Impact of Different Fertilizer Forms on Yield Components and Macro–Micronutrient Contents of Cowpea (Vigna unguiculata L.)

Abstract

Organic materials, whose importance is increasing day by day in terms of soil fertility, plant nutrition, and sustainable agriculture in the world, need to be shown to be more effective against chemical fertilizers in order for farmers to adopt and use them more. The study was carried out to determine the effects of different organic fertilizer forms (farmyard manure (FM1 = 2500, FM2 = 5,104,000, FM3 = 7500, and FM4 = 10,000 kg ha⁻¹), leonardite (L1 = 5000 and L2 = 10,000 kg ha⁻¹) and vermicompost (V1 = 2500, V2 = 5000, V3 = 7500, and V4 = 10,000 kg ha⁻¹)) on the yield components and some macro and micronutrient contents of the cowpea (Vigna unguiculata L.). The study, which was carried out under the Kahramanmaraş Mediterranean ecological conditions in 2020–2021, was conducted according to the experimental design of completely randomized blocks with three replications. As a result of the study, it was found that the differences between the fertilizer forms were significantly effective in terms of all the examined characteristics. It was determined that the seed yields varied between 3043.3–4126.7 kg ha⁻¹, and according to the results of the two-year study, 10,000 kg ha⁻¹ vermicompost would be sufficient to obtain the highest cowpea yield (4126.7 kg ha⁻¹) under Mediterranean climate conditions.

1. Introduction

The cowpea (Vigna unguiculata L.), a member of the Fabaceae family, is among the most important legumes widely grown worldwide, especially in Africa, South America, some parts of Asia, and the United States, as a protein source [1] for food and feed [2]. The African continent is the homeland of V. unguiculata, which belongs to the genus Vigna, grown in our country, which has several important species, and their origins are found in many different parts of the world. In Türkiye, 75% of the cowpea, which is mostly grown in the Aegean and Mediterranean regions, is grown in the Aegean region [3]. This cowpea species, which is grown in western and southern Anatolia in Turkey, is an annual herbaceous plant that can grow up to 30–300 cm [4]. One of the most important factors affecting the yield of the cowpea in the regions where it is grown is the useful macro and micronutrients in the soil. In cases where these nutrients are insufficient, the nutrients needed by the plants are met by external fertilization, and plant production is made sustainable [5].

The use of mineral or organic fertilizers is directly related to soil fertility, soil carbon sequestration, greenhouse gas (GHG) emissions, and crop yields. Mineral fertilizers play an important role in rapidly increasing soil fertility and crop yield due to their high nutrient content and ease of availability. However, one of the most important factors here is the selection of the right fertilizer form and dose. It is understood that this type of fertilization is not the most effective or sufficient when considering the preservation of soil fertility and a crop with a high nutritional value in terms of sustainable agriculture, as an excessive application of mineral fertilizers can lead to low nutrient productivity and the degradation of the soil and environment [6,7,8]. Therefore, this situation increases the demand for the use of alternative organic fertilizers. Organic matter is of great importance in terms of soil fertility and sustainable agriculture. The success of sustainable agriculture largely depends on the availability of cost-effective and high-quality organic fertilizers. Organic fertilizers have great functions by increasing the oxygen content of the soil, providing good root development, increasing the water-holding capacity of the soil, and reducing the need for water by preventing salinization. In short, not only do they increase plant nutrition and yield, but they also protect the soil resources and increase their value [9,10,11,12]. In addition, organic fertilizers play a direct role in plant growth as the source of all necessary macro and micronutrients in their available forms during mineralization, as well as in the improvement of the physical and chemical properties of soils [9,13], and they increase soil fertility by activating the soil microbial biomass.

Organic fertilizers include compost (village compost, town compost, water hyacinth compost and vermicompost), farmyard manure (cattle manures, sheep penning and poultry manures), green manures (leguminous plant and non-leguminous plant), biofertilizers (algal biofertilizer, fungal biofertilizer, bacterial biofertilizer or plant-growth-promoting rhizobacteria (PGPR), etc.) [14]. Among the available organic fertilizer sources, vermicompost is a potential source due to the presence of ready-made plant nutrients, growth promoters, and a number of beneficial micro-organisms such as nitrogen stabilizers, phosphorus solvent, and cellulose-decomposing organisms [15].

Leonardite, another organic fertilizer, is a clayey organic sedimentary rock that stratified as a result of the decomposition, humification, oxidation, and metamorphosis of plant and animal remains in swamps and lake environments in prehistoric times under temperature, pressure, and anaerobic conditions over millions of years [16,17]. Leonardite is used as a good soil conditioner in plant production due to its low toxic element content, high humic acid content, and plant nutrient content [16,18].

Farm manure accelerates the activity of micro-organisms by affecting the physical, chemical, and biological properties of the soil. At the same time, it prevents soil compaction by increasing the aeration feature of the soil. In this way, it allows the plant roots to develop more easily. It has also been reported that within crops, vegetables and other hoe crops benefit more from barnyard manure than wheat [19].

