The rapid growth of the global population places increasing pressure on agricultural lands, making it essential to improve production efficiency and maintain yield stability to ensure food security. Mineral fertilizers have long been used to sustain nutrient balance and increase crop productivity; however, their excessive use may pose risks to ecosystem health and human well-being (Wang et al., 2020; Liu et al., 2021). Therefore, the use of organic and biofertilizers has gained increasing importance in sustainable agricultural systems (Glala et al., 2013; Vishwanatha et al., 2022). Environmentally friendly alternatives such as seaweed extracts, humic acid and vermicompost have attracted considerable attention due to their positive effects on soil fertility and plant growth (Colla and Rouphael, 2015; Choudhary et al., 2025). In addition, foliar fertilization has emerged as an effective strategy for improving plant nutrition by enabling rapid nutrient uptake through leaves and correcting deficiencies during critical growth stages (Singh et al., 2013; Rajasekar et al., 2017).
       
Sustainability in crop production is also closely linked to livestock production systems. The increasing demand for animal-based foods has led to a steady rise in global meat consumption, which is expected to reach 51.5 kg per capita by 2050 (Safdar et al., 2023). This trend increases the demand for forage crops and intensifies competition for land between human food and animal feed production (FAO, 2017). Among forage legumes, soybean [Glycine max (L.) Merr.] stands out as a strategic crop due to its high protein content, palatability and adaptability, making it an important protein source for both human nutrition and animal feeding (Zhang et al., 2015; Qin et al., 2022; Blount et al., 2017).
       
This study was conducted to determine the effects of different organic foliar fertilizers on plant growth, morphological traits and chlorophyll content during the early growth stages of fodder soybean [Glycine max (L.) Merr.], which holds a strategic position among forage crops.

The experiment was conducted in 2025 in a climate-controlled growth chamber at the Department of Field Crops, Faculty of Agriculture, Van Yüzüncü Yıl University, using a completely randomized design (CRD) with four replications. The physicochemical properties of the experimental soil are presented in Table 1.


       
Two forage soybean [Glycine max (L.) Merr.] varieties, ‘Yemsoy’ and ‘Yeşilsoy’, were used as plant materials. The experiment was conducted under a 16 h light/8 h dark photoperiod. Diammonium phosphate (DAP; 18% N-46% P) was applied to all pots as a basal fertilizer before sowing. The treatments consisted of four groups: (1) control (DAP only), (2) DAP + seaweed extract (OCEAN), (3) DAP + vermicompost (RİVASOL) and (4) DAP + humic acid (GLADİATÖR). Foliar fertilizers were applied starting at the V2 (two-leaf) growth stage and repeated three times at 15-day intervals according to the manufacturers’ recommendations. Sowing was performed on 15 January 2025 and irrigation was uniformly applied based on soil moisture conditions.
       
Morpho-physiological measurements were conducted 15 days after the final foliar application. In each pot, the single plant was measured for height from the point where the cotyledons abscised after emergence to the apical meristem using a ruler and the total number of leaves per plant was counted manually. Stem diameter, leaflet width and leaflet length were measured using a digital caliper. Relative chlorophyll content (SPAD values) was recorded using a SPAD-502 chlorophyll meter. Leaf area was calculated using the easy leaf area software and the leaf area index (LAI) was determined following the methods described by Watson (1947) and Easlon and Bloom (2014).

According to the results of the analysis of variance (ANOVA), the effects of fertilizer treatments and measurement time on all examined morpho-physiological parameters were statistically significant at the 1% probability level (P<0.01). For plant height, the main effects of variety, fertilizer and measurement time, as well as the variety × fertilizer and fertilizer × measurement time interactions, were significant at the 1% level. In contrast, for leaf area, leaflet width and leaflet length, all main factors, all two-way interactions and the three-way interaction of variety × fertilizer × measurement Time were found to be significant at the 1% level. The effect of variety on stem diameter and number of leaves was not significant; however, the interaction between fertilizer and measurement time significantly influenced these traits at the 1% level. For SPAD values, the Fertilizer × Measurement Time interaction was significant at the 1% level, whereas the three-way interaction (Variety × Fertilizer × Measurement Time) was significant at the 5% level (Table 2).


