PHOSPHORUS SORPTION CHARACTERISTICS OF PRODUCTIVE AND UNPRODUCTIVE NIGER SOILS

High variability in pearl millet [Pennisetum glaucum (L.) R. Br.) stan ds across short (2-20 m) distances is a serious limitation to farmers' yields in Niger and other parts of the Sahel. This study was undertak en to determine if differences in P availability of adjacent productiv e and unproductive soils contribute to variability in millet yields. S urface (0-15 cm) samples of paired productive and unproductive soils f rom 16 sites in the Maradi region and three sites in the Niamey region of Niger were fertilized with 0, 5.5, 10.9, 21.8, and 43.7 mg P kg - 1 soil. Soil solution extracts revealed that unproductive soils adsorb ed more P than did adjacent productive soils. Mixing the soil from 0- to 2-, 2- to 5-, 5- to 10-, and 10- to 15-cm depth increments had the effect of dramatically reducing soil solution P when compared with the P available in the separate increments. The logarithmic concentration of solution P (log [P(s)]) was related to the logarithmic concentrati on of labile P (log [P(L)]) and the logarithmic concentration of oxala te-extractable Fe (log[Fe(o)]; R = 0.96). Log[P(s)] correlated equally well with log[P(L)] and the logarithmic concentration of exchangeable Al (log [Al(ex)]). Using a P(s) value of 0.20 mg kg - 1 soil as the s tandard for P sufficiency, most productive soils were capable of suppl ying adequate P at a fertilization rate of 9.2 mg P kg- 1 soil (22 kg P ha - 1). However, unproductive soils supplied almost no P at this ra te. Millet yields correlated well with Al(ex) (r = - 0.87) and amount of P required to reach the 0.20 mug mL-1 P(s) level (r = - 0.81). Tota l P was related to quantities of dithionate-citrate-bicarbonate Fe oxi des (r = 0.91). Differences in P availability caused by differential P sorption appear to be a major cause of millet stand variability in Ni ger soils.