Measurement of the acid neutralizing capacity of agroforestry tree prunings added to tropical soils

Laboratory incubation in the UK of an Oxisol from Burundi and an Ultisol fr om Cameroon with 3.1% by weight of prunings of young shoots of Calliandra c alothyrsus, Cassia siamea, Flemingia congesta, Grevillea robusta, Gliricidi a sepium, Leucaena diversifolia and Leucaena leucocephala resulted in incre ased soil pH and decreased exchangeable aluminium content. The greatest inc rease in pH and corresponding decrease in exchangeable aluminium occurred d uring the first 14 days of incubation and the decrease continued at a slowe r rate until 42 days incubation. The acid neutralizing effect decreased aft er 42 days but was still important at the last sampling time at 98 days. Po lyphenol to nitrogen ratio was not well correlated with observed change in soil pH whereas the total base cation (calcium, magnesium and potassium) co ntent proved to be a good predictor of these changes in the Ultisol, but no t in the Oxisol. The proposed mechanism giving rise to acid neutralization is complexation of protons and aluminium by organic anions. The total base cation content of the prunings ranged from 0.94 to 225 mol(c)/kg and the bu ffer capacity of the Oxisol was 48 mmol OH-/pH/kg.