Rice production on acid sulphate soils of Sri Lanka

Transcription

1 Rice production on acid sulphate soils of Sri Lanka P. Deturckl, K.D.N. Weerasinghe2, D.A.B.N. Gunarathna2, J.P. Lexa2 and K. Vlassak' KU Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee. University of Ruhuna, Faculty of Agriculture, Mapalana,Kamburupitiya, Sri Lanka. Abstract Implementation of a flood protection scheme on acid sulphate soils in the Nilwala Ganga in South Sri Lanka has lead to severely-reduced rice yields. Agronomical and fertilizer experiments suggest that transplanting of three-week-old healthy rice seedlings was superior to direct seeding, which is the usual practice in South-West Sri Lanka. NPK fertilization increased the grain yield significantly. The response of rice, however, decreased with increasing fertilizer rate. At low fertilization, an acceptable yield was obtained by increasing the plant density, which is an interesting alternative for resource-poor farmers. Finely-ground locally available apatite proved to be a useful P source for rice. Addition of Gliricidiu muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields. Rice varieties developed in Sri Lanka for adverse soil conditions were also evaluated. BW 267-3, BW 297-2, BW and BW were among the highest-yielding varieties. Introduction The West and South-West coastal belt of Sri Lanka encloses an area of ha of low-lying lands (Balasuriya 1987) some of which have only been marginally productive for agriculture due to frequent flooding and salt water intrusion. In the seventies, the government of Sri Lanka decided to drain these marshes and reclaim them for rice cultivation. The Nilwala' Flood Protection Scheme near Matara, is one of these drainage projects. The objectives of this scheme were: - To protect the town of Matara, and other villages from floods; - To extend the total area cultivable with rice; - To ensure double cropping of rice on lands with elevations > 0.6 m above mean sea level (MSL); - To increase the rice yield through introduction of improved cropping practices. The implementation of the Scheme involved the reconditioning of existing drainage canals and the construction of new drainage canals, bunds, control regulators, access roads and pump houses to drain excess water from the lowest rice lands. No facilities for irrigation were provided. With the completion of the engineering work, the occurrence of major floods was 137

2 eliminated and the drainage condition of the soils in the project area improved. Contrary to the plans, on about 1000 ha with an elevation between 0.3 and 0.6 m above MSL rice yields sharply decreased. Cultivation of rice was not longer possible on another 300 to 500 ha. Balasuriya (1987) and Dent (1987) drew attention to the problem of acid sulphate soils in the area, the latter recommending a detailed soil survey as a basis for managing the problem. Soil survey An area of about 2000 ha on the right bank of the Nilwala river was selected for a semi-detailed soil survey (0.3 observations ha-'). A detailed survey (3 observations ha-', 1 :5 000) was done in sector 24 of the scheme (50 ha) in which a range of soil and water conditions are represented. Recent air photographs (1 :25 000), engineering survey sheets and an earlier soil map (1 :25 000) of the Nilwala Ganga basin (Jayawardane et al. 1980) were used as base maps. The soils were classified at the great group level according to Soil Taxonomy (1990). Subgroups were identified in the field in concurrence with a proposal for the classification of acid sulphate soils by Pons et al. (1986). The important physico-chemical parameters of the surface layer (0-30 cm) and the pyrite content of the sub-surface layer (> 50cm) were determined on 60 samples. The ph (1 : 1 water suspension) of the air-dried surface horizon was generally < 3.5. The available AI concentration ranged between 12 and 250 mmo1 AI kg-' with an average of 100 mmo1 AI kg-'. Due to the high organic matter and active iron content, submergence of these soils leads to a strong accumulation of ferrous iron in the soil solution. Pyrite was found in the subsurface of sulfic great groups and subgroups in concentrations ranging from 2 to 8 per cent. It is clear that the low productivity of the rice lands in the Scheme is mainly due to excessive drainage and oxidation of a pyrite-rich subsoil. The ensuing strong acidity of the soil directly affects the rice plant as a result of aluminum and iron toxicities and indirectly decreases the availability of P and other nutrients. Besides the depth of the sulfuric horizon or sulfidic materials, other land qualities affecting the productivity of rice include the availability of water, the occurrence of salinity and the frequency and duration of flooding. Soils with a sulfuric horizon or sulfidic materials within 50 cm of the soil surface are not considered suitable for rice production. In order to compile a set of recommendations for the management of acid sulphate soils with a sulfuric horizon or sulfidic materials between depths of 50 to 150 cm from the soil surface, a series of agronomical, fertilizer and varietal screening experiments was carried out. In the experiments presented in this paper, the level of the watertable was not controlled, except to prevent acute oxidation of the sulfidic subsoil. Cultural pract&es for rice on acid sulphate soils Crop establishment In South-West Sri Lanka, pregerminated rice seeds are broadcast onto puddled soils without much standing water. Because of the watertable fluctuations in the Nilwala 138

