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Summary The effects of fertilization of forage crops vary with soil types, relative levels of fertility within soil type, ratios of available nutrients, crops, and climatic conditions. Increased forage yields result from application of the plant-growth nutrient which is most limiting in the soil. The chemical composition of forages may be altered by fertilization. Application of a nutrient in quantities greater than those required for maximum yield response usually results in luxury consumption of the nutrient. The nutrients whose level in plants may be increased by large applications include nitrogen, phosphorus, potassium, calcium, sulfur, and cobalt. The nitrogen and phosphorus levels in leguminous forage usually were not influenced by heavy applications of these nutrients. Biological assays of forages produced with different fertilizer treatments have yielded varying results. Several studies with sheep and rabbits have indicated that applications of phosphorus fertilizer increase the biological value of forages when these forages are fed alone or in highly simplified rations. Application of limestone to the soil has been shown to exert a favorable effect on the biological value of forage. Forages grown on light soils have lower biological value for guinea pigs than do forages grown on heavy soils. Other, more numerous reports indicate no difference in biological value of forage due to level of soil fertility.