The Influence of Sowing Date and Seeding Density on the Yield of Soybean Glycine max (L.) Merrill

Abstract

The current study aimed to determine the optimum agronomic conditions—specifically sowing date and seeding density—for soybean cultivation in a temperate climate. A field experiment was conducted to evaluate three sowing dates based on soil temperature (S1—9 °C, S2—12 °C, S3—15 °C) and three seeding densities (D1—50, D2—70, D3—90 seeds·m⁻²). A field experiment was conducted in the years 2017–2019 in eastern Poland (Central Europe). Yields were strongly influenced by weather conditions. In 2019, the average yield was 2.61 Mg·ha⁻¹, making it the most favorable year, whereas 2018 was the least favorable, with an average yield of 1.41 Mg·ha⁻¹. Seeding density also affected soybean yields—the highest yield was obtained at the medium density (D2—2.36 Mg∙ha⁻¹). On the other hand, the highest thousand seed weight (159.30 g·m⁻²) was achieved at the lowest density (D1). Plant height and pod length depended on the sowing date. The tallest plants (69.96 cm) and the longest pods (4.55 cm) were obtained with early sowing (S1). The number of seeds per pod ranged from 1.8 to 2.7, with the highest values recorded in 2017, mainly with early sowing (S1) and low density (D1). It is recommended that sowing strategies be flexibly adjusted to the meteorological conditions of a given season. The findings indicate that appropriate selection of sowing parameters can significantly enhance the efficiency and stability of soybean yields under the variable climatic conditions of Poland.

1. Introduction

Soybean (Glycine max L. Merrill) is one of the most important leguminous crops cultivated worldwide. In temperate climates, including Poland and other Central and Eastern European countries, soybean cultivation is gaining increasing significance in the context of agricultural diversification, improvement of crop rotation systems, and the reduction of reliance on imported soybean meal. Despite its numerous advantages, the success of soybean cultivation in new geographic regions depends on a range of agronomic, climatic, and soil-related factors [1,2,3]. Climate change in Poland generally contributes to expanding soybean cultivation, as milder winters and a warmer, longer growing season allow the crop to mature even in regions previously considered too cold. Soybean, a heat-loving crop, benefits from a longer growing season and higher spring and summer temperatures, creating better conditions for maturing, especially for medium-early cultivars. The optimal sowing date is primarily influenced by local climatic conditions [4,5]. Among the agronomic factors influencing yield, sowing date and seeding density are of particular importance [6,7,8,9]. Appropriately selected sowing dates, based on soil temperature, promote uniform emergence and proper plant development. Sowing too early can inhibit emergence, while sowing too late can lead to a shorter growing season and yield reduction. Hence, well-selected sowing time and seeding density allow plants to make the most of the available growing season, resulting in an extension of both the vegetative and generative phases [10,11]. However, early-sown soybean seeds are more vulnerable to extremely low temperatures and late spring frosts, which can inhibit germination and seedling emergence. Conversely, delayed sowing increases the risk of drought-related damage [12,13]. A well-chosen sowing density allows plants to optimally utilize space, light, and water, which translates into an increased pod number, seed weight, and total yield. At moderate seeding densities, plants have sufficient space and access to light, which promotes uniform development and better root establishment. Research [14,15] indicates that in the north-central United States, maximum yield is generally achieved when sowing occurs in mid-May, but yields drop significantly if planting is delayed until late May or early June. Egli and Bruening [16] reported that early sowing combined with lower seeding density promotes individual plant development, with more pods and greater thousand-seed weight. However, under conditions of a cold spring—when early sowing may delay emergence and lead to uneven plant growth—a higher plant density can offset yield losses due to poor space utilization. In contrast, late sowing, which shortens the vegetative growth period, requires appropriate plant density adjustment [7,12,17,18]. In such cases, a higher plant density may be beneficial, as it compensates for reduced individual plant development by increasing the number of plants per unit area. Given the growing interest in soybean cultivation under temperate climatic conditions and the limited regional data available, the primary objective of this study was to determine how sowing date, based on soil temperature thresholds, and varying seeding densities affect the seed yield of the soybean cultivar Abelina under the specific environmental conditions of east-central Poland. Optimal coordination of these parameters enables the maximization of environmental resource use—such as light, water, and nutrients—while minimizing the adverse effects of abiotic stresses such as drought or high temperatures during critical growth stages.

