Effects of structure liming on clay soil

Structure lime, comprising 80-85% ground limestone (CaCO3) and 15-20% slaked lime (Ca(OH)2), is applied to clay soils to counteract losses of particulate phosphorus (PP) through aggregate stabilisation. This thesis evaluated the effect of structure lime on soil aggregate stability, aggregate size distribution, draught requirement in tillage and crop yield. Structure lime at the standard application rate of 8 t ha-1 increased aggregate stability at 1-2.5 years after application by 15-35% compared with an unlimed control. On average, structure liming proved to be an effective measure to increase aggregate stability and thereby reduce the risk of PP losses. However, significant trial-treatment interactions indicated different soil reactions in different trials, with clay content, soil organic matter content, initial pH and clay mineralogy being decisive variables. Sitespecific application of structure lime is therefore needed. Follow-up studies six years after structure liming showed declining effects on aggregate stability. A tentative recommendation is that clay soils with pH below 7 and clay content above 25% should be given priority in structure liming schemes. Structure liming resulted in a finer tilth and reduced the draught requirement in cultivator tillage by 7%, thus lowering fuel consumption and reducing associated CO2 emissions. Crop yield responses were inconsistent, with changes in spring barley grain yield of ±10%. Decreased availability of micronutrients through binding in limed soil can possibly explain the observed yield decreases. Yield increases were likely attributable to a finer tilth