Effects of Plant Density and Type of Fertilizer on Nutritional Quality of Flaxseed (Linum usitatissimum L.)

Abstract

Flax (Linum usitatissimum L.) is a multipurpose crop known for its highly nutritious seeds. This study aimed to determine how plant density and fertilizer type influence the nutritional composition of flaxseed, addressing a gap in the literature where combined effects on seed quality have not been thoroughly evaluated. Two plant densities (D1, 500 plants m⁻²; D2, 300 plants m⁻²) and four fertilizers (control, C; inhibited urea, I; organic, O; and urea, U), were compared. According to the results, the interactions of these factors influenced most of the traits. The highest yields were reported at D1I. Organic fertilization improved protein (7%) and fat (28%) content and reduced fiber (46%) and carbohydrates. In contrast, inorganic fertilization increased fiber, NDF (10%) and ADF (8%) content. Mineral composition was also affected, with O and I increasing K, Mg, Fe and Cu by 31%, 6%, 14% and 24% for O and 48%, 1%, 17% and 18%, respectively, for I. The correlation matrix analysis revealed a positive relationship between protein and fat content, whereas both traits were negatively correlated with fiber and carbohydrates. Overall, optimized density and fertilizer can improve the quality of flaxseed, supporting its use as functional food and feed ingredient.

1. Introduction

Flax (Linum usitatissimum L.) is a multipurpose crop [1] with potential applications as a mitigation strategy against climate change [2]. It is an annual winter crop belonging to the Linaceae family. Plants are characterized by a taproot system, erect stems that reach up to 1 m and narrow, linear, leaves with smooth margins [2]. Its long history of cultivation highlights its importance since ancient times, with two main products (seeds and fibers) [3,4]. Nowadays, flax is cultivated in many countries [5], with Canada and China being the main producers [6,7]. Linen is a high-quality fiber derived from flax [8,9], whereas shorter stem fibers are lower in quality and are used in various industrial applications [10] as an eco-friendly alternative [11]. In addition to fibers, seeds are characterized by high nutritional value [12]. They contain 20% protein and 30% fiber, and they are rich in polyunsaturated and monounsaturated fatty acids [13], which contribute to their antioxidant properties [14]. As a result, interest in flaxseed consumption has increased in recent years [13]. They are used both for human consumption [15] and as animal feed [16]. In human diets, flaxseeds are characterized as superfoods [17] that prevent diseases, mainly those that are correlated with poor diet [18,19]. In animal feed, the addition of flaxseeds can improve overall health in cattle [20], goats [21] and hens [22], while an increase in productivity is also reported in milk and egg production [23,24].

Agronomic practices influence flax performance. Apart from agronomic traits and yield, this effect also influences product quality of the final products [25]. Two of the most important practices in flax production are sowing density and fertilization [26]. Higher density improves both seed [27] and fiber [28] yield, as well as fiber quality [29]. In contrast, lower densities improve the growth of secondary branches [30], while a slight increase in oil yield has also been reported [31]. Fertilization is also a crucial factor influencing flax productivity. A wide range, (20 to 150 kg N per ha) has been reported as enough to overcome flax needs [5,25,32]. Fertilization enhances several agronomic traits and increases seed yield [12]. In addition to crops’ performance, fertilization also affects the quality of seeds [33]. The use of inorganic fertilizers can improve the nutritional profile of flaxseeds [34], while organic fertilizers can increase protein and lipid content [33]. The use of tomato pomace, a by-product after the processing of industrial tomato [35], has been suggested as an alternative source of organic fertilization, improving flax growth and yield [12].

Despite extensive research on how plant density and fertilization influence flax growth and yield, their combined effect on the nutritional composition of flaxseed remains insufficiently studied. Most studies have focused mainly on seed oil content and fatty acid profile, while information on seed nutrient and mineral composition under different sowing densities and types of fertilizers remains limited. The aim of this study was to evaluate the effects of two plant densities and three fertilization types, across two cropping seasons. According to the hypothesis, both factors would significantly influence seed nutritional quality. Therefore, the objective of this study was to determine how optimized combinations of sowing density and fertilizer type affect the nutritional profile of flaxseed.

