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Nutrient Effectiveness in Relation to Rates Applied for Pot Experiments: II. Phosphorus Sources¹

ABSTRACT

Three greenhouse pot experiments were conducted with corn (Zea mays L.) grown on infertile soils to evaluate four fertilizers as sources of P. The granular sources were concentrated superphosphate (CSP, 90% of P water-soluble), monoammonium phosphate (MAP, 100% of P water-soluble), a mixture of 30% of the P as CSP and 70% as dicalcium phosphate (DCP, water-insoluble), and a 10-90% mixture of CSP and DCP. Rates of 0, 50, 200, 400, and 800 mg of P/pot from each source were compared in Exp. 1; and 0, 60, 120, and 480 mg rates of P in Exp. 2 and 3. These P sources and rates were compared at various levels of N and K. The order of effectiveness was the same at all levels of applied N and K: MAP CSP > 30% CSP + 70% DCP > 10% CSP + 90% DCP. This indicates that on these soils, P was the chief limiting nutrient at all levels of N and K. However, yield levels and the precision of determining differences in effectiveness among sources relative to experimental error increased greatly at higher rates of applied N and K.

These results show that effectiveness of P sources and also source differences relative to experimental error are greater at adequate than at deficient levels of nontest nutrients or other growth-limiting factors. Adequate levels of nutrients for greenhouse pot experiments are much higher per pot equivalent than the normal recommended application rates for crops grown under field conditions.

¹ Contribution of the Soils and Fertilizer Research Branch, TVA, Natl. Fert. Develop. Center, Muscle Shoals, AL 35660.

² Research Soil Chemist and Agronomist, respectively.

Received for publication June 24, 1977. Accepted for publication November 11, 1977.

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