PHOSPHORUS SORPTION CHARACTERISTICS OF PRODUCTIVE AND UNPRODUCTIVE NIGER SOILS

High variability in pearl millet [Pennisetum glaucum (L.) R. Br.) stands across short (2-20 m) distances is a serious limitation to farmers' yields in Niger and other parts of the Sahel. This study was undertaken to determine if differences in P availability of adjacent productive and unproductive soils contribute to variability in millet yields. Surface (0-15 cm) samples of paired productive and unproductive soils from 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 adsorbed 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 concentration of labile P (log [P(L)]) and the logarithmic concentration of oxalate-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 standard for P sufficiency, most productive soils were capable of supplying 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 rate. 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). Total P was related to quantities of dithionate-citrate-bicarbonate Fe oxides (r = 0.91). Differences in P availability caused by differential P sorption appear to be a major cause of millet stand variability in Niger soils.