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

Abstract

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.

1. Introduction

Flax (Linum usitatissimum L.) is one of the world’s most important natural fiber crops and ranks among the top oilseed species [1,2,3]. It is a highly versatile plant, as every part of the plant has an economic use. Major flax-producing countries include Canada, the United States, China, Argentina, India, and several European nations [4,5]. In recent years, Kazakhstan has significantly expanded its flax cultivation area, with plans to reach nearly 1 million hectares by 2026 [6]. Flax holds an important position in the global market due to its technical-grade oil, and the seeds are widely used for oil extraction [7]. Flaxseed oil is recognized as a high-quality edible vegetable oil of considerable nutritional and commercial importance [8,9], and is also regarded as an essential nutraceutical that is increasingly incorporated into functional foods due to its proven health benefits [10,11]. With growing interest in flax-derived functional foods and dietary supplements, there is increasing demand for strategies to enhance crop productivity [12].

Oil flax is predominantly grown in dryland ecological zones, where it is valued for its tolerance to cold and drought stress [13,14]. Improving nutrient management is critical for maintaining productivity under such environmental conditions. Mineral fertilizers are widely used to optimize seed yield and composition in oilseed crops, as they provide essential macronutrients required for growth and yield formation [15,16].

In addition to mineral fertilizers, biofertilizers play a significant role in improving plant growth. They enhance photosynthesis, hormone levels, ion uptake, nucleic acid synthesis, and protein formation, ultimately contributing to improved plant development [17]. However, soil fertilization alone is often insufficient, particularly under dryland conditions, making foliar fertilization an important complementary strategy. Because leaf tissue has the same morphological structure as root tissue, plants may rapidly absorb dissolved minerals. As a result, foliar fertilization can effectively manage soil micro-element deficit [18].

Humans need more than 50 important nutrients, such as proteins, fatty acids, energy sources, and macro- and micronutrients [19], making nutrient-rich crops particularly important. Crop yield and quality can be improved using various fertilization methods such as soil application, fertigation, and foliar feeding [20,21]. Foliar fertilization is especially useful when soil conditions limit nutrient availability [22]. Foliar application of macro- and micronutrients can quickly alleviate nutrient deficiencies and reduce the risk of toxicity due to the small quantities required compared with soil fertilization. This method is particularly effective for supplying critical micronutrients such as boron (B) and zinc (Zn). Multi-nutrient foliar applications have been widely reported to enhance yield performance across different crop species, including cereals such as wheat [23] and maize [24], often exceeding the effects of single-nutrient treatments.

Although the benefits of individual fertilization strategies are well recognized, there is limited information on how the combined application of soil-applied macronutrients with foliar-applied macro- and micronutrients, along with biofertilizers, affects flaxseed yield, oil content, protein content, and overall quality, particularly under semi-arid conditions. Therefore, the aim of this study was to evaluate the effects of integrating soil-applied macronutrients with foliar applications of macro- and micronutrients and biofertilizers on the productivity and seed quality of oil flax grown in the semi-arid regions of Kazakhstan.

2. Materials and Methods

2.1. Experimental Design and Treatments

The field experiment was conducted in the growing seasons of 2024 and 2025 at the experimental site of the Kazakh Research Institute of Agriculture and Plant Growing, which is situated in a semi-arid climate zone in southeast Kazakhstan (43°13′ N, 76°40′ E; 740 m a.s.l.). The oil flax variety ‘Karabalyksky’ was used in this study. Sowing was conducted on 1 April 2024 and 5 April 2025, and harvested on 21 July 2024 and 25 July 2025, respectively. The soil of the experimental site is classified as light chestnut soil (corresponding to Kas-tanozem in the WRB system), developed on loess-like loam, with a medium loamy texture and a humus horizon of 0–46 cm. With total nitrogen, phosphorus, and potassium concentrations of 0.15%, 0.21%, and 1.67%, respectively, the arable layer has a humus content of 1.60–1.90%. The soil reaction was alkaline, with a pH of 7.8.

