Levels of nitrogen, phosphorus, potassium, calcium, magnesium, zinc, manganese, copper and selenium in grasses grown under rainfed conditions on different soils in the Santa Rita Experimental Range and other areas in southern Arizona were evaluated to determine whether these nutrients are deficient, sufficient or toxic to cattle. In the greenhouse, two grasses grown on five different soils representing four soil orders were fertilized with nitrogen and phosphorus and the uptake of a number of nutrients was determined. For the grasses collected from the field, the nutrient contents generally decreased with time to maturity and the highest levels were attained during August when the grasses were young, and March, the start of the spring growth. The nitrogen and phosphorus contents were lower than the requirement for grazing cattle indicating the need for supplementing protein and phosphorus. The contents of potassium, calcium, zinc, manganese, copper and selenium were adequate for grazing animals. Although the magnesium level in some cases was lower than the requirement, it was considered adequate because its deficiency is now known in warm season grasses. The nutrient contents in the grasses grown on Comoro soils were generally higher than when grown on an adjacent Sonoita soil. Soil analysis revealed no consistent pattern of the levels in these grasses and in Comoro and Sonita soils on which they were grown, except for nitrogen. The nutrient contents in the heads and leaves of two species of grasses were higher than in their stems. The greenhouse experiments showed that the vegetation on the N + P treatment was more vigorous than on the nitrogen treatment and the control, respectively. For the two grasses grown on the five different soils, the nitrogen and magnesium contents mostly were not significantly different between the three treatments, probably due to the dilution effect resulting from the vigorous vegetative growth on the N + P treatment. The phosphorus content in the grasses grown on the N + P treatment was significantly higher than the other two treatments. The nitrogen and phosphorus levels in the soils before and after planting were similar indicating that nitrogen and phosphorus fertilization helped maintain their levels in the soils. Other nutrients in the soils decreased after planting. Most of the nutrients were higher in the surface soils than in the subsurface soils, probably due to the ability of grasses to recycle nutrients from the subsoil.

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