Valuation of farm land depends, in the main, on farm income, which in turn depends on crop yields and farm prices. Crop yields, to go back still another step, rest largely on soil and climate. Consequently, a careful measurement of soil productivity, as determined by crop yields, is a fundamental aspect of farm valuation.

Measurement of soil productivity in terms of yield differences has been hampered by the influence of management. Two farms of equal quality may produce unequal yields, not because of soil differences but because of managerial differences. In order to measure soil variation some method of controlling the management variable has to be followed. One method is to obtain a group of representative, randomized yield samples, the mean of which will represent average or typical management. Another solution, the one followed for the most part in this study, is to take yield samples of various soil conditions within an individual field, thus holding management constant.