Mathematical forecast of the effect of mineral fertilizers on cotton productivity on the gray soils of different salinity in conditions of Turkestan region

Soil salinization remains a critical constraint for cotton cultivation in the southern regions of Kazakhstan, particularly in the Turkestan region. This study aimed to develop a model quantifying the combined influence of soil salinity levels and mineral fertilizer doses on cotton yield and fiber quality. Field trials were conducted on light gray soils with varying degrees of salinity on the production fields of the "Sabyr" farm, Maktaaral district. Standard agrochemical methods assessed soil properties, crop yield, and fiber quality. Results demonstrated that mineral fertilizers enhanced cotton productivity across all salinity levels, with the most significant yield response observed on slightly saline soils. The highest yield (6.49 t/ha) was recorded with N150P100K80 on slightly saline soils, while medium saline soils produced 5.41 t/ha under the same treatment. Phosphorus-potassium application without nitrogen (P100K80) resulted in the lowest yield gains. Regression modeling revealed a negative relationship between excessive nitrogen and potassium application and yield (Y), although their combined interaction partially offset this effect. Notably, the interaction between nitrogen and salt content (NtS) also negatively affected yield. The regression model demonstrated high reliability (R = 0.940), confirming its predictive accuracy. Correlation analysis indicated a moderate to strong relationship between potassium nutrition and key fiber quality indicators. Increasing potassium doses improved tensile strength (r = 0.61), maturity factor (r = 0.48), and fiber length (r = 0.62), while negatively affecting micronaire (r = -0.38), fiber fineness (r = -0.60), and water column resistance (r = -0.60). Fiber quality also correlated positively with total salt content in the arable layer (r = 0.37–0.61). Economic analysis showed that N150P100K80 was most profitable on slightly saline soils (1352.5 thousand KZT/ha), whereas N100P150K80 was optimal on medium saline soils (1035.8 thousand KZT/ha). These findings underline the importance of site-specific nutrient management under saline conditions.

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