Sustainable agriculture in the tropics: the case of potassium under maize cropping in Togo

The transfers of native and applied K in a rhodic Ferralsol were studied in an agrosystem of southern Togo to propose sustainable cultivation strategies for K in kaolinitic soils. Potassium balance was measured over two years in field conditions under continuous maize cultivation with two K fertilisation levels (0 and 137 kg K ha^–1 yr^–1). Postassium leaching below the root zone, determined using ceramic cup samplers and Darcy's law, was on average 7.5 kg K ha^–1 yr^–1 with K fertilisation, i.e. 2% of the quantity of K applied, and 4.5 kg K ha^–1 yr^–1 without. The low leaching values resulted from a K concentration lower than 130 M in the soil solution. The low K concentration in the soil solution was related to selective adsorption of K increased by a low content of exchangeable K, with a Gapon selectivity coefficient ranging from 7.9 and 11.5 M ^–0.5. So the level of exchangeable K must first be increased to raise K concentration in the soil solution. The fixation and release of K was analysed using the isotopic exchange method with ⁴²K-ions and compartmental analysis of the kinetics of isotopic exchange. Potassium fixed in a form non available within one year accounted for 78% of the difference between the two treatments. The annual amount of K fertilisation must thus be based on the quantity of K removed in the grain and crop residues, with an extra addition to account for K fixation. Given a crop residue content of 85 kg K ha^–1 yr^–1 in the fertilised treatment, the return of crop residues is essential if sustainability is to be achieved with traditional cropping systems where little K fertiliser is added.