2. Materials and Methods

2.1. Description of the Experiment

A field experiment was conducted at Łączka (N52°15′, E21°95′), east-central Poland, from 2017 to 2019, using a randomized block design with three replicates. The test soybean cultivar was Abelina developed by Saatzucht Donau Ges.m.b.H. & CoKG (Probstdorf, Austria) and represented in Środa Śląska (Poland) by Saatbau Polska Sp. z o.o. Abelina, an early ‘000’ maturity group cultivar exhibiting vigorous early growth, purple flowers, dark-hilum seeds with high protein and fat content, and very early maturity. For proper growth and development, soybean requires inoculation with Rhizobium bacteria. Saatbau provides sowing-ready seeds coated with Bradyrhizobium japonicum and an adhesive which preserves and protects against sunlight. The inoculated seeds are dried and packaged in bags to prevent clumping, with specialized components allowing inoculation 6–8 weeks before sowing. The experiment was conducted in Haplic Luvisol soil (World Reference Base for Soil Resources, 2014) [19] with average organic carbon and total nitrogen, medium phosphorus, high potassium, and low plant-available magnesium content (

Table 1. Selected soil properties in the layer 0–0.25 m before experiment initiation in 2017 and 2018.

Year	pH	Corg	Nt	Fet	Bt	Pav	Kav	Mgav
	(in KCl)	g·kg−1	g·kg−1	g·kg−1	g·kg−1	(mg·kg−1)	(mg·kg−1)	(mg·kg−1)
2017	7.0	9.5	0.77	998	0.71	55.5	132.7	26.3
2018	7.1	9.1	0.75	995	0.65	57.0	130.5	26.1
2019	7.2	9.3	0.81	997	0.74	56.2	131.6	26,4

org—organic, t—total, av—available.

).

A two-factor field experiment was set up as a split-plot arrangement of plots with three replicates, with the sowing date as the main (first-order) factor and the sowing density as the second-order factor. Three sowing dates were tested based on average daily soil temperature at 5 cm depth:

• S1—when the temperature reached 9 °C,
• S2—when the temperature reached 12 °C,
• S3—when the temperature reached 15 °C.

Three seeding densities were also tested:

• D1—50 seeds·m⁻²,
• D2—70 seeds·m⁻²,
• D3—90 seeds·m⁻².

The harvest area per plot was 9 m². Each experimental plot measured 2.0 × 4.5 m. The length of each row was 4.5 m. Soybeans were sown in 9 rows spaced 22 cm apart at a depth of 5 cm. The distance between sub-blocks (different sowing dates) was 2.5 m, which minimized interactions such as shading effects. The distance between plots within a sub-block was 1 m, reducing lateral influences. Border rows served as isolation and were excluded from analyses.

Soybeans were sown after 15 April, once average daily soil temperature exceeded the specified threshold for each sowing date (

Table 2. Soybean sowing dates in 2017–2019.

Date	Years
	2017	2018	2019
S1	15 May	24 April	28 April
S2	17 May	01 May	07 May
S3	19 May	08 May	12 May

). Soil temperature was measured in April and May at 5 cm depth using an elbow-type soil thermometer at 07:00, 13:00, and 19:00 local time.

Fertilization was adjusted based on the available phosphorus and potassium content in the soil. In spring, prior to sowing, nitrogen was applied at a rate of 20 kg·ha⁻¹ N, phosphorus at 35 kg·ha⁻¹ P₂O₅, and potassium at 95 kg·ha⁻¹ K₂O. Phosphorus and nitrogen were supplied using Polidap fertilizer, whereas potassium was applied in the form of 60% potassium salt (potassium chloride).