The content of plant-available nutrients was as follows: nitrate nitrogen (NO₃⁻–N) 19 mg kg⁻¹, available phosphorus 10 mg kg⁻¹, and exchangeable potassium 197 mg kg⁻¹. Available micronutrient concentrations were: boron 0.09 mg kg⁻¹, zinc 0.07 mg kg⁻¹, iron 1.61 mg kg⁻¹, manganese 2.3 mg kg⁻¹, and copper 0.2 mg kg⁻¹.

The alkaline soil reaction is known to limit the availability of several micronutrients, particularly boron and zinc, which supports the rationale for foliar application in the present study. The experiment followed a single-factor randomized complete block design with three replications. Five treatments were evaluated: T1 (control, absolute zero fertilization N₀P₀K₀, i.e., soil without any additional nutrient applications), T2 (N₆₀P₆₀K₆₀), T3 (N₆₀P₆₀K₆₀ + foliar macro- and micronutrients), T4 (N₆₀P₆₀K₆₀ + foliar biofertilizer), and T5 (N₆₀P₆₀K₆₀ + combined foliar biofertilizer and macro- and micronutrients). Each plot covered 25 m², with 50 cm spacing between plots, 50 cm between blocks, and a 1 m protective buffer around the experiment. The flax variety ‘Karabalyksky’ was sown at a rate of 40 kg ha⁻¹ at a depth of 2–3 cm.

Basal fertilization consisted of 60 kg ha⁻¹ each of nitrogen, phosphorus, and potassium, applied using urea (46% N), monoammonium phosphate (12% N, 52% P₂O₅), and potassium sulfate (50% K₂O), respectively. The foliar biofertilizer Terra Sorb Complex was used at 2 L ha⁻¹, containing 20% free amino acids, 5% organic nitrogen, 5.5% total nitrogen, 1.5% boron, 0.1% manganese, 0.25% copper, 0.1% zinc, 1% iron, 0.8% magnesium, 0.001% molybdenum, and 25% organic matter. It was applied at 2 L ha⁻¹ during the 5–6 leaf stage, 8–9 leaf stage, budding, and flowering phases. Foliar macro- and micronutrient application consisted of a tank mixture containing urea (46% N) 5 kg ha⁻¹, magnesium sulfate (Mg-16%, SO₃-32%) 1 kg ha⁻¹, monopotassium phosphate (P₂O₅-52%, K₂O-34%) or potassium sulfate (K₂O-51%, SO₄-45%) 1 kg ha⁻¹, and the micronutrient formulations Bortrac 150 (11% B, 4,7% N +adjuvants) and Zintrac 700 (40% Zn, 1% N+ adjuvants) 0.1 kg ha⁻¹ each. This mixture was applied at the same four growth stages as the biofertilizer.

Protein content was measured using the Kjeldahl method, and oil content was determined using infrared spectroscopy [25]. Oil yield was calculated using the formula: Oil yield (kg ha⁻¹) = oil content (%) × seed yield (kg ha⁻¹). Nitrogen (N) concentration was determined using the Kjeldahl method, while phosphorus (P) and potassium (K) contents were analyzed following wet digestion using standard spectrophotometric and flame photometric techniques, respectively [26]. Seed yield (kg ha⁻¹), 1000-seed weight (g), plant height, capsule number per plant, and dry weight (biomass, g) were recorded following harvest.

2.2. Experimental Site and Climatic Conditions

Weather conditions during the flax growing season (April–August) in 2024 and 2025 were characterized using monthly mean air temperature and total precipitation data obtained from a nearby meteorological station (Figure 1). Climatic data were compared with long-term averages to assess deviations from normal conditions. During the 2024 growing season, mean air temperatures were generally slightly higher than the long-term average, particularly from May to August. Precipitation was above the long-term average in early growth stages (April–May), providing favorable moisture conditions for crop establishment and vegetative growth. In contrast, June and July experienced markedly lower rainfall than the long-term average, indicating periods of moisture stress during reproductive development. In 2025, growing season temperatures were consistently higher than the long-term average across most months, with pronounced warming in May and July. Precipitation during 2025 was substantially lower than the long-term average throughout much of the growing season, especially in June and July, resulting in relatively dry conditions during flowering and seed filling. Overall, the two growing seasons differed markedly in thermal and moisture regimes, enabling assessment of treatment effects under contrasting temperature and precipitation conditions.