2. Materials and Methods

2.1. Experimental Site and Design

A two-year field experiment was conducted at the Agricultural University of Athens (37°59′ N and 23°42′ E; 30 m altitude) in 2023 and 2024. The soil was clay loam, with a pH of 7.39 (1:1 H₂O) and soil organic matter (SOM) 1.67%. Total nitrogen and calcium carbonate (CaCO₃) were 0.143% and 15.34%, respectively, while phosphorus (Olsen P) and potassium (K) were 13.6 mg kg⁻¹ and 233 mg kg⁻¹.

Meteorological data were collected throughout the experimental period via an automatic weather station (Davis Vantage Pro2 Weather Station; Davis Instruments Corporation, Hayward, CA, USA). The weather station was located in the experimental field, measuring the mean air temperature and the total rainfall, which are presented in Figure 1. For the first year, mean temperature was 14.4 °C and total rainfall was 123.7 mm. For the second year, 16.0 °C and 174.0 mm were the mean temperature and the total rainfall, respectively.

The experiment followed a split-plot design with two factors and four replicates. Plant density (D1, D2) corresponded to the main plots, and fertilizer type (C, I, O, U) corresponded to the sub-plots. The different treatments were randomly assigned within each replication. The total experimental area was 837 m². The main plot (91 m²) was plant density. High (500 plants per m²—D1) and low (300 plants per m²—D2) densities [12], were studied. Sub-plot (21 m²) was the fertilizer type. Three types of fertilizers were used, in a dose of 174 kg ha⁻¹, according to the mean value, purposed by the literature [5]. Urea 46-0-0 [EUROCHEM HELLAS S.A., Athens, Greece] (U) and urea with nitrification—N-((3(5)-methyl-1H-pyrazol-1-yl) methyl) acetamide (MPA; 0.07%)—and urease—N-(2-Nitrophenyl) phosphoric triamide (2-NPT; 0.035%)—inhibitor 46-0-0 [EUROCHEM HELLAS S.A., Athens, Greece] (U + UI + NI) were used for the inorganic fertilization, while an organic fertilizer was also used. The organic fertilizer was a mix of compost with tomato pomace, with a total of 28.7 g kg⁻¹ N, 15 and 30 mg kg⁻¹ phosphorus and potassium, respectively, 44% organic matter and a pH of 7.38. There was also an unfertilized treatment, representing the control (C).

2.2. Agronomic Practices

Flax (Linum usitatissimum L. cv Everest) seeds were sown on the 25th of November, for both years. The distance between each row was 25 cm and 35 cm for D1 and D2, respectively. No irrigation was performed, as the crop was rainfed. No infestations were reported, and weed management was performed weekly by hand. Seedlings emerged 12 days after sowing (DAS), and the vegetative and flowering stages lasted for approximately 60 and 25 days, respectively. Harvest took place on 9 May 2023 and 10 May 2024, by hand, when seed moisture reached 12%.

2.3. Measurements

After harvest, representative seed samples were collected from 20 plants of each plot and ground via a 1 mm Wiley mill (Thomas T4274.E15 Steel Model 4 Wiley Mill; Arthur H. Thomas, Philadelphia, PA, USA). The analysis of chemical composition was conducted according to the Association of Official Analytical Chemists [36], including the seed dry matter (DM-method 943.01), the ash (A-method 924.05) and the crude fat (CFa-method 920.39). Crude fibers (CFi), neutral detergent fiber (NDF-method 930.15) and acid detergent fiber (ADF-method 973.18) were calculated using an ANKOM 200 fiber analyzer (ANKOM Technology Corporation, New York, NY, USA) and the Kjendal nitrogen (N-method 984.13), via a Vapodest auto-analyzer (C. Gerhardt GmbH & Co. KG, Königswinter, Germany). In order to calculate the crude protein (CP) content, N was multiplied by 6.25. Total carbohydrates (CHO) and non-fiber CHO (NFC) were calculated by the following equation [37].

