Impact of Intact and Powdered Leaf Amendments from Forestry Species on Okra (Abelmoschus esculentus L.) Germination and Growth

Abstract

Soil is the primary source of nutrients for plants, mainly through the clay–humus complex derived from mineral components and decomposed organic matter. The decomposition of the litter—composed only of leaves—contributes significantly to the nutrient cycle. This study evaluated the effects of the intact leaves and the leaf powder resulting from three agroforestry species: Daniellia oliveri (DO), Terminalia macroptera (TM), and Piliostigma thonningii (PT) on the germination and growth of okra (Abelmoschus esculentus L.). The experiment was conducted under pot conditions using 10, 20, and 30 g doses of intact or powdered litter. Germination rate was measured over a period of 16 days, and plant height, number of leaves, and collar diameter were measured over a 90-day period. According to the results, all litter treatments maintained germination rates above 50%, indicating the absence of phytotoxic effects. Germination was significantly affected by litter treatment under both intact litter and powdered litter applications (p < 0.001). Several leaf powder mixtures achieved 100% germination at 16 days, particularly the combinations DO + TM at 30 g, DO + PT at 20 g, TM + PT at 30 g, and DO + TM + PT at 10 and 30 g. Intact leaves amendments promoted significantly greater vegetative growth than powders. Mean plant height was higher under intact leaves (18.04 cm) than powders (14.36 cm; p < 0.001). Daniellia oliveri produced the greatest plant height (23.33 cm at 20 g), whereas the three-species powder mixture resulted in the lowest values (9.50–10.23 cm at 90 days). Collar diameter was not significantly affected by treatment type. Overall, intact leaf litter was more effective than powders in promoting okra vegetative growth.

1. Introduction

Litter decomposition is a process during which nutrients immobilized in the litter are released and made available. Since organic carbon in litter is the main source of energy for decomposers, the amount of C in litter decreases over time; however, the loss of C in litter is determined by the growth rate and efficiency of decomposers [1,2]. The interaction between decomposers, litter quality and abiotic factors drives litter decomposition, in which litter is broken down into smaller pieces and finally mineralized into inorganic compounds [3]. During the litter decomposition process, the chemical composition of the litter changes due to the degradation of structural and soluble compounds [1,2]. As a result of this process, the availability of nutrients in the soil increases, which, in turn, has an impact on soil fertility. The rate at which plant debris decomposes can vary depending on the composition of the plant species and the particle size of the plant debris. Three factors—climatic, topographical, and geological—also influence the growth of different plant species, which in turn affect the composition of the litter and the nature of the humus. Through its exchange capacity and the balance of its microbial populations, soil provides plants with nutrients in specific proportions [1,2,4]. Also, soil fertility depends on the ease with which the root can benefit, in sufficient quantities, from the various factors necessary for plant growth: heat, water, all the chemical elements required by the plant, and organic growth substances [4,5].

Based on this analysis, plant residues can be considered an important source of organic matter and nutrients that may contribute to improving soil fertility and promoting more sustainable agricultural practices. The decomposition rate of plant litter depends on several factors, including the quality of the organic material, particle size, and plant species composition [2]. These characteristics influence soil mineralization processes, microbial activity, and the rate of nutrient release [6]. At the same time, differences among plant species, as well as the use of litter mixtures, can significantly alter decomposition processes and plant growth responses in different agroecosystems [7,8].

Daniellia oliveri, Terminalia macroptera, and Piliostigma thonningii are important agroforestry species, widespread in the Sudanese and Guinean savanna regions of Africa. These species are well-adapted to the conditions of tropical savanna ecosystems and are frequently used by local communities as a source of firewood, fodder, for soil fertility improvement, and in various traditional medicinal practices. In addition, their high litter production and frequent presence in agricultural landscapes make them valuable sources of organic residues, with significant potential for sustainable soil fertility management and the development of low-input agricultural systems [9,10,11].

