Effects of management systems on soil structure and other physical properties

The effects of different management systems on soil structural and other related physical properties were measured on a mixed cropping farm in Canterbury. In the study, four sites were chosen on a Wakanui silt loam soil (yellow-grey earth) representing long-term arable cultivation (AC), subsoiling in combination with two pasture (SP), minimum tillage (MT) and permanent pasture (PP). Soil structural development and stability, soil density and porosity, penetration resistance, soil water movement, soil organic matter and iron and aluminium oxide contents were measured. Relationships between selected properties were examined. A secondary study was conducted to examine the influence of soil constituents (especially organic matter and iron and aluminium oxides) on aggregate stability. In this study two other soils, Craigieburn high country yellow-brown earth and a Rapaki brown granular loam, compared with the Wakanui soil. Pedological criteria were used to assess soil structural development, whilst wet sieving and high energy moisture characteristic methods were adopted to assess structural stability. Both structural development and stability was lower in the longterm arable cultivation soil than in any of the others. The minimum tillage soil had better level of structure development and a higher aggregate stability than the conventional cultivation soil. A period of two years ryegrass in association with subsoiling produced a more stable soil structure, especially in the top 5 cm depth, compared with ploughing. In the main study, the low aggregate stability of the soil in the AC site was associated with a low soil organic matter content. A highly significant correlation was to exist between these, two parameters when data from all the Wakanui sites was examined. However, the correlation was not significant in the secondary study involving the Craigiebum and Rapaki soils. In these soils, iron and aluminium oxides appeared to be more responsible for soil aggregation than organic matter content. Compared with undisturbed pasture, long-term arable cultivation increased total and macroporosity and saturated hydraulic conductivity, and reduced soil bulk density and penetration resistance within the 5-20 cm soil depth. Nevertheless, this kind of management had an opposite effect on these soil properties in the surface (0-5 cm) layer and below the ploughed layer (>20 cm). Surface infiltration, subsoil porosity and hydraulic conductivity were decreased, and subsoil bulk density and penetration resistance were increased in these layers. In comparison to the arable cultivation, subsoiling plus pasture resulted in lower bulk density and penetration resistance values in the subsoil (>20 cm) layers. This was attributed to the loosening effect of the subsoiler on the compacted lower layers. Water transmission was also improved, as indicated by higher hydraulic conductivity within the 30-50 cm soil depth of the SP soil. In addition, subsoiling created a more uniform soil profile, in terms of soil density, penetration resistance and hydraulic conductivity, compared with arable cultivation and minimum tillage practices. The SP profile had similar properties to those in the PP site.