surface water is used. Water pumped up from deep aquifers contains somewhat more K than surface water, and human activities may increase K concentrations in shallow-well water up to 100 mg K L -1 or more (Handa, 1988).

16 128 A. Dobermann et al. / Field Crops Research 56 (1998) At IRRI, irrigation water provides between 120 and 200 kg K ha-] per rice crop in the dry season alone (Dobermann, A., unpublished data). A range of 0.2 to 20 mg S L-] in irrigation and river water was reported (Freney et al., 1982), but most of the irrigation waters analyzed in Malaysia and Indonesia contained less than 3 mg S L -] (Lefroy et al., 1991). At many sites, we may expect that the magnitudes of S input are similar to K and that S input from irrigation exceeds S input from rainfall. At five sites in Sri Lanka, for example, estimates of S supply by irrigation water were in a range of 14 to 43 kg S ha-1 per season and exceeded rainfall inputs by a large margin (Amarasiri and Latheef, 1982). Leaching losses depend on soil solution concentrations and percolation rates, both of which vary considerably. Examples of K leaching losses were 0.1 to 0.2 kg ha -] day -] in Bangladesh (calculated from Abedin Mian et al., 1991), 0.56 kg ha -] d -1 in Brazil (Beltrame et al., 1992) and 1 to 2 kg ha -] d- ] at IRRI (Dobermann, A., unpublished data). Many rice soils have heavy texture and, under field conditions, reduced percolation rates due to puddling and formation of a plow pan. Increased solution P concentration after flooding may increase the potential for P losses due to leaching compared to non-flooded soils (Xiao, 1988), but field measurements are rare. The field study in Bangladesh reported P leaching losses of 1.6 kg ha-] year-1 in a silty soil (Abedin Mian et al., 1991). o K (kg ha ~ season -1) 4O 2O O -8O O -NPK N +NP +NK +NPK Treatment In many cases, inputs of P, K, and S from rainfall and irrigation seem to be comparable or even smaller than nutrient losses due to leaching and a net gain in nutrients is not realized (Abedin Mian et al., 1991; Haque, 1995), but we need more complete nutrient balance studies to verify this observation. Input and leaching of K vary widely and both significant net gains or net losses are possible. Contribution of sources other than soil P and fertilizer P to P uptake of rice in Asia is mostly small, whereas S deposition from rainfall and irrigation may provide enough S to meet plant demand at many sites P and K balances in irrigated rice systems We used a simplified approach for calculating partial net K and P balances based on fertilizer input, aboveground plant uptake, and recycling with the straw in LTFEs at 10 sites (Dobermann et al., 1996c,d). Average negative P balances of -7.0 to -8.0 kg P ha-t per season were estimated for the -P treatments in the LTFEs, whereas +P treatments had an average net P gain of 3.7 to 4.9 kg P ha -1 per season (Dobermann et al., 1996c). Phosphorus deficits were largest in the + NK treatments (-4.4 to kg P ha -1 per season) and resulted in a fast depletion of labile soil P reserves at many sites. The fertilizer P rates used (17 to 25 kg P ha -1 ) were large enough to maintain the P balance or even to build up soil P in most of the LTFEs, confirming I I Fertilizer K Recycled K K uptake K balance Fig. 6. Partial net K balance for one rice crop in six different fertilizer treatments. Values shown are averages and standard deviations (error bars) of 10 sites in 5 countries sampled in Stubble was recycled at five sites and all straw was removed at six sites, reflecting standard farmer practices for each location (Dobermann et al., 1996d).