According to scientific reports, crop residues are now excluded from green manures, as they do not contribute any material from outside the field. According to the Soil Science Society of 1965, a green manure is plant material incorporated into the soil after the mature stage, with the aim of improving soil fertility [5]. Green manure approaches can also lead to long-term increases in soil organic matter and microbial biomass, further improving nutrient and nitrogen retention and uptake efficiency [12]. In many traditional agricultural practices, practical knowledge of the effects of waste is well known. For example, plant litter was used for mulching in low-tech agriculture, gardening and modern horticulture, to protect against weed infestations, conserve moisture, reduce evapotranspiration and improve ecosystem function [13,14,15].

Currently, several studies have been carried out on the effect of litter and litter powders of certain plant species on crops. Several studies have shown that plant-based mulch and powders derived from plant material can significantly influence seed germination and plant growth [16,17]. Different plant species may exert either stimulatory or inhibitory allelopathic effects on crops, including okra (Abelmoschus esculentus L.) [18]. Furthermore, application of plant powders and litter has been reported to alter germination, root growth, and seedling development through the release of bioactive compounds into the soil [19]. However, information regarding the direct comparison between the effects of intact litter and powdered litter on the germination and growth of okra remains limited. Therefore, further studies are needed to clarify the underlying impact and mechanism involved.

Okra (Abelmoschus esculentus L.) is an important crop species widely used for food, medicinal, and industrial purposes. Its high levels of carbohydrates, proteins, vitamins A and C, iron, phosphorus, potassium and magnesium have been demonstrated [20]. Okra is an important crop due to its nutritional value and its multiple uses in food and agriculture. Germination is considered a critical factor in the plant’s life cycle. In fact, it prepares the seedling for rooting, adaptation to the environment, and subsequent productivity [21]. Thus, evaluating the effects of litter on okra germination and growth is important for understanding its influence on crop development. Furthermore, we hypothesized that litter amendments from Daniellia oliveri, Terminalia macroptera, and Piliostigma thonningii would not exert phytotoxic effects on okra germination; litter mixtures would produce different responses compared with single-species amendments and intact litter would promote greater vegetative growth than litter powders because of its slower decomposition and more gradual nutrient release.

Therefore, the main objective of this study was to (i) evaluate the effects of intact litter from leaves from Daniellia oliveri, Terminalia macroptera and Piliostigma thonningii on okra germination and growth; (ii) assess the impact of leaf litter on okra germination and growth; and (iii) compare the effects of intact and leaf litter in order to determine the most effective form for improving okra growth and development.

2. Materials and Methods

2.1. Biological Materials

The biological material used in this study consisted of the litter composed of leaves of three woody species: Daniellia oliveri (Fabaceae), Terminalia macroptera (Combretaceae), and Piliostigma thonningii (Fabaceae), as well as seeds of okra (Abelmoschus esculentus L., Malvaceae).

It occurs in wooded and shrub savannahs, generally on sandy soils, from sea level up to 1500 m altitude [9,11,22]. It is a medium-sized deciduous tree reaching up to 25 m in height. The species produces abundant litter and is commonly found in the Sudano–Guinean savannahs [10].

Terminalia macroptera is widely distributed in West and Central Africa, from Senegal to Ethiopia [9]. It is characteristic of wooded savannahs and is also found on rocky and lateritic slopes. The species is a small deciduous tree reaching about 13 m in height and is known for its large leaves and important litter production [9].

Piliostigma thonningii occurs in regions receiving 600–1200 mm of annual rainfall and grows from sea level to 2200 m altitude [9]. It develops on different soil types but prefers clay and loamy soils. The species is commonly found in wooded savannahs, gallery forests and secondary forests. It is a deciduous tree or shrub reaching about 10 m in height and represents an important source of fodder and plant litter in African savannah ecosystems.

The species used in this study are common in the Sudano–Guinean savannahs of Cameroon and represent important sources of plant litter in local ecosystems.

Okra (Abelmoschus esculentus L.) is an annual plant cultivated in tropical and subtropical regions. The experiment used West African okra seeds grown using traditional methods in the Adamawa region. It is propagated by seed and harvested for its immature fruits, generally 3 to 5 days after fertilization. The species is widely appreciated for its adaptability to dry conditions and its nutritional value. Before sowing, seeds were sorted to eliminate damaged or diseased seeds.

