Modeling the Stress Relaxation of Agricultural Soils in Borno State, Nigeria

Abstract. The impact of organic materials incorporated into three soils before compaction was studied in Borno State, Nigeria. The study concentrated on determining the recovery of the soil-organic matter mixtures compacted at different moisture contents. The soils were Ngala town clay (Vertisol), Nguma clay loam and University of Maiduguri Research Farm sandy soil. These three soils represent the major soils used for crop production in Borno State, Nigeria. The soil samples were collected from the top 0-20 cm of the soil profile, air dried and ground to pass a 2 mm sieve. The organic materials used were cow dung, chicken dung and groundnut haulms. These organic materials were air-dried and mixed with the soil samples to raise their organic matter levels to 2, 4 and 6%. Each of the soil-organic matter mixtures was compacted at four different moisture content levels of 5, 10, 15, 20 %. The loads used for compaction were 177, 288 and 398 MPa. Soil bulk density, penetration resistance, saturated hydraulic conductivity and modulus of recovery were measured after compaction. Results showed that for a particular level of compaction and organic matter level, the bulk density and penetration resistance values increased with increase in moisture content up to a point and there after decreased with further increases in moisture content. The increases were however, organic matter dependent. Hydraulic conductivity increased with increase in percentage of organic matter. Sandy loam soil produced the highest values of saturated hydraulic conductivity followed by clay loam and clay soils. Increase in organic matter increased soil recovery. At any given percentage of organic matter, recovery increased with increase in compaction level. This also implies that recovery reduces with increase in load. Groundnut haulms yielded the highest values of recovery followed by cow dung and chicken dung. This was for all the soils irrespective of compaction level.