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Nitrogen, A Key Factor in Fertilizer Phosphorus Efficiency¹

ABSTRACT

The action of N in promoting fertilizer P utilization was studied in the field and greenhouse over a period of 4 years using P³² tagged superphosphate as the tool for uptake evaluation. Wheat and oats were used as indicator crops on several soils, employing different rates, times, and methods of application, and different forms of N and P.

Fertilizer N stimulates plant use of fertilizer P throughout a wide range of soil conditions. The NH₄⁺ ion apparently exceeds NO₃^- in this capacity, especially during early stages of plant growth, the maximum influence of either occurring with intimate association of the fertilizer N and P. In greenhouse plant production, a rate of slightly > 20 pounds of N per acre is sufficient to bring about the maximum proportionate change in fertilizer P use over soil P. With a given low rate of P application, the N effect may be as much as doubling the rate of fertilizer P use with little change in soil P use. The phenomenon apparently accounts for the reported greater early utilization of P from ammonium phosphates than from non-ammonium bearing but otherwise similar orthophosphate compounds.

There seems little doubt of a physiological function of N stimulating root activity in the absorption of fertilizer P, in consideration of several earlier works and the observations here. This, coupled with observed root concentration in the zone of joint N-P placement could largely account for the greater fertilizer P uptake under consideration. Whatever the full explanation, this is a principle of utmost significance in plant nutrition, practical application of which probably can be obtained only with a current annual crop where fertilizing the crop at a low rate, rather than the soil at a high rate, is practiced.

A related incidental finding is the high efficiency associated with side-band placement of N + P fertilizer in small grain seedings. Such a placement has advantage over placement with the seed in that the entire quantity of N required can be supplied with the P application without endangering germination and stand under limited soil moisture conditions, while at the same time permitting the maximum N effect on fertilizer P to be manifest.

Contribution from the Agron. Dept., Nebr. Agr. Expt. Sta., Lincoln, Nebraska. Published with the approval of the Director as Paper No. 718 Journal Series. Presented before Div. IV, Soil Sci. Soc. Amer., Davis, California, Aug. 18, 1955. Received Aug. 1, 1955. Grateful acknowledgment is made to the Olin Mathieson Chemical Co. for a grant-in-aid supporting this work; to the Fert. & Agr. Lime Sec. of S.W.C.R.B., A.R.S., U.S.D.A. for supplying P²² tagged fertilizer; and to T.V.A. for supplying some of the fertilizer materials.

² Assoc. Professor of Agronomy and Ass't. Agronomist, respectively.