Two long-term experiments on the effects of plant residue mulch and organic matter incorporation on the growth of corn and soil fertility were carried out in Ultisols (a Reddish Brown Lateritic Soil) of Central Thailand, starting from 1976 and 1981, respectively.
Plant residue mulch (5t/ha/year) combined with fertilizer application was remarkably effective in maintaining a high yield of corn throughout the experimental period. Without mulch application, the corn yield fluctuated to a great extent and decreased markedly in drought years even under chemical fertilizer applicaUon. The effect on the growth and yield was mainly ascribed to the maintenance of a suitable amount of soil moisture and the behavior of soil nitrogen.
The effect of compost application (20t/ha/year) on the yield was negligible in the first eight years. However, thereafter, gradually the yield increased compared with that in the absence of compost application. Organic carbon and total nitrogen contents in the soil increased. Soil biomass nitrogen and the content of available nitrogen (mineralizable nitrogen and inorganic nitrogen) increased with the decrease in the rate of nitrogen fertilizer application and increase in the amount of organic matter application. Soil bulk density, water permeability, plasticity, soil hardness, aggregate formation were improved by long-term incorporation of organic matter to the soil.
Though many technical problems still remain to be solved before the technology can be transferred, proper soil management consisting of the application of either plant residue mulch or compost would at least provide some economic answers to sustainable upland crop production under rainfed conditions in Thailand.