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Influence of Organic Matter on Soil Aggregation and Erodibility of Some Malaysian Soils
Thesis Abstract:
The dominant role of organic matter in promoting aggregate stability and the strong relationship between the latter and soil erodibility were consistently observed by many researchers. These suggest that organic matter and soil erodibility have a strong relationship. However, this is not always true. Some workers found a weak relationship between these two variables. In this context, experiments were conducted to examine the role of organic matter and its constituents in soil aggregation and erodibility of some Malaysian soils.
Ten soils, composed of five soil series under different land uses and varying organic matter contents, were subjected, in situ, to simulated rain. The soil losses were taken as relative erodibility values. The aggregation characteristics indices, namely: percent aggregation, mean weight diameter after dry sieving (MWDd ), mean weight diameter after wet sieving ((MWDw), percent water-stable aggregates >0.5 mm (%WSA >0.5), stability index (SI), clay ratio, and T2/T2 of the soils were determined. T2 and T20 were percentage oflight absorption after the soil were dispersed in water for 2 and 20 minutes, respectively. (MWD), percent WSA > 0.5 and Sl were taken as macroaggregate stability and clay ratio and T20/T2 as microaggregate stability. The soils were also analyzed 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 were referred to as humus.
It was found that humus acted as a more effective binding agent and consequently showed a better relationship with soil erodibility compared to the total organic matter. This implies that the forms of organic matter were more important than its total amount in relation to soil aggregation and erodibility.
Malaysian soils tended to have a greater amount offulvic acid and polysaccharide. Their relative importance with respect to soil aggregation and soil erodibility was in the following order: fulvic acid > polysaccharide > humic acid.
Aggregate size distribution and aggregate stability were significantly correlated to soil erodibility. However, the macroaggregate stability had a more dominant effect than the microaggregate stability. Of the macroaggregate stability indices tested, percent WSA >0.5 was found to be the most efficient index for soil erodibility.
Land use types which cause differences in organic matter status of the soils directly affected the aggregation characteristics and soil erodibility value. The results showed different erodibility values of the same soil series subjected to different land uses.