Long-term tillage and maize monoculture effects on a tropical Alfisol in western Nigeria. II. Soil chemical properties

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Intensive monocropping of maize is widely practised in densely populated regions of west Africa. A wide range of tillage systems are used for seedbed preparation without the benefits of research information from long-term experiments relating tillage method to soil properties. Soil chemical effects of eight tillage and residue management treatments were studied for 8 years or 16 consecutive maize monoculture crops for a western Nigerian soil with a coarse-textured surface horizon. Treatments established on a newly cleared land were: (1) notill ± crop residue mulch, (2) notill ± chiselling in the row zone at 50 cm depth, (3) mouldboard ploughing ± harrowing, (4) disc ploughing ± rotovation, (5) notill − residue mulch, (6) mouldboard ploughing at the end of rains or summer ploughing, (7) mouldboard ploughing ± residue mulch, and (8) mouldboard ploughing and harrowing ± ridging or ridge till. Soil chemical properties of the 0–5-cm and 5–10-cm depths were measured during the dry season every year beginning with base line characterization soon after land clearing in 1980. Soil pH declined with cultivation duration from 6.6 in 1981 to 5.4 in 1987 for the 0–5-cm depth, and from 6.5 in 1981 to 5.5 in 1987 for the 5–10-cm depth. Mean soil pH for the 0–5-cm depth declined from 6.7 to 5.6 in notill ± residue mulch, and from 6.7 to 5.4 with mouldboard ploughing. Soil content of Bray P increased with cultivation duration due to application of phosphatic fertilizers. There occurred a significant decrease in soil organic carbon (SOC) and total soil nitrogen (TSN), and an increase in C:N ratio with cultivation duration. Mean SOC content in 1987 of the 0–5-cm depth was 13.9 g kg −1 for the notill treatment compared with 11.0 g kg −1 for the plough-based methods. Soil concentration of Ca and Mg and total cation exchange capacity (CEC) increased with cultivation duration. Maize grain yield was significantly correlated with SOC, exchangeable Ca 2+, and CEC. Continuous cropping decreased soil chemical quality, and the rate of decreases was more with plough-based than notill methods.