Germination and initial growth of four cover crops as a function of liquid organic fertilization

Germinação e crescimento inicial de quatro plantas de cobertura em função da adubação orgânica líquida

Abstract

The application of compounds rich in humic substances in agriculture may improve the soil attributes and development of plants, but with unknown effects in seed germination, which may be inhibited or induced. Therefore, we aimed to study the germination and initial growth of four cover crops as a function of liquid organic matter (LOM) application. The experiment was carried out in a completely randomized design and a 4 x 3 factorial scheme (four legume species: Mucuna aterrima, Mucuna cinereum, Crotalaria spectabilis e Crotalaria breviflora and three levels of LOM: absence (0.0 g L^-1), low concentration (28.0 g L^-1), and high concentration (56.0 g L^-1). The analyses were: germination (%), index speed germination, average germination time (days), and seedling root and shoot lengths (cm). The use of LOM determined an increase in seedling germination and growth of Mucuna aterrima and Crotalaria spectabilis, especially Crotalaria spectabilis seeds, which when exposed to 2.5% LOM presented a 40% increase in germination, reaching values of 98%.

Keywords:
humic substances; legumes; green manures; biostimulant

Resumo

A aplicação de compostos ricos em substâncias húmicas na agricultura pode proporcionar a melhoria dos atributos do solo e no desenvolvimento das plantas, porém com função desconhecida na germinação das sementes, podendo inibir ou induzir o seu processo. Portanto, objetivou-se estudar a germinação e o crescimento inicial de quatro plantas de cobertura em função de aplicação de matéria orgânica liquida (LOM - liquid organic matter). O trabalho foi conduzido no Laboratório de Análise de Sementes do CCAE-UFES, em delineamento inteiramente casualisado, em um esquema fatorial 4 x 3 (quatro espécies de leguminosas: Mucuna aterrima, Mucuna cinereum, Crotalaria spectabilis e Crotalaria breviflora e três níveis de LOM: ausência (0,0 g L^-1); baixa concentração (28,0 g L^-1) e alta concentração (56,0 g L^-1). Foram analisados: germinação (%), índice de velocidade de germinação, tempo médio de germinação (dias) e os comprimentos de raiz e de parte aérea das plântulas (cm). A utilização de LOM determinou aumento na germinação e no crescimento das plântulas de Mucuna aterrima e a Crotalaria spectabilis, destacando-se as sementes de Crotalaria spectabilis, que apresentaram, com a aplicação da dose de 2,5% de LOM, um aumento de 40% na germinação, atingindo valores de 98%.

Palavras-chave:
substâncias húmicas; leguminosas; adubos verdes; bioestimulante

1. Introduction

The management of agroecosystems with cover crops is one of the strategies with great potential for maintaining or increasing productive stability in agricultural systems (Angeletti et al., 2024; Giacalone et al., 2021; Liu et al., 2022). These plants can contribute to climate regulation, to reduce environmental impacts from agriculture, and also to reduce the dependence of farms on agricultural inputs (Giacalone et al., 2021; Liu et al., 2022; Monteiro et al., 2017; Santos et al., 2018). A considerable number of plant species may be used for this purpose. However, the plants of the family Fabaceae (legumes) can add to the benefits of this practice the increase of soil nitrogen concentrations, reducing the use of agricultural inputs (Angeletti et al., 2019; Foster et al., 2017).

Several species of the Fabaceae family have the potential to be used as cover crops, especially Crotalarias sp., because they are suitable for crop rotation in different environments, providing high biomass production and great nitrogen fixation capacity (Foster et al., 2017). Additionally, Crotalaria spectabilis and Crotalaria breviflora are species that have the potential to combat nematodes in infested areas (Inomoto et al., 2023; Rosa et al., 2015). Other species that are similarly of great agronomic interest are Mucuna cinereum and Mucuna aterrima, which when used in agricultural consortia can contribute to increased production and improvement of soil fertility (Lima-Filho et al., 2023; Masikati et al., 2014).

