Effects of Different Mineral, Foliar Macro- and Micronutrient, and Biofertilizer Fertilization Strategies on Oil Flax (Linum usitatissimum L.) Yield and Seed Quality Under Semi-Arid Rainfed Conditions

Efficient nutrient management is essential for enhancing flax productivity under semi-arid conditions. A two-year field experiment (2024–2025) was conducted in southeast Kazakhstan to evaluate the effects of integrated foliar fertilization with macro- and micronutrients and biofertilizers on the growth, seed quality, and yield of flax (Linum usitatissimum L.). The experiment was arranged in a randomized complete block design with five treatments: T1 (control, absolute zero fertilization (N0P0K0), i.e., soil without any additional nutrient applications), T2 (N60P60K60), T3 (N60P60K60 + foliar macro–micronutrients), T4 (biofertilizer), and T5 (N60P60K60 + foliar macro–micronutrients + biofertilizer). Integrated foliar fertilization significantly increased nitrogen, phosphorus, and potassium concentrations in vegetative biomass and seeds, leading to higher nutrient uptake and improved nutrient use efficiency compared with mineral fertilization alone. Treatments combining foliar macro- and micronutrients with biofertilizers (T3 and T5) enhanced plant establishment, biomass accumulation, and dry matter allocation to reproductive organs. Seed yield increased from 0.58 to 0.89 t ha−1, while protein and oil contents ranged from 27.0 to 28.4% and 39.8–41.8%, respectively. The combined foliar treatment showed the highest and most stable performance, likely due to improved nutrient uptake and plant growth. These findings indicate that integrated foliar fertilization is an effective and sustainable strategy for improving flax yield stability, nutrient efficiency, and seed quality under semi-arid conditions.