Comparative Effets of Forest and Pasture on Some Physical Properties of Latosolic Soils

A study was made to determine; some of the physical properties of two Hydrol Humic Latosol soils, the interrelationships of the physical properties measured, the relationships between land use and the variation in the physical properties measured, and the possible effects of these properties on the difference in productivity of stands of flooded-gum eucalyptus (E. salign) on these soils. Two soil types, Honokaa and Pohakea were selected for the reason that both provided areas supporting a high and low productivity of stands of E. saligna and an adjacent pasture. The results of this study show that a significant correlation exist between aggregate stability and organic matter in the forest plots. No significant correlation has been observed in the pasture plots. The lack of significance in the correlation coefficients in two of the forest and most of the pasture plots may be due to the high iron and aluminum oxides. In these soils, Honokaa and Pohakea the iron and aluminum oxides probably play a greater role in stabilizing the aggregates formed than the cementation effects of organic matter. Both the forest and pasture plots show a significant correlation (1% level) between organic matter and soil moisture content and between soils dry bulk density and soil moisture content. A conclusion can be drawn from the foregoing interrelationships of some of the soil physical properties studied that in these soils the organic matter content has a greater role in aggregation than in stabilizing the soil aggregates formed. This is indicated by the low absolute value of the correlation coefficients between aggregate stability and organic matter but a high (significant) correlation coefficients between organic matter and soil moisture content. Also, the soils studied have a high percent aggregate stability and high organic matter content. The results further show that there are no significant differences in the soil physical properties studied between the forest and pasture plots except in the percent aggregate stability. The high aggregate stability in the pasture is significant considering that it was on the land only eight years compared to twenty-eight years for the forest. The specific role played by the pasture in the formation of water-stable aggregates could not be determined in this study. A conclusion can be drawn that, factors other than organic matter have a profound influence on aggregate stability. The higher per cent aggregate stability under the pasture is attributed to the effect of grass roots. Also, the high percent aggregate stability in the soils studied is attributed mainly to the high iron and aluminum oxides. The results and observations show that putting the soils studied under pasture is better than putting it under forest. Aside from the greater beneficial effect of grasses on aggregate stability it is easier to establish on the land and also the benefits are derived much earlier than forest. Although the high tree productivity plots have a higher aggregate stability and available moisture than the low tree productivity plots it is suspected that the cause for the low productivity plots in F # 2 is different from that in F # 4. The wetter condition of F # 2 as well as the drier condition pf F # 4 may be unfavorable to the growth of trees. Furthermore, F # 4 has a red cap (6” red layer) and a shallow profile