16ref. Sum.(En,Es)

The effects of two treatments - natural succession and successional monoculture - on volcanic soil fertility following forest felling and vurning were studied over 5 yr in Costa Rica. For the successional monoculture maize (Zea mays), cassava (Manihot esculenta) and laurel(Cordia alliodora) were chosen. Owing to microvariability problems, each treatment was divided intro two groups of different fertility. Data on above-ground biomass and soil organic matter, total N, available P. percent acid saturation, extractable Ca, Mg, K and Al + H were analysed statistically. Percent acid saturation was considered a suitable index to evaluate tropical volcanic soil fertility (high saturation + high fertility) and vice versa. Defined in these terms, soil fertility was little affected by successional monoculture or natural succession when the original conditions of acid saturation were low (<2 percent at depth 0-5 cm. <20 percent at depth 5-25 cm). However, soil fertility was significantly affected when the original conditions were of high acid saturation (>7 percent at depth 0-5 cm. >50 percent at depth 5-25 cm); soil fertility was reduced under both successional monoculture and natural succession, but the reduction was less under the latter. A non-statistical comparison with a bare plot is also included