Total carbohydrate (CHO) = 100 − (CFa + CP + A) (%)

(1)

Non-fiber CHO (NFC) = 100 − (CP + NDF + CFa + A)

(2)

Following the previous measurements, the seeds’ elemental composition was also estimated. This measurement was carried out by a quadrupole Inductively Coupled Plasma Mass Spectrometer (ICP-MS), Agilent Technologies 7850 series instrument (Santa Clara, CA, USA). Data acquisition and processing were performed using MassHunter 5.1 software (Agilent Technologies, Santa Clara, CA, USA).

2.4. Statistical Analysis

For the statistical analysis of the results, StatGraphics Centurion XVII v.17.2.00 (Statgraphics Technologies Inc., The Plains, VA, USA) package was used. Correlation matrix was created in Statistica v.7.0 (StatSoft, 2011). A three-way ANOVA (including the factors year, sowing density and fertilizer type) occurred for the evaluation of data. The multiple comparison of means was performed via Tukey HSD (Honestly Significant Difference) test, while Pearson’s correlation was performed in order to make the correlation analysis. All comparisons were performed at a 5% (p ≤ 0.05) significance level.

3. Results

3.1. Seed Yield, Composition and Macronutrient Profile

According to the results, the interaction of plant density and type of fertilizer statistically significantly affected the studied traits (

Table 1. Effect of plant density and fertilization on yield, seed dry matter and ash content.

	Yield (Kg ha−1)		Dry Matter (DM) (%)		Ash (A) (%)
Density
	D1	D2	D1	D2	D1	D2
C	2270 ± 90 bc	1549 ± 90 c	94.67 ± 0.02 e	94.59 ± 0.02 de	2.97 ± 0.04 a	2.91 ± 0.02 a
I	2678 ± 60 ab	2610 ± 37 ab	94.18 ± 0.02 b	94.14 ± 0.01 b	3.28 ± 0.04 c	3.09 ± 0.05 b
O	1801 ± 32 ab	1718 ± 30 ab	94.14 ± 0.01 b	94.43 ± 0.01 c	3.07 ± 0.04 b	3.37 ± 0.02 c
U	2141 ± 35 ab	2126 ± 29 a	94.57 ± 0.02 cd	93.84 ± 0.07 a	3.18 ± 0.03 b	3.11 ± 0.02 b
FYEAR	0.01 ns		1.99 ns		2.32 ns
FDENSITY	31.06 ***		41.58 ***		0.05 ns
FFERT	94.61 ***		98.06 ***		25.95 ***
FYxD	2.52 ns		0 ns		0.23 ns
FYxF	0.72 ns		1.06 ns		0.91 ns
FDxF	17.63 ***		100.15 ***		18.51 ***
FYxDxF	1 ns		1.04 ns		1.44 ns

Values represent a two-year mean (2023–2024). Statistically significant differences at a significant level of p = 5% are symbolized with different letters in the column. The term “ns” indicates non-significant differences. Symbols “***” indicate significance at the p < 0.001 levels. F_YEAR, F_DENSITY, F_FERT, F_YxD, F_YxF, F_DxF and F_YxDxF represent the F-test ratios from ANOVA of year, density, fertilization and their interactions, respectively.

). The highest yields were reported in D1I and D2I. C had lower yields, with D2C showing the lowest values. The use of I reported a mean of 43% increase in the crop’s yield, while U reported an increase of 16%. Organic fertilization (O) slightly reduced yield in D1, whereas in D2, it increased yield by 11%. As presented in Table 1, dry matter was significantly affected by the interaction of plant density and type of fertilizer used. Fertilization decreased DM by 0.39% in D1 and by 0.48% in D2. Higher values were reported for D1C, while the lowest values were reported for D2U. Ash content was also affected by the interaction of density and fertilization, with D1I and D2O having the highest values. In these treatments, ash content increased by 10.4% and 15.8% compared to C (Table 1).

