Preserving the fertility of the chernozem when applying irrigation on the territory of the Republic of Moldova is a major problem, which can be achieved in particular by monitoring the quality of irrigation water supply and optimizing the irrigation regimes appropriate to each type of soil and crop, as well as by applying some measures to improve the organic matter contents of the soil. There is a bioindicator defined in the methodology used in the research, according to which irrigation regimes can be optimised in terms of soil fertility maintenance. Such an index was chosen by soil microbiological activity which can be characterized by its intensity. That is, by measuring the concentration of carbon dioxide (CO2) in the soil atmosphere. Thus, [3] there has been established the extent to which different soil characteristics and environmental factors can alter the CO2 production processes in soil atmosphere and what the pathways for regulating this process may be. According to the research of these authorsthere was determined that one of the external factors may be soil moisture. Thus, considering that microorganisms are the main factor on which the potential fertility of soils depends, it is logical to conclude that in order to improve fertility it is necessary to create optimal conditions for their activity. In the light of the above this means that the concentration/content of CO2 in the soil atmosphere should be at a maximum level. In the case of irrigation, it is necessary to keep the moisture regime within the optimum range for the activity of soil micro-organisms.