 
Initial measurement period
 
During the initial measurement period (15 days after the V2 growth stage), the highest plant height values in both soybean varieties were obtained from the vermicompost treatment. Plant height reached 21.9±0.3 cm in Yemsoy and 23.3±0.3 cm in Yeşilsoy, representing increases of approximately 34.4% and 36.3% compared with the control, respectively. For leaf area, the highest values in both varieties were recorded under the seaweed treatment, showing an increase of about 81.8% compared with the control. Vermicompost and humic acid applications also increased leaf area but were statistically grouped together. Stem diameter was not significantly affected by fertilizer treatments during the early growth stage and no differences were observed between varieties. In terms of leaf number, vermicompost, seaweed and humic acid treatments were classified within the same statistical group in both varieties, showing slight increases compared with the control. Leaflet width and length were highest under the seaweed treatment in both varieties. In Yeşilsoy, leaflet width reached 24.5±0.3 mm, while in Yemsoy it reached 23.9±0.3 mm, representing increases of approximately 33.9% and 29.9% over the control, respectively. Similarly, leaflet length increased to 29.5±0.3 mm in Yemsoy and 35.2±9.88 mm in Yeşilsoy under the seaweed application. SPAD values were also highest under seaweed treatment. In Yemsoy, SPAD reached 36.4±0.27 (38.9% higher than the control), while in Yeşilsoy the highest values were obtained from seaweed (35.9±0.3) and vermicompost (35.5±0.3), corresponding to increases of 31.0% and 29.6%, respectively (Fig 1).


 
Second measurement period
 
During the second measurement period, pronounced differences among treatments were observed in both soybean cultivars, with vermicompost emerging as the most effective application in promoting plant growth. The highest plant height values were recorded under vermicompost application, reaching 37.6±0.3 cm in the Yemsoy cultivar and 38.7±0.3 cm in the Yeşilsoy cultivar. In contrast, the lowest plant heights were obtained from the control treatment, with values of 24.7±0.3 cm and 26.3±0.3 cm, respectively, corresponding to increases of 52.2% in Yemsoy and 47.1% in Yeşilsoy compared with the control. In terms of leaf area, vermicompost application in the Yemsoy cultivar (0.48±0.28) resulted in a 54.8% increase relative to the control, whereas the highest leaf area in the Yeşilsoy cultivar was obtained from the seaweed treatment (0.45±0.26), representing an increase of 66.6% over the control. Regarding stem diameter, the seaweed application produced the highest values in both cultivars (3.1±0.3 mm in Yemsoy and 3.4±0.3 mm in Yeşilsoy), corresponding to increases of 55% and 70%, respectively, compared with the control. For leaf number, vermicompost application resulted in the highest value in both cultivars (5.3±0.3 leaves per plant), providing a 15.2% increase relative to the control. Leaflet width was highest under vermicompost application in the Yemsoy cultivar (38.0±0.3 mm, 30.6% increase), while the seaweed application was most effective in the Yeşilsoy cultivar (33.9±0.3 mm, 29.8% increase). Similarly, leaflet length reached its maximum under the seaweed application in both cultivars, measuring 48.1±0.3 mm in Yemsoy and 29.6 ±0.3 mm in Yeşilsoy, which corresponded to increases of 25.9% and 39.6%, respectively, compared with the control. With respect to SPAD values, vermicompost application in the Yemsoy cultivar produced the highest chlorophyll content (45.8±0.3), representing a 48.7% increase over the control, whereas the seaweed application in the Yeşilsoy cultivar yielded the highest SPAD value (41.1±0.3), corresponding to a 35.0% increase (Fig 2).


 
Third measurement period
 
During the third measurement period, increases in all growth parameters continued and the effects of the treatments became more pronounced. In terms of plant height, the highest values in both soybean cultivars were obtained from the vermicompost treatment, reaching 50.3±0.3 cm in Yemsoy and 50.0±0.3 cm in Yeşilsoy, whereas the lowest values were recorded in the control treatment (34.7±0.3 cm and 37.3±0.3 cm, respectively).
       