On the other hand, the percentage of germination of “non-domesticated” species, which did not undergo a genetic selection process over time, as well as those described above, is low (Lima-Filho et al., 2023). Furthermore, the studies on the conditions that may favor the increase of their germination are still scarce. The germination process is influenced by several environmental variables, among which the availability of oxygen, humidity, temperature and the influences of growth regulating phyhormones (Baskin and Baskin, 2014).

In this context, the application of compounds rich in humic substances, resulting from the microbiological decomposition of plant and animal tissues in the soil, may be a strategy to increase the germination rates of different plant species. These potential biostimulant (Nardi et al., 2016) substances are mainly composed of humic and fulvic acids, which have a large number of functional groups, assist in the transport of ions leading to increased cellular permeability, and promote the increase of plant enzymatic reactions (Rodrigues et al., 2017).

The literature points out several benefits that the application of humic substances can provide for agriculture (Pittarello et al., 2018; Pukalchik et al., 2017; Zanin et al., 2017). However, the volume of information about the influence of this material on seed germination is still very scarce, and there are no further studies of this nature with plants potentially used as green fertilizers.

The objective of this study was to evaluate the germination and initial growth of four cover crops as a function of liquid organic matter application.

2. Material and Methods

This work was conducted in the Seed Analysis Laboratory in the Department of Plant Production of the Universidade Federal do Espírito Santo (CCAE-UFES), located in Alegre, ES. The experiment was conducted in a completely randomized design and 4 x 3 factorial scheme (four species of green manure: Mucuna aterrima, Mucuna cinereum, Crotalaria spectabilis and Crotalaria breviflora, and three concentrations of liquid organic matter: absence = 0% _(v/v) (0 mg L^-1), low concentration = 2.5% _(v/v) (28 g L^-1), and high concentration = 5.0% _(v/v) (56 g L^-1)), with four repetitions.

The physicochemical characteristics and the available macro and micronutrient levels of liquid organic matter (LOM) are shown in Table 1.

Table 1
Characterization1 of the applied organic matter.

Physicochemical characteristics	Unit	Quantity
Water-soluble total organic carbon	g L-1	67.2
Salinity index	-	1.0
Density	g cm-3	1.12
Electric conductivity	mS cm-1	0.01
Humic acids	%	20
Fulvic acids	%	10
Nitrogen	dag kg-1	0.6
Potassium	dag kg-1	0.1
Calcium	dag kg-1	0.02
Magnesium	dag kg-1	0.1
Iron	mg kg-1	5.1
Zinc	mg kg-1	0.5
Copper	mg kg-1	3.6
Manganese	mg kg-1	1.1
Boron	mg kg-1	178

• ¹
  Source: Sampaio (2013).

In each experimental unit 25 seeds of the correspondent cover crop were used. The cover crops seeds were firstly disinfested with 70% alcohol for one minute and then immersed in 2.5% sodium hypochlorite for five minutes. Subsequently, the seeds were washed with distilled water for three consecutive times, followed by sowing on three sheets of Germitest paper in gerbox-type boxes, humidified with an amount of solution equivalent to 2.5 times the dry paper mass. The experiment was conducted in a room with a controlled temperature of 25 °C, and the following variables were analyzed:

Seed water content was determined with two replicates, by the oven-drying method, at 105 ± 3 °C, for 24 hours, as instructed in the Rules for Seed Analysis (Brasil, 2009).

Germination was recorded daily by recording the percentage of normal seedlings (Brasil, 2009), and the results were expressed as the percentage of germination.

Index speed germination (ISG) was determined according to the Maguire method (Maguire, 1962), concomitant with the germination test, and the number of seeds that exhibited a protrusion of the primary root equal to or greater than 2 mm were recorded daily until the 15^th day.