3 Ganga floodplain, transplanting of vigorous seedlings may have several advantages: - Accumulation of a floodwater layer of more than 5 cm after direct seeding will lead to low germination, need for resowing and delay in the cropping schedule; - Transplanting reduces the drainage requirement in the beginning of the growth season, which limits the risk of acidification, and aluminum and iron toxicity; - Vigorous seedlings have a higher tolerance to adverse conditions prevailing in the beginning of the cropping cycle. I We compared the effect of transplanting and direct seeding on the growth and yield of rice. Pregerminated seeds were spread evenly on the saturated soil surface at a rate of 120 kg ha-'. Twenty days later, healthy 3-week old seedlings, raised in a nursery, were transplanted on an adjacent plot at a density of 130 to 140 plants m-2 (3 seedlings per hill at 15 x 15 cm). Both treatments were fertilized according to the recommendations of the Department of Agriculture for mineral soils of the Low Country Wet Zone (Nagarajah 1986). The variety used was BG 379-2, a popular, high-yielding 4-month variety. Pesticides were applied when required. The effect of crop establishment on the yield of rice grown on a Sulfic Histic Hydraquent is presented in Table 1. The higher yield of the transplanted crop was expressed in a higher plant length, number of fertile tillers, panicles m-2, spikelets per panicle, and a reduction in the percentage of empty seeds. The effect of transplanting was more pronounced in zones with high drought and flood risk. In areas with better water control, the difference in net production may not be sufficient to recommend transplanting because of the labour needed for transplanting. Plant density In January 1990, the fertilizer prices in Sri Lanka were doubled. Resource-poor farmers are now unwilling to apply the recommended fertilizer rates, especially in areas with a high risk of crop failure. A field experiment was conducted on a Typic Sulfihemist to find out if increased plant density could compensate for a reduced level of fertilization. Seedlings of BW 272-3, a high yielding 35month variety, were transplanted with a spacing of 20 x 20 cm at three densities: 50, 100 and 150 plants m-2, corresponding to 2,4 and 6 seedlings per hill, respectively. No fertilizers were applied. Plant density had a significant effect on the growth and yield of rice (Table 2). Higher plant density reduced the number of tillers per plant, but was adequately remunerated by an increased number of panicles m-2. The plant height, number of seeds per panicle, Table I Effect of crop establishment on the yield of rice grown on a Histic Sulfic Hydraquent, Kiralakele, wet season Crop establishment Yield Plant Fertile Panicle Spikelets Empty Net height tillers density per seeds prod.* panicle t ha-' cm nrper nrm-2 nr % t ha-' plant Transplanting Direct seeding * Net production = yield -costs 139

4 Table 2 Grain yield, productive tillers, and panicle density of rice grown on a Typic Sulfihemist under different plant density levels, Kiralakele, 1988 dry season Plant density (no m-') Grain yield Productive tillers Panicle density (t ha-') per plant per m b 2.7 a 2.9 ab 1.9b 3.4 a 1.7 b 154c I83 b 227 a In a column, means followed by a common letter are not significantly different at the 5% level by Duncan's Multiple Range Test percentage empty seeds and 1000-seed weight were not affected by plant density. On lands marginally suitable for rice production, we recommend increasing the plant density to obtain a reasonable grain yield, even in the absence of NPK fertilizer. Nguu and De Datta (1979) also reported that, without fertilizer or with low levels (60 kg N ha-'), the grain yield increased either linearly or curvilinearly with increased plant density. Fertilization of acid sulphate soils The beneficial effects of NPK fertilization on the yield of rice grown on acid sulphate soils are well documented (Attanandana and Vacharotayan 1986). Balasuriya (I 987) reported that rice generally responded favourably to NPK fertilizer on the oligotrophic and mesotrophic soils ofthe South-West coastal belt of Sri Lanka but specific fertilizer recommendations for acid sulphate soils are not available in Sri Lanka. Data from our field experiments with a low (50 kg N ha-', 10 kg P ha-' and 20 kg K ha-') and intermediate (90 kg N ha-', 20 kg P ha-' and 50 kg K ha-') rates confirm that fertilization has a significant positive effect on yield. Application of the intermediate NPK level, however, did not increase the yield in comparison with the low NPK level. The yield response to the low and intermediate NPK doses were 19 and 10 kg grain per kg NPK, respectively. Even on fields with high fluctuations in the watertable, a low NPK fertilizer application is essential. Seedling establishment, submergence tolerance, and tillering improved after a small basal NPK dressing. On soils with better water management, a higher quantity of NPK fertilizer will be beneficial. Rock phosphate Rock phosphate has been successfully introduced in paddy production on acid sulphate soils of the Mekong Delta (Le Van Can 1982). According to Chien et al. (1990) the agronomic effectiveness of rock phosphate is high on acid sulphate soils due to the low ph, high organic matter content and high P-fixing capacity of these soils. Sri Lanka has a deposit of rock phosphate at Eppawala with an estimated reserve of 40 million ton. This deposit has been little exploited because the P solubility was thought to be too low for direct application to annual crops. Dahanayake and Subasinghe (1 99 1) suggested that the agronomic effectiveness of the rock phosphate could be increased by mechanical separation of the primary apatite crystals from the second- 140