Protein content was also affected by the interaction of the two factors (p < 0.05). The use of urea in D2 increased CP by 10% compared to the control. Fertilization increased CP in both D1 and D2 by 6, 7.3 and 7.1 for I, O and U, respectively, (

Table 2. Effect of plant density and fertilization on crude protein, crude fat and crude fibers.

	Crude Protein (CP) (%)		Crude Fat (CFa) (%)		Crude Fibers (CFi) (%)
Density
	D1	D2	D1	D2	D1	D2
C	21.04 ± 0.16 ab	20.70 ± 0.35 a	25.52 ± 0.03 a	25.67 ± 0.14 ab	30.93 ± 0.51 d	30.50 ± 0.28 d
I	22.16 ± 0.15 cd	22.09 ± 0.16 bc	26.68 ± 0.05 ab	26.16 ± 0.03 ab	25.42 ± 0.34 c	24.84 ± 0.20 c
O	22.64 ± 0.14 de	21.95 ± 0.22 cde	32.65 ± 0.15 c	26.45 ± 0.05 ab	16.84 ± 0.19 a	25.43 ± 0.23 c
U	21.74 ± 0.27 c	22.93 ± 0.20 e	30.70 ± 0.02 c	26.81 ± 0.02 b	21.63 ± 0.12 b	24.68 ± 0.22 c
FYEAR	0.01 ns		1.32 ns		0.49 ns
FDENSITY	0.03 ns		2222.31 ***		177.9 ***
FFERT	19.32 ***		1140.85 ***		429.55 ***
FYxD	0.25 ns		2.89 ns		0.01 ns
FYxF	0.53 ns		0.9 ns		2.01 ns
FDxF	6.64 ***		717.5 ***		116.51 ***
FYxDxF	0.92 ns		1.1 ns		0.87 ns

Values represent a two-year mean (2023–2024). Statistically significant differences at a significant level of p = 5% are symbolized with different letters in the column. The term “ns” indicates non-significant differences. Symbols “***” indicate significance at the p < 0.001 levels. F_YEAR, F_DENSITY, F_FERT, F_YxD, F_YxF, F_DxF and F_YxDxF represent the F-test ratios from ANOVA of year, density, fertilization and their interactions, respectively.

). Organic fertilization had a strong impact on crude fat and crude fiber content. In D1 the addition of organic fertilization increased CFa by 27.9%, and decreased CFi by 45.6%, compared to C. Urea also increased CFa by 20.3% and decreased CFi by 30.1% in D1. The rest of the treatments were statistically significantly different from the control, in a range of 1.9%–4.5% and 17.9%–19.1%, for CFa and CFi, respectively, (Table 2).

NDF and ADF content were also statistically significantly affected by the interaction of plant density and type of fertilizer. Inhibited fertilizers increased NDF content by 9.3% and 9.8% for D1 and D2, while for the ADF content this increase was 9.1% and 6.8% for the high- and low-density values (

Table 3. Effect of plant density and fertilization on NDF and ADF content.

	NDF (%)		ADF (%)
Density
	D1	D2	D1	D2
C	29.86 ± 0.37 a	29.26 ± 0.31 a	21.09 ± 0.27 a	21.72 ± 0.23 ab
I	32.66 ± 0.14 b	32.14 ± 0.11 b	23.00 ± 0.15 bc	23.19 ± 0.08 cd
O	33.94 ± 0.19 cd	34.47 ± 0.08 d	26.22 ± 0.19 e	25.58 ± 0.08 e
U	28.88 ± 0.07 a	32.39 ± 0.74 bc	21.41 ± 0.05 abc	23.64 ± 0.53 d
FYEAR	0.02 ns		0.02 ns
FDENSITY	9.52 **		12.41 ***
FFERT	74.02 ***		123.91 ***
FYxD	0.01 ns		1.48 ns
FYxF	1.01 ns		1.54 ns
FDxF	16.37 ***		12.31 ***
FYxDxF	0.87 ns		0.94 ns

Values represent a two-year mean (2023–2024). Statistically significant differences at a significant level of p = 5% are symbolized with different letters in the column. The term “ns” indicates non-significant differences. Symbols “**” and “***” indicate significance at the p < 0.01 and p < 0.001 levels, respectively. F_YEAR, F_DENSITY, F_FERT, F_YxD, F_YxF, F_DxF and F_YxDxF represent the F-test ratios from ANOVA of year, density, fertilization and their interactions, respectively.