Average germination time (AGT) was calculated according to Labouriau and Valadares (Labouriau and Valadares, 1976).

Lengths of the shoot and root system were determined after 15 days of sowing with the aid of a millimeter ruler. Shoot measures were performed on the region between the collar and the apex of the last leaf of ten seedlings, root measures were performed between the collar and the end of the largest root. The measures were expressed in cm plant^-1.

Shoot and root dry mass of the seedlings was determined after 15 days of sowing, with the aid of an analytical balance (0.0001 g). The seedlings were sectioned by separating the shoot and root parts, packed in Kraft paper bags, kept in a convection oven at 65 ºC for 72 hours (constant mass). The aforementioned measures were expressed in mg seedling^-1.

In this study, orthogonal contrasts were used to evaluate the factors under study (Table 2) (Chew, 1977). The mean values of the treatments were submitted to analysis of variance to obtain the residual sum of squares and later calculations of the contrasts. Considering this is an exploratory study, treatment means were plotted as a function of LOM levels to enable the determination of seed response type, albeit it is not possible to obtain the regression coefficients. In addition to the orthogonal contrasts, four further contrasts were added to compare the best treatments.

Table 2
Orthogonal contrasts1 used in the evaluation of the dependent variables, considering the different species and doses of liquid organic matter.

Plant 2	LOM 3	C1	C2	C3	C4	C5	C6	C7	C8	C9	C10	C11	C12	C13	C14	C15
P1	D0	-1	-1	0	-2	0	0	0	0	0	0	0	-1	0	0	0
P1	D1	-1	-1	0	1	-1	0	0	0	0	0	0	1	0	0	0
P1	D2	-1	-1	0	1	1	0	0	0	0	0	0	0	0	0	0
P2	D0	-1	1	0	0	0	-2	0	0	0	0	0	0	-1	0	0
P2	D1	-1	1	0	0	0	1	-1	0	0	0	0	0	1	0	0
P2	D2	-1	1	0	0	0	1	1	0	0	0	0	0	0	0	0
P3	D0	1	0	-1	0	0	0	0	-2	0	0	0	0	0	-1	0
P3	D1	1	0	-1	0	0	0	0	1	-1	0	0	0	0	1	0
P3	D2	1	0	-1	0	0	0	0	1	1	0	0	0	0	0	0
P4	D0	1	0	1	0	0	0	0	0	0	-2	0	0	0	0	-1
P4	D1	1	0	1	0	0	0	0	0	0	1	-1	0	0	0	1
P4	D2	1	0	1	0	0	0	0	0	0	1	1	0	0	0	0

• ¹
  C1 = Mucuna sp. vs Crotalaria sp.; C2 = Mucuna aterrima vs Mucuna cinereum; C3 = Crotalaria spectabilis vs Crotalaria breviflora; C4 = absence vs. presence of LOM within Mucuna aterrima; C5 = low dose vs high dose of LOM within Mucuna aterrima; C6 = absence vs presence of LOM within Mucuna cinereum; C7 = low dose vs high dose of LOM within Mucuna cinereum; C8 = absence vs presence of LOM within Crotalaria spectabilis; C9 = low dose vs high dose of LOM within Crotalaria spectabilis; C10 = absence vs presence of LOM within Crotalaria breviflora; C11 = low dose vs high dose of LOM within Crotalaria breviflora; C12= absence vs low dose of LOM within Mucuna aterrima; C13 = absence vs low dose of LOM within Mucuna cinereum; C14= absence vs low dose of LOM within Crotalaria spectabilis; C15 = absence vs low dose of LOM within Crotalaria breviflora;
• ²
  P1 = Mucuna aterrima; P2 = Mucuna cinereum; P3 = Crotalaria spectabilis; P4 = Crotalaria breviflora;
• ³
  Absence = 0.0% LOM, Low = 2.5% LOM, and High = 5.0% LOM.