The organic matter content is insufficient in almost all of our country’s soils [20]. Organic fertilizers have an extremely important role in eliminating this negativity. The scarcity of organic matter in our soils and the lack of nutrients reveal the importance of giving farm manure and other organic fertilizers to the soil. Many of these fertilizers are abundant in nature [21]. Although their nutrient content is not low, they are important in terms of adding organic matter to the soil and improving the physical properties of the soil. By accelerating the microbiological activity in the soil, it increases the structure, aeration, and water-holding capacity of the soil, as well as provides macro and micronutrients to the soil [9,22]. Meeting the nutritional needs of the increasing population brings with it the necessity of providing a high efficiency per unit area in agricultural areas, and for this reason, it is extremely important to apply fertilizers in order to increase productivity in plant production. Although fertilization is important in these respects, the unconsciously increasing use of chemical fertilizers and pesticides to increase the amount of products, the increasing dependence on these inputs, and the problems caused by this has put the long-term sustainability of the agricultural production system at risk and has caused economic losses [23]. The use of more fertilizers, pesticides, and water than necessary in conventional agriculture over the last fifty years, with the goal of more production, has created negative effects on mother earth and its people. These negativities have led to the emergence of human- and nature-friendly agricultural production techniques, which form the basis of sustainable life. The unconscious and uncontrolled use of synthetic chemicals in conventional agriculture and the deterioration of the natural balance are threats to the food chain and all living things. Organic fertilizers are needed to prevent these losses, to obtain healthy food without polluting natural resources and without disturbing the natural balance, and to increase the yield and especially the quality per unit area.

Organic fertilizers contain higher levels of relatively available nutrients that are essential mainly for plant growth, as well as for soil improvement. In addition, organic fertilizer is considered an important source of humus, a carrier of macro and microelements, and it is also used as a fertilizer in the production of many leguminous plants, as it increases the activity of beneficial micro-organisms [24]. Considering the benefits of organic fertilizers, it is well known that they take more time but have more stable and sustainable effects compared to chemical fertilizers. Especially as a supply of organic matter to the soil for the plant species to be planted after it, the root, stem, and stem residues of the cowpea decompose after harvest and provide organic matter and the nutrients it contains to the soil [25]. However, before the cowpea, which is a legume, can provide sufficient nitrogen and nutrients, the plants should be assisted by fertilization, especially during the first starvation period after planting [26]. In addition, the cowpea needs not only nitrogen but also important macronutrients for growth, such as phosphorus and potassium, as well as microelements. For this reason, fertilization can be very useful, especially in these periods, to meet the nutritional needs of the plant and to provide soil improvement that will encourage plant growth afterwards. Recently, great importance has been given to the use of organic fertilizers in cowpea production to reduce plant and soil contamination with synthetic elements and to reduce chemical fertilizer applications [24].This study was carried out to determine the effects of different organic fertilizer forms on the morphological and yield characteristics and some macro–micronutrient contents of the cowpea (V. unguiculata L.) in low-organic-matter-ingredient soil.

2. Materials and Methods

In this study, which was carried out under the Kahramanmaraş, Türkiye ecological conditions in 2020–2021, the Karagöz cowpea (Vigna unguiculata L.) variety was used as the test material. Karagöz is a horizontal type of cowpea variety. The plant is of medium height, has abundant flowering, and is high yielding. The fruits are shiny, showy, and awnless. The fruits are flat, thin, and long. The pod length varies between 15–18 cm on average. The fruits are green in color, and the seed color is light brown. It is suitable for fresh consumption. It has a long harvest time and is a high-quality variety. Fertilizers, the forms, doses, and properties of which are given in

Table 1. The used fertilizer forms, doses, and properties in the experimental design.

Applications	Application Doses		Properties of Fertilizer Forms (*)
			Organic Matter (%)	Total Nitrogen (%)	Total Phosphorus P2O5 (%)	EC (dS/m)	pH
Control	Control	0 kg ha−1	-	-	-	-	-
Chemical fertilization (CF)	CF	60 kg ha−1	-	20 N	20 P	-	-
Farmyard manure (FM)	FM1	2500 kg ha−1	61.00	0.35	0.10	2.10	7.70
	FM2	5000 kg ha−1
	FM3	7500 kg ha−1
	FM4	10,000 kg ha−1
Leonardite (L)	L1	5000 kg ha−1	55.00	1.40	0.17	1.30	6.00
	L2	10,000 kg ha−1
Vermicompost (V)	V1	2500 kg ha−1	56.10	2.20	0.46	3.60	6.50
	V2	5000 kg ha−1
	V3	7500 kg ha−1
	V4	10,000 kg ha−1

(*): Organic fertilizer analyses were carried out at the ÜSKİM Central Laboratory of Kahramanmaraş Sütçü İmam University.

, were evaluated as trial factors [27]. The organic fertilizers were subjected to analysis to identify their properties at the ÜSKİM Central Laboratory of Kahramanmaraş Sütçü İmam University, and the results are presented in Table 1. For comparison, as the average amount of chemical fertilizer applied under farming conditions, in each block, 60 kg ha⁻¹ nitrogen (N) and phosphorus (P) compound (20–20-0) chemical fertilizer were applied to only one plot, and no fertilizer was applied to another plot, which was accepted as control.