For leaf area, the seaweed treatment resulted in the highest values in both cultivars (0.81±0.01 in Yemsoy and 0.78±0.03 in Yeşilsoy), showing marked increases compared to the control. Stem diameter was most strongly enhanced by vermicompost application, with values of 3.6±0.3 mm in Yemsoy and 3.5±0.3 mm in Yeşilsoy, indicating a clear improvement over the control treatment. Regarding leaf number, vermicompost was distinctly superior in both cultivars, producing 9.03±0.25 leaves per plant in Yemsoy and 8.53±0.21 leaves per plant in Yeşilsoy, which represented the highest increases relative to the control. Leaflet width also reached its maximum under vermicompost application (48.3±0.3 mm in Yemsoy and 47.4±0.3 mm in Yeşilsoy), resulting in significant differences compared to the control in both cultivars. In contrast, leaflet length was most effectively promoted by the seaweed treatment, with values of 58.4±0.3 mm in Yemsoy and 46.1±0.3 mm in Yeşilsoy, substantially exceeding those of the control. In terms of SPAD values, vermicompost application produced the highest chlorophyll content in both cultivars (46.3±0.3 in Yemsoy and 44.8±0.3 in Yeşilsoy), yielding significantly higher results than the control (Fig 3).


 
Overall evaluation based on the mean values of three measurement periods
 
The overall evaluation based on the arithmetic means of three measurement periods clearly revealed differential responses of the Yemsoy and Yeşilsoy soybean cultivars to the applied organic fertilizers. The general results indicated that vermicompost application produced the highest effects on almost all morpho-physiological parameters when compared with the control treatment. In particular, plant height showed marked increases under vermicompost application, reaching 38.77% in Yeşilsoy and 44.27% in Yemsoy relative to the control. Similarly, substantial improvements were observed in leaf area, with increases of 80.00% in Yeşilsoy and 77.42% in Yemsoy, further confirming the superior performance of vermicompost. Leaf number followed a comparable trend, reinforcing the overall effectiveness of this treatment. From a physiological perspective, a similar pattern was observed, as vermicompost application most strongly stimulated chlorophyll synthesis, increasing SPAD values in the Yemsoy cultivar by 44.97% compared to the control. However, seaweed application was more prominent for certain traits. Notably, stem diameter increased by 30.43% in the Yeşilsoy cultivar and leaflet length increased by up to 54.33% in the Yemsoy cultivar relative to the control, indicating that seaweed can be more effective for specific parameters. Although humic acid application contributed positively to all evaluated traits compared to the control, the magnitude of these increases generally remained within the range of 12–30%, placing it behind vermicompost and seaweed treatments in overall effectiveness. When cultivar-specific responses were considered, Yeşilsoy showed higher baseline performance under control conditions, but the highest values and percentage increases were mostly observed in Yemsoy, indicating a stronger response to vermicompost and seaweed applications (Fig 4).


       
Global challenges such as climate change, soil degradation and rapid population growth have intensified the need for sustainable and efficient plant nutrition strategies that minimize the environmental risks associated with conventional fertilization practices (Schjørring and Cakmak, 2014; Prasad and Shivay, 2020). The findings of the present study demonstrate that organic-based foliar fertilizers induce multidimensional, time-dependent and genotype-specific improvements in plant morphology and physiology (Sohail et al., 2025; Xu et al., 2025). The analysis of variance confirmed that the applied foliar fertilizers exhibited cumulative effects across growth stages, with their influence becoming more pronounced over time. A notable observation was the absence of statistically significant differences among treatments for stem diameter during the first measurement period. This response can be attributed to the physiological priorities of soybean during the early seedling stage. Large-seeded legumes such as soybean primarily rely on the rapid metabolic activation and breakdown of cotyledonary reserves to meet initial energy requirements rather than external nutrient stimuli (Meneguzzo et al., 2021; Xu et al., 2025). During this establishment phase, early growth is mainly directed toward vertical elongation driven by apical dominance to maximize light interception (Meneguzzo et al., 2021). Furthermore, the limited leaf surface area at this stage likely restricts nutrient absorption from foliar applications, resulting in more pronounced treatment effects only during later measurement periods when leaf area and photosynthetic activity have increased (Li et al., 2018; Bărdas et al., 2023).
       
