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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 from Cameroon with 3·1% by weight of prunings of young shoots of Calliandra calothyrsus, Cassia siamea, Flemingia congesta, Grevillea robusta, Gliricidia sepium, Leucaena diversifolia and Leucaena leucocephala resulted in increased soil pH and decreased exchangeable aluminium content. The greatest increase in pH and corresponding decrease in exchangeable aluminium occurred during the first 14 days of incubation and the decrease continued at a slower rate until 42 days incubation. The acid neutralizing effect decreased after 42 days but was still important at the last sampling time at 98 days. Polyphenol to nitrogen ratio was not well correlated with observed change in soil pH whereas the total base cation (calcium, magnesium and potassium) content proved to be a good predictor of these changes in the Ultisol, but not 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 2·25 molc/kg and the buffer capacity of the Oxisol was 48 mmol OH−/pH/kg.