Nitrogen fertilizers in the form of urea are commonly applied in Uzbekistan in order to increase cotton yield in low fertile semi arid soils. The NO ₃ formed through nitrification of those nitrogen fertilizers by soil microorganisms is susceptible to loss by leaching and denitrification and may contribute to NO ₃ pollution of ground and surface waters. Treatments of fertilizers with nitrification inhibitors have been suggested as a technique to reduce the nitrification rate. Several nitrification inhibitors were introduced by the Research Institute of Fertilizer of Uzbekistan. There is still risk on negative effects of those compounds on soil biological activities. The soil microorganisms are thus of great importance to the nitrogen nutrition of the crop vegetation. They are sensitive to changes in the surrounding soil. The objective of this research was to determine the effects of fertilizer application combined with the nitrification inhibitor potassium oxalate (PO) developed in Institute of Fertilizers on soil microbial population and activitiesand net nitrification in a nitrogen poor soil in Uzbekistan under cotton cultivation. Sites used in this study represent continuously cultivated (more than 50 years) cotton fields located in the north-eastern part of Uzbekistan. The soil type is calcareous calcisol with low content in organic matter. Field-moist sub samples (1kg) of each treatment replicate were placed in pots and treated with N as Urea at a rate of 150 mg kg soil. P was supplied as ammonium phosphate at a rate of 140 mg P kg soil and Potassium chloride at a rate of 60 mg K kg soil. PO was added to urea and ammonium phosphate at a rate of 2%. The control pots received neither PO nor fertilizer. The tested pots were then placed in incubators maintained at 27 C for 45 days. After 45 days pots were removed from the incubation and were analysed for microbiologic tests and net nitrification rate. Populations of oligotrophic bacteria, ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, mineral assimilating bacteria, oligonitrophilic bacteria, and Azotobacter were determined by the most probable number method. Nitrification inhibitor PO combined with urea increased the number of oligonitrophilic bacteria, utilization of mineral forms of nitrogen by ammonifying bacteria, decreased the number of nitrifying bacteria, denitrifying bacteria, and increased the cellulose degradation activity of soil. The application of fertilizer alone increased net nitrification seven times compared to the control. Nitrification inhibitor PO combined with urea decreased the net nitrification compared with fertilizer alone. In conclusion, our experiment showed that PO combined with mineral fertilizer is a most promising compound for inhibiting nitrification, which increased N fertilizer availability and efficiency to the cotton in semi arid soils.