As vegetative growth progressed, vermicompost application consistently exhibited superior performance across nearly all evaluated parameters. During the second measurement period, vermicompost increased plant height by 52.2% in Yemsoy and 47.1% in Yeşilsoy, while in the third period it produced the highest values for leaf number (9.03±0.25 leaves plant^-1 in Yemsoy) and leaf area (80.00% increase in Yeşilsoy), underscoring its key role in promoting overall biometric development. This comprehensive enhancement can be attributed to the high microbial activity of vermicompost and its content of growth-regulating substances that stimulate cell division and expansion (Ataklı Bice et al., 2022; Kumar et al., 2024). From a physiological perspective, vermicompost markedly promoted chlorophyll biosynthesis, as evidenced by a 44.97% increase in SPAD values in the Yemsoy cultivar compared with the control. This response suggests that micronutrients such as zinc (Zn) and copper (Cu) present in vermicompost effectively activate photosynthetic processes and electron transport mechanisms (Ataklı Bice et al., 2022; Beyk-Khormizi et al., 2023; Iqbal et al., 2024).

In contrast, seaweed application exerted a distinct influence on plant architecture, particularly during early growth stages, where it increased leaf area by 81.8% in both cultivars. In later stages, seaweed application resulted in a remarkable 70% increase in stem diameter in Yeşilsoy, indicating a substantial contribution to structural robustness. These effects are consistent with previous reports attributing such responses to alginates, betaines and cytokinin-like phytohormones in seaweed extracts, which stimulate leaf blade expansion and strengthen vascular tissues (Ali et al., 2021; Shukla et al., 2019).
       
Although humic acid positively influenced all traits compared with the control, improvements generally ranged from 12-30%, making it less effective than vermicompost and seaweed. Yemsoy showed a stronger response to organic inputs than Yeşilsoy, highlighting the role of genotype in fertilization efficiency. Overall, seaweed can enhance biometric stability and plant architecture, while vermicompost effectively increases vegetative biomass and chlorophyll content. These findings support environmentally friendly agricultural objectives (Zulfiqar et al., 2020) and underscore the importance of organic foliar fertilizers during the early vegetative stage for long-term soybean productivity.
               
Early-stage biometric advantages are not transient; rather, they establish the foundation for yield potential throughout the plant life cycle. Enhanced vegetative growth primarily maximizes the plant’s photosynthetic capacity. Increased leaf area and chlorophyll content achieved through vermicompost application enable maximum light interception and energy production prior to critical reproductive stages such as flowering and seed filling. Previous studies have emphasized that a rapidly increasing leaf area index (LAI) during early growth in soybean is strongly and positively correlated with final grain yield, particularly before flowering (Board and Modali, 2005; Ferreira et al., 2024). Simultaneously, the thicker stems and increased plant height associated with seaweed application strengthen the plant’s ability to support greater biomass and, more importantly, enhance the vascular system responsible for efficient translocation of photosynthates from leaves to developing pods and seeds (Ambika and Sujatha, 2016; Kocira et al., 2018; Shukla et al., 2021). Therefore, the positive effects of organic foliar fertilizers observed during early growth stages represent a crucial first step toward achieving a successful soybean harvest.

This study demonstrated that vermicompost showed the highest performance in key growth traits, including plant height, leaf number and chlorophyll content, while seaweed was particularly effective in improving structural attributes such as leaf area and stem diameter. The stronger response of the Yemsoy cultivar highlights the importance of integrating cultivar selection with fertilization strategy in sustainable agriculture. These organic foliar applications optimized both physiological capacity and morphological resilience, with vermicompost recommended to enhance vegetative biomass and photosynthetic capacity and seaweed to improve structural stability and leaf expansion. Positive effects observed during the early vegetative stage are expected to contribute to harvest index and final yield, emphasizing the strategic role of early-stage organic foliar fertilization in sustainable soybean cultivation.

This study was supported by the Scientific Research Projects Unit (BAP) under project number FYD-2024-11475.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.
 
Informed consent
 
All animal procedures for experiments were approved by the Committee of Experimental Animal care and handling techniques were approved by the University of Animal Care Committee.

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.