2.1. Characteristics of the Trial Area Soils

Some physical and chemical properties that were determined as a result of the analysis of soil samples taken from 0–30, 30–60, and 60–90 cm depths of the trial soils are given in

Table 2. Some physical and chemical properties of the experimental area soil at different depths (*).

Depth (cm)	Soil Class	Observed Parameters
		Saturation (%)	pH	Salinity (%)	Lime CaCO3 (%)	Organic Matter (%)	K (mg kg−1)	P (mg kg−1)
0–30	Clay	85.80	7.28	0.30	1.00	2.08	266.80	10.46
30–60	Clay	86.35	7.31	0.26	1.10	1.79	291.70	4.92
60–90	Clay	83.60	7.30	0.23	2.90	1.23.	293.90	3.65

(*) Soil analyses were carried out at the Soil Analysis Laboratory of the Eastern Mediterranean Transition Zone Agricultural Research Institute.

. As seen in Table 2, based on the useful soil depth of 0–30 cm, it was determined that the experimental soils were clay, very slightly alkaline, very slightly calcareous, slightly salty, moderate in organic matter, and very rich in potassium and phosphorus content.

2.2. Climatic Characteristics of the Trial Site

Kahramanmaraş province is located in the eastern Mediterranean Region of Türkiye, and the Mediterranean climate is dominant. The summers are dry and hot, and the winters are warm and rainy. The temperature, precipitation, and humidity values for 2020–2021 and the long term (1926–2016) when the experiment was conducted are given in Figure 1.

As seen in Figure 1, it was determined that the temperature values (12.5 and 10.4 °C) in March of both years in which the sowing process was carried out were higher than the long-term average (10.3 °C) of the same month. It was determined that the temperature values (25.1 and 25.6 °C) in June of both years in which the harvesting process was carried out were higher than the long-term average temperature (24.8 °C) of the same month. It was determined that the total precipitation in the months of sowing and harvesting was lower than the long-term average (658.4 mm). While the total moisture content of the first year of the experiment was found to be slightly higher than the long-term average, it was found to be low in the second year.

2.3. Method

The experiment was carried out according to the experimental design of completely randomized blocks with three replications. In the experiment, each plot was set as 4 rows, plot length 5 m, inter-row spacing 70 cm, intra-row spacing 10 cm, and plot area 2.8 m × 5.0 m = 14.0 m². A distance of 1.5 m was left between the blocks and 1 m between the plots. After the seedbed preparation was made in the experimental area, the parceling process was completed, and before planting, 20.20.0 compound fertilizer was applied as 6.0 kg N and 6.0 kg P per hectare as chemical fertilizer. In addition, all of the organic fertilizer forms and doses used as trial factors were applied to the relevant plots before planting. Sowing was performed in the first week of March in both experimental years, and the plants in the remaining area were harvested in the first week of July after the edge effects were removed from each plot. In the study, hoeing was performed twice as a maintenance operation, and irrigation was performed as necessary.

2.3.1. Determination of Growth Parameters and Protein Ratio

In the study, the growth parameters such as plant height (cm), stem thickness (mm), first pod height (cm), number of branches (piece/plant), number of pods (pieces/plant), number of seeds (pieces/pod), 1000 seed weight (g), seed yield (kg ha⁻¹), and protein ratio (%) were measured according to Anonymous [28].

2.3.2. Detection of Chlorophyll Contents

A hand-help Chlorophyll Meter SPAD-502 Plus (Konica Minolta Camera Co., Osaka, Japan) was used to estimate chlorophyll content (represented by the measured SPAD value) according to the optical density difference at two wavelengths. This instrument weighs 200 g (excluding batteries), has a 2 mm × 3 mm measurement area, and calculates an index in SPAD units. The measurement with high accuracy of the SPAD-502 Plus is ± 1.0 SPAD units [29]. SPAD makes simple, rapid, and non-destructive measurements on leaves of a smaller size to provide a relative indicator of leaf chlorophyll concentration compared to the extraction method [30].

2.3.3. Detection of Macro and Microelements

Cowpea seed samples obtained during harvest from each plot were brought to the laboratory in paper bags, washed with distilled water, after which the excess water was removed with blotting paper, dried at 65 °C for 48 h (until they reached a constant weight), and ground. During the preparation of plant samples for analysis, due care was taken against possible contamination. Approximately 0.3 g of the ground plant parts were taken and wet-burned with 10 mL of nitric acid solution in a microwave sample shredder with a Cem Mars 6 Model 40 combustion unit [31]. The final volumes of the thawed samples were brought up to 100 mL with ultrapure water and filtered through blue-banded filter paper. Phosphorus, potassium, calcium, magnesium, boron, iron, copper, zinc, and manganese concentrations of plant nutrients in the obtained filters were determined by the ICP-OES device [32].

2.4. Statistical Analysis of Data

A data set was created with the findings obtained in the field and laboratory studies, variance analysis was performed with the Costat (v. 6.03) statistical package program to reveal the effectiveness of the factors, and the difference between the factors was determined with the Least Significance Test (LSD) at the 5% significance level. Correlation analysis was performed with the SSPS (v. 23.0) statistical program to reveal the relationship between the examined features.

