In Thailand and the Philippines, and many other countries in the region, cropping systems on marginal land are being relied on increasingly for agricultural production. However, acid infertile soils (Ultisols and Oxisols) - which are generally low in organic matter, ion-exchange capacity and thus available nutrients - are dominant in these areas. Earlier ACIAR projects have demonstrated that such soils benefit from increased organic matter return. This provides a reservoir of plant nutrients, improves the soil's nutrient- and moisture-holding capacity, reduces erosion and suppresses weed growth.

This project - aimed at the development and implementation of management practices that maintain soil fertility and crop production and avoid significant land degradation - will concentrate on three rainfed cropping systems:

. lowland rice in northeastern Thailand;

. upland corn and rice in the central Philippines; and

. wheat in southern Queensland and northern New South Wales.

Objectives of the project are to determine, for the three systems:

. strategies to optimise management of crop residues, legume leaf litter and inorganic fertilisers; and

. the suitability of nutrient and carbon balances as performance indicators of sustainability.

Field experiments in Thailand and the Philippines will compare different management systems for organic matter residues. They will look at the level and nature of organic matter in the soil, the changes in soil chemistry that result from changes in organic matter levels, nutrient dynamics, and effects on production. Field experiments in Australia will investigate the changes that have occurred with long-term cropping under different regimes, and monitor changes in newly implemented cropping regimes. Complementary laboratory and glasshouse studies, both in Australia and overseas, will further investigate the breakdown of organic matter, its movement within the soil profile, and its effect on soil properties.

In Thailand and the Philippines, collaborators will apply organic residues that differ in their breakdown (mineralisation) rates. They will use the shrub legume Flemingia congesta as a source of slowly mineralising organic matter, and apply other sources of organic matter separately and in combination, or provide no organic matter input.

In Thailand, the annual legumes mungbean and cowpea, and in the Philippines, the shrub legume Gliricidia sepium, will supply organic matter that is rapidly mineralised. In both countries the residues of the main crops will be either returned to the soil or removed.

While the application of organic residues can result in short-term immobilisation of plant nutrients, the judicious application of mineral fertiliser during these periods can overcome the reduction in available nutrients. This must be carefully controlled, or soil biological activity will be stimulated, preventing long-term build up or organic matter.

Overcoming this problem is a key aspect of the project.

The scientists will analyse nutrient levels and calculate balances for all nutrients - including carbon - into, within, and out of each system. Combined with chemical analysis of the soil, this will allow monitoring of changes in the nutrient-holding capacity of the soil and assessment of system sustainability. They will also evaluate, as indicators of change in carbon pools and thus system stability and sustainability, two techniques that measure carbon isotope ratios.

The development of such techniques for calculating an index of the sustainability of agricultural systems, with particular reference to nutrient dynamics, is seen as an important potential output of the project. This will assist researchers to monitor the effects of various treatments, and policy makers to set goals for the development and rehabilitation of degraded areas.

The project will benefit Southeast Asian countries by increasing the efficiency of utilisation of crop residues and inorganic fertilisers and by reducing soil degradation. It will also enhance their research capability through the interaction with Australian scientists and introduction of new techniques. The quantification of nutrient dynamics and the development of methods to enhance efficiency and soil sustainability will be equally applicable to Australian systems.