Plant nitrogen 1.1 Uptake of N 1. N is required by plants in large amounts Most plant material contains 2-4%N and 40%C. 2. N is usually the limiting nutrient in unfertilized systems 3. Most N is taken up from the soil in the form of nhat or no a small amount of NH can be absorbed through the leaves N2 can be used by legume plant via biological nitrogen fixationI Plant nitrogen 1.1 Uptake of N 1. N is required by plants in large amounts. Most plant material contains 2-4% N and 40% C. 2. N is usually the limiting nutrient in unfertilized systems. 3. Most N is taken up from the soil in the form of NH4 + or NO3 - . A small amount of NH3 can be absorbed through the leaves. N2 can be used by legume plant via biological nitrogen fixation