The effects of conservation tillage on the soil/plant system

The effects of conservation tillage (direct drilling) on the soil-plant system were compared with those produced by conventional cultivation in a long term monoculture winter wheat trial on a Claremont silt loam in South Canterbury, New Zealand. There was a greater potential for reduced harvest grain yields from direct-drilled (DD) than from conventionally- cultivated (PL) soil in both wet and dry years. This was not the result of lower plant populations, but a reflection of the lower tiller and head densities on the DD than the PL soil. This was probably the result of reduced crop N-availability in uncultivated soil, which also possibly caused the lower N concentration in the harvest grain from DD than PL soil. Root density was greater in the surface (0-5 cm) DD than PL soil throughout the growing season. A trend was also apparent towards a greater root density immediately below the depth of cultivation in DD than PL soil early in the growing season. As compared with the PL soil, the soil chemical fertility status was not adversely affected by the adoption and continued long-term use of a DD regime. The DD surface (0-5 cm) soil had a greater organic carbon (C) and total nitrogen (N) content than the PL soil, with no differences apparent at greater depths. Both treatments had similar C:N ratios at all depths, but the in situ N-mineralisation rate was faster in PL than DD soil, although the estimated potential for mineral-N production throughout the growing season was similar for both cultivation treatments. There was a more marked concentration gradient of available phosphorus and potassium throughout the equivalent depth of cultivation in DD than PL soil, although the total content to 30 cm depth was similar for both treatments. Early in the growing season, the PL topsoil (0-20 cm) had a lower bulk density than the DD soil, although this trend was reversed at 25-35 cm depth. These differences did not persist throughout the growing season, with similar bulk densities at most depths for both treatments following harvest. A similar, but opposite trend was observed for total porosity, with higher values throughout most of the PL than DD topsoil, with the greatest difference between treatments observed early in the growing season. The differences in total porosity between treatments were mainly the result of a greater aeration capacity (i.e. volume of pores> 300 µm e.s.d.) in the PL than DD soil, although these physicogenic pores were unstable and decreased markedly in volume throughout the growing season. The volume of macropores was greater in PL than DD soil, although the continuity of the larger macropores was less than in the uncultivated soil. There was little difference between treatments in the volume of transmission pores (300-30 µm e.s.d.), storage pores (30-0.2 µm e.s.d.) or the residual soil water content. The soil water content was greater in the surface of DD than PL soil throughout most of the growing season. The greater water content in PL than DD soil at approximately 10-20 cm depth during the winter and spring suggested there was slower drainage through the lower depths in the cultivated than uncultivated soil. Soil strength values were greater for DD than PL soil at all depths sampled, except at approximately 25-30 cm depth when the soil was close to field capacity. These differences did not appear to adversely affect seed germination and emergence or root growth to depth in the uncultivated soil. The stability of aggregates sampled from 0-20 cm depth was greater for DD than PL soil. The retention of crop residues had a greater effect on increasing the stability of surface aggregates from PL than DD soil. Earthworm populations were greater in DD than PL soil, with a greater difference between treatments for mature than immature earthworms Infiltration rates measured using double-ring infiltrometers in the field appeared to give an unreliable estimate of the true infiltration rate due to extensive lateral flow of water. When measured using large, undisturbed soil monoliths, similar infiltration rates and saturated hydraulic conductivities were recorded for DD and PL soil. The breakthrough curves obtained for both DD and PL soil during the leaching of surface-applied "equilibrated" and "non-equilibrated" tracers under saturated and unsaturated conditions were markedly different to those predicted by convective-dispersive flow theory. This was attributed to extensive preferential flow occurring in both DD and PL soil through macropores continuous throughout the sample depth. Under saturated conditions, the rate of tracer leaching was greater in DD than PL soil, due to a greater number of continuous macropores in the uncultivated soil. Under unsaturated conditions, similar rates of solute leaching were observed for both cultivation treatments. The efficiency of solute leaching was greater under unsaturated than saturated conditions due to a longer residence time of the displacing solution under conditions of simulated rainfall.