Surface soil structure, the soil water balance and the effects of tillage

This study considers the effects of multiple-pass tillage on the surface soil structure of a Templeton silt-loam soil In Canterbury, New Zealand. The effects of pre-tillage soil water content (PTSW) and type of tillage operation are assessed for the freshly-tilled soil. A numerical simulation model (CONSERVB, van Bavel and Hillel, 1976) is evaluated as a method to assist in the identification of the soli properties which are most significant in determining evaporative loss of soil water. PTSW and Intensity of tillage operations interact to determine the aggregate size distribution resulting from multiple-pass tillage. Intensive tillage of a dry soil produces a high proportion of small wind-erodible soil aggregates and particles. The avoidance of intensive tillage reduces the likelihood of a PTSW effect occurring. Aggregates produced from tilling this soil at a water content near the lower plastic limit (LPL) are less mechanically stable (when dry) than those produced from tilling dry soil. Aggregate stability must be considered when assessing the most appropriate PTSW for the desired tillage objectives. Tillage-induced random roughness was quantified using a geostatistical method. Intensive tillage reduces aggregate size resulting in a smoother soil surface with a lower surface area. Intensive tillage decreases the macro-pore volume mainly through a decrease in the volume of aeration pores (pores >300µm diameter). PTSW does not have significant effects on macro-porosity or available water holding capacity. Near-saturation hydraulic conductivity is significantly reduced by intensive tillage as a result of decreased macro-pore volume. The Jackson (1972) model was evaluated by sensitivity analysis and found unsuitable for assessing the effects of tillage on unsaturated hydraulic conductivity. The output from the Jackson model showed extreme sensitivity to the 0 to -1.0 kPa matric potential section of the water characteristic input. Tillage-induced changes in soil porosity are reflected by changes in soil volumetric heat capacity and thermal conductivity. Tillage-induced soil structure changes affected shortwave albedo but to a smaller extent than previous studies indicated. The shortwave albedo on the tilled soil was low, due to the high organic matter content and rough surfaces. Predictions of evaporation, soil water content and soil temperature from the numerical simulation model CONSERVB were compared with field measurements from the tilled Templeton silt-loam soil. The CONSERVB model accurately simulated bare soil evaporation when the unsaturated hydraulic conductivity input function was determined by calibration. Simulated soil water and temperature profiles were generally good although water content near the soil surface was sometimes under-estimated and surface soil temperature was over-estimated in warm conditions. The CONSERVB model could be used In future to help in predicting benefits and risks from tillage operations. Identifying the tillage-sensitive soil properties which have the greatest influence on evaporative soil water loss is a research priority.