2.2. Study Site

This work was carried out at the Laboratoire de Biodiversité et Développement Durable for germination tests and at ReviTec, University of Ngaoundéré (Adamawa, Cameroon), for growth experiments. Germination tests were conducted under laboratory conditions.

The Sudano–Guinean zone of Adamawa lies between 6 and 8° N latitude and 10–16° E longitude. It is characterized by high plateaus with an average altitude of approximately 1000 m. The climate includes two seasons: a rainy season from May to October and a dry season from November to April. Annual rainfall ranges from 1500 to 1800 mm, while temperatures vary between 24 and 32 °C. Vegetation is mainly composed of shrub and tree savannahs dominated by Daniellia oliveri and Lophira lanceolata. The herbaceous layer includes Hyparrhenia spp., Andropogon spp., Panicum spp., Pennisetum spp. and Imperata cylindrica [23].

2.3. Soil Characteristics

Soil used in the experiment was collected from the experimental area of the University of Ngaoundéré campus. Surface soil (0–20 cm depth) rich in organic matter was sampled, air-dried at room temperature, thoroughly mixed, and sieved through a 2 mm mesh to obtain a homogeneous substrate and to remove roots, stones, and plant debris. The same soil was used for all experimental pots to ensure uniform experimental conditions.

The soil was moderately acidic, with a pH of 5.6, measured using a digital pH meter in a soil–water suspension (1:2.5, w/v). Prior to the experiment, the physicochemical properties of the soil were analyzed and standardized across all experimental units. Organic carbon content (7.24%) was determined using the Walkley–Black method, while total nitrogen (0.12%) was measured using the Kjeldahl method. The phosphorus concentration (0.05%) was determined using standard colorimetric methods. Exchangeable potassium (0.32 cmol kg⁻¹), calcium (2.10 cmol kg⁻¹), and magnesium (0.84 cmol kg⁻¹) were determined by atomic absorption spectrophotometry after ammonium acetate extraction.

Soil moisture content was maintained at approximately 60–70% of field capacity throughout the experimental period by regular watering.

2.4. Collection and Preparation of Litter

The choice of Daniellia oliveri, Terminalia macroptera and Piliostigma thonningii was based on their abundance, socio-economic importance and litter production capacity in the Sudano–Guinean zone of Adamawa.

Litter from the three species was collected under tree crowns on the Ngaoundéré-Dang University campus during the ripening period. In the experiment, leaf litter consisting of mature, dry leaves was used. To obtain the leaf litter powders, the plant material was shredded and passed through a 2 mm sieve to produce a homogeneous material. Part of the litter was used intact, while another part was ground into powder. Several mixtures, including the control treatment (T), were prepared using intact and powdered litter (

Table 1. Composition of intact litter and litter powder treatments.

Category	Treatment	Composition
Intact litter treatments	T1	100% Daniellia oliveri (DO)
	T2	100% Terminalia macroptera (TM)
	T3	100% Piliostigma thonningii (PT)
Litter powder treatments	T1	100% Daniellia oliveri (DO)
	T2	100% Terminalia macroptera (TM)
	T3	100% Piliostigma thonningii (PT)
	T4	50% DO + 50% TM
	T5	50% DO + 50% PT
	T6	50% TM + 50% PT
	T7	33.33% DO + 33.33% TM + 33.33% PT
Control (T)		No litter

, Figure 1 and Figure 2).

2.5. Effects of Intact Litter and Litter Powders on Okra Germination

Plastic pots measuring 17 cm × 16 cm were filled with sieved soil. Intact litter or litter powders were incorporated into the soil at doses of 10 g, 20 g and 30 g.

For the germination experiment, 30 okra seeds were sown in each pot at a depth of 1.5 cm on 18 April 2023. Pots were watered every two days. The experiment lasted 16 days, corresponding to the maximum germination period. The germinated seeds were counted daily throughout the entire experimental period.

The experimental design was a split-plot with three replicates. Dose represented the main factor, and litter type was the secondary factor. A total of 96 pots were used.

The germination rate (TG) was calculated as the percentage ratio between the number of germinated seeds (NGGs) and the total number of seeds sown (NGS) [24].

