According to classical concepts, soil fertility is an integrating indicator of soil properties. Correlation-regression analysis allows for the evaluation of the complexity of Agro-gray soil fertility in the East European Plain of Russia. The participation of all recorded soil properties and their equal contribution is the criteria for optimal assessment. The initial data array on soil properties divided into two clusters using the cluster analysis method. The absence of significant differences between clusters and some parameters in the model determined the need for correction. An acceptable level of fertility of Agro-gray soil established. The minimum requirements for the soil include 3.2% humus, 181 mg/kg of mobile phosphorus, and 144 mg/kg of exchangeable potassium pH and 1.5 mg-eq/100 g, respectively, the ratio of saturation of the soil with bases is not lower than 92%. With such a numerical combination of soil properties, the complexity of fertility ensured.