2.2. Research Material and Statistical Analyses

Biometric measurements were taken from a randomly selected sample of 20 plants per plot before harvest, at the full maturity stage (BBCH 99). The following morphological traits were examined based on a sample of 20 plants: plant height, height of the first pod, and pod length. Plant height was measured with a tape measure from the soil surface to the tip of the main stem when the plant was fully upright, with minimal wind influence. Before harvest, 20 plants were randomly selected from each plot to determine the number of pods per plant and the number of seeds per pod. After harvest, the yield from each experimental plot (9 m²) was converted to Mg per ha. The thousand-seed weight was determined at a seed moisture content of 15%.

The results were subjected to statistical analysis using a two-way analysis of variance (ANOVA) based on a split-plot design, according to the following model:

y_ijl = m + a_i + g_j + e_ij¹ + b_l + ab_il + e_ijl²,

where

• y_ijl—value of the analyzed variable,
• m—overall population mean,
• a_i—effect of the i-th level of the first-order factor (sowing date), i = 1, 2, …, a; a = 3,
• g_j—effect of the j-th replicate, j = 1, 2, …, n; n = 3,
• e_ij¹—error 1 (caused by the interaction between the first-order factor and replication),
• b_l—effect of the l-th level of the second-order factor (sowing density), l = 1, 2, …, b; b = 3,
• e _ijl²—random effect.

Tukey’s HSD test was used for mean comparisons at a significance level of α = 0.05. All statistical computations were performed using Statistica 13.3 software.

2.3. Weather Conditions

Soybean has relatively high thermal and moisture requirements [20]. It thrives in a warm climate—optimal temperatures for germination are around 10–12 °C, whereas optimal temperatures for plant growth and development range between 20 and 25 °C [21,22]. Sowing into overly cold soil (below 8 °C) can result in poor emergence, seed rot, and increased susceptibility to disease [23]. Soybean exhibits frost sensitivity and requires moderate and evenly distributed precipitation during the growing season. Sufficient soil moisture is especially critical during the flowering and pod-setting stages [24]. The temperature and precipitation conditions during the study period are presented in

Table 3. Air temperature and atmospheric precipitation values in 2017–2019.

Month	Precipitation			Air Temperature
	2017	2018	2019	2017	2018	2019
April	82	52	9	7.1	12.5	9.4
May	46	26	114	13.1	16.4	13.0
June	56	75	29	17.6	18.3	21.5
July	76	96	40	17.6	19.7	18.0
August	53	29	72	19.0	19.9	19.3
September	112	42	42	13.9	15.2	14.0
	Sum (Apr–Sept)			Mean (Apr–Sept)
	425	320	306	14.7	17.0	15.9

.

In 2018, the thermal conditions were the most conducive to soybean cultivation. The average temperature during the growing season was 17.0 °C, with the highest values recorded in June (18.3 °C), July (19.7 °C), and August (19.9 °C), which supported intensive plant growth and development. However, the total precipitation in 2018 was lower (320 mm) compared to 2017 (425 mm), with particularly low rainfall in April (52 mm) and May (26 mm), which may have adversely affected seedling emergence and early plant development.

In 2019, thermal conditions were also favorable (on average, 15.9 °C), and precipitation was more evenly distributed during the critical soybean growth stages, particularly in May (114 mm) and August (72 mm), which likely supported proper pod setting and filling. In contrast, 2017 had the lowest average temperature (14.7 °C), though it saw the highest total precipitation (425 mm). While lower temperatures may have slowed plant development, the abundant rainfall likely reduced the risk of drought stress.

Soil temperature is a key factor determining the sowing date and emergence of thermophilic crops such as soybean. The minimum soil temperature that allows for safe sowing of soybean is approximately 9 °C, while the optimal thermal conditions for germination and early plant development range between 12 and 14 °C. During the study years, the soil temperature at a 5 cm depth varied (Figure 1). In 2018, the soil warmed up most rapidly in spring, as the 9 °C threshold was exceeded around 10 April and temperatures above 12 °C were recorded by 1 May. These conditions enabled early sowing of soybean, increasing the likelihood of fully utilizing the growing season and reducing the risk of drought during later development stages (flowering, pod setting). In contrast, conditions in 2017 and 2019 were less favorable. Soil temperature only reached 9 °C by 15 May in 2017 and 28 April in 2019. The delayed soil warming in those years limited the possibility of early sowing and negatively affected emergence quality. It was therefore concluded that the most favorable soil temperature for early soybean sowing occurred in 2018, whereas in 2017 and 2019, sowing could only be safely carried out from late April to early May.