2.6. Effects of Intact Litter and Litter Powders on Okra Growth

Growth experiments were conducted using the same pot dimensions and litter doses (10 g, 20 g and 30 g). Five okra seeds were sown per pot on 11 May 2023.

After germination, only the two most vigorous seedlings were retained per pot. Pots were transferred to the ReviTec experimental site and monitored for 90 days.

Growth parameters measured every 15 days included:

‑

Plant height;

‑

Number of leaves;

‑

Collar diameter.

The experimental design was also a split-plot with three replicates, giving a total of 96 pots.

2.7. Statistical Analysis

All statistical analyses were conducted using R software 4.2.2. One-way, two-way and three-way analyses of variance (ANOVA) were applied to assess the effects of litter treatments, doses and evaluation time on germination and growth parameters of okra. When significant differences were detected, means were separated using Tukey’s Honest Significant Difference (HSD) test at the 5% probability level. Student’s t-test was used to compare the overall effects of intact litter and litter powders.

3. Results

3.1. Influence of Intact and Powder Litters on Okra Germination

The germination of okra seeds was significantly influenced by litter treatments (p < 0.001). According to our results, under intact litter treatments, a highly significant effect of treatment type and interaction between treatment and dose was observed.

These results indicate that the response of okra seeds depended both on litter type source and on the quantity applied (

Table 2. Analysis of variance (ANOVA) of okra germination under intact litter treatments.

Effect	Df	Sum Sq	Mean Sq	F Value	Pr (>F)	Significance
Dose	2	84.05754	42.02877	2.00307	0.15257	ns
Treatment	3	551.7001	183.9	8.76458	0.000251	***
Day	5	98880.19	19776.04	942.5157	5.07 × 10−32	***
Dose/Treatment	6	493.4798	82.24663	3.91983	0.005215	**
Dose/Day	10	171.4431	17.14431	0.81709	0.61497	ns
Treatment/Day	15	503.9116	33.5941	1.60108	0.13276	ns
Dose/Treatment/Day	30	629.4655	20.98218	0.99921	0.50213	ns
Residuals	30	629.4655	20.98218

Note: significance levels are indicated as follows: ** p < 0.01; *** p < 0.001; ns = non-significant.

).

All litter treatments produced germination rates higher than 50% (Figure 3), indicating the absence of phytotoxic effects on okra germination according to the criteria proposed by different researchers [25,26].

The higher germination rates, better than the control, were observed with D. oliveri, T. macroptera and Piliostigma thonningii litter (Figure 3). These results may be associated with more favorable litter quality and nutrient release during decomposition (Figure 3).

Similar results were observed for litter powder treatments regarding the ANOVA of the three factors (

Table 3. Analysis of variance (ANOVA) of okra germination under litter powder treatments.

Effect	Df	Sum Sq	Mean Sq	F Value	Pr (>F)	Significance
Dose	2	57.7042	28.8521	0.2836	0.7536	ns
Treatment	7	953.6098	136.2299	4.0261	0.00056	***
Day	5	207413.6	41482.72	1225.967	5.83 × 10−9	***
Dose/Treatment	14	2228.9	159.2072	4.7052	9.89 × 10−7	***
Dose/Day	10	242.8185	24.2819	0.7175	0.7064	ns
Treatment/Day	35	2687.114	76.7747	2.2687	0.00196	**
Dose/Treatment/Day	70	2364.904	33.7843	0.9984	0.5031	ns
Residuals	72	2436.518	33.8405

Note: significance levels are indicated as follows: ** p < 0.01; *** p < 0.001; ns = non-significant.

). According to the results, type of treatment (source of litter) had a highly significant effect on germination (p < 0.001).

Furthermore, germination changed significantly over time and the type of treatment during the experimental period (p < 0.01). A significant interaction was observed between dose and treatment (p < 0.001), indicating that the effect of litter dose depended on the type of litter treatment applied. On the other hand, the interaction between dose and day was non-significant (p > 0.05) (Table 3).

The results showed that mixtures of litter powders frequently produced higher germination rates than single-species powders. These findings suggest possible complementary interactions among litter types during decomposition and nutrient release (Figure 4).

