Changes in Fractions of Iron, Manganese, Copper, and Zinc in Soil under Continuous Cropping for More Than Three Decades

The effect of continuous cropping with maize and wheat on soil characteristics and various forms of micronutrient cations in an Incetisol over the years was studied in an ongoing long-term experiment in New Delhi, India. The soil samples collected in the years of 1993, 1995, 1997, 1999, 2001, 2003, and 2004 were analyzed for different fractions of iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) by following a sequential extraction procedure. The pH, electrical conductivity (EC), and calcium carbonate (CaCO3) content of the soil varied from 8.28 to 8.53, 0.40 to 0.43dSm-1, and 0.92 to 1.05%, respectively. Organic carbon content ranged from 0.38 in the control to 0.67% in 100% NPK + farmyard manure (FYM). Diethylenetriaminepentaacetic acid (DTPA)-extractable Fe and Mn (but not Zn and Cu) in soil declined from their respective initial (1971) values as a result of intensive cropping for more than three decades. It also resulted in a decrease in the concentrations of all the four metallic cations bound to organic matter, in addition to Fe and Zn, associated with carbonates in all the treatments in surface soil.