3. Results

Soybean seed yields were significantly affected by the study year, sowing date, and seeding density. Significant interactions were also observed between the study year and sowing date, as well as between the year and seeding density (

Table 4. Soybean seed yield (Mg ha⁻¹) by year, sowing date, and seeding density.

	Year			Mean
Sowing Date	2017	2018	2019
S1	2.54 B	1.86 A	2.86 A	2.42 A
S2	3.59 A	1.06 B	2.13 B	2.26 A
S3	1.47 C	1.30 B	2.83 A	1.87 B
Mean	2.53 ab	1.41 b	2.61 a
Seeding density	2017	2018	2019
D1	2.12 B	1.24 A	2.45 B	1.94 B
D2	2.67 A	1.48 A	2.93 A	2.36 A
D3	2.81 A	1.51 A	2.44 B	2.25 A

Means in rows followed by different lowercase letters (a, b) (for study years) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B, C) (for sowing dates, seeding densities, study year × sowing date interactions, and study year × seeding density interactions) differ significantly at p ≤ 0.05.

). On average, the highest yield, regardless of sowing date and seeding density, was recorded in 2019 (2.61 Mg ha⁻¹), which was significantly higher than the yield obtained in 2018 (1.41 Mg ha⁻¹). Soybean sown on the first and second dates (S1 and S2) produced superior yields (2.42 and 2.26 Mg ha⁻¹, respectively), both significantly greater than that obtained from the third sowing date (S3) (1.87 Mg ha⁻¹). Seeding density also had a significant effect on soybean yields. Plants sown at 70 and 90 seeds m⁻² (D2 and D3) achieved the highest yields (2.36 and 2.25 Mg ha⁻¹, respectively), while a significantly lower yield was recorded at 50 seeds m⁻² (D1—1.94 Mg ha⁻¹). In 2017, sowing was delayed (S1—15 May) due to soil temperatures remaining below 9 °C at the 5 cm depth for an extended period. However, in this year, the highest yield was obtained from soybean sown on the second date (S2), reaching 3.59 Mg ha⁻¹, which was significantly higher than the yields from S1 and S3. In 2018, the earliest sowing date (S1), characterized by the warmest April and May, resulted in significantly higher yields compared to S2 and S3. In 2019, the yields for S1 and S3 were similar (2.86 and 2.83 Mg ha⁻¹) and both were significantly higher than that of S2 (2.13 Mg ha⁻¹).

Analysis of the seeding density × study year interaction revealed that in 2017, yields were similar for D2 and D3, whereas in 2018, the soybean yields differed insignificantly across all tested densities. Conversely, in 2019, the highest yield was obtained at 70 seeds m⁻² (D2—2.93 Mg ha⁻¹) (Table 4).

The seed yield of soybean cv. Abelina varied significantly across the study years, depending on both the sowing date and seeding density (Figure 2). During the three-year study period (2017–2019), considerable variation in weather conditions was observed, which in turn affected the yield performance and stability of the crop. In 2017, under favorable meteorological conditions, the highest yield (4.21 Mg ha⁻¹) was achieved with the optimal sowing date (S2) coupled with the highest seeding density (D3). This indicates a positive response of soybean to increased plant density, especially under conditions conducive to growth. In the same year, the lowest yield (1.42 Mg ha⁻¹) was associated with the latest sowing date (S3) and the lowest density (D1), confirming the negative impact of delayed sowing and low plant density on production efficiency. The year 2018 was characterized by the lowest yields throughout the study period. Yields for all sowing date and density combinations were significantly lower than in the other years, likely due to unfavorable weather conditions, particularly drought and high temperatures during the growing season. The best result was observed for the S1–D2 combination (2.03 Mg ha⁻¹), suggesting that early sowing and higher plant density could partially offset adverse weather effects. The lowest yield that year, only 0.87 Mg ha⁻¹, was recorded for the S2–D1 combination. In 2019, the yields increased again, approaching the levels observed in 2017. The highest yield (3.42 Mg ha⁻¹) was recorded for the S3 sowing date coupled with the D2 density. This indicates that later sowing was the most effective in that season, possibly reflecting better alignment of sowing timing with the weather conditions during the growing period.

