Root-induced increases in soil pH and nutrient availability to field-growncereals and legumes on acid sandy soils of Sudano-Sahelian West Africa

A field experiment with millet (Pennisetum glaucum L.), sorghum [Sorghum bi color (L.) Moench], cowpea (Vigna unguiculata L.) and groundnut (Arachnis h ypogeae L.) was conducted on severely P-deficient acid sandy soils of Niger , Mali and Burkina Faso to measure changes in pH and nutrient availability as affected by distance from the root surface and by mineral fertiliser app lication. Treatments included three rates of phosphorus (P) and four levels of nitrogen (N) application. Bulk, rhizosphere and rhizoplane soils were s ampled at 35, 45 and 75 DAS in 1997 and at 55 and 65 DAS in 1998. Regardles s of the cropping system and level of mineral fertiliser applied, soil pH c onsistently increased between 0.7 and two units from the bulk soil to the r hizoplane of millet. Similar pH gradients were observed in cowpea, but pH c hanges were much smaller in sorghum with a difference of only 0.3 units. Sh ifts in pH led to large increases in nutrient availability close to the roo ts. Compared with the bulk soil, available P in the rhizoplane was between 190 and 270% higher for P-Bray and between 360 and 600% higher for P-water. Exchangeable calcium (Ca) and magnesium (Mg) levels were also higher in th e millet rhizoplane than in the bulk soil, whereas exchangeable aluminium ( Al) levels decreased with increasing pH close to the root surface. The resu lts suggest an important role of root-induced pH increases for crops to cop e with acidity-induced nutrient deficiency and Al stress of soils in the Su dano-Sahelian zone of West Africa.