Long-term tillage and maize monoculture effects on a tropical Alfisol in western Nigeria .1. Crop yield and soil physical properties

A wide range of tillage methods (e.g. mouldboard ploughing, discing, harrowing, chiselling, notill, ridge till, and their combinations) are used for continuous maize (Zea mays L.) cropping in western Nigeria without the benefits of experimental data on their comparative effects on soil properties and crop response. Therefore, the main objective of this experiment was to evaluate the impact of a range of tillage methods and continuous maize culture on grain and stover yields and degradative effects on soil properties. Tillage-induced changes in soil physical properties and maize yields were studied for 8 consecutive years from 1980 through 1987. Two crops were grown every year in the two rainy seasons. Season I lasted from April through July and Season 2 from August to November. The eight tillage treatments studied were: (1) notill + residue mulch, (2) notill + chiselling in the row zone to 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 and harrowing + residue mulch, (8) mouldboard ploughing and harrowing + ridging or ridge till. The experiment was established on a newly cleared land that had been under fallow (native vegetation regrowth) for about 6 years. With some exceptions, soil physical properties were measured in the dry season from 1980 through 1987, year by year. Maize grain yield was significantly affected by tillage treatments in 3 of 8 years only in Season 1. Notill + residue mulch produced the highest yield in both seasons. In addition, ploughing + residue mulch produced high yield in the first season only. Mean maize grain yield (averaged for 8 years) was 3.1 Mg ha(-1) in the first season compared with 1.4 Mg ha(-1) in the second. Maize gain yield in the first season increased from 2.3 Mg ha(-1) in 1980 to 4.4 Mg ha(-1) in 1984 and then progressively declined to 1.6 Mg ha(-1) in 1987. In the second season, grain yield was highly variable and depended on the rainfall amount and distribution. Complete crop failure occurred in Season 2 of 1983. The highest seasonal yield of 2.5 Mg ha(-1) was in 1984, and yield progressively declined to 0.6 Mg ha(-1) in 1987. Tillage treatments had no effect on soil bulk density of the surface 0 to 10 cm depth which progressively increased with cultivation duration. After 8 years, sand content was significantly lower and clay content significantly higher in the surface 0-10-cm layer of notill + mulch compared with plough-based and unmulched treatments. Equilibrium infiltration rate was rapid in all treatments, but significantly higher in notill compared with ridged treatment. Maize monocropping had drastic adverse effects on soil quality and crop yield. Adverse effects were more drastic in plough-based systems - residue mulch than with the notill system + residue mulch. (C) 1997 Elsevier Science B.V.