Organic Manure Significantly Promotes the Growth of Oilseed Flax and Improves Its Grain Yield in Dry Areas of the Loess Plateau of China

Abstract

Organic fertilizers could be useful for agricultural sustainability. Therefore, this study explored green cultivation techniques to improve the grain yield of oilseed flax in dry areas of the Loess Plateau of China. With no fertilization (CK) as the control, the effects of sheep manure (S1: 12.5 t ha⁻¹; S2: 25 t ha⁻¹), poultry manure (C1: 5.8 t ha⁻¹; C2: 11.6 t ha⁻¹), and chemical fertilizers (F1: N 112 kg ha⁻¹, P 75 kg ha⁻¹, K 67.5 kg ha⁻¹; F2: N 225 kg ha⁻¹, P 150 kg ha⁻¹, K 135 kg ha⁻¹) on the growth and development, the grain filling characteristics, and the yield of the Zhangya 2 oilseed flax (Linum usitatisimum L.) variety were compared and analyzed based on a two-factor split plot experiment. The results showed that the application of manure significantly increased the emergence rate (ER) of oilseed flax. Poultry manure increased plant height while sheep manure increased stem diameter. The dry matter production was higher in the 25 t ha⁻¹ sheep manure treatment by 2.47–40.11% compared with that of the other treatments, and it promoted the distribution ratio of dry matter to grains after anthesis. The observed relationship was in accordance with that presented by the logistic equation between grain weight and days after anthesis, and there were significant positive correlations between the 1000-grain weight and the average filling rate (V-ave), the maximum filling rate (V-max), and the growth at the maximum filling rate (W-max). The application of organic manure accelerated the grain filling rate (GFR); under the treatment with 25 t ha⁻¹ sheep manure, V-ave, V-max, and W-max increased by 4.84–22.72%, 1.16–17.54%, and 4.58–22.63%, respectively, and the grain yield and the net income per unit area increased by 6.35–39.25% and 3.04–95.07%, respectively, compared with those under the other treatments. Consequently, the treatment with 25 t ha⁻¹ sheep manure can significantly promote the growth and development of oilseed flax plants, optimize the grain filling characteristics, and increase the grain yield and net income, making it a suitable fertilization technique for oilseed flax in dry areas of the Loess Plateau of China.

1. Introduction

Oilseed flax (Linum usitatisimum L.) is an important economic crop in arid and semi-arid areas of northwest China because of its short growth period and strong stress resistance. In recent years, due to low efficiency and the adjustment of planting structures, sown areas of oilseed flax have declined. However, oilseed flax is still widely used as a raw material in the industry, medical treatment, healthcare, and other fields. It is rich in α-linolenic acid and other functional components that are increasingly becoming better known for maintaining good human health. Therefore, due to the continuous increase in domestic rigid demand, the gap between production and demand continues to widen, and the Chinese oilseed flax trade deficit is becoming more significant [1,2,3]. The use of chemical fertilizers is considered to be one of the most effective agronomic measures to increase crop yield [4,5]. However, long-term application of chemical fertilizers may lead to soil degradation, a decrease in crop yield or an increase in environmental pollution, and an increase in production costs [6,7,8]. Therefore, exploring fertilization measures that can improve soil fertility, increase yield and efficiency, and are environmentally friendly is of great significance for sustainable agricultural production in farmland. Due to organic manure now being available as an alternative nutrient resource, the utilization rate of chemical fertilizers is declining [9]. Therefore, how to optimize fertilization measures, to use organic manure instead of chemical fertilizer during the crop production, to improve the planting efficiency and make up for the gap in demand of oilseed flax, and to protect the farmland environment is an urgent problem that needs to be solved. Organic agriculture is a potential alternative to chemical fertilizer production systems, and organic manure can ensure safe agricultural production [10]. Organic manure can promote plant growth, increase crop yield, and bring considerable economic and environmental benefits. Studies have shown that nitrogen reduction combined with organic manure could increase the nutrient supply of rice (Oryza sativa L.) growth and play a positive role in yield stability [11,12]. The substitution of chemical fertilizers for organic manure could significantly reduce nitrogen loss, increase the aboveground biomass and grain yield of maize (Zea mays L.), and lead to higher economic benefits [13,14]. In addition, the application of organic manure significantly improved the agronomic characteristics of the sorghum (Sorghum bicolor L. Moench) plant; increased its leaf area index and total biomass accumulation at a later growth stage; and maintained a higher photosynthetic capacity, which was beneficial to grain filling and promoted a high yield [15,16]. However, previous studies on organic manures replacing chemical fertilizers on flax have mostly focused on aspects such as the effects of crop rotation combined with the organic manure on grain yield [17], the optimization of the water consumption characteristics of oilseed flax [18], and the improved quality of oilseed flax [19], while there are few reports on the study of organic fertilizers on oilseed flax grain filling characteristics in dryland agricultural areas. We hypothesized that high application amounts of organic manure can improve the growth of oilseed flax and increase or maintain the grain weight and yield of oilseed flax with farmyard manure replacing chemical fertilizer completely. But how much change will occur for the crop growth and yield increase potential under organic manure treatments in different climatic years? To verify the hypothesis and answer the questions, a two-year field experiment was conducted from 2021 to 2022 in dry areas of the Loess Plateau of China. Therefore, the aim of this study was to explore the effects of different farm yard manure (FYM) on the growth and development, grain filling, and yield of oilseed flax, in order to select the types and optimal doses of manure for higher yield on a sustainable basis and the efficient fertilization of oilseed flax in dry areas of the Loess Plateau of China.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted from April 2021 to August 2022 at the experimental base of the Oil Crops Institute of the Gansu Academy of Agricultural Sciences (104°49′ E, 35°48′ N) in Dingxi City, Gansu Province, China. This location was a typical hilly and gully region of the Loess Plateau, with an average altitude of 2050 m, an annual average temperature of 7.25 °C, an average amount of annual sunshine hours of 2472.6 h, a frost-free period of about 140 d, and an average annual rainfall of 391.7 mm. The experimental site was a terraced field, and the soil type was yellow cotton soil. The basic soil nutrients are shown in

Table 1. Basic physical and chemical properties of the tested soil.