2.3. Statistical Analysis

The data were analyzed using analysis of variance (ANOVA), with year and treatment considered as fixed effects. Treatment means were compared using the least significant difference (LSD) test at a significance level of p ≤ 0.05. All statistical analyses were performed using IBM SPSS Statistics 27, and results are reported as mean values with corresponding significance groupings.

3. Results

3.1. Oilseed Nitrogen, Phosphorus, and Potassium Content

Fertilization treatments significantly affected nitrogen (N), phosphorus (P), and potassium (K) concentrations in oilseeds during both the 2024 and 2025 growing seasons (

Table 1. Effect of soil mineral fertilization and foliar macrofertilization, microfertilization, and biofertilization on N, P, and K content (%) of oilseed flax.

Treatments	2024			2025
	N	P	K	N	P	K
T1	2.16 c	0.20 d	0.15 c	2.21 d	0.28 d	0.21 d
T2	2.24 c	0.28 c	0.16 bc	2.29 c	0.34 c	0.25 c
T3	3.12 a	0.60 a	0.17 a	2.36 b	0.42 b	0.29 b
T4	2.80 b	0.35 bc	0.16 bc	2.43 ab	0.44 b	0.34 ab
T5	2.88 ab	0.41 b	0.16 bc	2.47 a	0.52 a	0.37 a

Note: Means followed by the same lowercase letters within a column are not significantly different at p ≤ 0.05 according to the LSD test.

). Treatments involving foliar nutrient applications resulted in higher nutrient accumulation compared with the unfertilized control and sole mineral fertilization. In 2024, oilseed N content ranged from 2.16% in the control (T1) to 3.12% in T3, which showed a statistically significant advantage over all other treatments (p ≤ 0.05). Treatments T4 and T5 also significantly increased seed N concentration compared with the control. In 2025, absolute N concentrations were slightly lower; however, T5 recorded the highest oilseed N content (2.47%), followed by T4 and T3. Phosphorus concentration in oilseeds showed a strong response to foliar fertilization. In 2024, P content increased from 0.20% in the control to 0.60% in T3, which was significantly higher than all other treatments. In 2025, the highest P concentration was recorded in T5 (0.52%), followed by T4 and T3. Potassium content in oilseeds showed smaller but statistically significant differences among treatments. In 2024, K concentration ranged from 0.15% in the control to 0.17% in T3. In 2025, K concentration increased from 0.21% in the control to 0.37% in T5. Treatments combining foliar macro- and microelements with biofertilizers consistently resulted in higher K accumulation.

3.2. Nutrient Concentration in Vegetative Biomass During Growth Stages

Fertilization treatments significantly INFLUENCED N, P, and K concentrations in vegetative biomass during growth stages in both 2024 and 2025 (

Table 2. Dynamics of nitrogen (N), phosphorus (P), and potassium (K) content (%) in oil flax plants at different growth stages.

