Long-term effect of organic/inorganic input on the distribution of zinc and copper in soil fractions under a rice – wheat cropping system

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A long-term experiment under a rice – wheat system was used to investigate the effect of organic and inorganic fertilizer application on the zinc (Zn) and copper (Cu) in various soil fractions. Levels of Zn and Cu in the different fractions were comparable to the initial levels after 7 years of continuous cropping without any fertilization. However, addition of organic manures in the absence of inorganically applied Zn or Cu increased the water-soluble plus exchangeable, organic fraction and the manganese oxide and amorphous iron oxide fractions of Zn and Cu over their initial levels. Application of N150P75K75Zn25 + farmyard manure (FYM) or N75P37.5K37.5Zn25 + green manure lowered the Zn content in the carbonate and crystalline iron oxide fractions, with a concomitant increase in Zn associated with the water-soluble plus exchangeable, organic fraction and the manganese oxide and amorphous iron oxide fractions compared to the N150P75K75Zn25 treatment alone. Levels of Cu in the water-soluble plus exchangeable, organic fraction and the manganese oxide and amorphous iron oxide fractions were significantly higher in the N150P75K75Zn25 + FYM, N150 + press mud and N75P37.5K37.5Zn25 + green manure treatments than in the N150P75K75Zn25 treatment alone. A maximum increase in Cu of 1.4 – 1.5-fold was observed in the organic fraction. The residual fraction, and amorphous and crystalline iron oxide fractions constituted the major proportion of total Zn and Cu in soil.