Year	Organic Matter (g kg−1)	Total N (g N kg−1)	Available N (mg N kg−1)	Total P (g P kg−1)	Available P (mg P kg−1)	Total K (g K kg−1)	Available K (mg K kg−1)	pH
2021	10.4	0.81	48.9	0.69	27.4	29.3	108	8.14
2022	9.21	0.75	46.9	0.68	27.1	26.9	176	8.31

.

The total precipitation from sowing to harvest in 2021 and 2022 was 182.7 mm and 129.1 mm, respectively. The average maximum temperatures during the planting year in 2020 and 2021 were 28.1 °C and 29.7 °C, respectively, and the average minimum temperatures during the planting year in 2021 and 2022 were 2 °C and 0.4 °C, respectively (Figure 1).

2.2. Experimental Design

A two-factor split-plot randomized block experimental design was used in the field experiment, with different types of fertilizers as the main factor: C, rotten poultry manure (N, 1.94%; P₂O₅, 1.19%; and K₂O, 0.85%); S, rotten sheep manure (N, 0.9%; P₂O₅, 0.5%; and K₂O, 0.45%); F, chemical fertilizer; and CK, no fertilization. Different amounts of fertilizer were used as the sub-factor: under the two equal amounts of N, the amounts were 112 kg ha⁻¹ and 225 kg ha⁻¹; the levels of N, P, and K followed the same nutrient content for each type of fertilizer, and any deviations in the nutrient levels of organic manure were corrected using calcium superphosphate and potassium sulfate. The phosphate fertilizer levels corresponded to 75 kg ha⁻¹ and 150 kg ha⁻¹ (P₂O₅), respectively, and the potassium fertilizer levels corresponded to 67.5 kg ha⁻¹ and 135 kg ha⁻¹ (K₂O), respectively. The specific fertilizer amounts are shown in

Table 2. Different fertilizer and organic manure amounts and organic manure deviation nutrient supplement application amounts.

Fertilizer	Fertilization Level	Amount of Organic Manure Application	N	P2O5	K2O
		(t ha−1)	(kg ha−1)	(kg ha−1)	(kg ha−1)
Poultry manure	C1	5.8	0	6	18.2
	C2	11.6	0	12	36.4
Chemical fertilizer	F1	0	112	75	67.5
	F2	0	225	150	135
Sheep manure	S1	12.5	0	12.5	11.2
	S2	25	0	25	22.4
CK	No fertilizer	0	0	0	0

. A total of seven treatments were performed with three replications, a total of 21 plots with a plot area of 2 m × 4 m = 8 m² were used, and the main plot was 19 m long and 5 m wide. The interval between each plot was 1 m, and deep 60 cm plastic film was used to prevent fertilizer diffusion. Two rows of adjacent treatment areas were not investigated as protective rows, and 2 m protective rows were planted around the experimental site. The tested variety was Zhangya 2, with a sowing rate of 7.5 million seeds·ha⁻¹, sown in rows at a depth of 3 cm and a row spacing of 20 cm. The previous two years’ crop stubble was wheat, and the basic soil fertility was essentially the same. The seeds were sown on 8 April 2021 and 27 April 2022, and both were harvested on 11 August.

The organic manure was made by Shijiazhuang Fengdi Fertilizer Co., Shijiazhuang, China. Urea (46% N) was used for nitrogen, calcium superphosphate (16% P₂O₅) was used for phosphate, and potassium sulfate (51% K₂O) was used for potash, all made by Gansu Liuhua (Group) Co., Lingxia, China. The amounts of nitrogen, phosphorus, and potassium used for the chemical fertilizers were the local recommended amounts, and the types and amounts of organic manure were determined by taking the total N content as the standard for determining the amounts of the other nutrients, based on percentages obtained from local commonly used manure from survey results, a nutrient analysis of organic manure, and previous research. All fertilizers were buried a week before sowing. The other fields were managed in the same way as the local general field.

2.3. Measurement and Calculation

2.3.1. Emergence Rate

The emergence status of oilseed flax under each treatment was observed from the date of sowing, with 1 m × 4 m selected as the investigation range in each plot at the seedling stage, and the seedling emergence rate was recorded.

$$\mathrm{E}\mathrm{R}=\mathrm{S}\mathrm{E}/\mathrm{S}$$

Here ER represents the emergence rate (%), SE represents the number of seedling that emerged, and S represents the number sowed.

2.3.2. Plant Height and Stem Diameter

Five representative plants with basically the same growth were selected at the seedling stage, the budding stage, the anthesis stage, the kernel stage, and the maturity stage. The length from the top to the base was considered the oilseed flax plant height, and the diameter of the base was considered the stem diameter; these were measured using a tape measure and a vernier caliper, respectively.

2.3.3. Dry Matter Accumulation

In each growth stage of oilseed flax, 10 plants with uniform growth were collected from each plot, and the different organs (stem, leaf, and fruit) were killed for 30 min in a thermostat at 105 °C and then dried at 80 °C to a constant weight. The accumulated dry matter of the oilseed flax organs at each growth stage was weighed and recorded.

2.3.4. Grain Filling

From the first day of oilseed flax flowering, basically the same growth and robust flower binding markers were selected in each plot. From the 14th day after flowering, 50 plant capsules marked on the same day were taken and sampled every 7 days until they matured. The grain filling rate was calculated using the following formula:

$$\mathrm{G}\mathrm{F}\mathrm{R}=\mathrm{D}\mathrm{M}\mathrm{I}/\mathrm{I}\mathrm{D}$$

where GFR represents the grain filling rate (g·d⁻¹), DMI represents the dry matter increment for each determination (g), and ID represents the interval between days (d).

