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The objective of this study was to understand the causes of crop productivity decline on a soil cultivated by the slash and burn method. The contribution of ashes, fine roots, and soil organic matter (SOM) mineralization to the pool of available nutrients of a nutrient-poor Haplustox of the semi-arid zone of northeastern Brazil was documented. Ashes were the most important input of nutrients to the soil. The burning of the vegetation debris produced 11 Mg ha(-1) of ash containing considerable quantities of Ca and K, and some N, Mg and P. The ashes, in general, contained more Ca and Mg, and less N and K, than the estimated requirements of the cassava (Manihot esculenta) crop during the cultivation cycle, whereas the P in ashes was equivalent to the crop P uptake.

About 65% of the fine roots from the native vegetation decomposed in the soil during the first rainy season after the slash and burn, contributing Ca, N, and Mg to stores of available nutrients, with limited supply of K and P. The SOM content decreased with cultivation. The losses 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 saturation strongly decreased, promoting better growing conditions for the cassava crop, particularly during the first years of the cultivation cycle. The soil properties reverted to pre-burn conditions within two or three crop years, productivity declined and the field was abandoned to natural fallow.