Organic Matter Effects on the Strength Properties of Compacted Agricultural Soils

Dry bulk density, penetration resistance, and shear strength were measured in the laboratory for two Trinidadian soils (sandy loam and clay) mixed with peat and farmyard manure (FYM) at four levels (0, 4, 8, and 12% by mass) after they had been compacted using 5, 15, and 25 standard Proctor blows. The compaction was carried out at different moisture contents determined according to the consistency limits of the soils. Values of bulk density, penetration resistance and shear strength of the soils increased with increments in moisture content up to peak values after which the values decreased with further increments in moisture content. Generally, organic matter reduced these peak values for all the soils at all compaction levels, with peat producing lower peak values than FYM. There were significant interaction effects between level of added organic matter and type of organic material, between soil compaction level and type of organic material and between compaction level and soil type. These interactions were used to describe the effect of peat and FYM on peak penetration resistance and peak shear strength. Values of bulk density decreased with increasing organic matter contents at most levels of moisture content in all the soils. Values of penetration resistance and shear strength decreased with increasing organic matter contents at lower levels of moisture content while the reverse occurred at higher moisture levels. Regression models were used to describe these trends in results. Results from this study demonstrate that the effect of organic matter on the strength properties of compacted agricultural soils depends on the level of soil moisture at compaction and the type of organic material.