A study of hard-setting behaviour of structurally weak tropical soils
Some soils set to a hard structureless mass on drying and this behaviour may limit crop productivity. The purpose of this study was to assess the extent to which soil management and soil properties influence hard-setting behaviour in order to identify those soil properties which are most appropriate to characterise the limitations to management and crop growth imposed by hard-setting. Five sites from four vegetation zones of Nigeria were investigated. Within each site soils were sampled from no-till or forested plots and these were contrasted with cleared (deforested) amd ploughed plots. Hard-setting behaviour was determined by measuring the unconfined compressive and tensile strengths of small undisturbed cores (minicores) which had been equilibrated at a range of matric potentials. Rooting potential was assessed both by measuring needle penetrometer resistance or cores equilibrated at potentials of -6 and -100 kPa; and also by measuring penetrometer resistance in the field at a range of moisture contents. Soil bulk density, organic matter concentration, soil friability, aggregate stability and amounts of water suspendable solids were also measured to assess soil properties that are likely to be diagnostic of hard-setting behaviour. The limitations imposed by hard-setting behaviour on rooting and yields of maize and cowpea were assessed at IITA. The minicore strengths increased as the moisture content decreased but the increase was much more marked for mechanized cleared or tilled soils than for the less disturbed treatments of forestry and a no-till system. For all soils the most pronounced effects of moisture content deplection on soil strength occurred at potentials of < -100 kPa. Theoretical considerations indicated that the effective stress accounted for over half of the strength of minicores at -100 kPa and more than accounted for the strength of minicores at -1 MPa. The greater strength in tilled soils was attributed to a decrease in organic matter content, wet aggregate stability and friability and to an increase in bulk density. Needle and field penetrometer resistance results indicated that rooting potential was reduced on hard-setting soils and actual root measurements supported this view. Consequently maize and cowpea yields were reduced.