3. Results and Discussion

The climate and soil of the region support high-input intensive agriculture to achieve the maximum yield per unit area. Inputs such as chemical fertilizers, pesticides, weed pesticides, and irrigation water that are used excessively and unconsciously spoil the precious land and water resources of the region. Many early measures are needed to adequately protect both soil fertility and quality, as well as water resources. As it is known, the excessive use of chemical fertilizers, such as nitrate, causes chemical pollution, not only in soil resources but also in underground and surface water resources. For this reason, it is necessary to demonstrate the effectiveness of organic fertilizer applications both for the protection of these resources and for sustainable agriculture and crop productivity in the region. Among the various crops grown in the region, the interest in cowpea production is increasing. The application of plant nutrients in the form of organic fertilizer has a great potential to improve the plant growth, yield, and yield-related properties of the cowpea. Therefore, it is essential to evaluate the effectiveness of organic fertilizers in plant production, which will be an alternative to chemical fertilizers, in order to maintain the long-term productivity and sustainability of the region’s soils. As a result of the variance analysis of the data obtained from the two-year study, it was determined that there was no statistical (p > 0.05) difference between the years, and for this reason, the two-year data were combined and subjected to the analysis of variance in an attempt to reveal the differences between the applications. The results of the variance analysis showed that the differences between the applications were found to be statistically significant at the levels of 1% or 5% in terms of their effects on all the physiological and quality characteristics examined. The average values of the yield and some quality parameters that were observed in the cowpea using different fertilizer forms as well as the groups formed as a result of the LSD multiple comparison test are presented in

Table 3. The effects of different fertilizer forms on some observed yield parameters of cowpea.

Treatments		Plant Height (cm)		Stem Diameter (mm)		First Pod Height (cm)		Number of Branches (Pieces/Plant)		Number of Pods (Pieces/Plant)		Number of Seeds (Pieces/Pod)
Control		111.8	C–F	6.97	D	37.27	BC	4.63	BC	7.11	F	8.22	C
Chemical Fertilization		139.3	A	8.27	AB	39.33	AB	4.86	A	7.98	EF	8.66	BC
Vermicompost (V)	V1	106.4	EF	7.37	CD	33.81	C	4.37	BC	8.35	DE	8.80	A–C
	V2	109.5	D–F	7.72	BC	39.26	AB	4.14	CD	8.70	DE	8.49	C
	V3	118.3	B–D	8.05	AB	40.25	AB	4.42	BC	8.37	DE	8.64	BC
	V4	139.1	A	7.69	BC	36.99	BC	4.13	CD	9.92	BC	8.83	A–C
Farmyard manure (FM)	FM1	115.3	C–E	7.37	CD	34.42	C	3.95	D	10.58	AB	8.36	C
	FM2	102.5	F	7.62	B–D	36.81	BC	3.53	E	8.43	DE	8.37	C
	FM3	115.0	C–E	7.65	BC	40.26	AB	3.98	D	9.04	CD	8.21	C
	FM4	122.3	BC	7.68	BC	40.26	AB	4.33	BC	10.69	AB	9.39	A
Leonardite (L)	L1	127.0	B	7.35	CD	42.03	A	3.98	D	8.52	DE	8.51	C
	L2	119.8	B–D	8.41	A	37.63	BC	4.46	BC	11.48	A	9.25	AB
	CV	5.583	*	5.122	*	6.637	*	4.765	*	5.909	*	4.687	*

*: There is no statistically (p > 0.05) significant difference between the means shown with the same superscript, capital letters in the same column.

and

Table 4. The effects of different fertilizer forms on some observed yield and quality parameters of cowpea.

Treatments		Total Chlorophyll before Flowering (SPAD)		Total Chlorophyll after Flowering (SPAD)		1000 Seed Weight (g)		Protein Ratio (%)		Yield (kg ha−1)
Control		59.26	A	53.26	DE	17.70	A–C	23.92	A–C	3121.7	EF
Chemical Fertilization		51.09	D	56.45	A–E	18.12	AB	23.14	DE	3763.3	A–C
Vermicompost (V)	V1	53.71	B–D	56.09	B–E	17.64	A–C	22.69	E	3442.5	C–F
	V2	58.80	A	60.97	AB	17.34	C	23.42	CD	3575.0	B–E
	V3	56.30	AB	51.25	EF	18.12	AB	23.61	B–D	3573.3	B–E
	V4	55.53	A–C	47.10	F	18.00	A–C	23.42	CD	4126.7	A
Farmyard manure (FM)	FM1	52.92	B–D	54.95	C–E	17.57	BC	24.21	A	3669.6	A–D
	FM2	50.88	D	59.08	A–C	18.17	AB	23.65	B–D	3275.8	D–F
	FM3	55.89	A–C	61.90	A	18.01	A–C	23.51	B–D	4026.3	AB
	FM4	55.80	A–C	61.58	AB	17.74	A–C	23.45	CD	3043.3	F
Leonardite (L)	L1	52.25	CD	60.05	A–C	17.30	C	23.29	D	3110.8	EF
	L2	56.71	AB	58.70	A–D	18.35	A	24.00	AB	3867.1	A–C
	CV	4.278	*	5.994	*	2.359	*	1.334	*	8.055	*

*: There is no statistically (p > 0.05) significant difference between the means shown with the same superscript, capital letters in the same column.

