ORGANIC-MATTER EFFECTS ON SOIL STRENGTH PROPERTIES

Measurements of bulk density, moisture retention, aggregate stability and shear strength were taken for sandy loam and loamy sand soils with different organic-matter contents ranging from 1.23 to 5.64%. Organic matter in soils originated from either being under grass treatment for some years or mixing two forms of peat each at 4, 11 and 17% rates to a control soil. Aggregate stability was measured by the percentage water stable aggregates (WSA) as well as individual aggregate energy (IAE) measured using single drop impact test. Organic matter from grass significantly increased percentage WSA from 2.15 to 21.62% and IAE from 4 to 33 mJ. While percentage WSA reduced significantly from 2.50 to 0.88 with increasing peat content, the IAE reduced only slightly from 3.93 to 3.45 mJ. This suggests that peat reduces the overall soil stability without affecting the strength of individual aggregates. Organic matter from both grass and peat reduced bulk density and increased moisture retention. While organic matter from grass increased soil shear strength from 19.17 to 24.44 kN m-2, that from peat reduced it from 15.47 to 11.90 kN m-2. The effects of both forms of organic matter on bulk density and moisture retention were expected to have reduced their strengths. However, since organic matter from grass improved aggregate stability, it reduced soil dispersion in water and improved shear strength, while peat only made the soil aggregates fall apart and reduced their shear strengths. The results from this study show that the influence of organic matter on soil shear strength depends on whether or not it improves soil aggregate stability. © 1990.