The variation in germination responses between treatments may therefore be explained by differences in litter quality, decomposition rate and nutrient release. These factors probably influenced the bioavailability of mineral elements required during the germination process. Scientific reports reveal that seed germination can be significantly influenced by nutrient availability, particularly nitrogen supply [27].

The differences observed between intact litter and litter powders may also be related to differences in decomposition patterns (Figure 5). Intact litter produced significantly higher germination rates than litter powders (p < 0.001). This result suggests that intact litter may ensure a slower and more progressive release of nutrients, whereas litter powders may cause localized nutrient excesses or temporary inhibitory effects during the early stages of germination.

3.2. Influence of Intact Litters and Their Powders on Okra Growth

According to the results, the application of intact litter significantly influenced okra plant height depending on litter type and dose. Plant height increased progressively throughout the experimental period. At 90 days, all litter treatments produced higher plant height than the control, indicating a positive effect of litter incorporation on vegetative growth.

Among the intact litters, D. oliveri produced the greatest improvement in okra height at 10 g and 20 g, whereas P. thonningii showed the highest values at 30 g. The significant differences observed between treatments and doses suggest that okra growth responses depended on both litter quality and amendment level. Similar results were reported in other research reports that observed improved tomato growth following soil amendment with organic litter materials (

Table 4. Variations in 90-day okra height at different doses and treatments.

		Doses (g)
Litter	10	20	30	F value
T	13.41 a	13.41 a	13.41 a
DO	20.06 (4.42) bβ	23.33 (4.96) bγ	16.08 (2.76) aα	4.58 *
TM	16.06 (4.51) abα	19.08 (6.11) bα	16.66 (5.53) aα	0.52 ns
PT	15.85 (3.19) abα	13.51 (0.80) aα	21.66 (5.00) bβ	7.20 **
F value	3.55 *	7.66 ***	4.41 *

Note: Daniellia oliveri; TM: Terminalia macroptera; PT: Piliostigma thonningii. Different numbers with different Arabic letters in columns and Greek letters in rows indicate that the values are significantly different. * = p < 0.05; ** = p < 0.01; *** = p < 0.001; ns = non-significant.

) [28].

The positive effect of some litter types on plant height may be associated with improved nutrient availability during decomposition. Organic amendments can enhance soil fertility by supplying mineral elements essential for plant growth. The superior performance of D. oliveri at moderate doses may indicate a more balanced nutrient release compared with the other litter types (Figure 6 and Table 4).

The number of leaves varied slightly among treatments and doses, but these differences were not statistically significant (

Table 5. Variations in number of okra leaves at 90 days at the three doses and treatments.

Litter		Doses (g)
	10	20	30	F value
T	8.00 a	8.00 a	8.00 a
DO	8.60 (0.51)	8.33 (1.36)	7.83 (2.04)	0.50 ns
TM	7.66 (1.21)	7.50 (1.64)	8.00 (0.63)	0.26 ns
PT	7.83 (0.40)	7.83 (0.75)	8.83 (1.32)	2.40 ns
F value	2.43 ns	0.56 ns	0.77 ns

Note: Daniellia oliveri; TM: Terminalia macroptera; PT: Piliostigma thonningii. The different numbers assigned to the different Arabic letters in the columns and Greek letters in the rows indicate that the values are significantly different. ns = non-significant.

and Figure 7). Although some treatments showed numerically higher values than the control, intact litter amendments did not substantially improve leaf production in okra plants. The highest number of leaves was recorded with D. oliveri at 10 g (8.60 leaves) and P. thonningii at 30 g (8.83 leaves), whereas the control remained relatively stable across all doses.

These results are consistent with other reports [29], which showed limited effects of decaying leaf litter amendments on maize vegetative growth. However, they differ from findings that observed significant improvements in leaf number following organic litter application in maize [30].

The absence of significant differences between treatments and doses suggests that leaf production in okra was less sensitive to litter amendments than plant height. This may indicate that leaf development was controlled more by the plant’s genetic characteristics and general growing conditions than by the additional nutrient availability provided by the treatments included in this study (Figure 7 and Table 5).

Collar diameter varied slightly among treatments and doses, but no statistically significant differences were observed at 90 days (

Table 6. Variation in collar diameter of 90-day okra plants at different doses and treatments.

