Residual, direct and cumulative effect of zinc application on wheat and rice yield under rice-wheat system

Zinc (Zn) deficiency is prevalent particularly on calcareous soils of arid and semiarid region. A field experiment was conducted to investigate the direct, residual and cumulative effect of zinc on the yield of wheat and rice in permanent layout for two consecutive years, 2004-05 and 2005-06 at Arid Zone Research Institute D.I. Khan. Soil under study was deficient in Zn (0.8 mg kg^-1). Effect of Zn on yield, Zn concentrations in leaf and soils were assessed using wheat variety Naseer-2000 and rice variety IRRI-6. Three rates of Zn, ranging from 0 to 10 kg ha^-1 in soil, were applied as zinc sulphate (ZnSO4. 7H2O) along with basal dose fertilization of nitrogen, phosphorus and potassium. Mature leaf and soil samples were collected at panicle initiation stage. The results showed that grain yield of wheat and rice was significantly increased by the direct application of 5 and 10 kg Zn ha^-1. Highest grain yield of wheat (5467 kg ha^-1) was recorded with the direct application of 10 kg Zn ha^-1 while 4994 kg ha^-1 was recorded with the cumulative application of 10 kg Zn ha^-1 but the yield increase due to residual effect of Zn was statistically lower than the cumulative effect of Zn. Maximum paddy yield was recorded with the cumulative application of Zn followed by residual and direct applied 10 and 5 kg Zn kg ha^-1, respectively. Zn concentration in soils ranged from 0.3 to 1.5 mg kg^-1 in wheat and 0.24 to 2.40 mg kg^-1 in rice, while in leaves it ranged from 18-48 mg kg^-1 in wheat and 15-52 mg kg^-1 in rice. The concentration of Zn in soil and leaves increased due to the treatments in the order; cumulative residual direct effect control (without Zn). The yield attributes like 1000-grain weight, number of spikes, spike length and plant height were increased by the residual, direct and cumulative effect of Zn levels; however, the magnitude of increase was higher in cumulative effect than residual and direct effect of Zn, respectively. Under Zn-deficient soil conditions, yield of wheat can be increased by direct application of Zn, while yield of rice can be maximized by the cumulative use of zinc fertilization.