5 Grain yield (g/pot) O ERP mixture ERP crystale TSP P source and level Control (no P) Low1 1 (20 P/hd Lewl 2 (30 kq P/hd Figure I Response of rice grown on a potential acid sulphate soil to different levels of rock and triple super phosphate. ERP crystals = Eppawala rock phosphate pure apatite crystals ERP mixture = apatite crystals and matrix ary matrix. The apatite crystals have a phosphate content of 30 to 40 per cent and low R,O, and Cl values. The matrix consists of secondary phosphate minerals and siliceous and ferruginous components and has a phosphate content ranging from 10 to 30 per cent. Rice (variety AT 76-1) was grown in a greenhouse on a potential acid sulphate soil to evaluate its response to phosphate. Three P fertilizer products were compared (finely ground < 80 mesh) pure Eppawala apatite crystals, a mixture of secondary matrix and apatite crystals (70/30 ratio), and triple superphosphate (20 and 30 kg P ha-'). The different P sources and levels were incorporated into the soil one day before the rice seedlings were planted. Recommended rates of N and K were applied to all treatments. Plants which received the highest dose of triple superphosphate matured 9 days earlier and yielded significantly better than other plants (Figure I). This demonstrates that the current P recommendation (20 kg P ha-') is not adequate for acid sulphate soils. It is encouraging to note that a comparable rice yield was achieved by application of apatite crystals at 30 kg P ha-'. Replacement of triple superphosphate by Eppawala apatite crystals could increase the farmer's net return and save some valuable foreign currency, but direct application of untreated Eppawala phosphate on a low rate of apatite crystals or a low dose of ERP-crystals was not effective in increasing rice yield. Green manures Touré (I 982), working on acid sulphate soils in Senegal, reported that application of green manures led to strong reduction and a build-up of toxins, and adversely affected rice yield. However, favourable experience with rice grown in nutrient-defi- 141

6 Grain yield Wha) O Control Leucaena Qllricidia Leucaena*ERP QMricidia*ERP Fertilizer treatment Figure 2 Effect of application of green manures and rock phosphate on the grain yield of rice grown on a potential acid sulphate soil, Watagederd 1991 dry season. Control = 4 recommended NPK; GliricidiulLeucaeno = 3 t ha-' fresh biomass; ERP = Eppawala rock phosphate (20 kg P ha-') cient Ultisols (Deturck and Vlassak 1991) prompted us to use green manures with low C/N ratios. A field experiment was laid out on a Sulfic Hydraquent at Wategedera in the dry season. Fresh biomass of Gliricidiamaculata and Leucaena leucocephalu was incorporated at a rate of 3 t ha-', two weeks before transplanting. All plots received half of the recommended fertilizer dosage (Nagarajah 1986). Because green manures are low in P, 20 kg P ha-' was added in two of the treatments as well. Application of Gliricidia had a pronounced positive effect on the availability of N, Ca, Mg and especially, K but did not significantly lower the redox potential. The water soluble Fe concentration in green manure-amended treatments increased, but not significantly in comparison with the control. No symptoms of Fe toxicity or other physiological disorders were observed. A very good rice yield was obtained in the treatment with.gliricidiu and rock phosphate (Figure 2). Incorporation of Leucuenu was not as effective as Gliricidia. A combination of green manuring (3 t ha:'), rock phosphate application (20 kg P ha-'), and NPK fertilization looks like a suitable practice for securing high rice yields on potential acid sulphate soils. Improvement of rice on acid sulphate soils The performance of rice varieties and breeding lines developed in Sri Lanka for adverse soil conditions was evaluated on the acid sulphate soils of the Nilwala Ganga floodplain for several seasons by Pathirana and Chandrasiri (1991). They reported that BW 267-3, BW 297-2, BW and BW were among the highest-yielding varieties: they had a higher number of panicles per m2, more spikelets per panicle and less empty seeds. With adequate management, these varieties have a yielding capacity of 6 t ha-' on potential acid sulphate soils with a soil and water ph above

