Subsoiling and surface tillage effects on soil physical properties and forage oat stand and yield

Much of New Zealand's agriculture integrates animal and crop production on poorly drained, easily compacted soils. We hypothesized that soil properties affecting forage oat (Avena sativa, cv Awapuni) establishment on land compacted by 15 years of conventional cropping might be influenced by various subsoiling and surface tillage combinations. Plots on a Moutoa silty clay (Typic Haplaquoll) were paraplowed (P), deep subsoiled (V), shallow subsoiled (S), or were left as non-subsoiled controls (C), Subsequently, the surface 15 cm was surface-tilled (T) using a power rotary-tiller and firmed with a Cambridge roller or were not tilled (N). Oats were then sown with a cross-slot drill, Subsoiling greatly reduced soil strength. Cone indices showed disruption to 40 cm with P, 36 cm for V, and 30 cm for S, Approximately 60% of profile cone indices to a depth of 0.5 m from subsoiled treatments were less than 1.5 MPa, compared to approximately 30% for C. T slightly improved strength distribution in non-subsoiled controls but had little effect in subsoiled treatments. Subsoiling without T continued to show improved profile cone index cumulative frequency 233 days after subsoiling. Subsoiling after T in this high rainfall climate eliminated most of the separation in cumulative frequency of soil profile cone index values by two weeks after T. T reduced emergence from 142 to 113 plants per square meter and reduced yield from 5318 to 3679 kg ha(-1). Forage yield increased from 3974 to 4674 kg ha(-1) with subsoiling. Soil porosity, saturated and slightly unsaturated hydraulic conductivities (K-SAT and K-40) and air permeability were highly variable but generally increased with subsoiling. Oxygen diffusion rate (ODR) (using Pt microelectrodes) was also variable, but N and C treatments had consistently lower ODRs than T or subsoiled treatments, Generally, subsoiling without T produced better soil conditions and oat crop performance than the prevailing New Zealand practice of T without subsoiling.