Critical limit of zinc for response of rice to zinc application in acidic soils of Assam

Pot culture experiment was conducted with twenty five georeferenced bulk surface soil samples (0–30 cm) having variable zinc status and with four levels of zinc viz., 0, 2, 4 and 6 kg Znha⁻¹. The dry matter yield of rice (var-Ranjit) increased significantly over control with rates of zinc application. The highest dry matter yield was recorded in 4 kg Znha⁻¹. Bray's percent yield ranged from 57.18 to 95.7 and the highest Bray's percent yield of 95.7 was observed in the soil having DTPA-Zn of 0.4mgkg ⁻¹ and the lowest value of 57.18 was observed in the soil with DTPA-Zn of 2.49 mgkg⁻¹. The soils were also extracted by different extractants viz., DTPA, 0.5N HCl, 0.1N HCl, NH₄OAC, EDTA-(NH₄)₂CO₃, AB-DTPA, 0.04M EDTA and 2M MgCl₂ solution for available Zn, where DTPA showed the highest correlation with all the plant parameters followed by AB-DTPA, 0.05 N HCl and EDTA-(NH₄)₂CO₃, 0.04 EDTA and NH₄OAC. The critical limits of DTPA, AB-DTPA 0.5N HCl, EDTA-(NH₄)₂CO₃, 0.04 M EDTA and NH₄OAC in soil were found to be 1.24, 1.74, 1.25, 2.8, 0.74, 0.9 mg/kg respectively. The critical limit of Zn for rice was 40 mgkg⁻¹.