7 To seek a better understanding of the physiological mechanisms of stress tolerance, we compared the physio-chemical and microbiological properties in the rhizosphere of a tolerant and a susceptible variety. In a pot experiment, BG 94- I, a variety susceptible to adverse soil conditions and BW 267-3, a tolerant variety, were grown on an acid sulphate soil and their rhizosphere was sampled at regular intervals. The recommended rate of NPK fertilizer was added. In the susceptible variety, the plants were stunted and showed clear symptoms of iron toxicity. The total microbial population and the iron- reducing bacteria were found to be more abundant in the rhizosphere of the susceptible variety for the entire growth season (Table 3). However, the population of iron-reducing bacteria fluctuated and the difference between the two varieties was significant only at periods of high plant metabolism. Increased microbial activity in the susceptible variety lead to a slightly higher concentration of water soluble Fe in the rhizosphere and resulted in a higher Fe uptake. Analysis of the nutrient content of the shoot showed that the tolerant variety was better provided with essential nutrients. Benckiser et al. (1984) proposed that a multinutritional soil stress is the cause of iron toxicity of wetland rice. In susceptible varieties, a deficiency of essential nutrients would affect the cell permeability and synthesis of high molecular weight organic compounds. More plant-derived organic material, thus, would be released in the rhizosphere with a consequent explosive growth of the microbial biomass. Our data support this hypothesis. In developing new rice varieties for acid sulphate soils, incorporation of tolerance to aluminum and iron toxicity at the seedling stage merits consideration. In this respect, it is essential to breed rice lines with an efficient nutrient uptake mechanism and high oxidizing activity of the roots. Conclusion Our field survey and lab analyses confirm the occurrence of actual and potential acid sulphate soils in Sri Lanka. This has major implications for the reclamation of 30 O00 Table 3 Shoot iron content and total microbial population, iron-reducing bacteria, and iron-concentration in the rhizosphere of two rice varieties grown on an acid sulphate soil Rhizosphere parameters* Variety Total microbial Iron reducing Soluble iron Shoot iron population ** bacteria ** (mg 1-9 content % BG 94-1"' 22 io ~ ~ 570 a BW 267-3"" 9 io5 2.3 x. 1 0 ~ ~ 520 a 0.07 b 0.05 a * Meansof four samples (4,7,9 and 11 weeks after submergence) and three replicates ** counts per gram of soil *** Susceptible to soil stresses **** Tolerant to soil stresses In a column, means followed by a common letter are not significantly different at the 5% level by Duncan's Multiple Range Test 143

8 to 40 O00 ha of low-lying lands along the South West coastal belt. Before the government decides to implement other land reclamation and flood protection schemes, it is of paramount importance to do a thorough feasibility study, including a soil survey that pays specific attention to the question of acid sulphate soils. Two strategies can be recommended for the management of acid sulphate soils for rice production. On lands with moderate suitability, rice production can be intensified by a combination of practices: '- Broadcasting pregerminated seeds ( kg ha-'); - Applying an intermediate NPK fertilizer dose (N50, P10, K20 kg ha-'); - Using high-yielding varieties with moderate resistance to soil stresses. Crop management of lands with marginal suitability for rice production due to high risk of floods, drought or soil stresses should include these operations: - Transplanting vigorous seedlings; - Increasing plant density (up to 150 plants m-2); - Applying low amounts of NPK; - Replacing TSP with rock phosphate (minimum dose 30 kg P ha-'); - Incorporating green manures of low C/N ratio (3t ha-'); - Utilizing tolerant varieties with good yield potential. ' The actual yield that can be obtained with improved crop management, however, will very much depend on the level of water management. Oxidation of the pyrite-rich subsurface should be prevented at all costs and a certain amount of vertical drainage to flush out toxic reduction products is needed. Acknowledgements Experiments reported in this paper were conducted as part of the Sri Lanka - Belgium Ruhuna Agricultural Development Project and the Sri Lanka French Agricultural Research Development Project. The financial support of the Belgian and French Cooperation is gratefully acknowledged. References Attanandana, T. and S. Vacharotayan Acid sulphate soils: their characteristics, genesis, amelioration and utilization. South-East Asian Studies, 24(2), Balasuriya, I Reclamation and development of rice cultivation on coastal low-lying lands of Southern and Western Sri Lanka. Tropical Agriculture Research Series, 20, Benckiser, G., J.C.G. Ottow, 1. Watanabe and S. Santiago The mechanism of excessive iron-uptake (iron toxicity) ofwetland rice. Journal ofplant Nutrition, 7(1-5), Chien, S.H., P.W.G. Sale and L.L. Hammond Comparison of the effectiveness of phophorus fertilizer products. In: Phosphorus requirements for sustainable agriculture in Asia and Oceania, IRRI, Manila Dahanayake, K. and S.M.N.D. Subasinghe Mineralogical, chemical and solubility variations in the Eppawala phosphate deposit of Sri Lanka - a case for selective mining for fertilizers. Fertilizer Research, 28, Dent, D.L Environmental impact report on the Nilwala Ganga Flood Protection Scheme. Land Use Policy and Planning Division, Ministry of Lands and Land Development, Colombo 144

