Soil acidity and aluminium toxicity as affected by surface liming and cover oat residues under a no-till system

A 3-year field trial examined in a long-term no-till system the effects of surface-applied lime and cover black oat (Avena strigosa Schreb) residues on soil chemical attributes, root growth and grain yield of corn (Zea mays L.) and soybean (Glycine max L. Merrill) on a loamy, kaolinitic, thermic Typic Hapludox in Parana State, Brazil. The treatments consisted of dolomitic lime broadcast on the soil surface at 0 or 12 t/ha, with and without cover of black oat residues. Corn and soybeans were grown without rainfall limitation. Applying lime on the surface improved soil acidity and decreased aluminium (Al) toxicity to a 10-cm depth 1 year after application. Surface liming increased pH and the content of exchangeable Ca2+ to a 20-cm depth, and decreased Al toxicity to a 40- to 60-cm depth 3 years after application, indicating that the surface-applied lime moved deeper. Cover black oat residues did not favour the mobility of surface-applied lime to alleviate subsoil acidity and an increase in the Al3+ saturation level at the soil surface was found in unlimited plots with black oat residues. Root growth and grain yields of corn and soybean were not influenced by surface liming with or without cover black oat residue. Despite the soil acidity level, root length of corn and soybean ranged from 55 to 60% at 0- to 10-cm depth. The results suggest that Al toxicity is low in no-till systems during cropping seasons with adequate and well-distributed rainfall, but this effect is not related to the presence of cover oat residues.