Aggregate Stability in Relation to Some Physical and Chemical Properties of Drained and Reclaimed Paddies

Abstract

Soils from some drained and reclaimed paddies from the Philippines and Hawaii were studied to determine the factors which are important for structural regeneration. Structural regeneration is the limiting factor for the conversion of soils from paddy to non-paddy cultivation of crops. Soil aggregate stability was used as an index of soil structure. The soils from the Philippines were texturally coarser than their Hawaiian counterparts. In general the clay fraction of the Philippine soils were montmorillonitic in mineralogy whereas the Hawaiian soils were kaolinitic. The Hawaiian soils had higher clay, organic matter and free iron oxide contents than the soils from the Philippines. Aggregate stabilities of soils from the Philippines were highly dependent on the concentration of clay, organic matter, and free oxide contents. No similar dependence could be established for the Hawaiian soils. This apparent discrepancy was explained by the fact that the aggregate stability is dependent upon the concentration of cementing chemicals only at low concentration. The Philippine soils fall in the lower portion of the aggregate stability-concentration of cementing agent curve. In this region there is a strong dependence of aggregate stability on the concentration of a cementing agent. The Hawaiian soils form the plateau of this curve. Together, the soils from the Philippines and Hawaii form a group of soils having a wide spectrum of physical, chemical and mineralogical properties. A number of treatments (soil processes) were imposed on the surface soil of the drained Wailua paddy to demonstrate changes in aggregate stabilities with time. The treatments were as follows: T1, continuously dry; T2, alternate wet and dry with fertilization of NPK; T3, alternate wet and dry with fertilization of NP; T4, alternate wet and dry with fertilization of N; T5, alternate wet and dry alone, and T6, continuously wet. It was found that continuous dry treatment on the soil substantially increased the status of aggregation of the drained paddy soil. An increase in the percent aggregate stability was also observed when a fertilizer treatment was superimposed on the alternate wetting and drying treatment. This increase, however, was of a lower magnitude than the continuous dry treatment. The lowest increase in aggregation was shown by the alternate wetting and drying treatment alone. The continuous wet treatment was included to simulate a paddy condition. After 6 months, aggregate stability of soils under continuously wet condition did not materially change from the original aggregate stability of the drained paddy soil. In all treatments the greatest increase in aggregate stability was observed in the surface layer at two months but this depth dependence disappeared after six months.