The concept of the critical nutrient level in relation to plant growth has been described. The critical deficiency level (CDL) is the nutrient concentration in the tissue associated with a 10% growth reduction from maximum growth due to a deficiency. The critical toxicity level (CTL) is the nutrient concentration in the tissue associated with a 10% reduction in growth from the maximum due to toxicity. Critical Ma deficiency levels were established at 10 and 15 μg/g for blade 3 in cotton and sorghum, respectively, and from 10 to 15 μg/g for mature sugar beet blades. Critical Zn deficiency and toxicity levels in cotton (blade 3) were determined as 11 and 200 μg/g, respectively. Additional research in establishing the critical levels for other nutrients and crops is needed to provide nutrient ranges within which maximum plant growth and yield would result. Research is also needed in coordinating soil tests with plant tissue to determine initial soil test values that would sustain plant nutrient values within the adequate range throughout the growing season.