.

3.1. Effects of the Different Fertilizer Forms on Yield and Quality Components in Cowpea

As seen in Table 3, the plant heights ranged from 102.5 cm to 139.3 cm. The highest plant height values (139.3 cm and 139.1 cm) were obtained from the chemical fertilizer and V4 applications, while the lowest value (102.5 cm) was obtained from the FM2 application. It is thought that this beneficial effect of vermicompost on the plant height of cowpea may be due to the presence of plant-growth-promoting substances/hormones and micro-organisms in it that make them useful. These findings show that even a high level of farmyard manure [33] and leonardite applications cannot be converted into available nutrients required for cowpea growth in the short term compared to chemical fertilizers. For the rapid growth of the cowpea, the ready availability of nutrients obtained from chemical fertilization and the available nutrients at the highest dose of the vermicompost led to the formation of taller plants. The results of the study are partially in agreement with the results of Adediran et al. [34], who examined the effects of organic and inorganic fertilizer applications on the yield elements of sustainable corn and cowpea crops in Nigeria and reported that the highest plant height values were obtained from vermicompost applications.

The stem diameters in the study ranged from 6.97 mm to 8.41 mm under the effects of different fertilization forms and doses. The highest stem diameter was obtained from the L2 treatment, with 8.41 mm, while the lowest stem diameter was obtained from the control treatment, with 6.97 mm (Table 3). In the study, it was observed that the organic origin leonardite fertilizer had a beneficial effect on stem thickness compared to the control and chemical fertilizer, and similar results were reported by Nuon [35]. In addition, Nazli et al. [36] reported that the thickest stalks were obtained from leonardite fertilizer application compared to other organic and inorganic fertilizer applications as a result of their study investigating the effects of different organic materials on the nutrient uptake of silage maize.

As seen in Table 3, the first pod heights ranged from 33.81 cm to 42.03 cm. The highest first pod height was obtained from the L1 treatment, with 42.03 cm, while the lowest first pod heights were obtained from the V1 treatment, with 33.81 cm, and FM1, with 34.42 cm. Consistent with the study results, Uçar, Soysal and Erman [16] reported that leonardite applications had a positive effect on the height of the first pod of the chickpea, another legume plant.

In the experiment, the number of branches varied between 3.53 and 4.86 pieces/plant. The highest number of branches was obtained from the chemical fertilization treatment, with 4.86 pieces/plant, while the lowest number of branches was obtained from the FM2 treatment, with 3.53 pieces/plant (Table 3). In the study, it was seen that organic fertilizers had a partially negative effect on the number of branches in the plant compared to the control and chemical fertilizers. Contrary to the results of this study, Joshi et al. [37] reported that applied organic fertilizers (vermicompost, farm manure, poultry manure, neem cake, and castor cake) did not have any effect on the number of branches.

As seen in Table 3, the number of pods varied between 7.11 and 11.48 pieces/plant. The highest number of pods was obtained from the L2 treatment, with 11.48 pieces/plant. The lowest number of pods was obtained from the control treatment with 7.11 pieces/plant. In the study, it was observed that organic fertilizers had a positive effect on the number of pods up to a level. The results of the study were found to be in agreement with the findings of Uçar, Soysal and Erman [16], who stated that the leonardite fertilizer, which was determined to have the highest number of pods from these fertilizers, had a positive effect on the number of pods in the plant. In addition, similar to the results of this study, Joshi, Gediya, Patel, Birari and Gupta [37] and Yadav et al. [38] reported that the organic fertilization applied in their studies caused significant increases in the number of pods compared to the control.

As a result of the research, the number of seeds per pod changed between 8.21 pieces and 9.39 pieces. The highest number of seeds was obtained from FM4 treatment, with 9.39 pieces/pod. The lowest number of seeds per pod was obtained from the FM3, control, FM1, FM2, V2, and L1 treatments (8.21, 8.22, 8.36, 8.37, 8.49, and 8.51 pieces, respectively) (Table 3). The result of the study is in agreement with the findings of Didem and Hüsnü [19], who stated that farm manure had a positive effect on the number of seeds in the pod compared to other applications. In addition, in accordance with the results of this study, Joshi, Gediya, Patel, Birari and Gupta [37] and Yadav, Naleeni and Dashrath [38] reported that the fertilization applied in their studies with different organic fertilizer forms caused a significant increase in the number of seeds per pod compared to the control.

As seen in Table 4, the total chlorophyll of the treatments before flowering ranged from 50.88 to 59.26 SPAD values. The highest total chlorophyll was obtained from the control treatment, with a 59.26 SPAD value before flowering, while the lowest total chlorophyll was obtained from the FM2 application before flowering, with 50.88, and chemical fertilization treatments, with a 51.09 SPAD value. As a result of the experiment, the total chlorophyll content after flowering ranged from 47.10 to 61.90 SPAD values, and the highest amount of chlorophyll after flowering was obtained from the FM3 treatment, with 61.90, while the lowest chlorophyll content after flowering was obtained from the V4 treatment, with a 47.10 SPAD value (Table 4). Contrary to the results of this study, Alaboz et al. [39] reported that vermicompost applications did not have any effect on the chlorophyll content of pepper plants. This may be due to the different plant species responding to organic applications and the difference in the nutrient content of the applied organic fertilizers and the application doses. The result of this study is compatible with the study results of Badar, Khan, Batool and Shabbir [9], who stated that organic fertilizers had a positive effect on the chlorophyll content of the wheat plant. In addition, Jan et al. [40] reported that the use of organic fertilizers induces chlorophyll content in wheat plants and the ability to withstand stress conditions.