9 Deturck, P. and K. Vlassak Potassium management on wet Ultisols grown to rice. In: Rice production on acid soils of the tropics, P. Deturck and F.N. Ponnamperuma (editors). Institute of Fundamental Studies, Kandy Jayawardane, N.E., L.D. Jinadasa, J.A.D.K. Perera and K.A. de Alwis Soils of the Nilwala Ganga basin. Land Use Division Publication No. 80.3,Sp. and map 1: Irrigation Department, Colombo Le Van Can Rock phosphate in rice production on acid sulphate soils in Vietnam. In: Proceedings of the Bangkok symposium on acid sulphate soils, H. Dost and N. van Breemen (editors). International Institute for Land Reclamation and Improvement Publication No. 3 1, Wageningen Nagarajah, S Fertilizer recommendations for rice in Sri Lanka: a historical review. Journal of the Soil Science Society of Sri Lanka, IV, 3-29 Nguu, N.V. and S.K. De Datta Increasing efficiency of fertilizer nitrogen in wetland rice by manipulation ofplant density and plant geometry. Field Crops Research, 2, Pathirana, R. and P.A.N. Chandrasiri Evaluation of rice genotypes grown on acid soils of the Nilwala river basin, Southern Sri Lanka. In: Rice production on acid soils of the tropics, P. Deturck and F.N. Ponnamperuma (editors). Institute of Fundamental Studies, Kandy. Pons, L.J., M.E.F. van Mensvoort and Le Quang Tri A proposal for the classification of mineral Vietnamese acid sulphate soils according to Soil Taxonomy. Paper presented at the 3rd International Symposium on Acid Sulphate Soils, January 1986, Dakar Soil Survey Staff Keys to Soil Taxonomy, Fourth Edition. SMSSTechnical Monograph no. 6. Virginia Polytechnic Institute and State University, Blacksburg Tennakoon, D South-West coast drainage and land reclamation project: an assessment of results. Research Study no. 54. Agrarian Research and Training Institute, Colombo Touré, M Improvement of acid sulphate soils: effects of lime, wood ash, green manure and preflooding. In: Proceedings of the Bangkok symposium on acid sulphate soils, H. Dost and N. van Breemen (editors). International Institute for Land Reclamation and Improvement Publication No. 3 I, Wageningen 145

10

11 Fertilization of nitrogen, phosphorus, potassium and lime for rice on acid sulphate soils in the Mekong Delta, Vietnam Do Thi Thanh Ren, Vo Thi Guong, Nguyen My Hoa, Vo Quang Minh and Tran Thanh Lap Soil Science Department, Can Tho University, Vietnam Abstract The effects of NPK fertilizers and lime on rice yield are investigated in various types of acid sulpate soils after leaching of water-soluble acid. In Typic Sulfaquepts, N fertilizer had no effect on rice yield. Phosphorus application did increase yield, thermophosphate and superphosphate being more effective than apatite. Various application methods of phosphorus gave no difference on rice yield and P content of grain and straw. Supplying P at different growth stages made no difference to yields. Potassium had no effect on the first three crops and a negative effect on the fourth crop. Liming under good water management and with NPK fertilizers greatly increased rice yield in four consecutive crops. Even at low doses (1.0 t ha-'), lime showed a positive effect, especially from the second crop, and there was a clear residual effect from heavier lime applications. In Sulfic Tropaquepts, where the sulfuric horizon is between 50 and 80 cm, rice yields increased significantly at 50 to 100 kg N ha-' and 30 to 60 kg P205 ha-'. Potassium had no significant effect on yield. In Sulfic Tropaquepts, where the sulfuric horizon is deeper than 80 cm, the highest rice yield was obtained at 120 kg N ha-' and 30 kg P205 ha-'. There was no response to potassium fertilizer. Introduction For rice growing in acid sulphate soils the important adverse factors are toxicities, of iron and aluminum, and nutrient deficiencies, which lead to low yields and, often, crop failure. Sulfaquepts (Soil Survey Staff 1990) have a sulfuric horizon within 50 cm, an extremely low ph (below 3.5) and high concentrations of Al3+, Fe2+ and SO,2-. Where the sulfuric horizon occurs deeper than 50 cm, the soil is less toxic and crop production is better than on Sulfaquepts. Earlier studies (Nhung and Ponnamperuma 1966, Dent 1986) indicated that the soil must be improved first by leaching of water-soluble acid and, next, by liming and fertilization. Application of lime after preliminary leaching raises soil ph and leads to a decreased concentration of iron and aluminum in the soil solution. There is a clear response of rice to nitrogen and phosphate in combination with lime. The research presented here investigated the effect of N, P, K fertilizer and lime on the rice yield of the different kinds of acid sulphate soils in the Mekong Delta. 147

