Relationship of Diagnostic Nutrient Expressions to Soybean Phosphorus and Potassium Diagnoses

Research is lacking as to whether forms of nutrient expression (e.g., N/P vs. P/N, etc.) and placement of nutrient variables (N, P, K, etc.) and/or dry matter (DM) exclusively in the numerator vs. the denominator of function values affects nutrient diagnoses by the modified‐diagnosis and recommendation integrated system (M‐DRIS) using the Jones (J) and Traditional (T) methods. Consequently, a soybean [ Glycine max (L.) Merr.] cultivar‐by‐P‐by‐K‐by‐limestone soil fertility field study conducted on a Cresco loam (fine loamy mixed, mesic Typic Argiudoll) was used to determine whether these variations affect P and K diagnoses. Forms of expression affected T diagnoses, and placement of DM in the numerator vs. the denominator of functions had an effect on both T and J diagnoses. Effects of nutrient expression and DM placement on T diagnoses were caused by systematic errors in the M‐DRIS norms. These biases were largely alleviated when nutrient indices were calculated so that the variable under consideration always appeared in the numerator or the denominator of the function. Diagnoses by J were more accurate, however, when variables were placed in the denominator. Our results show that the effects of nutrient expression, calculating functions by T and J, and DM placement on M‐DRIS diagnoses can be largely alleviated when index values are calculated so that the variable under consideration is always in the numerator or denominator of the function.