Nitrogen, Phosphorus, and Potassium Concentrations in Cotton Organs under Humic-Based Biostimulants across Different Irrigation and Fertilization Regimes

How humic-based biostimulants affect cotton nutrition under different irrigation and reduced-fertilization regimes remains poorly documented on the irrigated serozem soils of Uzbekistan. This study evaluated the biostimulants Reflect and Geohumate in field trials conducted on typical serozem soil in Tashkent province. In the first experiment, the UzPITI-103 cultivar received seed and foliar applications of Reflect (200–400 mL/t) under two irrigation regimes (65–70–65% and 70–75–65% relative to field water capacity, FWC). In the second experiment, Geohumate, Bactofert, and their combination were evaluated on the Andijon-37 cultivar against a 40%-reduced N120P84K60 fertilizer background relative to a full N200P140K100 control. Nitrogen, phosphorus, and potassium concentrations in plant organs were determined at the budding, flowering, and boll-opening stages. Changes in NPK concentration under Reflect depended on organ, growth stage, application rate, and irrigation regime, with the most consistent positive responses observed in certain reproductive organs. Under the reduced-fertilizer background, the Geohumate and Geohumate+Bactofert packages matched or exceeded the fully-fertilized control across numerous tissue indicators. Yield exceeded the control by 3.1–5.3 c/ha for Reflect treatments and by 3.4–6.6 c/ha for Geohumate/Bactofert treatments, with corresponding gains in net profit and profitability. However, the tissue analyses are based on treatment means, and no biostimulant-free control was included at the reduced-fertilizer level; the results therefore reflect the performance of the tested technological packages rather than the isolated contributions of the biostimulant and the fertilizer reduction.