). For the ADF, I reported an increase of 9.1% and 6.8% for D1 and D2, respectively, compared to the control. Organic fertilizer had the higher values for both NDF and ADF; the former increased at a level of 13.7% and 17.8% for D1 and D2, while ADF was 24.3% and 17.8% higher at the higher and lower densities, compared to C. Urea did not significantly affect NDF or ADF in D1, whereas in D2 it increased NDF by 10.7% and ADF by 8.8% (Table 3).

For carbohydrates, both CHO and NFC were affected by the interaction of plant density and fertilization (

Table 4. Effect of plant density and fertilization on carbohydrates.

	CHO (%)		NFC (%)
Density
	D1	D2	D1	D2
C	50.47 ± 0.17 e	50.72 ± 0.34 e	20.61 ± 0.41 d	21.46 ± 0.43 d
I	47.89 ± 0.18 cd	48.66 ± 0.14 de	15.22 ± 0.23 bc	16.51 ± 0.14 c
O	41.64 ± 0.08 a	48.23 ± 0.25 cd	17.71 ± 0.23 c	13.76 ± 0.21 a
U	44.38 ± 0.30 b	47.15 ± 0.19 c	15.50 ± 0.24 bc	14.76 ± 0.69 ab
FYEAR	0.01 ns		0.01 ns
FDENSITY	265.13 ***		6.41 **
FFERT	258.4 ***		118.95 ***
FYxD	0.03 ns		0.01 ns
FYxF	0.35 ns		1.13 ns
FDxF	81.31 ***		22.19 ***
FYxDxF	1.37 ns		2.34 ns

Values represent a two-year mean (2023–2024). Statistically significant differences at a significant level of p = 5% are symbolized with different letters in the column. The term “ns” indicates non-significant differences. Symbols “**” and “***” indicate significance at the p < 0.01 and p < 0.001 levels, respectively. F_YEAR, F_DENSITY, F_FERT, F_YxD, F_YxF, F_DxF and F_YxDxF represent the F-test ratios from ANOVA of year, density, fertilization and their interactions, respectively.

). CHO decreased by 5.1%, 17.5% and 12.1% in D1 and by 4.1%, 4.9% and 7% in D2, for I, O and U, respectively. NFC content decreased by 14.1%–35.8%, compared to C for both D1 and D2.

3.2. Mineral and Micronutrient Composition

According to the results, plant density and type of fertilizer, did not affect statistically significantly sodium (Na), boron (B), calcium (Ca) and zinc (Zn) content in seeds (

Table 5. Effect of plant density and fertilization on mineral content in seeds.

