Phosphorus and sulphur efficiency in tropical cropping

• Department of Agriculture, Thailand
• Universiti Pertanian Malaysia, Malaysia
• Food Crop Research Institute, Indonesia

In Asia, population pressure is forcing the expansion of agriculture into areas with infertile soils not previously used for crop production, while intensified cropping is draining the natural resources of soil nutrients from traditional cropping areas. Apart from nitrogen, the major nutrients limiting crop production in many areas are phosphorus and sulfur.
Phosphorus is being used widely in South-east Asia, but national decisions about the most efficient rates and sources for farmers are being made on the basis of very limited information about the reactions and transformations of fertiliser phosphate in the appropriate tropical soil-crop systems. The widespread use of 'high-analysis' phosphorus and nitrogen fertilisers (together with increased crop removal) has led to an increasing incidence of sulfur deficiency in the region. An ADAB/Sulfur Institute Seminar held in Indonesia in 1983 reviewed the scope of the sulfur problem in South-east Asia and identified research needs.
This project is based on the conclusion from that meeting and on he need to link sulfur research to phosphorus research, since the two nutrients behave similarly in the soil and can be supplied together in compound fertilisers. It comprises three parallel sub-projects.
The first of these will study the dynamics of phosphorus and sulfur in upland and flooded cropping areas, with a view to increasing the efficiency with which crops utilise the nutrients. Scientists will characterise the phosphorous and sulfur status of soil samples collected from the major rice-producing areas of Indonesia and Thailand, and will also include some Australian soils. Small-scale greenhouse trials at Armidale using upland and lowland rice will examine the fate of the two elements applied to those soils, alone and in combination, using radioisotopes to separate nutrients from different sources. In the light of the results obtained, a survey of available fertiliser sources should identify possible new material suited to the particular system. After further greenhouse trials, field tests of the most promising material will consider both the initial response and residual value of the various fertiliser sources.
In the second sub-project, scientists will seek better criteria for establishing the phosphorus and sulfur status of plants and soils. Greenhouse experiments in Australia both in solution culture and in a range of tropical and subtropical Australian soils, will study the uptake of these nutrients by important food crops such as maize, soybean and sweet potato, as well as rice. They will also include measurements of tops and root growth, and will investigate critical levels of the elements in these food crops, with emphasis on the effects of other nutrients, plant part and age. Complementary field studies in Indonesia and Thailand will take into account additional factors such as climate, disease and insects. A nutrient uptake model for the crops under study will be constructed, with particular emphasis on phosphorus and sulfur supply-demand relations. This will help in identifying deficient areas, and also serve as a valuable tool to indicate appropriate fertiliser types.
Estimations of the accession rate of sulfer, and other nutrients, to agricultural areas in the region will form the third sub-project. Mixed anion-cation exchange resin columns will be constructed in Australia and used to 'trap' ions from rainfall. The scientists will also investigate the feasibility of including a lead peroxide pad to collect SO2. They will locate these columns at strategically placed meteorological stations in Indonesia, Thailand and Malaysia, exchanging them by mail at 2-month intervals to coincide with major changes in rainfall. River-water samples from the largest river draining the local catchment will be sent to Armidale with each exposed column for mineral analysis.
It is expected that this research will result in a new group of better-adapted fertiliser materials becoming available to farmers in South-each Asia.