17 A. Dobermann et al. / Field Crops Research 56 (1998) significant residual effects of fertilizer P. The partial net K balance was highly negative in all NPK combinations tested (-34 to -63 kg ha -~ per crop cycle, Fig. 6) and fertilizer K application at an average rate of 40 kg ha ~ in the + NK and + NPK treatments was not sufficient to match the K removal at most sites. Extractable K in the + NPK treatments was not significantly different from values in the control plots at seven sites, indicating no accumulation of soil K from application of K fertilizer at the rates used (Dobermann et al., 1996d). About 80 to 85% of the K, 30 to 35% of the P, and 40 to 50% of the S absorbed by rice remains in the vegetative parts at maturity (Yoshida, 1981; Mohapatra et al., 1993; Dobermann et al., 1996c,d). In the LTFEs, interquartile ranges of nutrient concentrations in straw at harvest were 0.5 to 0.8% N, 0.04 to 0.10% P, and 1.4 to 2% K, equivalent to 5 to 8 kg N, 0.4 to 1 kg P, and 14 to 20 kg K t-1 of straw on a dry matter basis. Straw is either removed from the field, burned in situ, piled or spread in the field, incorporated in the soil, or used as mulch for the succeeding crop (Ponnamperuma, 1984). Each of these measures has a different effect on the overall nutrient balance. Incorporating the remaining stubble into the soil returns most of the nutrients. Burning straw destroys diseases, but causes P losses of about 25%, K losses of 20% (Ponnamperuma, 1984), and S losses of 4 to 60% (Lefroy et al., 1994). Straw removal from the field is widespread and contributes significantly to the depletion of soil K resources observed at many sites. At two sites with a high yield level (7.9 to 8.0 t ha -1, IRRI and PhilRice), a slightly positive P balance in the + NPK treatment was only achieved when significant P was recycled with the remaining tall stubble (8.0 to 8.3 kg P ha-l), in addition to the 25 kg P ha-1 supplied as fertilizer (Dobermann et al., 1996c). Our analysis confirmed reports from southern China (Huang et al., 1990) and India (Tiwari, 1985; Mohanty and Mandal, 1989; Mongia et al., 1991; Prasad, 1993), indicating negative K balances and on-going K depletion in many irrigated rice systems. The K rates applied by most farmers are even lower than the rates used in the long-term experiments and, particularly on soils where straw is completely removed, K exhaustion may be even more rapid. The situation seems to be less dramatic for P because fertilizer P application is more common, P-containing N fertilizers are popular, and P losses are small Long-term fate of added nutrients Long-term fate of added K depends on soil texture (leaching) and clay mineralogy (fixation), but our literature search located only a few studies on recovery rates of fertilizer K applied to rice. A microplot study in China with 86Rb-labeled KC1 on brown-red earth gave a first-season recovery rate of 23% (Guo et al., 1992), but the limited soil data given do not allow a clear evaluation of this result. The initial reaction of water-soluble fertilizer P with soil solid phases is rapid but the mechanisms of the subsequent slow decline of P in solution have not been characterized in flooded soils (Kirk et al., 1990). Slow intra-aggregate P diffusion is probably a major mechanism affecting the slow decline phase and long-term plant availability of added P in aerobic soils (Nye and Staunton, 1994). In a flooded paddy soil, however, most aggregates are destroyed by puddling and we may expect that this process is much less important. This may explain why the loss of availability of applied phosphates in a submerged soil is slower than it is in an upland soil (Chang, 1976). Apparent first-season recovery of fertilizer P is usually in the range of 5 to 30% (De Datta et al., 1966; Janssen et al., 1987; Dongale and Kadrekar, 1991), but direct measurements are rare. General models describing the long-term crop response to fertilizer P have been developed (Wolf et al., 1987; Janssen and Wolf, 1988) and they appear to be useful for specifying fertilizer recommendations because the data needed for model calibration can be obtained from simple field experiments. The Wolf-model uses the first-season recovery as an input variable, which includes dynamic changes during that period. Therefore, it may also account to some extent for the dynamic changes in the P chemistry of flooded soils (Willett, 1986; Kirk et al., 1990) and for possible rhizosphere effects on the solubilization of fertilizer P Nutrient management practices and recommendation domains Some of the challenges to improved nutrient management are illustrated by results from several recent

