Both nutrient concentrations and nutrient ratios of plants have been associated with crop seed yields, but research examining their relative correlation to yield is scarce. Furthermore, research comparing nutrient diagnoses by the modified‐diagnosis and recommendation integrated system (M‐DRIS) against diagnoses resulting from use of M‐DRIS concentration means as critical nutrient levels (CNL's) is lacking. Consequently, a soybean[Glycine max(L.) Merrill] cultivar by phosphorus (P) by potassium (K) by limestone soil fertility field study conducted on a Cresco loam (fine‐loamy mixed, mesicTypic Argiudoll)was used to determine: (i) whether plant nutrient concentrations or ratios better accounted for differences in soybean seed yield; and (ii) whether diagnostic systems that use nutrient concentrations or nutrient balance are more accurate in diagnosing soybean P and K status. Correlation analyses demonstrated that both nutrient concentrations and nutrient ratios were related to yield, but partial correlations showed that concentrations explained more variability in yield than did ratios. Partial correlations also indicated that most nutrient ratios were correlated to yield simply because one of the nutrients in the ratio was related to yield. Potassium diagnoses by M‐DRIS were superior to those for the CNL when M‐DRIS concentration means were used as CNL's. However, these differences could be attributed to factors other than nutrient balance versus concentrations.