Continuous fertilization on non-humiferous acid Oxisols in Rwanda “Plateau Central”: Soil chemical changes and plant production

Crop rotation system of maize and beans, established at Rubona (Rwanda) from 1984 to 1992, was used to evaluate different types and rates of fertilizers in improving the productivity of acid Oxisols. Continuous cropping of maize followed by beans for a period of 8 years gave no yield in control plots. A single application of 2 t per ha of lime increased significantly (p = .01) the soil pH, Ca2+ content, cationic exchange capacity, and decreased the level of the exchangeable aluminium. This quantity of lime when applied every two years for a period of eight years led to overliming. The application of more than 8 t per ha of fresh farmyard manure (annually), combined to 300 kg per ha of NPK 17:17:17 (every six months) significantly improved soil organic C and crop production at Rubona. The high rate (351 per ha) of fresh manure or the combination of “lime, manure and NPK fertilizers” gave the best crop performance. Ten tons of farmyard manure (dry matter) per ha and per year seems to be the minimum acceptable amount which can effectively substitute for inorganic fertilizers.