As seen in Table 4, the 1000 seed weights ranged from 17.30 g to 18.35 g. The highest 1000 seed weight of 18.35 g was obtained from the L2 treatment, while the lowest 1000 seed weight of 17.30 g was obtained from the FM2 and 17.34 g from the V2 treatments. It was determined that leonardite applications had a more positive effect on the 1000 seed weight compared to other organic and chemical fertilizers. Similar results by Uçar, Soysal and Erman [16] have also been reported.

As seen in Table 4, the protein ratios ranged between 22.69% and 24.21%. The highest protein ratio was obtained from the FM1 treatment, with 24.21%. The lowest protein ratio was obtained from the V1 treatment, with 22.69%. The result of this study is in line with the findings of Didem and Hüsnü [19], who reported that farm manure had a positive effect on protein content. Additionally, in his study, Tamer reported that different organic fertilizer applications had a positive and significant effect on protein content, which is a quality parameter in the summer cowpea.

As a result of this study, it was determined that the seed yields varied between 3043.3 and 4126.7 kg ha⁻¹. The highest seed yield was obtained from the V4 treatment, with 4126.7 kg ha⁻¹, while the lowest seed yield was obtained from the FM4 treatment, with 3043.3 kg ha⁻¹ (Table 4). It was observed that as the amount of leonardite and vermicompost application increased, an increase in seed yield occurred, but a decrease was experienced in seed yield compared to other fertilizer applications depending on the increased rate of farm manure. Thus, in this study, it was seen that the vermicompost used for the cowpea provided a better microbial environment compared to the chemical fertilizers, farmyard manure, and leonardite, so it had a more positive effect on the yield and features that have a positive correlation with the yield. Contrary to the results of this study, Didem and Hüsnü [19] reported in their study that farm manure significantly increased the seed yield of the cowpea compared to the control. This may be due to the difference in the nutrient content of the farm manure used. In this study, the findings on the seed yield and seed yield characteristics under different treatments showed that the seed yield was significantly increased when vermicompost was applied (Table 4). It is thought that increased microbial activity due to beneficial microbes such as phosphorus solvents and nitrogen fixers in the vermicompost causes an increase in the nutrient content concentration of the vermicompost [15], and this leads to an increase in the seed yield in plants.

3.2. Effects of the Different Fertilizer Forms on Macro and Micronutrient Contents of Cowpea

In the analysis of variance, it was determined that the differences between the applications were statistically significant at the 1% or 5% levels in terms of their effects on all the macro and micronutrients examined. The average values and LSD groups of some of the macro and microelements examined as a result of the study are given in

Table 5. The effects of different fertilizer forms on some observed macro and microelements of cowpea.

Treatments		P (g kg−1)		K (g kg−1)		Ca (g kg−1)		Mg (g kg−1)		Fe (mg kg−1)		Mn (mg kg−1)		Zn (mg kg−1)		Cu (mg kg−1)
Control		5.15	AB	13.58	AB	1.14	AB	1.90	A	49.22	CD	17.69	A	35.92	A–C	9.35	B
Chemical Fertilization		4.96	A–D	13.46	BC	1.02	D	1.84	A–C	46.21	DE	16.71	CD	34.76	B–D	9.47	AB
Vermicompost (V)	V1	4.94	B–D	13.35	B–D	1.01	D	1.84	A–C	55.19	A	16.92	B–D	33.11	D	9.21	B
	V2	5.15	AB	13.89	A	1.10	A–C	1.89	AB	53.33	AB	17.63	A	34.17	CD	9.66	AB
	V3	4.86	DE	13.14	CD	1.09	A–C	1.79	BC	46.47	DE	17.68	A	34.87	B–D	9.21	B
	V4	4.88	C–E	13.03	D	1.03	CD	1.74	C	44.54	E	16.30	D	34.80	B–D	8.67	C
Farmyard manure (FM)	FM1	4.76	DE	13.32	B–D	1.08	A–D	1.78	C	48.34	CD	16.66	CD	34.25	CD	9.46	AB
	FM2	4.71	E	13.34	B–D	1.15	A	1.81	A–C	51.32	BC	17.17	A–C	34.07	CD	9.31	B
	FM3	4.79	DE	13.22	B–D	1.11	AB	1.80	A–C	48.14	CD	17.43	AB	34.00	CD	9.29	B
	FM4	5.10	A–C	13.41	BC	1.07	B–D	1.82	A–C	50.62	BC	17.41	AB	37.48	A	9.21	B
Leonardite (L)	L1	5.17	A	13.54	AB	1.11	AB	1.83	A–C	48.02	C–E	17.22	A–C	36.84	AB	9.84	A
	L2	4.95	A–D	13.45	BC	1.08	A–D	1.80	A–C	47.90	C–E	17.16	A–C	34.77	B–D	9.31	B
	CV	2.685	*	1.669	*	3.769	*	3.309	*	4.236	*	2.131	*	3.682	*	2.876	*

*: There is no statistically (p > 0.05) significant difference between the means shown with the same superscript, capital letters in the same column.