Treatments	2024			2025
	Vegetative Biomass			5–6 Leaves			7–8 Leaves			Budding			Flowering			Before Harvesting
	N	P	K	N	P	K	N	P	K	N	P	K	N	P	K	N	P	K
T1	1.04 c	0.10 c	0.53 c	1.24 c	0.67 c	0.46 c	0.93 c	0.23 c	0.42 c	0.89 c	0.19 c	0.35 c	0.81 c	0.16 c	0.29 c	0.72 c	0.13 c	0.25 c
T2	1.20 c	0.15 c	0.55 c	1.32 c	0.71 c	0.49 c	1.04 c	0.26 c	0.44 c	1.81 b	0.26 c	1.29 b	1.49 b	0.61 b	0.85 b	1.31 b	0.43 b	0.63 b
T3	2.96 a	0.43 a	0.67 a	1.43 a	0.90 a	0.59 a	1.17 a	0.43 a	1.24 a	3.80 a	0.57 b	1.43 a	2.12 a	0.92 ab	1.41 a	1.75 ab	0.86 ab	1.22 ab
T4	1.60 b	0.19 b	0.55 b	1.41 ab	1.11 a	0.52 b	1.19 a	0.46 a	0.48 b	2.60 ab	0.77 ab	1.43 a	1.82 ab	1.12 a	1.33 a	1.64 ab	1.80 a	1.07 ab
T5	1.96 ab	0.33 ab	0.58 ab	1.46 a	1.15 a	0.58 a	1.23 a	0.37 ab	0.59 ab	2.35 ab	0.58 b	1.40 a	2.28 a	0.93 ab	1.36 a	2.15 a	2.07 a	1.33 a

Note: Means followed by the same lowercase letters within a column are not significantly different at p ≤ 0.05 according to the LSD test.

). The unfertilized control (T1) consistently showed the lowest nutrient concentrations. Mineral fertilization alone (T2) resulted in only marginal increases.

Treatments involving foliar applications significantly enhanced nutrient concentrations. The highest N, P, and K contents in vegetative biomass were recorded under T3, where N concentration reached 2.96%. Treatments T4 and T5 also showed significant improvements.

At the 5–6 and 7–8 leaf stages, treatments T3 and T5 consistently produced higher nutrient concentrations. During budding, flowering, and pre-harvest stages in 2025, nutrient concentrations declined across all treatments; however, significant differences among treatments persisted. T3 recorded the highest N concentration at budding, while T5 showed the highest P and K concentrations at pre-harvest.

3.3. Effects of Fertilization Treatments on Nutrient Uptake

Foliar fertilizer applications significantly affected N, P, and K uptake in oilseed, green biomass, and total plant biomass in both experimental years (

Table 3. Effect of fertilization treatments on N, P, and K uptake from oilseed, green biomass, and total uptake (kg ha⁻¹).

Treatments	2024									2025
	Uptake of Oilseed			Uptake of Green Yield			Total			Uptake of Oilseed			Uptake of Green Yield			Total
	N	P	K	N	P	K	N	P	K	N	P	K	N	P	K	N	P	K
T1	15.5 ± 0.8 c	1.4 ± 0.05 c	1.0 ± 0.03 c	7.4 ± 0.4 c	0.7 ± 0.03 c	3.8 ± 0.2 c	22.9 ± 1.2 c	2.1 ± 0.08 c	4.8 ± 0.2 c	12.8 ± 0.6 c	1.6 ± 0.05 c	1.2 ± 0.04 c	4.1 ± 0.2 c	0.7 ± 0.03 c	1.4 ± 0.05 c	16.9 ± 0.8 c	2.3 ± 0.07 c	2.6 ± 0.1 c
T2	16.8 ± 0.9 c	2.1 ± 0.06 c	1.2 ± 0.04 c	9.0 ± 0.5 c	1.10 ± 0.05 c	4.1 ± 0.2 c	25.8 ± 1.4 c	3.2 ± 0.1 c	5.3 ± 0.3 c	14.8 ± 0.7 c	2.2 ± 0.07 c	1.6 ± 0.05 c	8.5 ± 0.4 c	2.7 ± 0.08 c	4.0 ± 0.2 c	23.3 ± 1.2 c	4.9 ± 0.2 c	5.6 ± 0.3 c
T3	27.7 ± 1.1 a	5.3 ± 0.15 a	1.5 ± 0.06 a	26.3 ± 1.2 a	3.8 ± 0.12 a	5.9 ± 0.3 a	54.0 ± 2.3 a	9.1 ± 0.25 a	7.4 ± 0.4 a	19.4 ± 0.9 ab	4.0 ± 0.15 ab	2.5 ± 0.1 ab	14.1 ± 0.7 b	6.9 ± 0.2 b	9.8 ± 0.4 b	33.5 ± 1.6 b	10.9 ± 0.3 b	12.3 ± 0.5 b
T4	23.2 ± 1.0 b	2.9 ± 0.08 b	1.3 ± 0.05 b	13.2 ± 0.6 b	1.5 ± 0.05 b	4.5 ± 0.2 b	36.4 ± 1.6 b	4.4 ± 0.15 b	5.8 ± 0.3 b	16.9 ± 0.8 b	3.0 ± 0.1 b	2.0 ± 0.08 b	11.8 ± 0.6 b	12.9 ± 0.4 a	7.7 ± 0.3 b	28.7 ± 1.4 b	15.9 ± 0.5 a	9.7 ± 0.4 b
T5	24.7 ± 1.1 ab	3.5 ± 0.09 ab	1.3 ± 0.05 b	16.8 ± 0.7 b	2.8 ± 0.08 ab	4.9 ± 0.2 ab	41.5 ± 1.8 ab	6.3 ± 0.2 ab	6.2 ± 0.3 ab	20.4 ± 0.9 a	3.7 ± 0.12 a	2.8 ± 0. 1 a	18.0 ± 0.8 a	17.3 ± 0.5 a	11.1 ± 0.4 a	38.4 ± 1.7 a	21.0 ± 0.6 a	13.9 ± 0.5 a