Taking the days after anthesis (t) as the independent variable and grain weight (y) as the dependent variable, a logistic equation, y = K/(1 + a·e^−bt), was used to fit the process of grain filling, in which K (theoretical maximum grain weight), a, and b were equation-fitting parameters. By deriving the equation, the following characteristic parameters could be obtained:

The days when maximum filling rate was reached (d): $\mathrm{T}-\mathrm{m}\mathrm{a}\mathrm{x}=\mathrm{l}\mathrm{n}\mathrm{a}/\mathrm{b}$;

The maximum filling rate (g·d⁻¹): $\mathrm{V}-\mathrm{m}\mathrm{a}\mathrm{x}=\mathrm{K}\mathrm{b}/4$;

The biomass at the maximum filling rate (g): $\mathrm{W}-\mathrm{m}\mathrm{a}\mathrm{x}=\mathrm{K}/2$;

The filling active period (d): $\mathrm{D}=6/\mathrm{b}$.

2.3.5. Grain Yield and Its Components

During the mature period, 20 oilseed flax plants were randomly sampled from each plot, and the number of branches of oilseed flax, the number of branches on the main stem, the number of effective capsules, the number of grains per fruit, and the 1000-grain weight were measured. It was harvested separately according to the plot, the actual yield was measured after drying, and the yield per hectare was converted according to the actual area. The economic performance was calculated using the following formula:

$$\mathrm{E}\mathrm{L}\mathrm{P}=\mathrm{G}\mathrm{Y}\times \mathrm{P}/\mathrm{S}\mathrm{Y}\times \mathrm{P}$$

where ELP represents the economic land productivity (CNY ha⁻¹), GY represents the grain yield (kg ha⁻¹), SY represents the straw yield (kg ha⁻¹), and P represents the unit price.

$$\mathrm{N}\mathrm{I}=\mathrm{E}\mathrm{L}\mathrm{P}-\mathrm{C}$$

Here NI represents the net income (CNY ha⁻¹), and C represents the total cost (CNY ha⁻¹).

2.4. Statistical Analyses

The value of each indicator was the mean of three replicates per treatment. Data pre-processing was carried out using Excel 2016. The figures were plotted using Origin 2021b (OriginLab Corp., Northampton, MA, USA). The R language was used for correlation results, a nutrient analysis of organic manure and previous research All which the data of each index passed the normality test, and all pairwise multiple comparisons of the treatment means were performed using the least significant difference (LSD) test, with significance determined at the 5% level.

3. Results

3.1. Seedling Emergence Rate

The results of our statistical analysis on the emergence rate of oilseed flax showed that the seedling emergence rate (ER) was significantly affected by different fertilizer types and amounts in 2021 and 2022 (p < 0.05, Figure 2). The ER of oilseed flax was 7.47% higher in 2021 than in 2022; the rates were, on average, 29.97% and 15.00% higher in the sheep manure (S) and poultry manure (C) treatments than in the chemical fertilizer (F) treatments and 34.38% and 18.97% higher than that of no fertilization (CK), respectively. This indicates that the organic manure significantly increased the seedling emergence rate of oilseed flax. For the different amounts of 25 t ha⁻¹ sheep manure (S2), the ER was significantly 12.19% higher than that of the 12.5 t ha⁻¹ sheep manure (S1) treatment and 6.15–36.62% higher than that of the other treatments. Under the chemical fertilizer treatment with 225 kg N·ha⁻¹ and 150 kg P₂O₅·ha⁻¹ (F2), the ER was significantly reduced by 26.63% compared with that of the treatment with 225 kg·ha⁻¹ and 75 kg P₂O₅·ha⁻¹ (F1) and significantly reduced by 3.47–18.01% compared with the other treatments. It could be seen that the seedling emergence rate of oilseed flax was closely related to fertilization, and organic manure effectively ensured a good seedling emergence rate of oilseed flax.

3.2. Plant Height and Stem Diameter

The results showed that the fertilizer types and amounts had significant effects on the growth of oilseed flax (Figure 3). The plant height and stem diameter of oilseed flax in 2021 were 3.32–7.13% and 10.24–11.35% higher than those in 2022, respectively. The plant heights under the S and C treatments were significantly higher, by 3.91% and 5.59%, respectively, than that under the F treatment, on average, and 15.73% and 17.60% higher, respectively, than that under CK during the whole growth period of oilseed flax within the two years. Compared with that under the organic manure treatments, the plant height of the F treatments at the seedling and budding stages increased, on average, by 4.85% and 2.96% (S) and by 4.49% and 3.16% (C), respectively, while it increased by 6.67% and 8.99%, respectively, under the S and C treatments compared with the F treatments at the anthesis to maturity stages.

The stem diameters under the S and C treatments were significantly higher, on average, by 13.69% and 5.59%, respectively, than those of the F treatments and by 21.32% and 12.67%, respectively, than that of the CK treatment. There were no significant differences in the stem diameters among the different treatments at the seedling stage of oilseed flax, but they were higher, on average, by 11.12% under the S treatment at the budding to kernel stages and increased by 11.80% and 4.94% under the S and C treatments, respectively, at the maturity stage compared with that of the F treatments. There were no significant differences in the plant heights between the S2 and C2 (11.6 t ha⁻¹poultry manure) treatments, but that for the C2 treatment was significantly higher, by 2.00–18.06%, than those for the other treatments. The stem diameter of the S2 treatment was higher, by 6.51%, than that of the S1 treatment and 7.61–25.14% higher than those of the other treatments. This indicated that high amounts of organic manure significantly promoted plant growth, in terms of height and stem diameter, in oilseed flax.