18 130 A. Dobermann et al. / Field Crops Research 56 (1998) case studies. For example, in a survey of 63 farms in Central Luzon, Philippines, rice yields in the 1994 DS ranged from 2.9 to 8.0 t ha-] (Dobermann and Oberthuer, 1997). Farmers applied 4 to 44 kg P ha- ' and 0 to 81 kg K ha -1, but the fertilizer rates applied were not related to measured soil test values (Fig. 7). Fertilizer rates and actual P and K uptake were not correlated, but all parameters covered a wide range. Most farmers burned the straw in piles after harvest, whereas the remaining stubble was incorporated into the soil with the next plowing. These practices recycle most of the K taken up by the plant, but the ash is usually not spread to the whole field after burning, resulting in patches with very high Si and K input within a field and possibly increased K leaching losses from those patches. In both dry and wet seasons the K balance was negative in most farms, with estimated average losses of 38 kg K ha-~ per crop. Many farmers preferred combination NP- or NPK-fertilizers whose fixed nutrient ratios did not allow proper adjustment of crop demand and soil supply. In two small rice-production domains, the Omon district (Mekong Delta, Vietnam) and in the province of Nueva Ecija (Central Luzon, Philippines), resinadsorption quantities of N, P and K after 7-day incubation had CVs among the farmers of 42 to 85% (Dobermann et al., 1996a). Rice farmers in both domains applied similar mean rates of P fertilizer (12 Fertilizer P (kg ha "I) O O 0 0 ee 8 ~ ee" Is'" 10 0 i i Olsen-P (rag kg -1) Fig. 7. Correlation between Olsen-P and fertilizer P application rate P in 60 farmers' fields of Central Luzon, Philippines. Each observation represents one field sampled in the 1994 dry season (Larazo, W., Oberthuer, T., Dobermann, A., unpublished data). to 13 kg P ha -l ) although the average RAQ P at 7 days was only 0.08 /xmol P cm -2 capsule surface in Omon compared to 0.15 txmol P cm -2 capsule surface in Nueva Ecija. In Omon, RAQ P at 7 days in 82% of all farmers' fields was lower than in the +NK treatment of the associated LTFE, which yielded 1.2 t ha-] less than the treatments with full N, P and K supply (Table 2). In Nueva Ecija, the RAQ K at 7 days in 60% of the farmers' fields was less than 1.16 /xmol cm -2 capsule surface. This same level was found in the K depleted + NP treatment of the LTFE, which, in 1993, yielded 2.6 t ha-1 less than the + NPK treatment (Table 2). In small rice production domains of five Asian countries, the magnitude of difference in grain yield from unfertilized plots among farms within each domain ranged from 2.8 to 4.9 t ha-' (Olk et al., 1996). Great between-farm variability in yield responses to P, K, Zn, and S were also found in on-farm studies at another site in the Philippines, but they could not be explained by static soil tests or the recent fertilization history (Angus et al., 1990). Because of high fertilizer subsidies, rice farmers in Java, Indonesia, have been applying triple superphosphate at rates higher than the rates suggested by the blanket recommendation during the rice intensification program. About 85% of the total lowland rice area now has a high soil P status (Sri Adiningsih et al., 1991) so that yield no longer shows a response to applied P, and the risk of pollution from P leaching has increased. Accumulation of available P and lack of P response as a result of high P fertilizer rates has also been reported from Taiwan (Lian, 1989). Whereas fertilizer practices would be expected to differ--from country to country, region to region, and village to village--in response to market forces, policies, and farmer goals, the conclusions drawn for N in irrigated rice (Cassman et al., 1998) apply equally to P, K and S. Potassium balances in many farmers' fields seem to be negative. In most irrigated rice areas, blanket prescriptions for the rate and timing of fertilizer applications are presently issued by national research and extension authorities for large domains (Table 7). Farmers often do not follow those blanket recommendations, but they also do not adjust their fertilizer rates according to a target yield, indigenous nutrient supply, or maintenance of the overall nutrient balance. Decisions about fertilizer

