Management of phosphorus for sustainable food crop production on acid upland soils in Australia, Philippines and Vietnam

LWR1/1994/014: Management of phosphorus for sustainable food crop production on acid upland soils in Australia, Philippines and Vietnam

Philippines, Vietnam

University of Queensland, Australia

Assoc Prof Pax Blamey
Phone: Tel +61 7 3365 2081
Fax: +61 7 3365 1188
Email: p.blamey@uq.edu.au

• National Institute for Soils and Fertilisers, Vietnam
• Central Mindanao University, Philippines
• Queensland Department of Primary Industries, Australia

$1,343,250

01/01/1996 - 31/12/1999

31/12/1999 - 30/06/2002

Dr Ian Willett

Acid upland soils occupy about 32% of land in the Philippines and about 70% of land in Vietnam, besides being widespread in other parts of humid tropical Asia and Australia. If managed carefully, these soils could be an important resource for cropping.
But nutrients in these soils are often washed away by leaching and runoff as well as being removed in crops. Plant growth is limited by the soil's inherently poor nitrogen, potassium, calcium, magnesium and molybdenum contents, and by manganese and aluminium toxicities which stunt root development. The upland soils usually have little organic matter or microbial activity and, therefore, their physical quality is degraded as well.
If farmers apply phosphatic fertiliser to such soils, more than half the application is rapidly fixed into insoluble forms. Fertiliser can be applied economically in bands that expose it to less of the soil's complexing agents. Alternatively, if farmers lime the soil first, then less of the iron and aluminium reacts with the phosphorus when it is added.
Soil can be amended with organic residues or rock phosphate fertilisers, or enriched with triple superphosphate. Suitable organic matter has been shown to make more phosphate ions available to plants.

This project aimed to study the movement of phosphorus, as phosphate ions, between its various forms in the soil as part of inorganic or organic (including microbial) compounds, or in the soil solution. Phosphorus (P) fertiliser, organic manures, green manure crops and lime were tested for their effects in attempting to amend acid soil sites in the Philippines, Vietnam and Australia.

Soil amendments were first assessed in each region to determine their viability in terms of cost, timing of application and available labour. The effectiveness of each method of soil amendment was subjected to economic analysis at farm level. Analysis of gross margins in past trials had shown that intermediately-yielding treatments could have the highest margins, because they cost less per unit gain in yield.
Experiment sites were chosen according to farmer cooperation, access and soil homogeneity. In the Philippines, researchers measured the effects of added organic matter and lime, and of agronomic practice, on yields and phosphorus uptake of maize, groundnut and soybean. In similar trials in Vietnam researchers worked on sites in heavy- and light-textured soils in two rainfall zones.
In Australia field trials determined yields and phosphorus uptake by a maize-groundnut-soybean rotation following a range of amendments with organic matter and added phosphorus fertiliser. Concurrent sorption studies in the glasshouse aimed to provide detailed understanding of the fluxes of phosphate ions. Staff overseas also analysed soil phosphate in its various nutrient pools, to contribute to a computer model of phosphorus dynamics.

Reviewers noted that the project had been successful in showing how P fertilisers can be used effectively and efficiently in upland acid soil areas of the tropics and subtropics. Questions about the importance of P fixation in reducing the efficiency of P fertilisation and about the usefulness of local organic matter sources for supply of P to plants had been answered.
Other questions, such as 'What is the role of rock phosphate in these soil-plant systems?', 'What are the residual effects of P fertilizers?', and 'What are the critical soil P tests for soils of different P buffering characteristics in this region?' were largely elucidated. Assessment of proposed synergistic effects of organic matter in reducing aluminium toxicity and P fixation as well as increasing P supply showed that the proposed effects were negligible. Thus the experimental program did not develop any new methods of managing soil and plant P, but laid a strong basis for efficient use of P fertilisers in these cropping systems.
These achievements mean that the project effectively advanced the understanding of P fertilisation of the acid upland soils of the region to the point where confident recommendations can be put forward for use by farmers in the region. Although the project concentrated on P sources and utilisation, omission experiments and soil testing were also used effectively to define the other fertilisers and lime that were required to sustain high crop yields.