The height at which the first pod formed on the soybean plant was influenced by the study year (

Table 5. Height (cm) at which the first pod formed by study year, sowing date, and seeding density.

	Year			Mean
Sowing Date	2017	2018	2019
S1	14.40 A	12.26 A	11.74 A	12.80 A
S2	13.68 A	7.35 B	11.96 A	11.00 B
S3	13.37 A	7.59 B	12.39 A	11.12 B
Mean	13.82 a	9.07 c	12.03 b
Seeding density	2017	2018	2019
D1	12.97 A	9.44 A	10.67 B	11.03 B
D2	13.87 A	9.41 A	11.64 B	11.64 A
D3	14.61 A	8.35 A	13.78 A	12.25 A
	Seeding density
Sowing date	D1	D2	D3
S1	12.09 a	13.57 a	12.74 a
S2	10.71 a	11.04 a	11.24 a
S3	10.28 b	10.3 b	12.76 a

Means in rows followed by different lowercase letters (a, b, c) (for study years and sowing date × seeding density interaction) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B) (for sowing dates, seeding densities, study year × sowing date interactions, and study year × seeding density interactions) differ significantly at p ≤ 0.05.

). The greatest height was observed in plants grown in 2017 (13.82 cm), whereas the lowest was recorded in 2018 (9.07 cm). Regardless of the study year, the earliest sowing date (S1) resulted in the greatest height compared to S2 and S3. Analysis of the study year × sowing date interaction revealed that in both 2017 and 2019, sowing date had no significant effect on the height of first pod formation. Only in 2018 did sowing at S1 significantly increase the height compared to S2 and S3.

In 2019, the highest seeding density (D3) significantly increased the height of first pod formation (13.78 cm) compared to D1 and D2 (10.67 and 11.64 cm, respectively). On average, the highest density (D3) resulted in the greatest height (12.25 cm), which was significantly greater than that observed for D1 (11.03 cm). A seeding density × sowing date interaction also significantly affected the height at which the first pod developed. Soybeans sown at the latest date (S3) under densities D1 and D2 developed their first pods at significantly lower heights (10.2 and 10.3 cm, respectively).

Thousand seed weight (TSW) was significantly influenced by study year, seeding density, and seeding density × sowing date interaction (

Table 6. Thousand seed weight (g ha⁻¹) by study year, sowing date, and seeding density.

Seeding Density	2017	2018	2019	Mean
D1	158.00 A	143.33 A	176.56 A	159.30 A
D2	162.44 A	142.33 A	158.44 B	154.40 B
D3	144.33 B	135.11 A	146.89 C	142.11 C
Mean	154.93 a	140.26 b	160.63 a
	Seeding density
Sowing date	D1	D2	D3
S1	169.89 a	150.22 b	139.44 c
S2	153.11 b	157.33 a	142.56 c
S3	154.89 b	155.67 a	144.33 c

Means in rows followed by different lowercase letters (a, b, c) (for study years and sowing date × seeding density interaction) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B, C) (for seeding densities, and study year × seeding density interactions) differ significantly at p ≤ 0.05.

). The highest average TSW was recorded in 2019 (160.63 g), which was significantly greater than in 2018 (140.26 g). This difference may be attributed to the more favorable distributions of rainfall and temperature during critical phases of plant development in 2019. Regarding seeding density, the highest TSW values were observed at the lowest plant density (D1—159.3 g, on average), in line with the established trend that reduced interplant competition promotes the development of larger seeds. Within each year, the differences in TSW among seeding densities were statistically significant. In 2019, these differences were particularly pronounced: D1—176.56 g, D2—158.44 g, and D3—146.89 g. The sowing date × seeding density interaction also significantly affected TSW. The highest values were recorded for early sowing at low density (S1–D1—169.89 g), while the lowest were observed for late sowing at high density (S1–D3—139.33 g).

