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Fertilizer Evaluation: II. Estimation of Availability Coefficients¹

ABSTRACT

Response curves produced when plants are grown on soils treated with different rates of fertilizers are used as the basis for estimating the ratio of the availability coefficient of a nutrient in one fertilizer to that in another. A least squares method is described and illustrated for fitting each of three types of response curves. Concurrent linear response curves are illustrated by data on yield of phosphorus. Parallel linear response curves for log response against log rate of application are illustrated by data on content of radioactive phosphorus. Concurrent Mitscherlich curves with the same initial and limiting responses but different rates of increase are illustrated by data on yield of corn grain.

¹ Joint contribution from the Iowa Agr. Exp. Sta. and the Tennessee Valley Authority. Journal paper No. J-2691 of the Iowa Agr. Exp. Sta., Ames, Iowa. Projects 890, 1183, and 1189.

² Graduate Assistant, Professor of Statistics, Professor of Soils, and Assistant Professor of Soils, respectively. The authors are indebted to Mr. W. L. Hill and associates, of A.R.S., U. S. D. A., for preparing the radioactive phosphorus fertilizers employed in the experiments utilized as a basis for certain examples given, to Dr. L. A. Dean and associates, of A.R.S., U.S.D.A., for making the P³² measurements reported, to Dr. P. F. Pratt, formerly of the Iowa Agr. Exp. Sta., for making the determinations of total phosphorus reported, and to Dr. L. B. Nelson, formerly of the Iowa Agr. Exp. Sta., for planning and supervising the experiments from which data on yield of phosphorus and P³² content of plants were obtained.

Received for publication April 15, 1955.