3. Results

The mean values of germination, index speed germination, average germination time, shoot length, and root length are shown in Table 3. It is noted that all seeds present low germination under the distilled water treatment, and only Crotalaria breviflora shows germination greater than 60% (Table 3).

Table 3
Mean values of germination, index speed germination, average germination time, shoot length, and root length.

Legumes	LOM 1	GER 2	ISG	AGT	SL	RL
		%	-	Days	------ cm ------
Mucuna aterrima	Absence	38	1.37	5.39	4.74	6.70
	Low concentration	47	1.55	6.33	6.11	4.88
	High concentration	20	1.44	6.00	2.13	1.83
Mucuna cinereum	Absence	57	2.01	5.56	10.16	11.43
	Low concentration	52	1.65	5.94	8.83	8.61
	High concentration	33	1.73	6.41	6.12	3.99
Crotalaria spectabilis	Absence	48	2.56	4.00	5.03	14.66
	Low concentration	98	0.75	4.08	6.10	18.21
	High concentration	48	2.02	4.05	3.46	11.20
Crotalaria breviflora	Absence	67	2.46	4.23	6.32	12.28
	Low concentration	34	2.42	4.07	3.13	9.70
	High concentration	10	2.09	5.77	0	0

• ¹
  Liquid Organic;
• ²
  Germination (GER); index speed germination (ISG); average germination time; shoot length (SL); root length (RL).

The germination of Mucuna sp. was higher than that of Crotalaria sp., with Mucuna aterrima being the species with the highest germination percentage among the Mucuna sp., and Crotalaria spectabilis with the highest germination among Crotalaria sp. (Table 4).

Table 4
Orthogonal and additional contrasts for germination evaluation, index spped germination, average germination time, shoot length, and root length.

Contrast 1	GER 2	ISG	AGT	SL	RL
C1	-34.47***	10.10***	-37.77***	-56.23***	114.51***
C2	-35.24***	4.11***	0.80	48.45***	42.56*
C3	-3.43***	6.58***	7.74	-20.58*	-88.36***
C4	-73.77***	0.98	6.17	-4.95	-26.76
C5	-1.04*	-0.43	-1.32	-15.92**	-12.20
C6	-1.16	-2.53*	4.92	-21.48*	-41.05*
C7	-0.78	0.33	1.88	-10.85*	-18.49p
C8	1.97*	-9.39***	0.50	-1.99	0.34
C9	-2.01***	5.10***	-0.12	-10.55*	-28.07*
C10	-3.72***	-1.64	5.50	-38.02***	-59.45**
C11	-1.08*	-1.31*	6.78*	-12.50*	-38.80***
C12	-36.37***	0.71	3.75	5.49	-7.28
C13	-0.19	-1.43*	1.52	-5.32	-11.28
C14	1.99***	-7.24***	0.31	4.28	14.20
C15	-1.32**	-0.17	-0.64	-12.76*	-10.32

• ¹
  C1 = Mucuna sp. vs Crotalaria sp.; C2 = Mucuna aterrima vs Mucuna cinereum; C3 = Crotalaria spectabilis vs Crotalaria breviflora; C4 = absence vs presence of LOM within Mucuna aterrima; C5 = low dose vs high dose of LOM within Mucuna aterrima; C6 = absence vs presence of LOM within Mucuna cinereum; C7 = low dose vs high dose of LOM within Mucuna cinereum; C8 = absence vs presence of LOM within Crotalaria spectabilis; C9 = low dose vs high dose of LOM within Crotalaria spectabilis; C10 = absence vs presence of LOM within Crotalaria breviflora; C11 = low dose vs high dose of LOM within Crotalaria breviflora; C12= absence vs low dose of LOM within Mucuna aterrima; C13 = absence vs low dose of LOM within Mucuna cinereum; C14= absence vs low dose of LOM within Crotalaria spectabilis; C15 = absence vs low dose of LOM within Crotalaria breviflora;
• ²
  Significant values at 0.1% (***), 1% (**) and 5% (*) by the t-test.²Germination (GER); index speed germination (ISG); average germination time; shoot length (SL); root length (RL).

