LIME-SLOTTING TECHNIQUE TO AMELIORATE SUBSOIL ACIDITY IN A CLAY SOIL .1. EFFECTS ON SOIL-PH AND PHYSICAL CHARACTERISTICS

In soils with subsoil acidity, root growth of plants sensitive to acid ity is restricted to surface layers resulting in reduced water and nut rient uptake and low yields of crops. In the present study, the use of the lime-slotting technique, with slots 0.15 m wide and 0.8 m deep, f or providing long-term improvements of the adverse soil chemical and p hysical characteristics of an acid, clay soil was investigated. Lime-s lotting using a rotary slotter resulted in a uniform increase in soil pH to the full depth of the slot (0.8 m), indicating a thorough mixing of the lime with the small soil fragments produced by slotting, as th ey are thrown up into the rotor shroud and redeposited in the slot. In contrast, ripping to 1.0 m depth and delve ploughing to 0.8 m depth o nly changed the soil pH close to the soil surface. The saturated hydra ulic conductivity, total porosity and air-filled porosity at a potenti al of - 10 kPa measured on soil cores was significantly (P < 0.05) inc reased within the slot, compared to the undisturbed soil. Values of un saturated hydraulic conductivities K(theta) and diffusivities D(theta) , calcualted from the saturated conductivity and water potential-water content relationships, also showed increases in the slot, at high wat er contents. Although a polishing of sections of the slot wall due to rubbing of the rotors was observed in the field, the saturated conduct ivities of the cores taken across the slot walls did not show signific ant (P < 0.05) differences compared with the undisturbed soil adjacent to the slot. The extent of protection of the loosened soil in the slo ts from recompaction during subsequent farm trafficking was evaluated in a vineyard. Soil strength measurement showed distinct differences b etween field plots which were non-ameliorated, lime-slotted or deep ri pped. Soil strength was estimated using penetrometer cone index measur ements, which were made when the soils were at similar water contents. Zones of high soil strength observed in the non-ameliorated soils, we re reduced in the ripped and slotted soil. However, the upper part of the ripped soil had soil strengths higher than the lower part, indicat ing repacking under trafficking. In contrast in the slotted soil, the slotted area had low soil strength to depth, indicating greater protec tion of the loosened soil during trafficking. This could be due to bri dging of the tractor wheels and implements across the slot on to the s tronger undisturbed soil on either side. Thus, lime-slotting is an eff ective technique for long-term amelioration of the adverse physical an d chemical properties of dense clay soils with subsoil acidity. This s hould encourage root growth to depth within the slots, and use of the water stored in the subsoil.