Establishment of the Critical Limit of Soil-Available Phosphorous for Maize Production in Low Acidic Ultisols of West Africa

The relationship between yields and the Bray I-extractable phosphorous (P) was studied on a 21-year-old experiment with maize (Zea mays) in an acid Ultisol of West Africa. Chemical fertilizer alone decreased soil organic carbon and increased soil acidity. The applications of cattle manure maintained the highest level of soil organic carbon, and soil acidity did not decrease. The extractable P varied from 4 mg kg-1 (original soil) to 25 mg P ha-1 in fertilized treatments. A linear correlation (P < 0.01; r = 0.85) was found between soil extractable P and maize yields. Each unit increase in soil extractable P tallied with 16% of yield increase. A critical limit of soil extractable P of 15.6 mg P kg-1 was identified. Below this limit, maize produced less than 2500 kg ha-1, 80% of the maximum yield. Responses to P fertilizer were not expected above this limit.