Biological Nitrogen Fixation (BNF) is an efficient source of fixed N2 that plays an important role in land remediation. One of the primary factors limiting the process of BNF is the soil moisture, which affects the physiological and biochemical processes in nitrogen-fixing algae. The intensity of the BNF process increases with the development of algal biomass, which requires first of all the presence of water. To determine the optimal soil moisture conditions that stimulate BNF in nitrogen fixing algae, it is necessary to start with experiments under laboratory conditions. These studies was performed in order to highlight the influence of soil moisture on the BNF by nitrogen fixing blue-green algae of genus Nostoc, as well as to establish the optimal dose of algal inoculum for 1 ha of land. The experiments were conducted under laboratory conditions at 220–250C at daylight. Soil moisture was maintained throughout the experiments in the range of 20%, 40%, 60%, 80% and 100%. The soil of all experimental variants was inoculated by live algal biomass in doses of 6 kg/ha, 12 kg/ha and 18 kg/ha. The experimental samples were compared with the control sample destitute of algal inoculum, maintained at the same moisture conditions. The pH values and total N in the soil were monitored during the experiments. The results showed that in all experimental samples the soil pH became weakly basic. Practically the same pH values (only deviations within the limits of 0,1–0,25 pH units) were recorded in experimental and control samples with 20% of soil humidity. In batches of 40%, 60% of soil moisture, the pH value in experimental sample was slightly higher compared to the control. In batches of 80% -100% of humidity, the pH values were slightly higher in the control variant. The soil pH values changed depending on the amount of algal inoculum. The highest pH values in the soil with 20% and 80% humidity were recorded after administration of 6 kg/ha and 12 kg/ha of algal biomass, while in the soil with 40% and 100% humidity – after administration of 18 kg/ha. The humidity directly affects the amount of total nitrogen in soil. The largest amount of nitrogen accumulated in soil (1, 01%) was observed after administration of 6 kg/ha of algal inoculum in the soil with moisture of 40%. In the case of 20% soil moisture, the largest amount of nitrogen was registered after inoculation of 12 kg/ha and 18 kg/ha. In the batches with soil moisture higher than 60%, the concentration of nitrogen accumulated in the soil was slightly reduced compared to the described above versions. In all experimental groups, the nitrogen tends to increase up to a certain limit, and then reduced. In the samples with a humidity of 20% the upper limit of soil nitrogen concentration increase was 0,86%; with the humidity of 40% – 1,01%, with the humidity 60% – 0,67%; in variants with 80% humidity – 0,83% and with 100% humidity – 0,85%. It is important to note that the highest values of soil pH were detected in the samples with the most intense nitrogen accumulation. Thus, to increase the quantitative indices of BNF is recommended to use 6 kg/ha of inoculum of nitrogen-fixing algae of genus Nostoc, maintaining the soil humidity at 40%.