LONG-TERM AND SHORT-TERM EFFECTS OF CROP RESIDUES ON ALUMINUM TOXICITY, PHOSPHORUS AVAILABILITY AND GROWTH OF PEARL-MILLET IN AN ACID SANDY SOIL

In a long-term field experiment millet straw application (+CR) increased soil pH and base saturation and strongly improved pearl millet (Pennisetum glaucum L.) growth on acid sandy soils. Aluminum (Al) toxicity may be responsible for poor millet growth in plots without crop residues (-CR). Laboratory experiments were conducted to verify this assumption. The concentrations of labile Al (8-hydroxyquinoline, 15 sec) in equilibrium soil solutions of top soil samples from field plots were 14.0 and 0.6-mu-M in unfertilized samples of -CR and +CR soil, respectively. The corresponding values for labile Al in fertilized (NPK) samples were 51.8 and 11.0-mu-M, respectively. A short-term (14 days) incubation of -CR soil with ground millet straw (0.1% w/w) increased soil solution pH and decreased total and labile Al in the soil solution by more than 44%. In a water-culture experiment with increasing concentrations of Al (0-60-mu-M) pearl millet proved to be very Al-tolerant compared to cowpea, peanut and soybean. A short-term (12 days) pot experiment with the incubated soil showed that root growth of pearl millet is not restricted by Al toxicity in the acid soils from Niger, but that after millet straw incubation root growth is considerably enhanced. Phosphorus (P) concentration in the soil solution was about three times higher in +CR (1.75-mu-M) than in -CR (0.52-mu-M) top soil. Since P is the most growth-limiting nutrient in those soils, the beneficial effect of crop residues on pearl millet is likely due to improvement of P nutrition by both increase in P mobility in the soil and enhancement of root growth.