FINE-ROOT MINERALIZATION, SOIL ORGANIC-MATTER AND EXCHANGEABLE CATIONDYNAMICS IN SLASH-AND-BURN AGRICULTURE IN THE SEMIARID NORTHEAST OF BRAZIL

The objective of this study was to understand the causes of crop produ ctivity decline on a soil cultivated by the slash and burn method. The contribution of ashes, fine roots, and soil organic matter (SOM) mine ralization to the pool of available nutrients of a nutrient-poor Haplu stox of the semi-arid zone of northeastern Brazil was documented. Ashe s were the most important input of nutrients to the soil. The burning of the vegetation debris produced 11 Mg ha(-1) of ash containing consi derable quantities of Ca and K, and some N, Mg and P. The ashes, in ge neral, contained more Ca and Mg, and less N and K, than the estimated requirements of the cassava (Manihot esculenta) crop during the cultiv ation cycle, whereas the P in ashes was equivalent to the crop P uptak e. About 65% of the fine roots from the native vegetation decomposed i n the soil during the first rainy season after the slash and burn, con tributing Ca, N, and Mg to stores of available nutrients, with limited supply of K and P. The SOM content decreased with cultivation. The lo sses ranged from 4 to 16%, 6-18%, and 10-20%, for C, N and organic P, respectively. The sum of exchangeable cations, base saturation and pH increased after the burn, whereas the exchangeable Al and Al saturatio n strongly decreased, promoting better growing conditions for the cass ava crop, particularly during the first years of the cultivation cycle . The soil properties reverted to pre-bum conditions within two or thr ee crop years, productivity declined and the field was abandoned to na tural fallow.