ORGANIC-MATTER EFFECTS ON THE STRENGTH PROPERTIES OF COMPACTED AGRICULTURAL SOILS

Dry bulk density, penetration resistance, and shear strength were meas ured in the laboratory for two Trinidadian soils (sandy loam and clay) mixed with peat and farmyard manure (FYM) at four levels (0, 4, 8, an d 12% by mass) after they had been compacted using 5, 15, and 25 stand ard Proctor blows. The compaction was carried out at different moistur e 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 va lues after which the values decreased with further increments in moist ure content. Generally, organic matter reduced these peak values for a ll the soils at all compaction levels, with peat producing lower peak values than FYM. There were significant interaction effects between le vel 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 effe ct of peat and FYM on peak penetration resistance and peak shear stren gth. Values of bulk density decreased with increasing organic matter c ontents at most levels of moisture content in all the soils. Values of penetration resistance and shear strength decreased with increasing o rganic matter contents at lower levels of moisture content while the r everse occurred at higher moisture levels. Regression models were used to describe these trends in results. Results from this study demonstr ate that the effect of organic matter on the strength properties of co mpacted agricultural soils depends on the level of soil moisture at co mpaction and the type of organic material.