3.3. Dry Matter Accumulation and Distribution

Fertilization significantly affected the dry matter accumulation (DMA) and showed an increasing trend in the two growing seasons of oilseed flax (Figure 4). The DMA of plants in the whole growth period of oilseed flax in 2021 was 7.63% higher than that in 2022, increasing by 12.58% from the budding stage to the kernel stage. The DMA in the whole growth period of oilseed flax indicated a ranking of S > C > F > CK, and the trend was the same in both years, with the DMA from the S and C treatments being 10.48% and 6.21% higher than that from the F treatment (p < 0.05, the same below), respectively, and 31.19% and 26.11% higher than that from the CK treatment. Among these treatments, the DMA under the S and C treatments was higher than that under the F treatment by 14.04% and 15.09% at the anthesis stage and higher by 13.13% and 7.64% at the maturity stage, respectively. The DMA of oilseed flax increased rapidly from the anthesis stage to the kernel stage. And in this period, it was higher under the S and C treatments than under the F treatment by 9.93% and 5.17% in 2022, respectively, and higher under the S treatment than under the F treatment by 8.21% in 2021. For the different amounts, the DMA of the S2 treatment significantly increased by 14.60% compared with that of the S1 treatment, and it was 2.47–40.11% higher than that of the other treatments.

With growth, the dry matter distribution ratio (DMD) of the oilseed flax stem increased at first and then decreased, the DMD of the leaf decreased gradually, and the DMD of the capsule increased gradually (Figure 5). The DMD of the oilseed flax stem was the largest at the anthesis stage, with an average ratio of 61.12–66.37% within two years, while that in the leaves at the seedling stage was 64.32–70.81%, and that in the capsules at the maturity stage was 47.41–52.22%. There were no significant differences in the organ distribution ratio between the C and F treatments, while the stem DMD of the S treatment decreased by 4.71%, on average, compared with that of the F treatment, but the DMD of the leaves and capsules was significantly higher, by 3.72% and 6.38%, than that of the F treatment, respectively (p < 0.05, the same below). Fertilization had a significant effect on the DMD of oilseed flax at the anthesis and kernel stages, the stem DMD of the C treatment was the highest for those two periods, the DMD of the leaves under the S treatment was 3.94% and 6.80% higher than that of the F treatment, and the DMD of the capsules was, on average, significantly higher, by 20.78% and 9.02%, than that of the C treatment and 12.38% and 4.83% higher than that of the F treatment, respectively. For the different amounts, the DMD of oilseed flax capsules under the S2 treatment had no significant difference from that under the S1 treatment, which was significantly higher, by 5.70–12.58%, than that of the other treatments. The results showed that organic manure could increase the distribution ratio of various organs at varying degrees. The treatment of 25 t ha⁻¹ sheep manure significantly increased the accumulation of dry matter before anthesis and promoted the distribution ratio of dry matter to grains after anthesis of oilseed flax.

3.4. Grain Filling Characteristics

3.4.1. Filling Rate

The responses of the oilseed flax grain filling rate (GFR) to the fertilizers varied with grain filling. The filling rate rapidly increased at 14 d after anthesis, and it reached the maximum at about 21 d after anthesis and then decreased continuously (Figure 6). Different years, fertilizer types, and amounts had significant effects on the GFR of oilseed flax 21 d after anthesis. There were significant interaction effects between Y (year) × F (types of fertilizers) and F × L (fertilizer amount level) (

Table 3. Analysis of the interaction effects of different treatments on the filling rate of oilseed flax.

Treatment	7 Days after Anthesis	14 Days after Anthesis	21 Days after Anthesis	28 Days after Anthesis	35 Days after Anthesis	42 Days after Anthesis
Y	0.09	6.35 *	18.13 **	39.2 **	40.11 **	37.04 **
F	6.89 **	6.96 **	22.43 **	52.22 **	27.5 **	43.01 **
L	0.46	0.54	13.75 **	12.35 **	10.42 **	29.81 **
Y × F	3.51 *	0.01	0.91	9.78 **	4.83 **	3.71 *
Y × L	3.31	3.09	1.34	0.84	0.41	0.21
F × L	2.07	2.06	9.57 **	10.7 **	8.68 **	19.71 **
Y × F × L	0.51	0.01	0.47	1.26	3.66 *	3.19

Y: year; F: types of fertilizers; L: fertilizer amounts level. *, ** indicate significant, or extremely significant between treatments at p < 0.05, or, p < 0.01, respectively. (The same below.).

). The GFRs of the S and C treatments were, on average, 9.14% and 4.51% higher than that of the F treatment, which was 18.63% and 13.60% higher than that of the CK within the two years, respectively. For the different amounts, the GFR of S2 treatment was significantly higher, by 8.25%, than that of the S1 treatment and 5.54–23.33% higher than that of the other treatments. The GFRs of the S and C treatments 21 d after anthesis were, on average, higher than that of the F treatment by 9.94% and 5.57% in two years, respectively. Among them, the GFR of the S treatment was, on average, 14.30% higher than that of the F treatment 42 days after anthesis, but there were no significant differences between the C and F treatments. The GFRs under S2 treatments 21 days and 42 days after anthesis were significantly higher, by 7.07–17.94% and 12.14–44.39%, respectively, than those under the other treatments.

3.4.2. Filling Characteristic Parameters

The difference in the characteristic parameters of oilseed flax filling under different fertilization treatments fitted by the logistic equation is shown in

Table 4. Effects of different fertilization treatments on the filling characteristic parameters of oilseed flax.

