CHANGES IN SOIL PROPERTIES DURING LONG-TERM FALLOW AND CONTINUOUS CULTIVATION AFTER FOREST CLEARING IN NIGERIA

Changes in soil chemical properties under natural bush regrowth, plant ed fallows (i.e. Guinea grass (Panicum maximum), Leucaena leucocephala , and pigeon pea (Cajanus cajan)) and under continuous cropping with m inimum tillage (i.e. maize (Zea mays L.)/cassava (Manihot esculenta Cr antz) intercrop and maize monoculture with residue returned or removed ), were monitored for 13 years after clearing of secondary forest on a kaolinitic Alfisol in Nigeria, West Africa. Under bush fallow, soil o rganic C (SOC) decreased during the first 7 years, then increased to t he original level of approximately 20 g kg(-1) at 12 years, whereas so il pH, exchangeable Ca and Mg and ECEC remained relatively constant. C hemical soil properties under Guinea grass and leucaena fallows compar ed favorably to those under bush fallow, whereas chemical properties o f the surface soil under pigeon pea fallow deteriorated. Under continu ous maize cropping, soil organic C decreased during the first 7 years, then reached a steady-state at about 65% of the level maintained by b ush fallow. The surface soil under continuous maize also became acidif ied. Maize grain yields of the first season declined from 6.0 t ha(-1) to 2.5 t ha(-1) during 7 years of cropping. Relative yield sustainabi lity of the three cropping systems was maize/cassava intercrop > maize monoculture with residue returned > maize monoculture with residue re moved.