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SummaryPhosphorus, followed by potassium, were the plant nutrients that most limited yields during the third decade of a long-term fertilizer experiment on groundnut, maize and cassava conducted on an Ultisol in Ghana. Application of sulphate of ammonia significantly decreased soil pH, thus reducing crop yields. Superphosphate or mulch applied alone consistently increased the yields of all three crops. Application of potash increased the yield of cassava throughout the cropping period, but the potash × mulch interaction depressed yields in two of the three cropping seasons. Use of crop rotation, grass mulch and fertilizer, including small amounts of lime, enabled crop yields and soil fertility to be maintained at a fairly high level under continuous cropping.

In the forest zone P is one of the major yield-limiting nutrients in most of the well drained soils. In this study, attempts were made to improve the phosphorus and calcium status of the soil for maize production through the use of ground Togo phosphate rock (TPR). Since TPR is unreactive, it is therefore necessary to allow for a time lag between time of application and nutrient availability. The possibility that legumes may enhance the dissolution of TPR was also considered. The experiment was started in the minor season of 1998. Initial soil properties showed that the soil was acidic with a very low content of available phosphorus. The experimental design was a split plot. Surface broadcast and broadcast incorporation were the main plot treatments. Four rates of TPR viz. 400, 600, 800, and 1,200 kg ha-1 and two checks (Control and 200 kg ha-1 SSP) constituted the subplot treatments. All the treatments (only in the major season) received 30 kg N ha-1 and 30 kg K 2 0 ha-1 at planting and a second dose of 20 kg N ha-1 six weeks after planting. SSP was applied directly to maize in the major season, while TPR was applied only to cowpea in the minor season. For the three year running, cowpea grain yield was similar for all the subplot treatments. Incorporation gave a significantly higher cowpea grain yield only in the first year. While maize grain yield was similar for all the treatments in the first year, SSP (4.47 t ha-1 ), 800 kg TPR ha-1 (4.59 t ha-1 ), and 1,200 kg (4.35 t ha-1 ) gave a significantly higher yield than the control (3.19 t ha-1 ) in the second year. Significant improvement of soil properties such as available P and exchangeable calcium by the TPR 800 and 1,200 kg ha-1 treatments suggests that these treatments can support a higher maize yield if rates of Nand K are increased.

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