Efficiency of Sulfur Bentonite and Ammonium Sulfate in the Nutrition System of Sunflower Hybrids on Typical Gray Forest Soils of the Republic of Tatarstan

The study was conducted on gray forest soils of Agrobiotechnopark of the Republic of Tatarstan from 2023 to 2025 in order to assess the effect of different doses of sulfur bentonite on productivity of Antemis and Terramis sunflower hybrids in comparison with ammonium sulfate (100 kg/ha) and to identify the optimal dose for its application on gray forest soils of the Republic of Tatarstan. Sulfur bentonite is a granular, sulfur-containing, prolonged action fertilizer with an elemental sulfur content of 90%. Field experiments included six mineral nutrition options: a control without fertilizers, a background of nitrogen-phosphorus-potassium fertilizers, a background with the addition of ammonium sulfate at 100 kilograms per hectare and a background with three doses of sulfur bentonite: 50, 100 and 150 kilograms per hectare. Meteorological conditions during these three study years varied significantly: 2023 was characterized by severe drought with a hydrothermal coefficient of 0.84; 2024 was characterized by conditions close to optimal with a hydrothermal coefficient of 0.99 and 2025 was characterized by excess moisture with a hydrothermal coefficient of 1.27. The maximum average yield over three years for Antemis hybrid was 2.94 tons per hectare with an ammonium sulfate application rate of 100 kilograms per hectare, while for Terramis hybrid this figure reached 2.78 tons per hectare. The application of sulfur bentonite at a rate of 150 kilograms per hectare resulted in a yield of 2.80 tons per hectare for Antemis hybrid and 2.65 tons per hectare for Terramis hybrid, which is comparable to the effectiveness of ammonium sulfate. Antemis hybrid outperformed the Terramis in yield by an average of 0.23 tons per hectare. A positive effect of sulfur-containing fertilizers on sunflower productivity in the gray forest soils of the Republic of Tatarstan was established. Sulfur bentonite can be considered as an alternative sulfur-containing fertilizer for sunflower cultivation on sulfur-deficient soils with an optimal application rate of 100-150 kilograms per hectare.