Micronutrition in the kernel
After oven-drying to a constant weight, the kernel was triturated. The triturations from peanuts grown in each district were mixed and used as one repetition. After adding 6 ml of 15 M nitric acid, the trituration was stored overnight. Then, 2 ml of 30% H2O2 was added and the mixture was digested using a microwave oven high-pressure digestion system (Mars 6 Classic; CEM Corp., USA). The micronutrient contents in the kernel were determined by ICP–OES.