19 A. Dobermann et al. / Field Crops Research 56 (1998) Table 7 Examples of blanket recommendations for fertilizer use (kg ha- 1 ) on irrigated rice N P K Country Comments Sources Pakistan Pakistan Philippines Philippines Indonesia Indonesia Indonesia low fertilizer level without K high fertilizer level with K DS rice, soils deficient in N, P, and K WS rice, soils deficient in N, P, and K Dept. of Agriculture actual use in Java actual use in outer islands (NFDC, 1989) (NFDC, 1989) (PhilRice, 1991) (PhilRice, 1991) (Sudjadi, 1995) (Sudjadi, 1995) (Sudjadi, 1995) application are often more affected by socioeconomic factors (market availability, subsidies, prices, availability of money) rather than by biophysical needs. Achieving and sustaining average yields of 7 to 8 t ha-1 will require improved nutrient management strategies that focus on increasing the use-efficiency of nutrients from both indigenous and external sources. Indigenous nutrient supply varies widely among ricefields within small domains and farmers do not seem to be aware of the variation. Therefore, the scale of nutrient management-recommendation domains must change from large regions to farms, single fields, or even single parcels within larger fields Soil information and recommendation domains A move from blanket recommendations for large regions to farm- or field-specific management will require a gradual transition. Over the short term, other cost-effective methods to help farmers achieve increased nutrient-use efficiency should be explored. Readily available soil information, such as maps, local 'soft' knowledge, or simple agronomic soilclassification systems may be used to improve fertilizer recommendations. Most countries in the region have programs to improve their soil data bases and, by using geographical information systems (GIS), much of this information is becoming available in digital format. In addition to classical soil taxonomy and mapping, some countries (e.g., Malaysia, Bangladesh, Philippines) have initiated thematic mapping of soil properties at detailed scales. A number of geostatistical studies at regional scales have also been reported (Ishida and Ando, 1994; Kumar et al., 1994; Oberthuer et al., 1995; Sylla et al., 1995; Dobermann and Oberthuer, 1997). Such improvements in the quantity and quality of soil information provide opportunities to develop fertilizer recommendations that are more specific to the needs of smaller regions. Biophysical limits to improving nutrient-use efficiency by using these methods do exist. Most soil maps, for example, are based on pedological principles of soil classification, i.e., morphological criteria, subsoil properties, and stable soil properties. These characteristics may limit their usability for agronomic decision-making at high productivity levels. In a survey of a 20,000-ha area in Central Luzon, for example, extractable nutrients ranged from 0.01 to 0.60 cmol c K kg -1 (CV=60%), 0.1 to 36 mg P kg- 1 (CV = 99%), and 0.2 to 84 mg S kg- 1 (CV = 70%). The existing reconnaissance soil maps, however, explained only 2 to 12% of the overall variation in extractable K, P, and S in the 0- to 20-cm soil layer. Nutrient uptake or rice yields were not related to mapped soil types (Oberthuer et al., 1996). In this case, the soil maps did not provide a reasonable basis for estimating indigenous soil-nutrient supply to improve fertilizer recommendations. As a more agronomic soil classification system, the Fertility Capability Classification (FCC), tries to assess soil fertility constraints in the form of presence or absence of various condition modifiers (Sanchez et al., 1982). The FCC has been used as a guide in P and K fertilizer management for rice in Taiwan (Lin, 1985), but it is an open question as to what extent it can contribute much to improve nutrient management in intensive rice systems at high yield levels. Most of the condition modifiers are not specifically tailored to the needs of lowland rice soils and it is difficult to quantitatively assess soil nutrient supply on such a basis. The system has not changed