The highest TSW values were generally obtained at the lowest seeding density (D1), regardless of sowing date or study year (Figure 3). Notably, the S1–D1 combination in 2019 (early sowing at medium density) achieved a superior TSW of 201.3 g. Conversely, the lowest TSW values were consistently recorded at the highest seeding density (D3), confirming the negative impact of increased interplant competition on seed quality.

In each study year and for most sowing dates, statistically significant differences in TSW were observed among seeding densities. Exceptions included certain combinations, such as S3 in 2018, where differences between densities were statistically insignificant.

Pod length in soybean was significantly influenced by both the study year and sowing date (

Table 7. Soybean pod length (cm) by sowing date in 2017–2019.

	Year			Mean
Sowing Date	2017	2018	2019
S1	4.35	3.94	5.35	4.55 A
S2	4.03	3.98	4.63	4.21 B
S3	4.27	3.71	4.54	4.17 B
Mean	4.22 b	3.88 b	4.84 a

Means in rows followed by different lowercase letters (a, b) (for study years) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B) (for sowing dates) differ significantly at p ≤ 0.05.

). The longest pods were recorded in the 2019 growing season (on average, 4.84 cm), which differed significantly from those in 2017 (4.22 cm) and 2018 (3.88 cm). The greater pod length in 2019 can be attributed to more favorable weather conditions during the flowering and pod maturation stages, including higher average temperatures and moderate rainfall. With respect to sowing date, the longest pods were formed at the earliest date (S1—4.55 cm), which was significantly greater than those for the intermediate (S2—4.21 cm) and late sowing dates (S3—4.17 cm).

Soybean plant height was significantly influenced by both study year and sowing date (

Table 8. Soybean plant height (cm) by sowing date in 2017–2019.

	Year			Mean
Sowing Date	2017	2018	2019
S1	79.83 A	47.5 A	82.55 A	69.96 A
S2	62.91 B	45.4 A	81.46 A	63.26 B
S3	78.6 AB	42.98 A	69.37 B	63.65 B
Mean	73.78 a	45.29 b	77.79 a

Means in rows followed by different lowercase letters (a, b) (for study years) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B) (for study year × sowing date interactions) differ significantly at p ≤ 0.05.

). The tallest plants were found in 2017 and 2019, with average heights of 73.78 cm and 77.79 cm, respectively. In contrast, the plant height in 2018 was significantly lower (averaging 45.29 cm), likely owing to the unfavorable weather conditions during that season, particularly a shortage of rainfall and lower temperatures during key vegetative growth phases. Analysis of the sowing date × study year interaction revealed that early sowing (S1) consistently contributed to the tallest plants across all three growing seasons (e.g., 82.55 cm in 2019). In 2017, plants sown at the intermediate sowing date (S2) were the shortest (62.91 cm—significantly shorter than those at S1), whereas in 2019, no statistically significant differences were observed between S1 and S2. Late sowing (S3) led to the greatest variation: in 2017, it produced plant heights comparable to S1, while in 2019, it resulted in significantly shorter plants (69.37 cm) compared to both S1 and S2.

The number of pods per plant differed significantly between study years and sowing dates (

Table 9. Pod number per plant by sowing date in 2017–2019.

	Year			Mean
Sowing Date	2017	2018	2019
S1	29.73 A	30.40 A	29.53 B	29.89 A
S2	16.24 B	32.45 A	37.77 A	28.82 B
S3	29.28 A	12.52 B	29.01 B	23.60 C
Mean	25.08 b	25.12 b	32.10 a

Means in rows followed by different lowercase letters (a, b) (for study years) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B, C) (for sowing date and study year × sowing date interactions) differ significantly at p ≤ 0.05.

). The highest average number of pods was recorded in 2019 (32.1), which differed significantly from the results in 2017 and 2018 (25.08 and 25.12 pods per plant, respectively). The improved performance in 2019 can be attributed to more favorable temperature and moisture conditions during the flowering and pod-setting phases. In terms of sowing date, the highest pod numbers per plant were recorded for early sowing (S1—29.89 pods per plant, on average), which was significantly higher than for the intermediate (S2—28.82) and late sowing dates (S3—23.60). Particularly low pod counts under late sowing (S3) were found in 2018 (12.52), indicating the crop’s sensitivity to delayed sowing under co-occurring unfavorable weather conditions. The sowing date × study year interaction was pronounced. In 2018, the highest pod number was recorded at the intermediate sowing date (S2—32.45), whereas the latest date (S3) resulted in the lowest pod count (12.52). Conversely, in 2019, the best outcome was achieved with sowing at the intermediate date (S2—37.77), confirming that under favorable environmental conditions, delayed sowing does not necessarily decrease pod number.

