Organic fertilizer use and the potential influence on cabbage (Brassica oleracea var. Capitata L.) plant and soil nutrient composition

Food security and sustainable crop production by small-scale and subsistence producers in South Africa relies heavily on organic fertilizers as an alternative to inorganic fertilizers. Recommendations on the use of organic fertilizers and the potential influence on plant and soil nutrient composition are not yet clear. Two field trials were conducted at Umsunduze Training Centre, KwaZulu-Natal during the 2005 and 2006 growing seasons to investigate the effects of three different organic fertilizers (chicken manure, kraal manure and compost) on the nutrient composition of cabbage and the soil. Four application rates were used for each organic fertilizer (chicken manure: 0, 6.25, 12.5 and 25 kg/10 m2; kraal manure: 0, 12.5, 25 and 50 kg/10 m2; compost: 0, 25, 50 and 100 kg/10 m2). Each treatment combination was replicated four times. Organic fertilizers were incorporated into the soil one month before planting. After harvesting, soil (2005 and 2006) and plant analysis (2006) were done. Nitrogen content of cabbage heads was the only nutrient significantly affected by chicken manure. Application of different organic fertilizers at different application rates did not influence most of the chemical properties of the soil during the first season (2005) but their influence was more evident in the second season (2006). Compost significantly affected most of the chemical properties of the soil, increasing the phosphorus, potassium, sulphur, calcium, total carbon and total cation content of the soil. Soil pH was increased and acid saturation decreased with organic fertilizer application at Rate 3. In most cases the two highest application rates (Rate 2 and 3) significantly influenced the chemical properties of the soil. Although two years is a relatively short period, it seems that compost and kraal manure improved the soil chemical properties more than chicken manure.