Year	Treatment	Grain Filling Fitting Equation	R2	F	D/d	V-Ave (g·d−1)	T-Max/d	V-Max (g·d−1)	W-Max/g
2021	C1	y = 4.55/(1 + 32.50e−0.204t)	0.994	788.21 **	29.41	0.111	17.06	0.23	2.27
	C2	y = 4.70/(1 + 29.89e−0.199t)	0.988	426.70 **	30.15	0.116	17.07	0.23	2.35
	S1	y = 4.60/(1 + 33.39e−0.203t)	0.994	772.94 **	29.56	0.112	17.28	0.23	2.30
	S2	y = 5.00/(1 + 30.23e−0.197t)	0.995	957.00 **	30.46	0.122	17.30	0.25	2.50
	F1	y = 4.41/(1 + 29.65e−0.202t)	0.989	460.76 **	29.70	0.109	16.78	0.22	2.21
	F2	y = 4.22/(1 + 29.56e−0.209t)	0.988	453.01 **	28.71	0.104	16.20	0.22	2.11
	CK	y = 3.88/(1 + 31.17e−0.206t)	0.986	353.97 **	29.13	0.096	16.70	0.2	1.94
2022	C1	y = 4.15(1 + 39.98e−0.231t)	0.992	613.32 **	25.97	0.103	15.97	0.24	2.08
	C2	y = 4.38/(1 + 29.78e−0.201t)	0.988	418.12 **	29.85	0.108	16.88	0.22	2.19
	S1	y = 4.28/(1 + 28.30e−0.198t)	0.986	437.01 **	30.30	0.106	16.88	0.21	2.14
	S2	y = 4.51/(1 + 32.15e−0.205t)	0.985	304.94 **	29.27	0.113	16.93	0.23	2.25
	F1	y = 4.22/(1 + 30.53e−0.208t)	0.99	524.87 **	28.85	0.105	16.44	0.22	2.11
	F2	y = 4.22/(1 + 33.10e−0.208t)	0.991	540.12 **	28.85	0.105	16.82	0.22	2.11
	CK	y = 3.87/(1 + 35.180e−0.213t)	0.987	493.88 **	28.17	0.096	16.72	0.21	1.94

V-ave: average filling rate (g·d⁻¹); T-max: days when maximum filling rate is reached (d); V-max: maximum grain filling rate (g·d⁻¹); W-max: biomass at maximum grain filling rate (g); D: filling active period (d). ** indicates extremely significant between treatments at p < 0.01. (The same below.).

. The results show that there were significant positive correlations between the average filling rate (V-ave), the maximum filling rate (V-max), the biomass at the maximum filling rate (W-max), and the 1000-grain weight and that the organic manure significantly affected the related parameters. The V-ave and V-max of the S treatment were, on average, 7.39% and 7.69% higher than those of the F treatment and 18.19% and 18.62% higher than that of the CK treatment, and those of the C treatment were 3.76% and 4.17% higher than those of the F treatment and 14.20% and 14.75% higher than those of the CK treatment, respectively. In 2021, the V-max of the S treatment was 2.94% and 8.18% higher than that of the C and F treatments, while that of the C treatment was 3.87% and 4.80% higher than that of the S and F treatments in 2022, respectively. For the S2 treatment, V-ave, V-max, and W-max were 7.97%, 7.11%, and 7.01% higher than that of the S1 treatment and 4.84–22.72%, 1.16–17.54%, and 4.58–22.63% higher than that of the other treatments, respectively. The filling active period (D) and days at maximum filling rate (T-max) were not significantly correlated with the 1000-grain weight (Figure 7). The D of the S treatment was, on average, 3.00% longer than that of the F treatment. There were no significant differences between the C and F treatments. In summary, the amount of 25 t ha⁻¹ sheep manure significantly increased the GFR, optimized the characteristic parameters in the process of oilseed flax grain filling, prolonged the days of active grain filling, and was beneficial in increasing the grain weight.

3.5. Yield and Components

Different fertilizer types and amounts had significant effects on the oilseed flax grain yield and the yield components. Their interactions were significant, and there were differences between years (

Table 5. Effects of different fertilization treatments on oilseed flax yield and its components.

Year	Treatment	Stem Number/Per	Branch Number of Main Stem/Per	Effective Capsule Number/Per	Seed Number Per Capsule/Seed	1000-Grain Weight/g	Grain Yield (kg·ha−1)
2021	C1	1.29 d	7.43 c	16.33 b	8.52 a	8.50 ab	1242.12 cd
	C2	1.33 d	7.71 bc	17.19 b	8.10 ab	8.12 c	1454.63 ab
	S1	1.67 b	7.97 bc	16.62 b	8.10 ab	8.52 ab	1335.83 bc
	S2	1.98 a	8.33 ab	19.52 a	7.62 bcd	8.73 a	1630.31 a
	F1	1.73 b	6.33 d	13.52 c	7.46 cd	8.25 bc	1331.17 bc
	F2	1.51 c	8.81 a	17.18 b	7.95 abc	8.32 bc	1044.72 e
	CK	1.62 bc	4.52 e	12.29 c	7.14 d	7.97 c	1116.06 de
2022	C1	1.69 b	5.33 ab	13.44 b	7.44 ab	8.17bc	964.12 bc
	C2	1.90 a	5.22 b	14.33 b	7.11 b	8.58 ab	1149.51 b
	S1	1.46 c	5.56 ab	14.33 b	8.00 ab	8.22 abc	1086.04 bc
	S2	1.23 d	6.11 a	16.33 a	8.89 a	8.52 abc	1339.24 a
	F1	1.67 b	5.11 b	13.11 b	7.89 ab	8.63 a	1084.41 bc
	F2	1.44 c	5.56 ab	13.00 b	8.33 ab	8.17 bc	944.64 c
	CK	0.98 e	4.78 b	11.44 c	8.11 ab	8.12 c	906.12 c
Y		0.02	195.88 **	103.13 **	1.24	0.08	52.65 **
F		3.39 *	27.41 **	41.49 **	1.09	5.25 **	15.20 **
L		3.10	22.79 **	46.2 **	0.25	0.11	6.30 *
Y × F		4.55 *	9.94 **	1.01	6.09 **	2.31	0.82
Y × L		0.46	7.14 *	9.59 **	1.42	0.67	0.14
F × L		0.24	8.56 **	3.34 *	1.62	3.50 *	19.26 **
Y × F × L		10.12 **	5.62 **	5.24 *	1.30	7.37 **	0.77

*, ** indicate significant, or extremely significant between treatments at p < 0.05, or, p < 0.01, respectively. Different lowercase letters represent significance at the 0.05 level between different treatments in the same column in different years.

