Author : Tajuddin Bin Adan

Summary : The dominant role of organic matter in promoting aggregate stability and the strong relationship between the latter and soil erodibility are observed consistently by many researchers. These findings suggest that, organic matter and soil erodibility should have a strong relationship too. However, this is not always true. Some workers found a weak relationship between the two. With this background, experiments were conducted to examine the role of organic matter and its constituents on soil aggregation and erodibility for some Malaysian soils. Ten soils comprising of five soil series under different landuses and varying organic matter contents were subjected, in situ, to simulated rain. The soil losses were taken as their relative erodibility values. The aggregation characteristics indices, namely, percent aggregation , mean weight diameter after dry sieving (MWD subscript d), mean weight diameter after wet seiving ((MWD subscript w), percent water-stable aggregates >0.5 mm (percent WSA >0.5), stability index (SI), clay ratio and T subcript 20/T subcript 2 of the soils were determined. T subcript 2 and T subcript 20 are percentage of light absorption after the soil being dispeared in water for two and twenty minutes respectively. (MWD subscript w), percent WSA > 0.5 and SI are taken as macroaggregate stability and clay ratio and T subcript 20/T subcript 2 as microaggregate stability. The soils were also analysed for total organic matter content, decomposed organic matter (humus) and constituents of organic matter (fulvic acid, humic acid, and polysaccharide). The sum of fulvic acid, humic acid, and polysaccharide is referred to as humus. It was found that, humus acted as a more effective binding agent and consequently showed a better relationship with soil erobility compared to the total organic matter. This implies that the forms of organic matter are more important than its total amount in relation to soil aggregation and erodibility. Malaysian soils tend to have a greater amount of fulvic acid.