12 ~ Table 1 Some chemical characteristics of soils used Soil Type phhz0 N C Total P Exch. K Exch.Al Soil Survey Staff % % YO P,05 mmolkg-l mmolkg-' Typic Sulfaquepts Sulfic Tropaquepts Sulfic Tropaquepts Sulfic Tropaquepts Sulfic Tropaquepts sulfic Tropaquepts Sulfic Tropaquepts Sulfic Tropaquepts Fertilizer use on Sulfaquepts N P K fertilizers Experiments were carried out at Hoa An station. The first row of Table 1 gives some chemical properties of the topsoil. The soil was ploughed and harrowed then leached twice with good quality water. Fertilizer was applied according to a split plot design with three replicates, using plots of 18 m2. Urea was given at three levels; O, 50 and 100 kg N ha-'. Superphosphate was applied at O, 45 and 90 kg P205 ha-', and potassium sulphate at O and 60 kg K,O ha-'. The experiment was repeated four times (two wet seasons and two dry seasons) on the same plot. In the first crop (wet season) all plants died within 15 days after transplanting. During the following dry season, about one third of the crop survived but there were no reliable yield data. Growth improved during the third and fourth crops, giving yields between 0.8 and 2.9 t ha-' (Table 2). Even without any ferilizer, more than 1 t ha-' was obtained. This is probably due to the leaching which is done before transplanting and the repeated leaching of the soil surface with irrigation water. Without K, N tended to increase rice yield, but the effect was rather small and the increase of yield was not significant. After applying 60 kg ha-' of K20, N had Table 2 Effect of N, P, K fertilizers on rice yield (t had) on a Typic Sulfaquept Treatment KO Kfd 3rd crop 4th crop 3rd crop 4th crop PO p45 p90 NO N50 NI00 NO N50 NI00 NO N50 NI I.56 I SO I.20 gh 1.46 fgh 1 S O efg 2.73 ab 2.87 a 2.30 abcd 2.21 bcd 2.70 ab 2.42 abc gh 0.76 h 0.82 h 2.00 cdef I.74 def 1.59efg 2.40 bcde 2.06 cdef I.84 cdef Numbers followed by the same letter are not significantly different at 5 per cent level 148

13 Table 3 Effect of sources and rates of P fertilizer on rice yield on a typic Sulfaquept (t ha-') Treatment 1st crop 2nd crop 3rd crop 4th crop wet season dry season wet season dry season N.P.K. Lo LI Lo Li LO LI LO LI A A A S S S T T T c 0.37 c 0.23 c 0.56 bc 0.28 c 0.33 c 1.14a 0.34 c 0.53 bc 0.96 ab 0.19~ 1.16 b 1.66 ab 1.58 ab 1.47 ab 1.93 a 1.90 a 1.11 b 2.00 a 1.99 a 0.7 f 1.1 e 1.3e 1.6d 1.4de 2.0cd 1.8cd 2.9a 1.9~ 2.1 c 2.6ab 2.2~ 2.8a 2.8ab 2.2 bc 2.4 bc 2.4ab 2.8ab 2.1a 3.2a Means followed by a same letter are not significantly different at the 5% level. Lo : Without lime LI : 2 tons lime ha-' A : Apatite S : Superphosphate T : Thermophosphate no effect and tended to decrease yields in the dry season (the fourth crop). K - N antagonism could explain the depressed grain yield. 45 kg P205 ha-' gave a significantly higher yield, but increasing P to 90 kg ha-' did not further increase the yield. Both panicles per m2 and the number of grains per panicles increased upon P application and decreased upon K application. Effect of sources and rates of P fertilizers The experiment was carried out for four consecutive crops (two wet seasons and two dry seasons) at the same location as the N P K experiment. Apatite was applied at three levels: 60, 120 and 180 kg ha-' of P205, thermophosphate and superphosphate were applied at 30, 60 and 90 kg ha-' of P205, and lime as CaC03 at O and 2 t ha-'. The experiment was based on a split plot design with three replications. As a basic fertilizer dressing, 50 kg ha-' N as urea and 30 kg ha-' K20 as KCI were applied. In the unlimed plots of the first two crops, all rice plants died. Later on, phosphate invariably helped the plant to recover quickly after transplanting and increased the number of tillers and the plant height. Fertilization with superphosphate and thermophosphate resulted in higher yields than with apatite (Table 3). In general, when the soil was improved by liming, plants showed a good response to 30 kg P205 ha-' to 60 kg P205 ha-'. Applying more than 60 kg P205 ha-' did not increase the yield very much. Double amounts of apatite were needed compared to the other P sources. The P content in rice was significantly higher in P-treated plots than in controls: 0.58 versus 0.43 percent P,O, in leaves at tillering, 0.45 versus 0.35 percent P,05 in leaves at panicle initiation, and 0.43 versus O. 17 percent P205 in the seeds. Phosporus application helped rice plants absorb more potassium at the tillering stage (3.4 percent compared with 2.8 per cent). The average efficiency of P uptake was 4 percent for apatite and 16 percent for 149

