The distribution and transformation of iron and manganese in soil fractions in a savanna Alfisol under continuous cultivation

Data on the responses of micronutrients in definable soil fractions to cultivation and management are required to design judicious fertilization practices to improve soil fertility in the savanna. Iron and manganese fractions are particularly sensitive to cultivation and management practices. The objectives of this study were to determine the sizes and changes in Fe and Mn fractions in a savanna Alfisol cultivated for 50 years and fertilized with (i) NPK, (ii) farmyard manure (FYM), (iii) FYM + NPK, (iv) a control plot, and (v) a natural site adjacent to the experimental field. The mean concentration of total Fe (Fe_T) ranged from 9.4 g kg^–1 in the surface layer to 45 g kg^–1 in the subsurface layer, whereas total mangenese (Mn_T) concentration ranged from 79 mg kg^–1 in the surface layer to 279 mg kg^–1 in the subsurface layer. The distribution of Fe_T followed the distribution of clay in the soil profile across the field. The distribution of Mn_T did not, however, follow the characteristic depth distribution of clay as observed for Fe_T, suggesting that Mn movement and distribution in this soil might be independent of clay movement and distribution. The concentrations of DTPA extractable Fe and Mn were much higher than the critical levels delineated for soils. Application of FYM increased the concentration of amorphous oxide bound Fe over the natural site and reduced the concentration of residual or inextractable Fe in the soil. Similarly, fertilization with FYM reduced the concentration of residual Mn, and increased the exchangeable, amorphous oxide bound and reducible Mn compared to the natural site. It seems that sole application of FYM or application in combination with NPK rather than NPK alone can mobilize non-labile Mn and Fe sources into labile and plant available forms of Fe and Mn in a savanna Alfisol.