Estimation of critical limit for zinc in rice soils

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Abstract
Twenty surface soil samples wore collectod from Nalnltal Tarai (foot‐ hills of Himalya) where ‘Khaira’ disease (Zn deficiency disease of rice) was prevalent. Rice (Oryza sativa L. variety IR ‐ 8) was grown in pots for 8 weeks after transplanting. Experiments were carried out to find the suitability of five soil Zn extractants viz. dilute acid (HC1 ‐ H2SO4) mixture, DTPA‐ (NH4) 2CO3, pH 7.3, dithizono, NH4 ‐ Ac, pH 4.6 and MgCl2. Critical limits of available Zn in soils were established for rice crop by old and new Cate and Nelson procedures. Zn extracted from the soil with NH4 ‐ Ac, pH 4.6, dithlzono, MgCl2, and DTPA‐(NH4) 2CO3, pH 7.3 was significantly correlated with the uptake of Zn by the rice plants. The correlation of Zn uptake with dilute acid mixture extractable Zn was not significant. The extractant which extracted more Zn gave higher values of critical limit and vice versa. It is concluded that all extracting solutions except dilute acid mixture were found to be suitable for predicting available Zn in rice soils of Taral.