Note: Means followed by the same lowercase letters within a column are not significantly different at p ≤ 0.05 according to the LSD test.

).

In 2024, total N uptake increased from 22.9 kg ha⁻¹ in T1 to 54.0 kg ha⁻¹ in T3. Treatment T5 also showed significantly higher N uptake (41.5 kg ha⁻¹) than the control. In 2025, although absolute uptake values were lower, T5 recorded the highest total N uptake (38.4 kg ha⁻¹).

Total P uptake in 2024 was highest in T3, followed by T5. In 2025, the highest total P uptake was observed in T5 (21.0 kg ha⁻¹), followed by T4.

Potassium uptake showed statistically significant but smaller differences among treatments. In 2024, total K uptake ranged from 4.8 kg ha⁻¹ in T1 to 7.4 kg ha⁻¹ in T3. In 2025, K uptake increased to 13.9 kg ha⁻¹ under T5.

3.4. Effect of Treatments on Plant Density, Growth, and Yield Components

Plant density was significantly affected by treatments in both years (

Table 4. Effect of integrated fertilizers on plant density, growth, and yield components.

Treatments	2024							2025
	Plant Density, Plants/m2	Plant Height, cm	Plant Biomass, g	Stem Biomass, g	Capsule Biomass, g	Seed Biomass, g	1000-Seed Weight, g	Plant Density, Plants/m2	Plant Height, cm	Plant Biomass, g	Stem Biomass, g	Capsule Biomass, g	Seed Biomass, g	1000-Seed Weight, g
T1	61.0 ± 1.5 c	61.0 ± 1.2 b	62.7 ± 1.8 c	35.8 ± 1.0 c	23.1 ± 0.9 b	14.4 ± 0.7 c	6.0 ± 0.2 b	67 ± 1.8 c	57 ± 1.5 b	65.3 ± 2.0 c	28.2 ± 1.1 c	20.9 ± 0.9 b	12.6 ± 0.6 c	5.6 ± 0.2 b
T2	70.0 ± 1.4 b	63.0 ± 1.1 ab	69.2 ± 1.6 b	42.4 ± 1.1 b	25.6 ± 0.9 a	17.7 ± 0.8 b	7.0 ± 0.2 a	74 ± 1.7 b	59 ± 1.3 ab	76.8 ± 1.9 b	37.6 ± 1.2 b	22.4 ± 0.8 a	14.3 ± 0.7 a	6.0 ± 0.2 ab
T3	72.0 ± 1.3 a	64.0 ± 1.0 a	71.2 ± 1.5 a	44.8 ± 1.0 a	27.4 ± 0.8 a	18.6 ± 0.7 a	7.2 ± 0.3 a	102 ± 2.1 a	62 ± 1.2 a	77.8 ± 1.8 b	39.7 ± 1.1 a	25.1 ± 0.8 a	16 ± 0.6 a	6.2 ± 0.2 a
T4	70.0 ± 1.4 b	63.0 ± 1.1 ab	69.3 ± 1.6 b	42.6 ± 1.1 b	25.8 ± 0.9 a	16.6 ± 0.7 b	7.0 ± 0.2 a	96 ± 1.9 a	59 ± 1.3 ab	76.7 ± 1.9 b	37.9 ± 1.2 b	22.8 ± 0.8 a	14.8 ± 0.7 b	6.0 ± 0.2 ab
T5	71.00 ± 1.3 ab	63.0 ± 1.1 ab	69.7 ± 1.6 b	42.5 ± 1.1 b	25.3 ± 0.9 a	17.5 ± 0.7 b	7.1 ± 0.2 a	100 ± 1.9 a	64 ± 1.2 a	86.3 ± 2.0 a	39.9 ± 1.1 a	22.7 ± 0.8 a	15.3 ± 0.7 a	6.2 ± 0.2 a

