LIME-SLOTTING TECHNIQUE TO AMELIORATE SUBSOIL ACIDITY IN A CLAY SOIL .2. EFFECTS ON MEDIC ROOT-GROWTH, WATER EXTRACTION AND YIELD

Subsoil acidity causes low crop production, which is often associated with shallow root development and restricted soil water extraction. In part I of this series, lime-slotting of an acid soil was shown to imp rove the soil physical and chemical characteristics for root growth. I n a lysimeter study on an acid soil, the effects of several soil ameli orative treatments on root growth, water extraction and yields of a me dic crop were evaluated. Large lysimeter cores of 0.75 m diameter and 1.35 m deep were used. The soil treatments included a non-ameliorated acid soil, lime-slotting with a 0.15 m wide and 0.8 m deep slot contai ning 20 t ha-1 of lime, lime-slotting combined with surface phospho-gy psum application at 10 t ha-1, and complete amelioration of the entire soil volume by mixing lime at 133 t ha-1 and repacking to a low bulk density of 1.1 t m-3. In the non-ameliorated acid soil, medic roots we re confined to the surface (0.1 m) layer, resulting in limited water e xtraction of 32 mm during a prolonged drying cycle, and a low dry matt er yield of 70 g m-2. In the lime slotted soil, roots grew within the slot to its full depth, although penetration into the undisturbed soil was restricted to the soil immediately adjacent to the slot. Conseque ntly, the root length per unit surface area (L(a)) at depths below 0.1 m depth was increased to 9.9 km m-2. During a drying cycle, water ext raction increased to 58 mm. The increased water extraction came from b oth the slotted soil and the undisturbed soil between slots. This led to an increase in dry matter yields to 270 g m2. In lime-slotted soils with surface gypsum applications, the root growth and crop water extr action patterns were similar to the lime-slotted soil. Repacking limed soil resulted in similar root lengths (L(a) 10.0 km m-2) as lime-slot ted soil. However, owing to more uniform distribution of roots in the repacked soil, water extraction was increased to 100 mm and yields inc reased to 590 g m-2. Yields of non-ameliorated soil were only 12% of t he repacked, limed soil. However, lime-slotting which involves looseni ng only 25% of the soil surface area and addition of only one-sixth of the amount of lime required for complete soil amelioration, led to ma rked increases in yield (46% of the yield of repacked soil). Future fi eld studies are required to evaluate the optimum limed-slot configurat ions required for different soils, crops and climatic regimes.