MULCHING EFFECT OF PLANT RESIDUES OF CHEMICALLY CONTRASTING COMPOSITIONS ON SOIL ORGANIC-MATTER CONTENT AND CATION-EXCHANGE CAPACITY

Effects of five types of plant residues [Acioa, presently Dactyladenia barteri, Gliricidia sepium, and Leucaena lecocephala prunings, maize (Zea mays) stover and rice (Oryza sativa) straw] applied as mulch on s oil organic matter (SOM) content and effective cation exchange capacit y (ECEC) were studied on an Alfisol in the humid tropics. Plant residu e mulch resulted in a decline in SOM and ECEC during two years of crop ping following six years of grass fallow. Rice straw mulch resulted in less and maize stover mulch in a greater decrease of SOM and ECEC tha n the other mulches. Decrease in SOM and ECEC is attributed to the mul ching effect on the soil micro-climate which enhanced the decompositio n of SOM accumulated during the grass fallow prior to the initiation o f the experiment. In order to maintain SOM for a tropical soil, plant residues with high lignin, polyphenols, and silica will have to be amo ng residue species when applied in continuous cropping systems.