Soil fertility erosion and the associated cost of NPK removed under different soil and residue management in Ghana

Two experiments were carried out at the University of Science and Technology, Kumasi, Ghana, to study the etlect of difterent tillage practices and mulching rates on ftertility erosion and its associated cost. In Experiment I. tillage treatments studied were hand tillage by hoe, plough-plant, plough-harrow-plant, and excessive-tillage (double ploughing + 3 times harrowing + 3 times spiketooth harrowing and planting). In Experiment 2, the mulching treatments, using dry Panicum maximum, were bare plot (To), no mulch + maize (T1), 2 t/ha maize (T2), 4 t/ha mulch + maize (T3), and 6 t/ha mulch + maize (T4). The test crop in both experiments was maize (Zea mays L.). Both studies were carried out on runoff plots on a slope of 3.5 per cent. The eroded sediments were analyzed for NPK and organic matter. Enrichment ratios were calculated to give indices of fertility erosion. The cost of NPK removed by erosion was calculated by the Replacement Cost method. In Experiment 1, hand tillage and all tillage in excess of plough-plant caused significant losses of soil, water, NPK and organic matter. In all cases, the excessive tillage recorded the highest losses whilst plough-plant had the least. In most cases, the eroded sediments were richer in NP~ and organic matter than the parent soil. The NPK removed by erosion represents a hidden cost to agricultural production. The seasonal cost in cedis per hectare of NPK losses due to erosion in a maize production enterprise, expressed as a 15-15-15 NPK fertilizer, were 15528, 7354, 2163 and 805 for excessive, hand, plough-harrow-plant and ploughplant tillage. respectively (I US Dollar = ¢2,200). In Experiment 2. mulching significantly reduced soil, water, NPK and organic matter losses, with the magnitudebf reduction being greater as mulch rate increased at 2 t/ha intervals from 0 to 6 t/ha. Enrichment ratios ranged from 0.9 to 2.3 for organic matter, 0.9 to 1.8 for N, 1.5 to 1.3 tor P, and 1.5 to 2.9 for K. The cost in cedis per hectare of NPK removed were 14416, 8908, 5712, 4692 and 2584 for T0, T1, T2, T3 and T4, respectively. The results for both experiments showed that although losses of soil may be small, the concentration of nutrients in the eroded sediment could be high.