. The data in Table 5 on the intake of macro and micronutrients show that the intake of these nutrients was significantly affected by different practices.

As seen in Table 5, the highest P content in the seed was obtained from the L1 application, with 5.17 g kg⁻¹, while the lowest P content was obtained from the FM2 application, with 4.71 g kg⁻¹. In this study, it was observed that all the other applications, except the first dose of leonardite, decreased the phosphorus content compared to the control. In accordance with the results of the study, Kumari and Ushakumari [15] reported that the highest amount of phosphorus intake was obtained from the application of vermicompost in the cowpea. Micro-organisms in organic fertilizers mineralize organic phosphorus into soluble forms. These reactions take place in the rhizosphere, and organisms convert more phosphorus into a useful form than is required for their growth and metabolism, allowing plants to take up the excess amount [15].

While the highest K content in the seed was obtained from the V2 application, with 13.887 g kg⁻¹, the lowest K content in the seed was obtained from the V3 application, with 13.025 g kg⁻¹ (Table 5). Contrary to these results, Kumari and Ushakumari [15] noted a higher K availability in vermicompost-treated plots compared to farm-manure-treated plots. However, supporting the results of this study, Basker et al. [41] concluded in their study that vermicompost increases K availability by shifting the balance between K forms from relatively non-existent forms to more suitable forms.

As a result of this study, the highest Ca amount in the seed was obtained from the FM2 application, with 1.148 g kg⁻¹, while the lowest Ca amount was obtained from V1, with 1.012 g kg⁻¹, and the control application, with 1.017 g kg⁻¹ (Table 5). Farm manure plays a direct role in plant growth as a source of all the necessary macro and micronutrients in available forms during mineralization and improves the physical and chemical properties of soils [42]. However, although farmyard manure contains all the needed macro and micronutrients, the soil will increase the presence of some micro and macronutrients, which can already be found in excess, and may cause an antagonistic or synergistic effect between these elements. Therefore, it can also prevent or increase the intake of some elements [43].

As seen in Table 5, while the highest Mg content in the seed was obtained from the control application, with 1.898 g kg⁻¹, the lowest Mg content in the seed was obtained from the applications of V4 (1.742 g kg⁻¹) and FM1 (1.784 g kg⁻¹). It was determined that organic and inorganic fertilizer applications in this study caused decreases in Mg content compared to the control. Contrary to the results of this study, Kumari and Ushakumari [15] reported that enriched vermicompost applications increased the Mg content in the plant. Additionally, the reason for the decrease in Mg content in this study may have been due to the antagonistic effect between K and Mg, as stated by Xie et al. [44].

While the highest Fe content in the seed was obtained from the V1 application, with 55.19 mg kg⁻¹, the lowest Fe content was obtained from the V4 application, with 44.54 mg kg⁻¹ (Table 5). In this study, it was observed that the doses of the fertilizer forms used had a significant and different effect on the Fe content of the cowpea at lowering and increasing rates compared to the control. Kumari and Ushakumari [15], on the other hand, found that there was no significant difference in micronutrient uptake between treatments. However, compared to the FYM-treated plots, the vermicompost-treated plots showed an improved micronutrient uptake.

As seen in Table 5, while the highest Mn amount in the seed was obtained from the control, V3 and V2 applications (17.69, 17.68, and 17.63 mg kg⁻¹, respectively), the lowest Mn amount in the seed was obtained from the V4 application, with 16.30 mg kg⁻¹. For Mn micronutrient intake, it was observed that the other treatments, except the V3 and V2 treatments, caused a decrease in Mn content compared to the control. Compared to the other organic fertilizers, vermicompost is rich in nutrients such as high levels of N, P, K, Ca, and Mg, as well as micronutrients such as Fe, Zn, Cu, and Mn. In a previous study, it was stated that there was no significant difference in micronutrient (Mn) intake between the applications of applied fertilizers. However, compared to FYM-treated plots, vermicompost-treated plots showed a better micronutrient uptake [15].

While the highest Zn content in the seed was obtained from the FM4 application, with 37.48 mg kg⁻¹, the lowest Zn content in the seed was obtained from the V1 application, with 33.11 mg kg⁻¹ (Table 5). Similar results were reported by Sharma et al. [45]. Contrary to the results of this study, [15] reported that organic and chemical fertilizer applications did not have any effect on zinc content. This may be due to the nutrient content of the fertilizers used and the difference in application doses.

As seen in Table 5, while the highest Cu content in the seed was obtained from the L1 application, with 9.84 mg kg⁻¹, the lowest Cu content in the seed was obtained from the V4 application, with 6.87 mg kg⁻¹. It was determined that the results of the study were in agreement with the results of Yolcu [46], who stated that leonardite applications significantly increased the Cu content of Vicia sativa L.