	Control (C)		Inhibited (I)		Organic (O)		Urea (U)
Density
(mg/kg)	D1	D2	D1	D2	D1	D2	D1	D2
K	7158 ± 95 a	9819 ± 168 b	10,567 ± 302 bc	9298 ± 992 b	10,139 ± 539 bc	11,763 ± 1433 c	7088 ± 132 a	7419 ± 118 a
Na	571 ± 43	732 ± 196 ns	527 ±20 ns	1057 ± 522 ns	547 ± 69 ns	631 ± 51 ns	504 ± 93 ns	593 ± 74 ns
B	16.2 ± 0.2 ns	15.3 ± 0.5 ns	15.8 ± 0.3 ns	15.3 ± 0.7 ns	15.8 ± 0.2 ns	18.9 ± 3.0 ns	15.5 ± 0.2 ns	14.6 ± 0.4 ns
Mg	3728 ± 144 a	3761 ± 181 ab	3773 ± 141 a	3658 ± 197 ab	3749 ± 143 a	4184 ± 252 b	3490 ± 100 a	3380 ± 115 a
Ca	306 ± 20 ns	334 ± 20 ns	275 ± 16 ns	298 ± 21 ns	276 ± 16 ns	346 ± 21 ns	350 ± 29 ns	498 ± 185 ns
Fe	57 ± 2 a	55 ± 2 a	70 ± 2 c	62 ± 3 ab	58 ± 2 a	69 ± 6 bc	57 ± 1 a	57 ± 1 a
Zn	56 ± 3 ns	62. ± 4 ns	57 ± 3 ns	56 ± 4 ns	55 ± 3 ns	56 ± 5 ns	51 ± 2 ns	52 ± 1 ns
Cu	13.3 ± 0.3 a	14.1 ± 0.7 a	15.9 ± 0.3 b	16.2 ± 0.6 b	16.9 ± 0.2 c	16.9 ± 0.6 b	15.2 ± 0.4 b	15.2 ± 0.2 ab
	K	Na	B	Mg	Ca	Fe	Zn	Cu
FYEAR	0.11 ns	2.28 ns	3.11 ns	0.4 ns	0.46 ns	0.09 ns	3.06 ns	2.06 ns
FDENSITY	3.01 ns	2.17 ns	0.06 ns	0.28 ns	1.94 ns	0 ns	0.77 ns	0.99 ns
FFERT	11.32 ***	0.54 ns	1.66 ns	3.56 *	1.57 ns	5.01 **	2.37 ns	16.97 ***
FYxD	1.54 ns	3.31 ns	2.54 ns	0.52 ns	0.1 ns	2.77 ns	0.06 ns	0.4 ns
FYxF	0.1 ns	0.89 ns	0.89 ns	2.11 ns	1.09 ns	0.09 ns	2.18 ns	0.1 ns
FDxF	3.1 *	0.59 ns	1.48 ns	1.26 ns	0.35 ns	3.56 ns	0.43 ns	0.24 ns
FYxDxF	0.87 ns	0.56 ns	0.86 ns	0.74 ns	1.21 ns	0.47 ns	1.03 ns	0.21 ns

Values represent a two-year mean (2023–2024). Statistically significant differences at a significant level of p = 5% are symbolized with different letters in the column. The term “ns” indicates non-significant differences. Symbols “*”, “**” and “***” indicate significance at the p < 0.05, p < 0.01 and p < 0.001 levels, respectively. F_YEAR, F_DENSITY, F_FERT, F_YxD, F_YxF, F_DxF and F_YxDxF represent the F-test ratios from ANOVA of year, density, fertilization and their interactions, respectively.

). On the other hand, the interaction of the two factors affected the K content. The highest K concentrations were observed in D2O, with an increase of about 20% compared to C. D1I and D1O increased K content by 47.6% and 41.7% compared to D1C (Table 5). Magnesium (Mg) content was affected only by the type of fertilizer used. O increased the level of Mg in seeds by 0.6% and 11.2% for D1 and D2, respectively. In contrast, urea reduced Mg content by 6.4% and 10.2% for D1 and D2, respectively. In D2, I also reduced Mg content by 2.8%, whereas in D1 it caused a slight increase (1.2%) (Table 5). Iron (Fe) content was also significantly affected only by fertilization type, with a mean increase of 17.3%, 13.9% and 1.8% for I, O and U, respectively. Copper (Cu) content was also statistically significantly affected by fertilization. Cu content was 17.6%, 23.7% and 10.9% higher in I, O and U, compared to the control, as presented in the table below.

4. Discussion

The factor “year” did not statistically significantly affect the results, as temperature and rainfall did not differ substantially between the two experimental periods. Sowing density and type of fertilizer are known to influence flax growth and yield [5,32]. The hypothesis of this study was that both factors influence flaxseed nutritional quality. The results confirmed this hypothesis, as most traits were statistically significantly affected by the interaction of these factors.

4.1. Yield and Seed Composition

Yield was significantly affected by the interaction between plant density and fertilization. The highest yields were reported at the high density combined with the use of inhibited fertilizer. The slow release of urea contributed to higher yields [38]. Low plant density also favored yield by reducing competition among plants [39]. The combined use of inhibited fertilizers and high density has been shown to increase yield [12].

