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A Simple Statistical Procedure for Partitioning Soil Test Correlation Data Into Two Classes¹

ABSTRACT

Most soil test laboratories divide soil test results into two or more classes for the purpose of making fertilizer recommendations. This is usually done for the practical reason that it reduces the number of different recommendations necessary. However, the basis for defining the different classes (e.g., Very Low, Low, Medium, etc.) is often subjective or arbitrary. This paper explains a simple, yet statistically sound, method for setting the class limits. The procedure is to split the data into two groups, using successive tentative critical levels to ascertain that particular critical level which will maximize overall predictive ability (R²), with the means of the two groups (classes) as the predictor values. The paper presents an actual example, using data believed typical of this kind of problem. Several continuous correlation models were also fitted to the same data. None gave as high an R² as a single Low-High split defined by the suggested procedure. Similar results have been obtained with a majority of 200 sets of soil test correlation data, indicating that the new procedure may be widely applicable.

¹ Paper no. 3372 of the Journal Series of the North Carolina State Univ. Exp. Sta., Raleigh, N.C. This study was supported by the Agency for International Development, Department of State, Washington, D.C.

² Research Associate, International Soil Fertility Evaluation & Improvement Program, Dept. of Soil Science, and Professor, Dept. of Statistics, North Carolina State Univ., Raleigh, N.C.

Received for publication January 18, 1971. Accepted for publication March 18, 1971.