14 ~ ~ ~ Table 4 Effect of application time on rice yield on severely acid sulphate soil Treatments Yield (t ha-') Control - without P 0.7 b 90 kg P205 ha-' Superphosphate (S) 1 day before transplanting (DBT) 2.7 a 60 kg P20, ha-' (S) 1 DBT and 30 kg at I5 days after transplanting (DAT) 2.5 a 45 kg P20s ha-' (S) 1 DBT and 45 kg at I5 DAT 1.8a 45 kg P205 ha-' (S) I DBT and 45 kg at 30 DAT 2.1 a 45 kg P2O5 ha-' (S) I DBT, 22.5 kg at 30 DAT and 22.5 kg at 45 DAT 2.0 a 90 kg P2OS ha-' Di-ammonium phosphate (DAP) I DBT 2.3 a 60 kg P20s ha-' (DAP) 1 DBT and 30 kg at 15 DAT 1.9a 45 kg P2Os ha-' (DAP) 1 DBT and 45 kg at 15 DAT 1.8 a 45 kg P2Os ha-' (DAP) 1 DBT and 45 kg at 30 DAT 1.9a 45 kg P2Os ha-' (DAP) 1 DBT, 22.5 kg at 30 DAT and 22.5 kg at 45 DAT 2.3 a cv (Yo) Means followed by a same letter are not significantly different at the 5 per cent level superphosphate and thermophosphate. The effect of liming was dramatic in the first and second crop, but decreased with the third and fourth crops. During the fourth crop, the unlimed plot gave a good yield which may be attributed to the repeated leaching and flooding. Time of P application The effect of application time on the rice yield was tested on the same soil type at Hoa An station. The treatments are described in Table 4, which also shows that rice yields were not significantly different between the various treatments. Application methods of P fertilizer Thirteen different methods of application of phosphate fertilizer were tested. The methods are described in Table 5, which also shows that the various application methods gave no difference in rice yield. Table 5 Effect of application methods on rice yield on severely acid sulphate soils Application methods Control- Without P Superphosphate broadcast Superphosphate applied in row Superphosphate dip the roots in fertilizer and broadcast Superphosphate dip the roots in fertilizer and apply in row Superphosphate soak the roots in 5'1, solution and broadcast Superphosphate soak the roots in 5'/00 solution and apply in row Di-Ammonium phosphate broadcast Di-Ammonium phosphate applied in row Di-Ammonium phosphate dip the roots in fertilizer and broadcast Di-Ammonium phosphate dip the roots in fertilizer and apply in row Di-Ammonium phosphate soak the roots in 1%' solution and broadcast Di-Ammonium phosphate soak the roots in Io/oo solution and apply in row cv (Yo) Yield (tha-i) 0.78 b 2.79 a 2.67 a 2.41 ab 2.19ab 2.30 ab 2.55 ab 2.33 ab 2.01 ab 2.40 ab 2.67 a 2.43 ab 1.95b Means followed by a same letter are not significantly different at the 5 per cent level 150

15 , Table 6 Average yields obtained in the various lime treatments. The numbers after the R indicate during which crop the lime was applied I Lime Yield (t ha-') dose (t ha-') RI (1) RI, (2) RI (2) R123 (3) RI, (3) RI (3) Rl234 (4) RI, (4) RI (4) I I 0.07d 0.74~ 0.64~ d 1.45d 1.17b O d 1.71b 1.20bc d 1.38d 1.53b I.o 0.25d 1.74b 1.59b ~ 1.61d 1.57b ~ 2.73a 1.45b b 2.54~ 1.31b I b 3.01a 2.48a b 3.31b 1.64b a 3.15a 2.55a a 4.35a 1.99a The 3rd crop had insufficient data for statistical calculations Figure between parenthesis indicates first, second, third and fourth crop The effect of lime on rice yield The effects of applying lime at different times and at different rates were studied. It was found that applying lime at different times before transplanting had no significant effect on yield. In Table 6, the average yields obtained at different rates of single and repeated lime application are shown. In the first crop, with liming at low dose of t ha-', yield did not increase significantly compared to the no-lime treatment. But liming at 3, 6, 10 t ha-' gave a significant increase in yield. In the second, third and fourth crop, repeated application even at 1 t ha-' gave an increase in yield though the residual effect of these low doses was small. Yields also increased in the repeated application of 3,6, 10 t ha-'. The residual effect of lime was strong in the rates of 6, 10 t ha-'. The effect of liming at 6 t ha-' in the first crop persisted into the 3rd crop; liming at 10 t ha-' persisted ' into the 4th crop. Note that yields tend to be highest in the dry season crops (second and fourth) compared to the wet season crops. Repeated application seems to be more effective than a single application of the same amount of lime, but the differences are very small. The effect of liming on chemical changes Table 7 shows some soil analytical data at the beginning of the first and second crop after liming. Regardless of the lime level (and even in the unlimed plot) the initial ph of the surface soil of the second crop is about one unit higher than at the first crop, and extractable aluminum has decreased strongly. This difference may be attributed to the prolonged flooding and soil reduction in the wet season, followed by a repeated leaching of the surface soil with irrigation water. Possibly, the high extractable AI values during the first crop may have included some soluble aluminum. Lime levels up to 3 t ha-' have little effect on ph, extractable AI and exchangeable Ca. In the plants, however, liming increases the Ca content and decreases the Fe content, even at the lowest rate of application. The phosphorus content is not affected, but N contents are higher at high lime doses. Results in Table 8 indicate that the yield increase at low dosage is mainly due to improved calcium nutrition and depressed uptake of iron. 151

