ZINC Fractions in Selected Zambian Soils and Maize and Wheat Responses Zinc application

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Chirwa, Meki
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Incidences of plant and soil zinc (Zn) deficiency have been reported in many countries. The University of Zambia soil analysis laboratory has observed that a high percentage of soil samples analyzed fall below the critical limits for Zn. Field experiments were conducted at the University of Zambia School of Agricultural Sciences Field Station to assess the response of one crop of wheat and two crops maize to soil and foliar application of Zn fertilizer. One half of each plot was assigned to treatments to receive soil application of Zn while the other half received foliar application. Zinc treatments were applied to designated plots at 25, 50, 75 or 100 kg ZnS04 ha"' in a randomized complete block design with three replications. At seven weeks after planting maize (Zea mays MRI 724) or wheat (UNZA WV \), foliar application of Zn fertilizer at 2.5, 5.0, 10 or 20 kg ZnS04 ha'' was made to designated plots, again in a randomized complete block design with three replications. Grain yield and uptake were determined. Laboratory experiments were conducted to define various pools of soil Zn. Twenty two different soils were fractionated using batch extraction schemes. These soils were further cropped in the greenhouse to determine Zn uptake by wheat at six weeks. Plant uptake and soil Zn pools were used in the correlation analysis. The results showed that methods of application were significantly different for both maize and wheat, (P = 0.0001) and (P = 0.0043), respectively. The maize grain yield from the soil applied fields were higher (1.78 t ha'') than from the foliar applied fields (1.14 t ha''). The wheat grain yield was higher in the soil applied fields (3.69 t ha'') than in the foliar applied fields (2.74 t ha"'). Both crops responded to Zn application and the Zn concentrations of the crops were more in the treated fields than in the controls except for the wheat foliar applied field. The mean uptake of Zn by maize in the plots with soil applied Zn ranged from 31.97 to 77.23 mg while that from foliar application ranged from 22.74 to 80.52 mg. The mean uptake of Zn by wheat in the plots with foliar applied Zn ranged from 5.13 to 10.11 mg while that of the soil applied Zn ranged from 8.89 to 14.63 mg. Results from fractionation study showed that the distribution of Zn was as follows: sesquioxide (45.5 percent) > residual (34.1 percent) > carbonate bound Zn (17.1 percent) > organically bound (9.5 percent) > exchangeable pool (5.5 percent). The sesquioxide bound Zn supplied significantly more Zn (P = 0.056; = 0.19) to wheat crop. The supply of Zn by other pools was non significant. The results obtained showed that soil application of Zn fertilizer could be more effective than foliar application. It was also observed that higher rates of Zn fertilizer application induced lower yields. In addition, the DTPA soil test method appears to be week in detecting plant available Zn from the sesquioxide Zn pool which was the major supplier of Zn in this study.
Soil Zinc-Zambia