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Nutrient Effectiveness in Relation to Rates Applied for Pot Experiments: I. Nitrogen and Potassium¹

ABSTRACT

An N source experiment was conducted on infertile Mountview sil (Typic Paleudult) to evaluate four N sources at multiple rates of applied N and P. Granular ammonium nitrate (AN), sulfur-coated urea (SCU), oxamide (Ox), and isobutylidene diurea (IBDU) were evaluated for corn (Zea mays L.) at N rates of 0, 400, 800, and 1,200 mg/pot (5 kg of soil), each at P rates of 0, 60, 120, 480, and 960 mg/pot. Yield response to applied N was in the order AN >> SCU > Ox > IBDU at the higher rates of applied P. At 60 and 120 mg of applied P/pot, however, P was too deficient for satisfactory evaluation of the N sources. In an experiment with flooded rice (Oryza sativa L.), P limited yields less than did N. In other N-P and P-K factorial experiments, rates of other nutrients and length of growth period also greatly affected crop response to applied N and K. These results show that satisfactory evaluations of N or K sources are possible only at adequate rates of nontest nutrients. Nutrient rates adequate for small greenhouse pots are much higher than rates equivalent to normal rates recommended for crops grown under field conditions. Length of growth period and other growth-limiting factors are equally important.

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

² Agronomist and Research Soil Chemist.

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