Note: Means followed by the same lowercase letters within a column are not significantly different at p ≤ 0.05 according to the LSD test.

). In 2024, plant density ranged from 61.0 plants m⁻² in T1 to 72.0 plants m⁻² in T3. In 2025, the highest densities were recorded in T3 (102 plants m⁻²) and T5 (100 plants m⁻²), while T1 consistently showed the lowest values.

Plant height differed significantly among treatments. In 2024, the tallest plants were recorded under T3 (64.0 cm). In 2025, plant height increased across all treatments, with T3 and T5 producing the tallest plants (62–64 cm).

Total plant biomass varied significantly. In 2024, T3 produced the highest total biomass (71.2 g plant⁻¹), whereas T1 recorded the lowest. In 2025, biomass accumulation increased substantially, with T5 achieving the highest total biomass (86.3 g plant⁻¹).

Stem biomass followed a similar trend, with the highest values recorded under T3 in 2024 and under T3 and T5 in 2025. Capsule biomass was significantly higher under T2–T5 than under T1 in both years, with the highest values generally recorded under T3. Seed biomass also responded strongly, with T3 and T5 producing the highest values in both years.

The 1000-seed weight differed significantly among treatments, with T3 and T5 consistently producing heavier seeds than the control.

3.5. Effects of Fertilization Treatments on Seed Yield and Quality

Seed yield was significantly influenced by fertilization treatments in both years (

Table 5. Effect of integrated fertilization treatments on yield and quality of oil flax.

Treatments	2024			2025
	Yield, t ha−1	Protein, %	Oil Content, %	Yield, t ha−1	Protein, %	Oil Content, %
T1	0.72 c	27 c	39.8 c	0.58 c	26.2 c	40.2 c
T2	0.75 bc	27.3 bc	40.1 bc	0.65 bc	26.4 bc	40.4 bc
T3	0.89 a	28.4 a	41.1 ab	0.81 ab	26.9 b	41.6 ab
T4	0.83 ab	27.5 b	40.8 b	0.72 b	26.7 b	40.9 b
T5	0.86 a	28.2 a	41.3 a	0.84 a	27.7 a	41.8 a

Note: Means followed by the same lowercase letters within a column are not significantly different at p ≤ 0.05 according to the LSD test.

). In 2024, yield ranged from 0.72 t ha⁻¹ in T1 to 0.89 t ha⁻¹ in T3. In 2025, yields were lower overall, with the highest yield recorded in T5 (0.84 t ha⁻¹), followed by T3 (0.81 t ha⁻¹).

Protein content ranged from 27.0% in T1 to 28.4% in T3 in 2024, while in 2025, the highest protein content was recorded in T5 (27.7%).

Oil content increased under fertilization treatments in both years, ranging from 39.8% in T1 to 41.3–41.8% in T5.