20 132 A. Dobermann et al. / Field Crops Research 56 (1998) much since its introduction more than 10 years ago and it has not been developed further A more promising strategy may be to develop slightly modified FCC versions tailored to the environmental conditions found in specific countries and to use them as part of simple agronomic soil-classification systems, such as one proposed for Cambodia (White et al., 1997). 5. Technological leverage points At present yield levels, the massive quantities of P, K and S removed in rice production necessitate maintenance of adequate nutrient balances in irrigated systems. Nutrient interactions and imbalances are important determinants of PE and AE of N, P and K in irrigated rice. These determinants will become even more important toward achieving the yield increases needed in the irrigated rice systems of Asia. Therefore, strategies that only aim at adding more fertilizer without considering P and K supply from indigenous sources are inadequate for maintaining long-term high yields in irrigated rice. Over the long run, both depletion and excessive accumulation of these soil nutrients have negative effects on soil quality, on environment, on nutrient-use efficiency, and on sustaining overall system productivity. Phosphorus and K are not subject to many of the loss mechanisms that complicate N management. Evidence from long-term experiments indicates that if input levels match plant requirements, it is possible to sustain soil fertility. Long-term effects of intensive rice cropping on soil P and K status can be accurately estimated from knowledge of the major components of nutrient balance. Therefore, we must develop management strategies based on understanding of crop requirements, soil nutrient supply, longterm fate of added fertilizer, and overall input/output balance. Compared to N, changes in P and K are more predictable and routine characterization of soil nutrient supply is needed only at intervals of several years. Compared to N or K, P is less mobile in the soil and external inputs other than fertilizer are usually small, making projections of the soil P status easier. Applied P tends to accumulate in the soil and long-term cumulative effects can be substantial (Kirk et al., 1998). Both under-use and overuse of P on irrigated rice have been reported. Therefore, crop P removal, soil P supply, and residual effects of fertilizer-p should be the major elements of long-term strategies for sustainable P management. Of the nutrients reviewed here, improved K management has the greatest potential for improving the overall productivity of intensive, irrigated rice systems. Occurrence of soil K deficiency and of response to applied K depend on the duration of intensive rice cropping, yield level, amount of NPK fertilizer applied, K-buffering capacity of the soil, straw management, and net K inputs from sources other than fertilizer. In practical K management, particular emphasis must be given to site-specific factors, such as clay mineralogy, soil texture and K inputs from irrigation or rainwater. Diagnosis of S deficiency in the field is the key leverage point for S management in rice. Soil tests or plant tissue analysis can be used for diagnosis, with the former indicating potential S deficiency in advance. If detected early enough, S deficiency is relatively easy to overcome by using S-containing fertilizer (sulfur-coated urea, ammonium sulfate, single superphosphate) as part of an integrated nutrient-management package. In addition, S input from rain and irrigation may provide a large proportion of the demand at many sites. Straw management is a key leverage point for the three nutrients reviewed here, particularly for K. Straw is removed from the field because it is valuable harvest product or burned to facilitate land preparation for the succeeding crop. Differences in straw management practices have to be considered in long-term fertilizer management strategies. Technically, we need to develop mechanized technologies for harvest and straw recycling. Little improvement in fertilizer use efficiency can be expected from current blanket recommendations for fertilizer use, and we should endorse more sitespecific approaches in the irrigated lowlands of Asia. Such approaches will have success only if they integrate all essential nutrients and their interactions, if the additional labor required is restricted to a minimum, and if the economic gain is great enough. It is further essential that scientific results are translated into simple, robust, and sound recommendations. This is a specialized and difficult task in which scientists and extension specialists must work together.

