## 4 Conclusions  
Uncultivated and surface-irrigated Calcisols have been investigated in the northern part of the Fergana Valley in terms of morphology, chemistry, organic matter content and quality, soil texture, and chemical composition. Dark, Typical, and Light Calcisols showed intensive transformation under the surface irrigation. This study presents a comparative analysis of three primary types of Calcisols (Dark, Light, and Typical), examining their conditions before (uncultivated soil) and after agricultural use (surface-irrigated agricultural soil). Irrigation has been found to increase SOC stocks in Typical Calcisols (from 113.8 to 126.3 t/ha) and Light Calcisols (from 62.8 to 100.1 t/ha), while decreasing SOC stocks in Dark Calcisols (from 175.3 to 160.1 t/ha). Overall, the content of biophilic elements is lower in irrigated soils, with their distribution in the soil profile showing a closer alignment with a functional relationship (r\({}^{2}\) = 0.98 to 0.99). Conversely, the distribution of SOC and TN in uncultivated Calcisols is more heterogeneous (r\({}^{2}\) = 0.67 to 0.97) with a wider confidence interval. Uncultivated (reference) Calcisols are characterized by higher C/N ratios in surface horizons compared to post-irrigation soils. For uncultivated Calcisols, the C/N ratios vary from 14 to 25; after irrigation they from 10 to 13.5. Changes in the SOC transformation processes have been identified; in irrigated soils, the carbon ratio CHA/CFA is lower (<1) compared to the uncultivated type (-1). Modifications in the soil water regime due to irrigation have resulted in transformations in the individual compositions of amino acids. All studied types of Calcisols exhibit changes in particle-size distribution, particularly in the silt fraction (0.01-0.05 mm), with a difference of 10-20% between uncultivated and irrigated soils. This variation is associated with the processes of colmatage through the accumulation of fine fractions and the replacement of sub-fractions in the sand fraction. The highest concentrations of nutrients are found in the upper soil horizons (phosphorus up to 231 mg/kg, potassium up to 2350 mg/kg). Increased content of mobile forms of P and K are found in surface horizons of soils subjected to irrigation, indicating their pedagogenic and agrogenic origins rather than inheritance from parent materials. Surface irrigation results in the apparent accumulation of water-soluble magnesium (Mg\({}^{2+}\)) (1.6-2.1 mg/100 g) and potassium (K\({}^{+}\)) (0.6-0.9 mg/100 g), while Ca\({}^{2+}\) remains the predominant cation (more than 50% of base saturation) in the base cation composition. This predominance of calcium is favorable for the formation of soil agrogenic properties. In summary, it was established that Calcisols of different types in the Fergana Valley significantly change their properties in the process of surface irrigation, especially regarding SOC stocks, particle-size distribution, quantity and profile distribution of biophilic elements, and base saturation. Nevertheless, it should be noted that the quality and quantity of irrigation water may have a special influence on soil characteristics, which requires separate specialized studies. Conceptualization, A.T. and E.A. ; methodology, A.T.; software, T.N. ; validation, G.Y., M.K. and K.A. ; formal analysis, R.M. ; investigation, Z.A. ; resources, A.T.; data curation, G.S; writing--original draft preparation, E.A. ; writing--review and editing, U.M. and M.I. ; visualization, T.N., H.H. and M.O. ; supervision, E.N. ; project administration, A.T.; funding acquisition, A.T. and E.A. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported by the international project \"Development of theoretical and practical basics of soil and plant geochemistry in Fergana Valley\" in 2013-2018.