DM content was influenced by the interaction of density and fertilization. Fertilized plots showed lower DM, probably because increased soil nitrogen enhanced protein synthesis [40]. Higher plant density resulted in higher DM. At lower densities, increased nutrient availability and reduced intra-plant competition may contribute to differences in DM [5]. In contrast to DM, CP was higher in fertilized treatments. An increase in N content in soil increases CP [40]. In addition, lower densities decreased competition between the plants, leading to an increase in CP. According to Trukhachev et al. (2023) [41], similar findings were reported, while the use of both organic and inorganic fertilizers increased protein content in flaxseeds. In another study, Makenova et al. (2024) [4] reported that the use of organic fertilization can increase protein content in flax. The high protein concentration also increases the value of flaxseed as an ingredient in animal feed. Diets supplemented with flaxseed have been shown to improve growth performance and overall health in pigs [42].

In parallel with the increase in protein content, significant variations were also observed in ash content, reflecting the mineral composition of the seed. Ash content was statistically significantly affected by the interaction between plant density and fertilization. The higher ash content likely reflects improved mineral uptake. Organic fertilizer enhances soil microbial activity [43], which increases nutrient availability to plants. Inhibited fertilizer has higher use efficiency [44], reflecting the higher A content, by supporting mineral uptake.

Crude fat was strongly affected by fertilizer, with the greatest increase reported in O. Higher density increased CFa by 27.9% compared to the control. A notable increase (20.3%) in U was also observed. The high increase in CFa under O may be attributed to the gradual release of nitrogen and enhanced microbial activity in soil. Both organic and inorganic fertilization increased lipid content in flaxseeds [41]. In contrast to CFa, CFi content was strongly decreased under O. In D1, the decrease was 45.6% for O, while the use of U also resulted a significant decrease in Cfi, at a level of 30.1%. Fiber content is negatively, or slightly positively correlated, with protein and fat content, while protein and fat content are positively correlated [45]. Organic fertilization could make flaxseed more suitable for animal feed due to the lower fiber content in seeds, whereas higher CFi values could be beneficial for human consumption. Flaxseeds are characterized as functional foods, due to their high fiber content [46,47]. Furthermore, flaxseed supplementation in ruminant diets has shown positive effects [48].

The carbohydrates (CHO and NFC) were also affected by the interaction of the two factors. Fertilizer application decreased these traits, with the greatest reduction observed under O and I. The increase in CP, CFa and CFi caused a reduction in carbohydrate content [49]. In higher densities the competition for nutrients is higher [27], leading to lower carbohydrates.

The interaction of plant density and fertilization also significantly affected the fiber fractions (NDF and ADF). Organic fertilization increased both NDF and ADF, followed by I. The slower release of nutrients increased the synthesis of structural polymers, possibly due to increased photosynthetic activity. These results agree with previous studies, where the use of fertilization increased NDF and ADF content in flaxseed, mainly in high-density situations [50].

4.2. Mineral Composition

The mineral composition of the seeds was also affected by plant density and fertilization. While Na, B, Ca, and Zn were not influenced, significant differences were observed for K, Mg, Fe and Cu. Potassium was influenced by the interaction of the two factors, while Mg, Fe and Cu were only influenced by fertilization. Fertilization affects Fe and Cu content in flaxseeds [51]. The use of organic fertilization and inhibited urea increased micronutrient concentration in seeds. Organic fertilization improved soil microbial activity, increasing the efficiency of the fertilizer and the availability of trace-elements. Inhibited fertilizers also improve nitrogen use efficiency [44], which may enhance nutrient uptake, through improved root development [52].

Correlation analysis indicated strong relationships among several nutritional traits. As presented in

Table 6. Correlation coefficients between the quality traits of flaxseed.

