The purpose of this paper was to show the economic rationale behind growing legume plants in crop rotation. To pursue that objective, this paper presents a modified profitability accounting method for agricultural production which takes biological benefits into consideration. The following sequence of crop rotation was used in this study: forage pea – winter rape – winter wheat. An assumption was made that, from an economic point of view, the after-effect of legume crops on soil and yields of subsequent crops is an important factor which, however, is disregarded in calculations. Research suggests that legume crop growing brings measurable benefits in following years. As regards forage pea, rape and wheat, biological benefits represented 2%, 19% and 12% of total income, respectively, in the study period. Feedback from respondents suggests that 25% of the interviewees do not reduce nitrogen fertilization input in the years after growing legume crops. In turn, as much as 83% of farmers surveyed do not reduce their phosphorus and potassium fertilization rates for subsequent crops. However, agricultural producers usually fail to take account of additional biological benefits brought about by legumes when assessing their economic competitiveness against cereals and rape.