Soil fertility assessment and determination of potential ameliorants for an Alfisol under long-term continuous cultivation in southwestern Nigeria

Soil fertility declines following continuous cultivation along with the use of fertilizers causing concern for sustainable food productivity. This study assessed chemical properties of degraded Alfisol, and also evaluated potential of organic residues and fertilizers in improving them. Soil samples (0 to 30 cm) of degraded and fallow fields were collected and analyzed. One kilogram each of soil from degraded field was weighed into incubation cups; mixed with NPK 15-15-15 (NPK), tithonia compost, poultry manure, and Gliricidia sepium leaves, and their combinations at 120 kg N/ha incubated for six weeks. Treatment without amendment was also included. After incubation, chemical properties of the soil were determined. Thereafter, maize was planted for six weeks. Maize biomass and NPK uptake was determined. Compared with fallow field, soil pH in the continuously cultivated plot declined by 12% while total nitrogen, organic carbon, phosphorus, Zn, and effective cation exchange capacity (ECEC) declined by 44, 55, 73.5, 83 and 70%, respectively. After incubation, organic residues or their combinations with NPK had higher pH as compared with control. Incubation of tithonia increased organic carbon, total nitrogen, ECEC, Ca, Mg and Mn than NPK alone by 12, 11, 16, 20, 11 and 7%, respectively. Maize shoot weight, N and K uptakes were also improved with addition of tithonia as compared to NPK and untreated control. Continuous cultivation reduced soil fertility levels. Incorporation of tithonia compost was better than NPK in improving soil chemical properties and maize growth. Key words: Alfisol, soil chemical properties, organic residues, maize, incubation, Nigeria.