Establishing sufficiency ranges and critical levels of nutrients are important for a correct evaluation of plant nutrition through leaf diagnosis. This study aimed to propose critical levels and sufficiency ranges of macro and micronutrients based on leaf diagnosis of soybean plants. The database used was generated from 86 samples of soybean leaves collected during flowering in Piauí and Maranhão, Brazil. Results for macro and micronutrients and grain yield were used to calculate critical levels using reduced normal distribution and boundary line methods. Nutrient levels for 90% maximum grain yield were used to define critical levels, and sufficiency ranges for 95–99% yields. Critical levels for N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn were determined. The boundary line method provided better distribution for diagnosing nutrient deficiencies, excesses, and adequate levels. The sufficiency ranges by the boundary line method yielded reliable leaf diagnostic thresholds for soybean nutrient evaluation.

‍