21 A. Dobermann et al/ Field Crops Research 56 (1998) At present we have little scope for managing soil variation within a single ricefield, a dimension that can improve P, K and S use efficiency. Breeding of new high-yielding cultivars should, however, include selection for tolerance to slight differences in soil fertility or homogeneous fertilizer application to minimize effects of within-field variation on crop growth. 6. Future research directions 6.1. Basic research needs The soil moisture regimes in rice-rice, ricewheat, and rice-maize systems are characterized by tremendous variation, but effects of such moisture changes on K availability and rice response have not received much attention. Research is needed to clarify: (1) processes of fixation and release of fixed K during alternate drying and wetting cycles; (2) extent of impeded K diffusion in the rhizosphere of rice and its effect on K uptake at very high yield levels; and (3) possible changes in soil solution K activity as a result of long-term irrigated rice cropping in relation to the quality of irrigation water and steady-state redox potential. A better understanding is needed of the processes affecting long-term fate of fertilizer P and K in irrigated rice systems, including more information about the influence of wetting and drying cycles on the cumulative recovery of fertilizer nutrients. To better adjust fertilizer rates, simple models describing recycling of P, K and S from crop residues and other organic inputs are needed. This information particularly relates to the question of how different straw management or soil tillage practices affect the amount of nutrient recycled and the dynamics of nutrient release. The influence of xylem concentrations of S or other minerals as possible limiting factors for grainfilling or grain quality needs further research. We also need to clarify the physiological processes governing partial replacement of K in the plant by Na and its effect on crop K demand Applied research needs Applied research must provide the tools necessary for practical use of long-term strategies for P, K and S management founded on nutrient balance concepts. Tools such as the resin capsule, models for estimating crop nutrient requirements based on interactions of N, P and K (Janssen et al., 1990), and models for predicting the long-term fate of added P and K fertilizer (Wolf et al., 1987) should be fully developed for rice. They may provide a basis for introducing farm- or field-specific nutrient-management approaches. Research on dynamic soil tests for P, K, S and other elements should continue to (1) develop procedures for in situ soil testing using ion-exchange resin capsules; (2) extend the calibration database for P and K by on-farm studies in different environments; (3) clarify whether the resin capsule can be used to predict indigenous N supply; and (4) establish critical resin-capsule levels for diagnosis of S and Zn deficiencies. Another area of needed research is to quantify temporal changes in soil-nutrient supplying capacities within a growing season, between two or more growing seasons, and over several years. This information is needed to specify optimal growth stages for soil sampling and to identify suitable intervals at which soil testing should be repeated. Long-term experiments may potentially provide this information, but detailed measurements of the P, K and S dynamics over very short and long time periods have not been made. Critical data about complete nutrient balances in irrigated rice systems are still lacking. Better quantification of nutrient leaching losses as a function of general soil properties is needed. Little information exists at watershed or irrigation-service area levels on nutrient transfer into and out of rice fields along the elevation gradient Research for increased yield targets (8-10 t ha - 1) More research is needed to clarify the physiological mechanisms that determine crop nutrient requirements at high yield levels. The envelope functions illustrated in Fig. 1 represent a first approximation of the relationship between P and K uptake and grain yield in irrigated rice. More data are needed for estimating the nutrient uptake requirements of P, K and S for achieving yields of more than 8 t ha -1. Despite the existence of considerable literature on