During the analyzed period, a clear influence of both sowing date and seeding density on the number of pods formed by soybean plants was observed. Generally, the highest number of pods was obtained at lower seeding densities (D1 and D2) and with early (S1) and optimal (S2) sowing dates, particularly under the favorable weather conditions of 2019 (Figure 4). In 2017, the highest pod counts were recorded at density D2 with sowing date S3 (33.10), as well as at D3 with S3 (33.31). The sowing date S2 in this year resulted in notably lower pod numbers, especially at density D3 (12.50). In 2018, the differences between combinations were smaller, although the negative effect of the late sowing date (S3) was evident, particularly at density D1 (10.77). The year 2019 stood out, with the highest pod numbers across all combinations. The greatest pod count was achieved at density D2 and sowing date S2 (43.33), confirming the beneficial effect of moderate density and optimal sowing timing under favorable conditions. Conversely, the lowest pod number in 2019 was observed for the combination D2–S1 (19.23), possibly due to environmental stress or increased plant competition with this sowing date.

The average number of seeds per pod for each study year indicated the highest value in 2017 (2.50) and the lowest in 2018 (1.99) (

Table 10. Seed number per pod by seeding density in 2017–2019.

	Year			Mean
Seeding Density	2017	2018	2019
D1	2.55 A	1.98 A	2.42 A	2.32 A
D2	2.46 A	2.09 A	2.26 AB	2.27 A
D3	2.48 A	1.99 A	2.15 B	2.17 B
Mean	2.5 a	1.99 c	2.28 b

Means in rows followed by different lowercase letters (a, b, c) (for study years) differ significantly at p ≤ 0.05. Means in columns followed by different uppercase letters (A, B) (for sowing date and study year × sowing date interactions) differ significantly at p ≤ 0.05.

). The differences between years were statistically significant (denoted by different lowercase letters following the annual means). Regarding seeding density, in 2019, densities D1 and D2 showed a higher number of seeds per pod than D3, with these differences being statistically significant. In 2017 and 2018, no significant differences between densities were observed.

Between 2017 and 2019, moderate variation in the number of seeds per pod was observed, influenced by agronomic and weather conditions. In 2017, values were the highest among the analyzed years and relatively uniform across all combinations (Figure 5). The greatest number of seeds per pod was recorded at density D1 and sowing date S1 (2.68), whereas the lowest was at D2–S3 (2.30), although these differences were statistically insignificant. In 2018, characterized by less favorable weather conditions, the number of seeds per pod declined, particularly in combinations with higher seeding densities (D2 and D3). The lowest values were noted for the D3–S3 (1.83) and D3–S1 (1.77) combinations, suggesting a strong influence of environmental stress (e.g., drought) on seed setting efficiency. In 2019, compared to 2018, this parameter improved, with average values increasing across most combinations. The highest number of seeds per pod was observed for the D1–S2 combination (2.48), possibly indicating the beneficial effect of low density and optimal sowing timing on pod and seed development. The lowest values occurred at highest seeding density for the D3–S3 combination (1.96).

4. Discussion

A three-year study on the soybean cultivar Abelina demonstrated a significant effect of sowing date and seeding density on plant growth, development, and yield performance. The obtained results align with current knowledge on soybean agronomy under the climatic conditions of Poland and Central Europe.