3.3. The Relationships among the Observed Characteristics

In this study, the Pearson correlation analysis test was performed to reveal the relationships among all the investigated properties of Vigna unguiculata L. seeds that were subjected to different organic fertilizer forms and doses. It was determined that there were statistically (p < 0.01 or p < 0.05) significant positive and negative relationships between the effects of fertilizer forms and doses that were applied among the many properties examined (

Table 6. The correlations among the observed parameters of cowpea under the effects of different fertilizer forms.

Observed Parameters	PH	SD	FPH	NB	NPP	NSP	TCBF	TCAF	TSW	PR	Y	P	K	Ca	Mg	Fe	Mn	Zn	Cu
Plant height (PH)	1	0.320	0.309	0.420 *	0.190	0.120	−0.120	−0.330	0.090	−0.100	0.305	0.073	0.316	0.308	0.384 *	0.310	0.104	−0.135	0.273
Stem diameter (SD)		1	0.128	0.376 *	0.410 *	0.080	−0.121	−0.013	0.270	−0.010	0.330	0.151	0.164	−0.019	0.100	−0.152	−0.206	0.183	0.294
First pod height (FPH)			1	0.022	−0.120	0.133	0.081	0.411 *	−0.078	0.053	−0.021	0.251	0.305	0.132	0.265	0.072	−0.187	0.010	0.207
Number of branches (NB)				1	−0.022	0.048	0.187	−0.285	0.046	−0.130	0.095	0.086	0.078	−0.329 *	0.151	0.333 *	−0.136	−0.235	0.094
Number of pods per plant (NPP)					1	0.340 *	−0.034	0.083	0.017	0.388 *	0.282	−0.080	−0.074	0.054	−0.079	−0.401	−0.139	0.149	0.107
Number of seed per pod (NSP)						1	−0.040	0.067	0.336 *	−0.150	−0.040	−0.340	0.024	−0.125	−0.195	−0.210	−0.021	−0.342 *	−0.051
Total chlorophyll before flowering (TCBF)							1	−0.110	−0.060	0.118	−0.040	−0.210	−0.166	−0.279	−0.149	−0.030	−0.430 **	−0.236	−0.176
Total chlorophyll after flowering (TCAF)								1	−0.170	0.101	−0.160	0.340 *	0.186	−0.035	0.109	−0.460	−0.012	0.175	0.017
1000 seed weight (TSW)									1	−0.030	0.247	−0.070	0.124	−0.152	0.016	0.211	−0.060	−0.046	0.200
Protein ratio (PR)										1	0.265	−0.020	−0.013	−0.202	−0.033	0.001	−0.353 *	−0.041	0.079
Yield (Y)											1	−0.040	−0.012	0.106	−0.072	−0.103	−0.141	0.112	0.235
Phosphorus (P)												1	0.486 **	0.036	0.496 **	0.227	0.214	0.655 **	0.422 *
Potassium (K)													1	0.463 **	0.649 **	0.210	0.429 **	−0.048	0.732 **
Calcium (Ca)														1	0.472 **	−0.060	0.604 **	0.079	0.480 **
Magnesium (Mg)															1	0.314	0.547 **	0.191	0.478 **
Ferrium (Fe)																1	0.185	−0.071	0.159
Manganese (Mn)																	1	0.142	0.323
Zinc (Zn)																		1	0.106
Cupper (Cu)																			1

* Correlation is significant at the 0.05 level; ** Correlation is significant at the 0.01 level.

). As presented in Table 6, in this study, it was observed that plant height was significantly and positively associated with the number of branches (r = 0.420*) and magnesium (r = 0.384*), and the stem diameter was positively correlated with the number of branches (r = 0.376*) and the number of pods per plant (r = 0.410*).

The first pod height had a positive association with total chlorophyll after flowering. As seen in Table 6, while the number of branches was negatively associated with Ca (r = −0.329*), it was positively correlated with Fe (r = 0.333*). The number of pods per plant was positively correlated with the number of seeds per pod (r = 0.340*), and protein ratio (r = 0.388*), while negatively correlated with Fe (r = −0.401*). In this study, it was determined that the number of seeds per pod had a positive correlation with the 1000 seed weight, while it had a negative correlation with P (r = −0.335*) and Zn (r = 0.342*). It was observed that the total chlorophyll before flowering was negatively correlated with Mn (r = −0.340*). In addition, the total chlorophyll after flowering had a significant negative correlation with Fe (r = −0.460*). As a quality parameter, the protein ratio was negatively correlated with Mn (r = −0.353*). As seen in Table 6, according to the correlations among the observed macro and micronutrient elements, it was determined that there was no antagonistic effect among them. Additionally, most of the elements had a significant positive correlation with each other.

4. Conclusions

According to the results of the two-year research, the differences between fertilizer applications were found to be statistically significant at the level of 1% or 5% in terms of all the properties examined. From the obtained results, it was concluded that using organic fertilizers can improve the nutritional status of cowpea plants and increase plant growth. According to the results of this study, it is recommended to apply 10,000 kg vermicompost per hectare where the highest seed yield value (4126.7 kg ha⁻¹) can be obtained as a fertilizer type applicable in cowpea cultivation in Kahramanmaraş and similar conditions. These results suggest that organic fertilizers should be given greater consideration in sustainable agriculture, as they can be considered not only as an environmentally sound substitute for the growing medium but also as a substitute fertilizer for organic crop production.