A fertility index as used by the Auburn University Soil Testing Laboratory for P, K, Ca, and Mg is described. It is reported as percentage sufficiency and is the amount extracted by soil test expressed as a percentage of the amount shown by research to be adequate for maximum yield of cotton or legumes on the soil sampled. It is calculated and reported by the computer in multiples of 10 from 0 to 9990. The index below 100 follows the curvilinear relationship between soil‐test values and yield without addition of the element. The primary objectives of the index are to report all nutrients determined on a common basis that can be understood by growers, and to provide a quantitative result that can be used in keeping records of soil fertility build‐up or depletion.