4. Discussion

The results clearly demonstrate that foliar fertilization with macro- and microelements, particularly when integrated with biofertilizers, significantly enhances nutrient accumulation in oilseeds and vegetative biomass compared with sole mineral fertilization or no fertilization. The higher N concentrations observed under combined treatments suggest improved nitrogen uptake efficiency and assimilation, likely due to better synchronization between nutrient availability and crop demand [27,28]. Similar benefits of integrated nutrient management have been reported in maize and soybean grown under semi-arid conditions [29,30].

The strong response of oilseed phosphorus concentration to foliar fertilization confirms the effectiveness of foliar P application in overcoming soil-related limitations to phosphorus availability. By bypassing soil fixation processes, foliar P improves uptake and translocation to seeds, particularly when combined with biofertilizers that enhance microbial activity and root function [31,32].

Although potassium concentration showed smaller absolute variation, the consistent increase under integrated treatments highlights its importance in supporting enzyme activation, assimilate transport, and seed quality formation. Improved K nutrition during flowering and seed filling is known to enhance carbohydrate translocation and oil synthesis [33,34].

Enhanced nutrient concentrations in vegetative biomass during early growth stages under foliar treatments indicate more efficient physiological activity and sustained nutrient uptake. The decline in nutrient concentrations during reproductive stages reflects nutrient remobilization toward developing seeds, a process that was more efficient under integrated nutrient management. Biofertilizers likely contributed to improved phosphorus mobilization and sustained nutrient availability throughout the growing season [35,36].

Higher nutrient uptake under combined foliar treatments translated into improved plant density, biomass accumulation, and yield components. Increased stem and capsule biomass under optimized treatments reflects improved source–sink relationships and assimilate partitioning, which are critical for yield formation in oilseed crops [37]. The consistent superiority of T3 and T5 across years indicates the robustness of these treatments under varying environmental conditions. These findings underscore the novelty of applying an integrated strategy combining mineral fertilization, foliar nutrients, and biofertilizers for oil flax, a crop where such approaches are less studied.

Finally, the observed increases in seed yield, protein content, and oil concentration under integrated fertilization highlight the importance of balanced macro- and micronutrient supply combined with biofertilizers enhanced nitrogen metabolism, enzyme activity, and lipid biosynthesis under these treatments support previous findings that integrated nutrient management improves both productivity and seed quality in oilseed crops [38,39,40], while providing practical guidance for sustainable fertilization strategies

Overall, our findings indicate that integrating soil-applied mineral fertilization with foliar applications of macro- and micronutrients and biofertilizers can effectively enhance nutrient use efficiency, stabilize yield, and improve seed quality in oil flax under semi-arid conditions.

5. Conclusions

This study demonstrated that integrated nutrient management, combining basal mineral fertilization with foliar applications of macro- and micronutrients and biofertilizers, substantially improves nutrient uptake, growth, yield, and seed quality of oil flax under field conditions. Across two growing seasons, foliar fertilization consistently enhanced nitrogen, phosphorus, and potassium concentrations in both vegetative biomass and seeds compared with sole mineral fertilization or the unfertilized control. Treatments combining foliar macro- and micronutrients with biofertilizers (T5) maintained nutrient availability during critical growth stages, resulting in higher nutrient uptake and more stable nutrient dynamics across years. Enhanced vegetative nutrition led to improved crop establishment, increased plant height, and greater biomass accumulation, with a notable shift in dry matter toward reproductive organs. These physiological improvements strengthened source–sink relationships, producing higher capsule and seed biomass, increased thousand-seed weight, and superior seed yield.

Seed yield and quality responded positively even under less favorable conditions, with T5 delivering the most stable yields, protein content, and oil concentration. Basal mineral fertilization alone provided limited benefits, highlighting the importance of synchronizing nutrient supply with crop demand through foliar feeding. Overall, integrated foliar nutrient management is an effective and sustainable strategy to enhance productivity, nutrient use efficiency, and seed quality in oil flax, particularly under semi-arid conditions.