Crotalaria spectabilis was the species that best responded to the application of LOM. The germination of this species was the lowest when submitted to the control treatment, whereas LOM application provided a strong stimulus in germination, especially at the low concentration treatment (Table 4).

As with the seeds of Crotalaria spectabilis, the germination of Mucuna aterrima seems to be benefited by the application of LOM. Although there was no statistical difference in the germination of Mucuna aterrima in the treatments with the application of LOM, germination presented a quadratic response to the increase of LOM concentration in the study range. Thus, a dose within this range can provide the maximum germination of this species (Figure 1).

Figure 1
Germination, index speed germination (ISG), average germination time (AGT), shoot length (SL), and root length (RL) in the seeds of Mucuna aterrima (A, C, E, G, I) and Mucuna cinereum (B, D, F, H, J) after the application of different concentrations of liquid organic matter.

The highest ISG among the species occurred in the seeds of Crotalaria sp., concomitantly to the lowest AGT of these species (Table 4). Among the species of Crotalaria sp., Crotalaria breviflora presented the highest IVG and lowest AGT, whereas, among the Mucuna sp. species, Mucuna cinereum presented a similar behavior (Table 4).

In general, the application of LOM caused the reduction of the ISG and the increase of the AGT, the only exception was for the seeds of Mucuna aterrima. This species presented a quadratic response to the increase in LOM concentration, indicating that within the study range, an increase of ISG can be obtained (Figure 1). In this sense, the application of LOM can stimulate the shoot length of Crotalaria spectabilis and of Mucuna aterrima, which is demonstrated by the quadratic response to the increased dose (Figures 1 and 2). Similar results occurred in the root length of Crotalaria spectabilis (Figure 2).

Figure 2
Germination, index speed germination (ISG), average germination time (AGT), shoot length (SL), and root length (RL) in the seeds of Crotalaria spectabilis (A, C, E, G, I) and Crotalaria breviflora (B, D, F, H, J) after the application of different concentrations of liquid organic matter.

The number of concentration levels does not allow for the obtainment of the equations regarding the response to the increase in LOM concentration. However, the type of response can indicate if the LOM can contribute to the improvement of the analyzed variables. In general, the germination of the species presented a quadratic response, concavity downward, indicating that there is a concentration within the study range that can provide the maximum germination of the species. A similar response was obtained in the ISG of Mucuna aterrima and Crotalaria breviflora, in the root length of Mucuna aterrima and Crotalaria spectabilis, and in the root length of Crotalaria sp.

4. Discussion

The low germination occurring in the treatment without LOM application can be explained by the fact that undomesticated plants naturally present low germination, which is one of the main problems in the agricultural management of these species (Lima-Filho et al., 2023).

The beneficial effects of LOM on germination and initial plant development may be attributed to the release of organic compounds that make up the humic substances present in this organic matter (Table 1), stimulating seed enzymatic activities. A large number of growth regulators is present in humic substances, some of which are physiologically active and may stimulate plant metabolism. In this scenario, cytokinins and indoleacetic acid stand out, once the first is responsible for cell division, and the second is an auxin that contributes to cell elongation and apical bud formation (Nardi et al., 2016), favoring seedling development.

The presence of hormones in organic materials, such as the one used in the present study, may have a biostimulating effect on the seeds (Monda et al., 2017) when applied in small concentrations. Akinci et al. (2009) observed that the application of 1% humic acids in nutrient solution stimulated seed germination and improved initial development of Vicia faba L.

