Case studies related to the management of soil acidity and infertility in the West-African Moist Savannah

Although the soil pH and base status of the soils in the West African Moist Savannah Zone (MSZ) are usually favourable, their buffer capacity is usually low, indicating that while soil acidity may not be a problem initially, inappropriate management of these soils may induce soil-acidity-related problems in the medium to long term. The current paper addresses 3 topics that are closely related to the management of soil pH (acidity) in the West African MSZ. A first experiment addressed the release of P from low reactivity phosphate rock (PR) by mixing it with various N fertilizers. Mixing ammonium-sulphate, urea, and calcium-ammonium nitrate with PR substantially enhanced the soil Olsen-P content, but not for soils with an initial pH above 5.5, while potassium nitrate did not change the Olsen-P content. Changes in soil pH could be predicted based on the production of nitrate from ammonium (nitrification) and the soil buffer capacity. A second experiment examined the changes in topsoil pH as affected by long term management based on the application of organic inputs derived from hedgerow trees (Leucaena leucocephala and Senna siamea), fertilizer, or both. Maize crop yields declined steadily over the 16 years studied, but the least so in the Senna + fertilizer treatment where in 2002 still 2.2 t ha-1 of maize were obtained. The fertilizer only treatment led to a yield of 0.4 t ha-1 in 2002, while the absolute control without any inputs yielded a mere 40 kg ha-1 in the same year. Nitrogen fertilizer use efficiency was usually higher in the Senna treatment compared to the control or the Leucaena treatment. Interactions between fertilizer and organic matter additions were negative for the Leucaena treatments in the first three years, and positive for the Senna treatment in the last 6 years. Trees had a positive effect on the maintenance of exchangeable cations in the topsoil. Exchangeable Ca, Mg and K - and hence ECEC - were only slightly reduced after 16 years of cropping in the tree-based systems, and even increased in the Senna treatments. Soil pHKCl values decreased at least 0.5 units in the control and Leucaena treatments, but only slightly in the Senna treatments. In general, the soils that received fertilizer during the trial were more acid (0.2 to 0.3 units) than the ones not receiving fertilizer. From the above, the Senna-based alley cropping system with fertilizers is the more resilient one, both in terms of crop yields and soil fertility status. A third experiment evaluated the potential of deep-rooting hedgerow trees to recycle basic cations from the subsoil and increase the topsoil pH. Topsoil Ca content, effective cation exchange capacity, and pH were substantially higher under Senna than under Leucaena leucocephala, Gliricidia sepium, or the no-tree control plots in sites with soils having a Bt horizon rich in exchangeable Ca. It was shown that this effect was largely related to the recovery of Ca from the subsoil by the Senna trees. The lack of increase in Ca accumulation under the other species was related to (i) potential recovery of Ca from the topsoil itself and/or (ii) substantial Ca leaching. The accumulation of Ca in the topsoil under Senna had a marked effect on the topsoil pH, the latter increasing significantly compared with the Leucaena, Gliricidia, and no-tree control treatments. (author)