Yield Decline Trends of Eroded Ultisols in Owerri, Southeastern Nigeria: Maize Response to Artificial Erosion

The artificial incremental removal of surface soil to varying depths to simulate erosion followed by subsequent evaluation of crop growth and performance under uniform management is a common agronomic technique to study erosion/soil productivity relationships. However, because its results tend to exaggerate yield decline rates it is being replaced by the more complex erosion phase approach. This paper is part of a series of elaborate studies conducted between 1996 and 2002 to document erosion induced productivity decline in Ultisols of Southeastern Nigeria. Specifically, it evaluated the impact of mechanical topsoil removal, in 1998 and 1999 cropping seasons on maize yield performance and compares the two methods of assessment. Three levels of topsoil removals (2.5, 5.0, 7.5 cm) were imposed on the non-eroded reference plateau of the erosion phase experiment reported in the paper I. Topsoil removal led to significant increases in bulk density, 1.64 Mgm-3 where 7.5 cm of topsoil was removed, 1.5 and 1.47 Mgm where 2.5 and 5.0 cm were removed respectively, 1.44 Mgm-3 in undisturbed plots. Declines in exchangeable acidity (from 3.4 Cmol/kg in control plots to 2.8 Cmol/kg in plots where 2.5 cm of top-soil was excavated) were observed in 1999. Significant reductions in soil organic carbon, by as much as 47% in all desurfaced plots, and available P (27.05 Mg/kg in 0 cm and 11.90 Mg/kg in the other treatments) were recorded. In both cropping seasons, artificial soil loss effects on maize yield parameters were significantly affected. Most yield variables were similar among the 2.5, 5.0 and 7.5 cm depths of desurfaced plots. However, significantly higher values were obtained in the undisturbed sites in the order 0>2.5≥5.0≥7.5 cm depths of topsoil removal. Relative grain yields in 1999 were in the other 100:41:9:6 for 0:2.5:5.0:7.5 cm depths of soil lost. The corresponding values were 5.18 t, 2.03 t, 0.48 t and 0.29 t/ha respectively. There was total crop failure in 1998 in plots from which more than 2.5cm of topsoil was excavated. Leaf Area Index (LAI) an indicator of canopy cover and photosynthetic efficiency was also negatively impacted by incremental soil loss. The shelling ratio (SR) was not affected by simulated erosion. The desurfacing technique exaggerated the magnitude of maize yield decline by a factor of 4. The erosion phased approach is recommended.