Nitrogen, an abundant and yet limiting nutrient for crop and food production, enters the plant as nitrate or ammonium, or as dinitrogen through biological fixation by procaryotes associated with the plant. Nitrogen incorporation into the soil-plant-animal system is ultimately restricted by rates of biological and industrial fixation. Biological fixation conserves fossil fuel, but fertilization is preferred in most present agriculture. Nitrogen-metabolism research has the practical objectives of allowing more efficient N-fertilizer utilization by plants, including those that fix N2 but benefit from fertilizer-N supplements.