The significance of crop residues to the maintenance of fertility under continuous cultivation at Samaru, Nigeria

Abstract

SUMMARYThe residues from a continuous millet–maize rotation, grown at two levels of phosphate and two levels of nitrogen fertilization, were either burned and the ash incorporated; chopped and incorporated unburned; or completely removed. After 3 years, differences in soil organic matter were small and largely non-significant, but the ash treatment had minimized the general fall in soil pH, and the ash and the unburned residue treatments had both conserved topsoil exchangeable K and Mg, which, where the residues had been removed, had declined by 44 and 12%, respectively. Fertilizer effects were also significant; single superphosphate had increased exchangeable Ca and lessened the fall in soil pH; calcium ammonium nitrate had depleted exchangeable Mg generally and had lowered pH and exchangeable Ca particularly where unburned residues had been returned.A balance sheet of soil cations (0–30 cm), drawn up from soil and crop analyses and known fertilizer inputs shows, first, that whereas the calcium added in single superphosphate increased the exchangeable reserves of this nutrient, that added as calcium ammonium nitrate did not and must be presumed lost by leaching; secondly, the decline in exchangeable K where residues were removed was much less than the total crop removal, indicating a probable substantial release from non-exchangeable form.In the fourth season, treatment-induced soil differences, in particular the ratios between exchangeable cations, significantly affected the chemical composition of the maize test crop but not its final yield.