In general, the presence of LOM in high concentration was detrimental to the germination and development of the green manure seedlings studied, and the seeds of Mucuna cinereum and Crotalaria breviflora did not respond satisfactorily to the application of LOM at any of the doses applied (Figures 1 and 2). These observations might be explained by the doses used being high or the characteristics of the seeds used. Commonly, there is great genetic diversity and high unevenness in the maturation of green manure seeds from the same lot, resulting in heterogeneity in seed germination (Silva et al., 2016).

It should be noted that the concentration of LOM applied is a factor that influences the biological response of the seeds, and even a phytotoxic effect can occur at high concentrations (Nardi et al., 2016). Corroborating with this affirmation, reduced germination of the seeds treated with LOM in high concentration was verified (Figures 1 and 2). Similarly, other authors have observed different effects of the use of humic substances on plant growth (Mota et al., 2015; Rodrigues et al., 2017) and confirmed that the positive effects on plant metabolism were generally obtained with the lowest doses applied. However, in the present work it was clear that the increase in the concentration of humic substances can result in the phytotoxic effect, hindering plant growth.

Results of phytotoxicity as a function of doses in concentrations higher than 0.5% were observed in studies on germination of Myracrodruon urundeuva Fr. All. (Mota et al., 2015) and of 158 mL kg^-1 in the germination of Zea mays L. (Rodrigues et al., 2017). However, there is little information on the physiological effects of increasing the concentration of humic substances on seed germination. On the other hand, it is known that the composition of the humic substances, as well as the physicochemical characteristics, are differentiated by the material that originated it and the way it was extracted (Nardi et al., 2016).

The phytotoxicity observed with the application of high doses of LOM (Figures 1 and 2) may be associated with a high boron concentration in the material (Table 1). Since boron is a micronutrient, the amplitude between the characteristic dose of deficiency and toxicity is very narrow. Mirshekari (2012) observed beneficial effects of boron application on the germination of Anethum graveolens seeds. However, when more concentrated doses were applied, a negative response occurred in the germination of this species.

When evaluating the application of different boron sources, associated or not with fungicides, Ribeiro et al. (1994) observed that the positive effect of boron seed treatment on corn germination occurred only in the treatments that received the fungicide application.

Finally, many studies show that the application of compounds rich in humic substances can benefit agriculture, which is mainly associated with improvements in soil conditions, increased enzymatic activities and their biostimulant effect on plant growth (Nardi et al., 2016; Pittarello et al., 2018; Pukalchik et al., 2017; Zanin et al., 2017). Our results demonstrate that the benefits of using organic compounds rich in humic substances can be extended to the germination process, optimizing the germination of seeds that commonly have low germination. Among the analyzed liquid organic matter doses, the use of 2.5% dose provided the best results in the germination and initial growth of the cover crops, mainly to Crotalaria spectabilis and Mucuna aterrima. However, considering the phytotoxicity effect that can occur when high liquid organic matter doses are used, in the next stage of this study an experiment will be developed in order to establish the ideal doses for each species.

5. Conclusions

Our results showed that the addition of liquid organic matter can benefit germination and initial growth of cover crops. The species that showed the best response to the application of liquid organic matter was the Crotalaria spectabilis, which achieved 98% seed germination when the 2.5% liquid organic matter dose was used. In addition, this dose of liquid organic matter also improved the growth of the shoot and root of this species. Therefore, 2.5% is the dose recommended to increase seed germination of Crotalaria spectabilis. The 2.5% liquid organic matter dose also improved the germination process and the root growth of Mucuna aterrima.

It is important to highlight that the use of high liquid organic matter doses caused phytotoxicity, so, further studies of this nature should be done in order to establish the ideal doses for each species.

Acknowledgements

The authors gratefully acknowledge the professors and employees of the Seed Analysis Laboratory of the Universidade Federal do Espírito Santo, for the support and instructions during the development of the present work. This work was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) (Finance Code 001) and by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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Publication Dates

• Publication in this collection
  31 Mar 2025
• Date of issue
  2025

History

• Received
  07 Sept 2024
• Accepted
  26 Jan 2025