Diagnostic Soil Nutrient Standards and Identification of Yield Limiting Nutrients in Onion ( Allium cepa ) Using DRIS

Soil samples collected from a survey of fifty onion growing fields in Karnataka were analyzed for various macro and micronutrients for establishing a data bank to develop soil nutrient norms. By using Diagnosis and Recommendation Integrated System (DRIS), the whole population was divided into two sub-groups namely, low and high yielding and selected nutrient expressions that have shown higher variance and lower coefficient of variation as diagnostic norms, viz K/N (1.229), S/N (0.238), Ca/N (20.62), P/Zn (37.41), Mg/K (0.6.859), Fe/Mg (0.004), Fe/Zn (5.736) etc. In addition, five nutrient ranges have been derived using mean and standard deviation as low, deficient, optimum, high and excess for each nutrient to serve as a guide for diagnostic purpose. The optimum organic carbon ranged from 7.1 to 11.0 g kg^-1, N from 115 to 178 mg kg^-1, P from 26 to 38 mg kg^-1 , K from 163 to 217 mg kg^-1, Ca from 2199 to 3398 mg kg^-1 , Mg from 802 to 1167 mg kg^-1 and S from 34 to 43 mg kg^-1. Among DTPA extractable micronutrients, the optimum iron ranged from 3.40 to 4.34 mg kg^-1, manganese from 5.84 to 6.66 mg kg^-1, zinc from 0.67 to 1.01 mg kg^-1 and copper from 1.70 to 2.11 mg kg^-1 for onion. The diagnosis of nutrient imbalance identified through DRIS indices indicated that organic carbon, phosphorus and zinc were the most common yield limiting nutrients in onion.