22 134 A. Dobermann et al. / Field Crops Research 56 (1998) the topic, systematic field studies on identification of critical plant tissue concentrations of P, K and S are few. Moreover, many of the critical levels in use for irrigated rice in Asia are based on field surveys conducted 25 years ago (Tanaka and Yoshida, 1970), when yield levels were considerably lower than today. Research is needed to identify critical levels and optimum ranges for rice crops yielding 8 to 10 t ha- ~ or more. Possible limitations in root K uptake due to rhizosphere processes in relationship to soil ph buffering need further clarification. Particulars of the rhizosphere chemistry (Ladha et al., 1998) and data from the LTFE at IRRI (Table 3) seem to indicate that such limitations may become important at very high yield levels. To verify this idea, basic research on rhizosphere processes should be accompanied by measurements of root-nutrient uptake rates under field conditions. Acknowledgements We acknowledge the valuable contributions made by our collaborators at the 10 LTFE sites, including Mr. Josue P. Descalsota (IRRI), Ms. Elisa M. Imperial (BRIARC), Dr. Pompe Sta. Cruz and Mr. Wilfredo B. Collado (PhilRice), Dr. S.P. Palaniappan (Coimbatore, TNAU), Dr. Pyare Lal (Pantnagar, G.B. Pant University), Dr. R. Djafar Baco, Ir. Reginald Le Cerff, and Ir. Rauf Mufran (Maros, MORIF), Dr. Pham Sy Tan (Cuu Long, CLRRI), Prof. Li Jiakang (Jinxian, Qingpu, Shipai), Dr. Lai Qingwang (Jinxian, Red Soil Institute-Jiangxi), Mr. Wang Yinhu (Qingpu, Soil and Fert. Inst.-SAAS), and Dr. Zhou Xiuchong (Soil and Fert. Inst,-GAAS). Furthermore, we are grateful to Ms. Mirasol F. Pampolino and Ms. M. Arlene A. Adviento for their excellent technical assistance in conducting the PST analyses and much of the data processing. References Abedin Mian, M.J., Blume, H.P., Bhuiya, Z.H., Eaqub, M., Water and nutrient dynamics of a paddy soil of Bangladesh. Z. Pflanzenernaeh.-Ernaeh. Bodenk. 154, Amarasiri, S.L., Latheef, M.A., The sulfur deposition by rainfall in some agriculturally important areas of Sri Lanka. Trop. Agric. 138, Anderson, G., Lefroy, R.D.B., Chinoim, N., Blair, G., Soil sulphur testing. Sulphur Agric. 16, Angus, J.F., St.-Groth, C.F.d., Tasic, R.C., Between-farm variability in yield responses to inputs of fertilizers and herbicide applied to rainfed lowland rice in the Philippines. Agric. Ecosyst. Environ. 30, Bajwa, M.I., Soil potassium status, potash fertilizer usage and recommendations in Pakistan. Potash Review No. 3. International Potash Institute, Basel. Basilio, P.R., San Valentin, G.O., Potassium quantity-intensity relationship in some lowland rice soils in Luzon. Philipp. Agric. 73, Begg, C.B.M., Kirk, G.J.D., Mackenzie, A.F., Neue, H.U., Root-induced iron oxidation and ph changes in the rice rhizosphere. New Phytol. 128, Belmehdi, A., Nyiri, K.F., Phosphorus fertilizer use in Asia and Oceania. In: Phosphorus Requirements for Sustainable Agriculture in Asia and Oceania. International Rice Research Institute, Los Banos, Philippines, pp Beltrame, L.F.S., Iochpe, B., Rosa, S.Md., Miranda, T.L.Gd., De-Miranda, T.L.G., Da-Rosa, S.M., The leaching of ions in paddy irrigated rice. Rev. Bras. Cienc. Solo 16, Bergmann, W Ern~itn'ungsstiSrungen bei Kulturpflanzen. Gustav Fischer Verlag, Stuttgart/New York. Biddulph, O., Biddulph, S.F., Cory, R., Koontz, H., Circulation patterns for P32, $35 and Ca45 in the bean plant. Plant Physiol. 33, 293. Blair, G.J., Lefroy, R.D.B., Chinoim, N., Anderson, G.C., Sulfur soil testing. Plant Soil 156, Blair, G.J., Mamaril, C.P., Ismunadji, M Sulfur deficiency in soils in the tropics as a constraint to food production. In: Priorities for Alleviating Soil-Related Constraints to Food Production in The Tropics. International Rice Research Institute, Los Banos, Philippines, pp Blair, G.J., Mamaril, C.P., Momuat, E Sulfur nutrition of wetland rice. IRRI Res. Pap. Ser. No pp. Blair, G.J., Momuat, E.O., Mamaril, C.P., Sulfur nutrition of rice: II. Effect of source and rate of S on growth and yield under flooded conditions. Agron. J. 71, Boje-Klein, G., Problem soils as potential areas for adverse soils-tolerant rice varieties in South and Southeast Asia. IRRI Res. Pap. Ser Boruah, H.C., Baruah, T.C., Nath, A.K., Response of rice to potassium in relation to its kinetics of release. J. Potassium Res. 9, Cassman, K.G., Pingali, P.L., Intensification of irrigated rice systems: learning from the past to meet future challenges. Geo. J. 35, Cassman, K.G., Olk, D.C., Brouder, S.M., Roberts, B.A., The influence of moisture regime, organic matter and root eco-physiology on the availability and acquisition of potassium: Implications for tropical lowland rice. In: Potassium in Asia. Proceedings of the 24th International Colloquium of the International Potash Institute. International Potash Institute, Basel, pp Cassman, K.G., Peng, S., Olk, D.C., Ladha, J.K., Reichardt, W.,

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