). Under the S and C treatments, the grain yields were significantly increased by 22.40% and 9.20% compared with that of the F treatment and increased by 33.31% and 18.94% compared with that of the CK treatment, respectively. The number of effective capsules under the S and C treatments was significantly higher, by 17.60% and 7.91%, than that under the F treatment and 40.77% and 29.16% higher than that under the CK treatment, respectively. There were no significant differences between the number of stems among the different treatments, and the number of branches on the main stem and grains per capsule under the S treatment were, on average, significantly increased by 8.36% and 3.06%, respectively, compared with the F treatment, while there were no significant differences between the C and F treatments. The 1000-grain weights of the S and C treatments were significantly higher, by 4.11% and 3.37%, respectively, than that of the F treatments in 2021, and there were no significant differences among the treatments in 2022. For the different amounts, the F1 treatment had the highest number of branches. The number of branches on the main stem, the effective capsules, and the seeds per capsule were highest for the S2 treatment and were significantly higher by 6.81–55.29%, 13.75–51.10%, and 1.36–8.56%, respectively, compared with those of the other treatments. The 1000-grain weight and grain yield of the S2 treatment increased by 3.05% and 22.61%, respectively, compared with those of the S1 treatment, which were significantly higher, by 2.15–7.21% and 14.03–46.85%, respectively, than those of the other treatments. It could be seen that the amount of 25 t ha⁻¹ sheep manure could significantly increase the yield components and the grain yield of oilseed flax.

3.6. Correlation Analysis between Grain Yield and Its Components

In order to clarify the contribution of different fertilizer treatments to the grain yield of oilseed flax, the grain yield and its components were analyzed via principal component analysis (Figure 8). The results showed that the accumulation of PC1 and PC2 represented 85% (2021) and 86.6 (2022) of the total variance, which covers most of the original variance in the sample. The trends of the two growing seasons were similar. The angle between the 1000-grain weight, the number of branches on the main stem, the effective capsules, and the grain yield was small, which indicated that they made great contributions to the grain yield, but there was not a significant correlation between the grain yield and the number of grains per capsule and number of stems (Figure 9). Therefore, the number of effective capsules, the number of branches, and the 1000-grain weight of oilseed flax could be increased by improving fertilization measures, which would then increase the grain yield of oilseed flax.

3.7. Economic Benefit Analysis

We can see from

Table 6. Cost benefit analysis of planting.

Year	Treatment	Planting Cost (CNY ha−1)			Yield Increase Rate (%)	ELP (CNY ha−1)	Net Income (CNY ha−1)
		Fertilizer	Other	Total
2021	C1	1740	750	2490	11.30	12,288.14	9798.14 b
	C2	3480		4230	30.34	14,238.32	10,008.32 b
	S1	2500		3250	19.69	13,137.90	9887.90 b
	S2	5000		5750	46.08	15,893.94	10,143.94 ab
	F1	822		1572	19.27	13,200.60	10,432.34 a
	F2	1644		2394	−6.39	9958.58	5172.07 d
	CK	0		750	0	10,029.01	9279.01 c
2022	C1	1740	1070	2810	6.40	9813.97	7003.97 d
	C2	3480		4550	26.86	12,097.51	7547.51 c
	S1	2500		3570	19.86	11,189.59	7619.59 bc
	S2	5000		6070	47.80	14,015.54	7945.54 ab
	F1	822		1892	19.68	11,176.52	8088.26 a
	F2	1644		2714	4.25	9207.94	4101.43 e
	CK	0		1070	0	8249.46	7179.46 d

Different lowercase letters represent significance at the 0.05 level between different treatments in the same column in different years. Other = manual and mechanical costs; yield increase rate = (grain yield of fertilization treatment − grain yield of CK)/CK.

that the total planting cost in two years was 750–6070 CNY ha⁻¹, the economic land productivity (ELP) was 8249.46–15,893.94 CNY ha⁻¹, and the net income was 4101.43–10,432.34 CNY ha⁻¹. The overall income from the sheep manure treatments increased. Under the S2 treatment, the grain yield increase rate and ELP were the highest in the two years, while the grain yield increase rate, ELP, and net income of the F2 treatment were the lowest. The net income directly reflected the economic benefit of oilseed flax planting, and the net income of the S2 treatment was not significantly different from that of the F1 treatment but was significantly higher, by 3.04–95.07%, than that of the other treatments.

