Amelioration of a hardsetting Alfisol through deep mouldboard ploughing, gypsum application and double cropping. I. Soil physical and chemical properties

Contrasting soil management techniques were applied to a hardsetting red-brown earth (Alfisol) used for flood-irrigated wheat (Triticum aestivum) production at Trangie, N.S.W., Australia. The individual and combined effects of deep mouldboard ploughing to a depth of 0.45 m, gypsum application (5 t ha^?1) and double cropping upon aggregate stability, bulk density, porosity, cone index and the non-limiting water range were evaluated. Dispersion and slaking of the surface soil were unaffected by the treatments when measured at the end of the second year of the experiment. Approximately 60% of the soil mass in the 0–0.15 m layer slaked on wetting, whereas less than 1% of the soil dispersed. Organic carbon (OC) levels of the surface soil were not affected by double cropping or gypsum application, but were reduced by mouldboard ploughing from 0.9% to 0.6% OC. The relationship between OC and macroaggregate (more than 250 μm) stability indicated that large increases in OC beyond 0.7% OC were required for relatively small increases in aggregate stability. Mouldboard ploughing increased clay content of the upper 0.15 m of the soil from 22% to 27%. This was associated with an increase in the frequency and depth of cracking which, however, diminished over time. The non-limiting water range (NLWR) was expanded in the uppermost 0.1 m by gypsum application from 0.15–0.30 to 0.09–0.28 m³ m^?3. Mouldboard ploughing expanded the NLWR at a depth of 0.2 m. Penetrometer resistance, on average, exceeded the critical value for wheat root growth at a water content of 0.15 m³ m^?3, which is substantially higher than the wilting point (0.09 m³ m^?3). Excessive resistance to penetration as opposed to inadequate aeration or water availability is the main agronomic impediment in these soils, at least in the initial stages of crop development. Penetration resistance within the 0.05–0.3 m layer was reduced during a drying cycle in the order: mouldboard ploughing>gypsum>double cropping. The reduced penetration resistance associated with mouldboard ploughing was due to higher water content to a depth of 0.2 m and reduced bulk density below this depth.