16 ~~ Table 7 Some chemical data for the 0-10 cm surface soil, just before planting the crops, as a function of the total amount of lime applied Soil test Total lime applied (t ha-') over 2 crops O crop AI I nd ph crop P' I I.7 I I AI ca ' -Available P (mg P kg-') '- Extractable AI (mmol( +)kg-') in 1 M KCI - Exchangeable Ca (mmol( +)kg-') in 1 M Ammonium acetate Economic effects Although the effect of liming on yield is strong, a benefit:cost analysis (including the cost of the blank fertilizer treatment) showed that most lime treatments were uneconomic. In the fourth crop, the B:C ratio increased from 0.6 at 0.5 t lime ha-' to 1.21 at repeated application of 3 t ha-', and to 1.5 at single application of 10 t ha-'. It should be remembered, however, that the favourable water management conditions have undoubtledly played an important role in the relatively high yields obtained and that the results reported here cannot be extrapolated directly to farmers' fields where water management might not be optimal. Fertilizer use on Sulfic Tropaquepts Twelve experiments were carried out on Sulfic Tropaquepts at different places in the Mekong Delta. Soil analysis data are given in row 2 to 8 of Table 1. Nitrogen was applied as urea at 4 levels (O, 60, 90 and 120 kg N ha-') or at 3 levels (O, 50, 100 kg N ha-'). Superphosphate and thermophosphate were applied at O, 30, 60 kg P205 Table 8 Chemical analysis of the rice plants at tillering stage (N, P, Fe) and at harvest (Ca) as a function of lime applied. N, Pand Fe in leaves, Ca in straw Plant test Total lime applied (t ha-') over 2 crops O IO st N(%) 2. I I crop P2O5 (YO) nd N(%) O crop P205 (YO) Ca (Yo) Fe(mgkg-I) I52

17 Table 9 Effect of different doses of nitrogen on rice yield (t ha-') on sulfic Tropaquepts kg N ha-' Exp. 1 Exp. 2 Exp. 3 O O ~ 2.1 d 2.3 d 2.6 b 3.1 c 5.3 c 6.2 b 3.3 a 6.1 a 6.9 a 5.1 b Numbers followed by the same letter are not significantly different at 5% level ha-' (experiments I, 2 and 3, respectively). In some experiments, phosphate was applied in combination of half superphosphate and half thermophosphate. Potassium chloride was applied at O, 30,45 and 60 kg K20 ha-'. Results (Table 9) showed that application of N gave significantly higher yields than the control treatment. The highest yield was obtained at the highest level of N application. Phosphate fertilization gave significantly higher yields than the control but increasing P applications over 30 kg ha-' did not always further increase yield, and there was no difference between the effects of superphosphate and thermophosphate. In all experiments, rice yields showed no response to potassium fertilizer. This could be due to sufficient K supply, especially under submerged conditions. The mineral fraction of soils in Trans Bassac area is about 50% illite (Brinkman et al. 1993), which is rich in potassium. Conclusion Nitrogen application on severely acid sulphate soils gave no effect on rice yield. There was a clear effect on slightly acid sulphate soils at 50 to 100 kg N ha-'. Phosphate application increased rice yield moreefficiently than other nutrients. The most efficient P fertilization was obtained at 60 kg P,05 ha-' for apatite and 30 kg P205 ha-' for superphosphate and thermophosphate. Raising P application to 60 kg P205 ha-' had a significant yield increase in only half of the experiments on slightly acid sulphate soils. Using various methods of P application made no difference on rice yield, as was the case when supplying P at different growth stages. Application of superphosphate, thermophosphate or a combination of half superphosphate and half thermophosphate gave no differences between them effect. In all experiments, K applications gave the same yield, except on severely acid sulphate soils where K had a negative effect on the fourth consecutive crop. Lime had a significant positive effect on yields, on Ca uptake in the plants and it depressed the uptake of Fe, even at low doses of 1.O t ha-'. The application of lime, however, is economically unattractive because of high prices in the Mekong Delta. 153

18 References Brinkman, R., Nguyen Bao Ve, Tran Kim Tinh, Do Phuoc Hau and M.E.F. van Mensvoort Acid sulphate materials in the Western Mekong Delta Vietnam. Submitted to Catena Dent, D Acid sulphate soils - a baseline for research and development. International Institute for Land Reclamation and Improvement Publication No. 39,82-83, Wageningen Nhung, Mai Thi My and F.N. Ponnamperuma Effect of calcium carbonate, manganese dioxide, ferric hydroxide and prolonged flooding on chemical and electrochemical changes and growth of rice in a flooded acid sulphate soil. Soil Sci. 102,29-41 Soil Survey Staff Keys to Soil Taxonomy. Fourth editon, SMSS Tech. Mag. 19, Cornell University, lthaca 154