Norwegian Journal of Agricultural Sciences : assessment of available zinc for rice in soils of the Tabora region, Tanzania

Four Zn extractants, EDTA-(NH)CO,, DTPA, HCI and EDTA, were screened for suitability in assessing the available Zn status of rice-growing areas of the Tabora region in Tanzania. Rice response to Zn application was evaluated in a glasshouse experiment and in three field trials. Zinc sulphate was applied in the glasshouse experiment while ZnSO, and ZnO were tested in the field trials. Extractable Zn in the potted soils was also determined at the end of the experiment. In addition the Zn status of 32 soils in major rice growing areas was assessed. EDTA-(NH,)CO, and DTPA-extractable Zn levels in the soil were significantly correlated with rice dry matter yield from the glasshouse experiment and gave critical Zn deficiency levels of 0.86 and 1.10 mg kg', respectively. Correlation coefficients for EDTA and HCI extractable Zn versus dry matter yield were not found to be significant. At the end of the experiment it was found that in all treatments where Zn was applied DTPA extractable Zn was more than three times the critical level established in this study. Zinc deficiency which was found to be a widespread problem in rice-growing areas of the Tabora region, covering 66 % of the areas, was corrected by application of either ZnSO, or ZnO. The optimum rate of application was 10 kg ha. We recommend the use of DTPA for assessing the available Zn status in these soils