Soil fertility changes and crop yields from the first 4 years of the Mandela trials.

Abstract

Abstract Long-term farming systems trials (the Mandela Trials) showed that organic farming systems initially had lower yields than conventional systems in the first 2 years (20% and 31% lower, respectively). In the third (drier) year, the yield gap was closed once soils had improved biologically and available soil phosphate had been supplemented with rock phosphate, and the organic system outyielded conventional by 11%. In the fourth (wetter) year, nitrogen-poor compost caused low yields in the organic system, and conventional yields were 27% higher than organic. Soil became less acid under organic management, and soil organic matter (SOM) levels improved, as did available potassium. The organic farming systems were supplied with less than a quarter of the nutrients supplied to the conventional system, but yields were comparable throughout. From the second season, crop rotation for both organic and conventional systems outyielded cabbage monocropping systems, but this was only statistically significant in the fourth year; the benefit of crop rotation was greater for organic systems, probably because in the fourth year the low nitrogen nutrition in the organic system affected monocropped cabbage severely, while rotated cabbage benefited from the previous cowpea (legume) crop. The rotation was: cabbage (heavy feeder) followed by sweet potato (light feeder) followed by cowpea (legume). Water use efficiency was better for the organic system with mulch helping to reduce evaporation and SOM improving the water and nutrient holding capacity of the soil. Soil microbiology was more diverse for the organic system and biological pest and disease control, although slower and not very effective against cutworm, was able to keep pests and diseases at acceptable levels.