The mineralization of nitrogen from compost in the tropics

The application of organic matter is very important in maintaining high and stable crop yields under tropical conditions. Most of tropical soils are characterised by low content of organic matter and low structure stability. Animal wastes contain many nutrients, including nitrogen, phosphorous, potassium and other minerals. In addition to supplying nutrients, the application of animal wastes also improves the chemical, physical and biological properties of soil. Raw animal waste is not accepted as an organic fertiliser, because it has an offensive odour, is unhygienic, and biological unstable. Also, the application of raw animal wastes may damage crop plants as a result of excess ammonium and the presence of phytotoxic substances such as phenolic acids and volatile fatty acids (Harada, 1990). According to Tyson and Cabrera (1993), composted poultry litter releases N more slowly than non composted poultry litter and, therefore, poses less environmental risk than non composted poultry litter. After 56 days, the proportion of organic N mineralised ranged from 0.4 to 5.8 % for the composted materials, and from 25.4 to 39.8 % for non composted materials.

An objective of composting is to convert animal wastes into products which are easy to handle and safe to health. Another objective is to convert the animal wastes into organic fertilisers which are safe for plant and soil (Harada et al., 1993). Harada (1992) suggested that the amount of manure and compost applied to the cropped land should be within a certain limit. Mineral fertilisers are recommended to use together with animal manure. Chong (1995) added that if mineral fertiliser should be replaced entirely by organic fertilisers, the application rate of organic fertilisers is much higher than normal rate. According to Gallardo-Lara and Nogales (1987), when high doses of compost (town refuse compost) were used, an inhibitory effect on seed germination may appear.

The large amounts of animal manure and/or compost tend to be applied in some cropped lands. However, heavy application of compost may cause environmental pollution. Not all of the nitrogen released from compost is taken up by crops, and the unutilised nitrogen is transformed into nitrate by nitrification. The nitrate leached down into the subsoil and groundwater, because soil has a poor ability retain nitrate (Smith and Peterson, 1982). Also an overloading of the soil by phosphorus can be a result of compost application.

The purposed of the experiment is to study the process of nitrogen mineralisation from two different sources of compost in comparison to mineral nitrogen.