The analysis showed that the highest average yield during the study years was achieved at a sowing density of 90 seeds·m⁻² (D3) and for the first sowing date (S1). Early sowing allows plants to make the most of the growing season, resulting in longer vegetative and generative phases. Consequently, plants have more time to develop their root systems, form pods, and mature seeds. With adequate soil moisture and moderate spring temperatures, early sowing helps avoid heat stress and drought during critical flowering and seed-filling stages. Meanwhile, high sowing density enables better utilization of the cultivation area. In years with favorable weather conditions (2017 and 2019), the highest yields were obtained for the combinations S2–D3 and S3–D2, respectively. These years had favorable meteorological conditions; that is, optimal temperatures and sufficient rainfall. In such conditions, the S2 sowing date allowed plants to avoid spring frosts, while the high density (D3) increased the number of yielding plants per unit area, without excessive competition. Conversely, the combination of delayed sowing and optimal density (S3–D2) enabled better individual plant development, providing sufficient space to prevent excessive competition during a shortened growing season. These findings are supported by studies from Zhang et al. [25], Tang et al. [26], and Siebers et al. [27], who examined soybean tolerance to heat stress. These results correspond with findings by De Bruin and Pedersen [28], Boquet [29], and Pospíšil [30], who pointed out that moderate seeding density facilitates optimal plant architecture development and more efficient utilization of environmental resources. In turn, Borowska and Prusiński [31] reported that significantly higher soybean yields were obtained from early sowing at the turn of April and May.

The highest number of pods per plant was recorded in 2019 under favorable weather conditions (an average of 32.1 per plant), particularly at the early sowing date (S1). On average, across the study years, pods formed at the earliest and optimal sowing dates (S1 and S2) were significantly longer, reaching lengths of 4.55 cm and 4.21 cm, respectively. Similarly, Jarecki and Bobrecka-Jamro [32] found that early sowing contributed to a higher number of pods per plant and greater thousand-seed weight (TSW) compared to later sowing dates.

Indicators such as pod number per plant and seed number per pod varied significantly in response to experimental factors. The highest number of pods per plant was observed in 2019, particularly at earlier sowing dates and moderate densities (D1-S2). The number of seeds per pod was less variable, suggesting a stronger influence of weather conditions and agronomic practices on pod number rather than on the internal structure of individual pods [33,34,35,36].

The strong influence of study years on all analyzed parameters highlights soybean’s sensitivity to meteorological conditions, especially during critical developmental stages. The year 2018, featuring weather conditions most divergent from the norm (drought and high temperatures), resulted in a significant decline in yield. Similar observations were reported by Jarecki and Bobrecka-Jamro [29], who emphasized that drought is a limiting factor for soybean yield during the flowering and pod-setting phases in Central and Eastern Europe. Mourtzinis and Conley [37] further confirmed these findings, highlighting the variability in soybean maturity and sensitivity to weather conditions across years and regions.

5. Conclusions

Soybean yield performance exhibited the greatest variation between years, reflecting the strong influence of meteorological conditions. The highest yields were recorded in 2019 (on average, 2.61 Mg ha⁻¹), whereas in 2018, this value dropped to 1.41 Mg ha⁻¹. Averaged over the study period, the earliest sowing date (S1) was the most favorable (2.42 Mg ha⁻¹). Seeding density also had a significant effect, with the highest yield obtained at the intermediate density (D2—2.36 Mg ha⁻¹). Plant height varied significantly, depending on the year and sowing date. The tallest plants were observed with the earliest sowing date (S1), particularly in 2019, with a mean height of 82.55 cm. Pod length remained relatively stable across years (2017 and 2018); however, pods formed with the earliest sowing date (S1 and S2) were significantly longer, averaging 4.55 cm and 4.21 cm. The number of pods per plant showed clear sensitivity to environmental and agronomic conditions. In 2019, under favorable weather conditions, the highest pod counts were recorded (on average, 32.1 pods per plant), particularly with an early sowing date (S1). The number of seeds per pod ranged from 1.8 to 2.7, with the highest values in 2017, primarily using the earliest sowing date and low seeding density (D1). Analysis of the results indicated that, under the climatic conditions of Poland, the most favorable production outcomes for soybean cultivar Abelina were achieved with the early and intermediate sowing dates (S1 and S2) and moderate (D2) and high (D3) seeding density, corresponding to 70 plants m⁻² and 90 plants m⁻². Both sowing date and seeding density should be flexibly adjusted to prevailing meteorological conditions and soil types. Optimization of these parameters may significantly enhance soybean cultivation efficiency and yield stability under variable climatic conditions.