4. Discussion

Reasonable fertilization measures are the foundation of high and stable crop yields. Organic manure could improve the physical and chemical properties of farmland soil and effectively ensure the balanced emergence rate of crops [20,21]. Through the study of wheat (Triticum aestivum L.) seedling emergence with different nitrogen sources, Khan et al. reported that the organic manure was more beneficial to seed emergence compared with inorganic N fertilizer [22]. The organic manure could improve soil water and heat conditions to promote rice seed germination [23], slow down the effect of low-temperature stress on crop emergence and seedling preservation [24], and even reduce exuberant weed growth in the crop field after the application of organic manure [25]. Therefore, organic manure is beneficial to plant emergence and seedling protection. In the current study on flax, it was found that the application of organic manure instead of chemical fertilizers could increase the emergence rate of oilseed flax, and the effect was obvious with the increase in the fertilizer amount [26]. In the current study, the treatments with sheep manure and poultry manure significantly increased the emergence rate of oilseed flax by 29.97% and 15.00% compared with that of the chemical fertilizers, respectively. The emergence rate of oilseed flax under the 25 t ha⁻¹ sheep manure treatment was significantly higher than that of the other treatments. The reasons may be as follows: First, mature organic manure is rich in organic matter, humus and iron, manganese, boron, and other trace elements, which enhance microbial activity and are conducive to seed germination [27]. Second, the application of organic manure could improve soil ventilation and water retention; effectively increase field capacity after sowing; and preserve heat, providing a relatively constant temperature and ensuring the emergence rate [27,28]. Factors affecting germination or seedling injury from seed-placed fertilizers include crop sensitivity, precipitation after seeding, organic matter, soil texture and soil moisture, fertilizer source and rate, etc. [29]. The emergence of seedlings in 2022 was low on the whole mainly because the soil moisture was generally poor when sowing in that year, which affected the emergence of seedlings. Soil moisture is important to take into account when applying fertilizer directly with the seed, while there is significant stand loss as a result of relatively dry soil conditions [30]. Fertilizer sources that cause the highest amount of damage to seedlings are those that release free NH₃, especially urea, and a large ion concentration around the roots of crops will move water out of the plant cells; therefore, salt damage from fertilizer is often more severe on dry soils [31]. The emergence rate was the lowest with 225 (N) kg ha⁻¹ of the chemical fertilizer, which may be due to excessive fertilization, which caused a large amount of ammonia in the soil, thus easily burning embryos or young roots and being not safe for seedling emergence and seedling preservation [32].

DMA and plant morphogenesis were the bases upon which the yield was assessed. Plant height and stem diameter, as important agronomic characteristics, are closely related to dry matter accumulation, thus affecting yield [33,34]. The application of organic manure could significantly improve plant growth and promote DMA [35,36]. A previous study showed that organic manure had no obvious effect on the early growth stage of oilseed flax, but with more advanced growth stages, the plant height, stem diameter, and DMA were significantly accelerated [19]. In our study, the application of organic manure significantly increased the plant height and stem diameter of oilseed flax, in which poultry manure increased the plant height and sheep manure was beneficial to the stem diameter of oilseed flax, which may be related to the higher levels of nitrogen in the poultry manure and the slow-acting property of sheep manure [37]. Furthermore, Cicek et al. reported that poultry manure had more effects on basil (Ocimum basilicum L.) plant height, while sheep manure affected its dry weight [38]. This indicates that the application of organic manure can promote the overall morphological formation of oilseed flax plants. In the current study, organic manure could promote the absorption of nutrients by oilseed flax plants, improve the quality of crop population development, and increase the accumulation of dry matter. The dry matter of each treatment showed an increasing trend at the early growth stage of oilseed flax, and there were significant differences after the anthesis stage: the organic manure significantly increased the DMA of oilseed flax in the whole growth period and promoted the DMA in the key growth period, which was basically consistent with the results of previous studies. The sheep manure and poultry manure treatment during the whole growth period significantly increased the DMA by 10.48% and 6.21% compared with the chemical fertilizer treatment, and the 25 t ha⁻¹ sheep manure treatment had the highest DMA. Therefore, a scientific and reasonable fertilization system is beneficial to the growth, development, and DMA of oilseed flax and improves the yield in the later stages. In addition, soil organic matter, alkali-hydrolyzable N, available phosphorus, and available potassium are important sources of plant nutrients [39,40], and the amount of soil nutrients increased significantly after organic manure treatment, which played a positive role in improving the soil activity [41]. The poultry manure and sheep manure are rich in nutrients, which increase the soil organic matter and the activities of the microorganisms; accelerate the mineralization rate of nitrogen, phosphorus, and potassium in the soil; and provide available phosphorus and potassium for crop growth in the current season to meet the crop growth needs, and the release of alkali-hydrolyzable N also increases with the mineralization process of organic manure [42]. After the application of sheep manure, the number of soil actinomycetes was significantly higher than that after poultry manure application. As an important indicator of soil fertility, actinomycetes could not only decompose and transform soil organic matter but also antagonize other harmful bacteria [43,44]. The above may be an important reason why the application of organic manure significantly increased the DMA of oilseed flax plants and for the significant effect of the sheep manure treatment; however, an investigation into those reasons is outside the scope of this article and should be the subject of further experiments. Rational fertilization was beneficial to coordinate the relationship between the crop source and sink, increased the crop DMA, and promoted the distribution of aboveground dry matter to capsule, thus increasing the yield [45]. In the current study, with advanced oilseed flax growth, the stem DMD increased at first and then decreased, the leaf ratio decreased, the capsule gradually increased, and the distribution ratio of the sheep manure and poultry manure to the various organs was higher than that of the chemical fertilizer. Among the treatments, the 25 t ha⁻¹ sheep manure treatment significantly promoted the accumulation of dry matter before anthesis and the distribution of dry matter to grains after anthesis, which was also confirmed by Cui et al. on oilseed flax [45]. It can be seen that the 25 t ha⁻¹ sheep manure treatment was beneficial in increasing the stock in the early stage and the later stage of oilseed flax, promoting the coordinated development of the yield components, and then increasing the yield of oilseed flax.