YIELD

DM

CP

A

CFa

CFi

NDF

ADF

CHO

NFC

K

Na

B

Mg

Ca

Fe

Zn

Cu

YIELD

−0.276 *

0.138 ns

0.218 ns

−0.183 ns

0.004 **

−0.012 ns

−0.263 *

0.099 ns

−0.228 ns

−0.102 ns

−0.278 ns

−0.025 ns

−0.103 ns

0.011 ns

0.04 ns

−0.253 ns

0.107 ns

DM

−0.649 ***

−0.196 ns

−0.139 ns

0.465 ***

−0.567 ***

−0.502 ***

0.330 **

0.494 ***

−0.053 ns

−0.186 ns

0.163 ns

0.142 ns

0.061 ns

0.317 ns

−0.107 ns

−0.102 ns

CP

0.262 *

0.329 **

−0.541 ***

0.451 ***

0.439 ***

−0.603 ***

−0.620 ***

0.066 ns

0.187 ns

−0.168 ns

0.022 ns

−0.134 ns

−0.347 ns

0.097 ns

0.120 ns

A

0.067 ns

−0.313 **

0.449 ***

0.353 **

−0.196 ns

−0.697 ***

0.078 ns

−0.175 ns

0.032 ns

0.005 ns

0.154 ns

−0.166 ns

−0.158 ns

0.323 ns

CFa

−0.884 ***

0.138 ns

0.383 **

−0.950 ***

−0.176 ns

0.194 ns

0.317 ns

−0.018 ns

0.056 *

0.183 ns

−0.069 ns

0.215 ns

0.127 ns

CFi

−0.427 ***

−0.591 ***

0.932 ***

0.454 ***

−0.198 ns

−0.297 ns

0.058 ns

0.018 ns

−0.158 ns

0.165 ns

−0.137 ns

−0.158 ns

NDF

0.930 ***

−0.283 *

−0.612 ***

−0.027 ns

0.184 ns

−0.200 ns

−0.117 ns

−0.190 ns

−0.333 ns

0.141 ns

−0.065 ns

ADF

−0.48 ***

−0.513 ***

0.010 ns

0.315 ns

−0.177 ns

−0.089 ns

−0.089 ns

−0.289 ns

0.197 ns

−0.068 ns

CHO

0.382 **

−0.184 ns

−0.306 ns

0.052 ns

−0.053 *

−0.134 ns

0.145 ns

−0.199 ns

−0.15 ns

NFC

−0.024 ns

0.074 ns

−0.034 ns

0.011 ns

−0.174 ns

0.156 ns

0.260 ns

−0.440 ns

K

0.036 ns

0.640 ***

0.723 ***

−0.145 ns

0.673 ***

0.381 *

0.373 *

Na

−0.081 ns

−0.024 ns

−0.015 ns

−0.166 ns

−0.063 ns

−0.154 ns

B

0.638 ***

−0.012 ns

0.443 *

0.058 ns

0.205 ns

Mg

−0.068 ns

0.616 ***

0.59 ***

0.174 ns

Ca

0.024 ns

0.001 ns

−0.080 ns

Fe

0.300 ns

0.333 ns

Zn

−0.056 ns

Cu

The term “ns” indicates non-significant differences. Symbols “*”, “**” and “***” indicate significance at the p < 0.05, p < 0.01 and p < 0.001 levels, respectively.

, crude protein is positively correlated with the crude fat content. In contrast, both traits are negatively correlated with crude fiber, carbohydrates and fiber fractions. Yield is positively correlated to the protein content, probably because of the higher uptake of N by the plant. Positive correlations between some micronutrients demonstrate related physiological uptake and transport mechanisms, which are enhanced by organic fertilization.

5. Conclusions

The present study aimed to evaluate how both plant density and different types of fertilizers affect the yield and quality traits of flaxseed. Organic fertilization and inhibited urea improved the nutritional value of the seeds, by increasing protein and fat content, while reducing fiber content. In contrast, urea led to higher carbohydrates and fiber content. According to this, organic fertilization (mix of compost with tomato pomace) combined with high plant density (500 plants ha⁻¹) can enhance the nutritional profile of flaxseeds, making them more suitable for animal feed, whereas inorganic fertilization could be more appropriate for human consumption, or animal feed in lower concentrations in meal. Overall, the findings highlight the potential of optimized fertilization and planting density as important agronomic tools to improve flaxseed quality.