Organic composition of green manure during growth and its effect on cation mobilization in an acid oxisol

Shoot samples of oil seed radish and black oat were collected during the winter at 60, 75, 90, and 120 days after planting (dap). At each cutting organic composition of the water soluble fraction (WSF) was determined and its effects on the mobilization of cations in acid soil were evaluated using PVC soil columns. A biological test based in root growth of pre-germinated wheat seeds was conducted for 7 days in soil columns after application of the WSF of plant materials. The relative organic composition of the plant materials changed between cutting times and the greatest values were observed at 60 dap for oil seed radish and 75 dap for black oat. In the later sampling dates, the relative values were decreased sharply. The identified organic compounds were aliphatic and phenolic organic acids. The application of the plant extracts in soil columns increased the pH and the concentrations of exchangeable calcium (Ca), magnesium (Mg), and potassium (K), and decreased exchangeable Al. Few differences between plant extracts were observed in the soil chemistry after extract applications. The extracts obtained at the 60 and 75 dap were more effective in alleviating soil acidity. The black oat plant material showed higher capacity for mobilization of Ca and the oil seed radish for Al. Wheat root growth, used as a biological indicator in the soil column system, was three times greater in the presence of all plant extracts when compared to the water control. The chemical changes in soil were not sufficient for discrimination of wheat root growth in the presence of different plant extracts. Higher efficiency for cation mobilization and acidity mitigation may be reached if plant material is cut before plentiful flowering stage.