Fertilization is an effective measure to increase the crop yield. An et al. [46] investigated the present situation of the resource utilization of livestock and poultry manure in Dingxi City, Gansu Province, China, and found that the annual production of sheep manure and poultry manure were as high as 2.539 million t/a and 4.92 million t/a, respectively, and suggested that the technology of returning manure to the field should be vigorously popularized, which can not only improve soil fertility but also save cost and increase yield, and the manure mature technology has been widely used in the local area [47]. Related studies have shown that organic manure instead of chemical fertilizers could significantly improve crop productivity, and the crop yield increasing effect of sheep manure is better than that of poultry manure [48,49]. In our research, the organic manure treatments significantly increased the oilseed flax grain yield, with the 25 t ha⁻¹ sheep manure treatment increasing the yield by 14.03% to 46.85% compared with the other treatments. Therefore, the use of nitrogen, phosphorus, and potassium was not the main factor affecting oilseed flax yield under the same nutrient conditions; the reason for the higher yield under the organic manure treatments may be due to the micronutrient elements in the organic matter, especially boron (B) and iron (Fe) [50]. It is worth noting that there are few reports about micronutrient elements in organic manures on crop yield formation, so further research is needed to reveal their contributions to crop yield. Li et al. [51] simulated the oilseed flax grain formation using the yield component method and found that the number of capsules per unit area was the biggest factor affecting the yield, and when the number of capsules per unit area reached a certain number and the yield was relatively high, the number of seeds per capsule and grain weight had significant effects on the yield. This study showed that the grain yield was significantly positively correlated with the 1000-grain weight, the number of branches on the main stem, and the number of effective capsules but not significantly correlated with the seed number per capsule. In the current study, the application of organic manure could significantly increase the number of branches on the main stem, the number of effective capsules, and the 1000-grain weight in the components of the oilseed flax yield. The fertilizer efficiency of organic manure was long, and the effect of slow nutrient release was helpful to reduce fertilizer nutrient loss, which could supply nutrients for the growth of oilseed flax even in the later stage, could promote the early growth of branches, could increase the number of effective capsules, and could then promote the formation of oilseed flax yield. Studies had reported that the application of organic manure combined with a chemical fertilizer could increase the maximum grain filling rate and average grain filling rate of buckwheat (Fagopyrum esculentum Moench) at the strong grain filling stage [52]; increase the grain yield and straw yield of sunflower (Helianthus annuus) [53]; increase the grain filling rate of oilseed flax seed; prolong the days of active grain filling; and increase 1000-grain weight, effective capsules per plant, and grain yield [45,54]. The results of this experiment showed that among the characteristic parameters of oilseed flax grain filling, the V-ave, the V-max, and the W-max were positively correlated with the grain weight. The organic manure significantly increased the GFR, W-max, V-ave, and V-max and prolonged the D, which indicates that the application of organic manure could significantly increase the grain weight and lays the foundation for the increase in yield, which is basically consistent with the research results of previous studies.

Not only reducing the amount of chemical fertilizers used in agricultural production while ensuring increases in crop yield but also increasing the economic benefits for farmers are key to judging whether a fertilization measure is feasible in agricultural production [55]. In the current study, the net income of the 25 t ha⁻¹ sheep manure treatment was second only to the chemical fertilizer at 112 (N) kg ha⁻¹, which is a significant increase of 3.04% to 95.07% compared with that of other treatments, while the net income of the 225 (N) kg ha⁻¹ chemical fertilizer treatment was the lowest, indicating that the application of sheep manure could obtain a higher income. The economic benefit was the highest for the 112.5 (N) kg ha⁻¹ chemical fertilizer treatment, but a large amount of chemical fertilizer would reduce the yield and income and bring harm to the agricultural ecological environment. Considering the economic cost and environmental benefit, the 25 t ha⁻¹ sheep manure treatment instead of a chemical fertilizer could be used as a more suitable green and efficient fertilization method in the test area. Organic agriculture is a sustainable production system that has a positive impact on biodiversity, the biological cycle, and soil microbial activities. The use of organic manure instead of chemical fertilizer to improve the adverse effects of the excessive application of chemical fertilizers leads to the need to develop green agriculture [56,57]. Although organic manure could reduce the economic benefit of oilseed flax to a certain extent, it had a significant effect on the grain yield and quality in oilseed flax. Considering that organic manure is a slow process in terms of soil fertility, long-term application may have a better effect on the grain yield and income of oilseed flax and on improvements to farmland environments. And only agronomic characteristics were analyzed in this study. Oilseed flax is one of the most important plant sources of the essential fatty acid α-linolenic acid for the human body, and the regulatory effects of organic manure on quality still need to be further explored. On this basis, long-term experiments should be carried out to clarify the long-term effects of organic manure on crops and the environment. In addition, in organic manure, it often occurs that the manure does not mature completely, and there are hidden dangers such as bacteria, disease, and insect eggs, which result in a certain negative effect on yield. Therefore, further optimization of these fertilization measures, reduction of the amount of organic manure under the conditions of this experiment, and combination with chemical fertilizer application can not only meet the short-term demand for available nutrients of crops but also realize the long-term supply of nutrients; it may better meet the growth of crops and the appeal for green environmental protection.

5. Conclusions

Different fertilizer types and amounts had significant effects on oilseed flax growth and development, grain filling, and yield. The organic manure effectively ensured the emergence rate of oilseed flax, poultry manure increased the plant height, and sheep manure was beneficial to thickening. Among the treatments, the 25 t ha⁻¹ sheep manure treatment promoted the growth and development of oilseed flax plants; the dry matter accumulation was 2.47–40.11% higher than that of the other treatments, optimized its grain filling characteristics, and increased its grain yield per unit area and net income, which were significantly higher, by 14.03–46.85% and 3.04–95.07%, respectively, than those of other treatments. Mature organic manure instead of chemical fertilizer can effectively reduce the amount of chemical fertilizer, can reduce the production input, and is expected to improve the farmland ecological environment. Therefore, the application of 25 t ha⁻¹ sheep manure treatment can be used as a suitable green, high-yield, and high-efficiency fertilization management technique in the experimental area.