		Doses (g)
Litter	10	20	30	F value
T	4.25	4.25	4.25
DO	3.75 (0.49)	4.38 (0.58)	3.98 (0.37)	2.54 ns
TM	3.7 (0.69) 8	3.63 (0.43)	3.71 (0.34)	0.13 ns
PT	4.11 (0.19)	3.91 (0.14)	4.31 (0.46)	2.60 ns
F value	1.82 ns	4.65 ns	3.47 ns

Note: Daniellia oliveri; TM: Terminalia macroptera; PT: Piliostigma thonningii. ns = non-significant.

and Figure 8). Although some treatments such as D. oliveri at 20 g (4.38 cm) and P. thonningii at 30 g (4.31 cm) showed numerically higher values, these increases were not significantly different from the control.

Changes in collar diameter of okra plants are represented in litter treatments at doses of 10 g, 20 g and 30 g. Collar diameter increased progressively throughout the experimental period for all treatments, although the rate of increase varied among litter types and doses (Figure 8).

Collar diameter varied among treatments and doses, but no significant differences were observed between treatments or doses at 90 days. This indicates that intact litter amendments did not substantially influence stem thickening in okra plants under the conditions of the experiment.

At the end of the experiment (90 days), okra plant height varied significantly according to treatment (

Table 7. Variations in height of okra plants at 90 days at different doses and treatments.

		Doses (g)
Litter	10	20	30	F value
T	13.41 abc	13.41 bc	13.41 bc
DO	20.75 (7.74) g	13.4 (2.67) bc	15.5 (3.83) b	3.15 ns
TM	16.16 (3.25) def	14.03 (3.31) cd	12.46 (0.81) ab	2.80 ns
PT	13.16 (3.00) abc	10.33 (0.81)a	13.4 (2.29) bc	3.01 ns
DO + TM	15.16 (2.63) bcd	16.33 (2.25) d	20.16 (8.23) c	1.54 ns
DO + PT	19.33 (6.94) ef	16.66 (2.33) d	16.16 (2.31) bc	0.88 ns
TM + PT	10.95 (0.94) ab	10.76 (1.79) ab	12.55 (1.42) ab	2.82 ns
DO + TM + PT	9.5 (2.64) a	9.66 (0.41) a	10.23 (0.63) a	0.35 ns
F value	5.34 ***	7.55 ***	4.5 ***

Note: Daniellia oliveri; TM: Terminalia macroptera; PT: Piliostigma thonningii. The different numbers assigned to the different Arabic letters in the columns and Greek letters in the rows indicate that the values are significantly different. *** = p < 0.001; ns = non-significant.

). Dose did not influence okra plant height, since the difference between the three doses (10, 20 and 30 g) was non-significant for each of the seven treatments. The interaction between dose and treatment was non-significant (Figure 9).

Between treatments, however, the difference in okra plant height was significant, with values ranging from 9.50 to 20.75 cm for the 10 g dose, from 9.66 to 16.66 cm for the 20 g dose and from 10.23 to 20.16 cm for the 30 g dose. Some bedding powders showed a significant improvement in the height of okra plants.

This action could be justified by the fact that the powders release the minerals gradually, which may ensure that they are available when the plant actually needs them.

Treatments containing D. oliveri alone or in combination with other species generally produced higher plant height values compared with the control, especially at 10 g and 30 g.

In contrast, the mixture composed of the three species (DO + TM + PT) consistently showed the lowest plant height values, which may indicate an antagonistic effect among the mixture components or a possible temporary immobilization of nutrients during decomposition.

At 90 days, the number of leaves on okra plants varied according to treatment and dose (

Table 8. Variations in the number of leaves on okra plants at 90 days at different doses and treatments.

		Doses (g)
Litter	10	20	30	F value
T	8.0 bc	8.0 bc	8.0 de
DO	8.83 (1.16) cdγ	8.16 (0.40) cβ	7.33 (0.83) abcα	4.62 *
TM	7.83 (0.40) bα	7.5 (0.54) abα	7.5 (0.83) abcα	0.57 ns
PT	7.33 (1.03) abα	7.0 (0.89) abα	7.16 (0.98) abα	0.18 ns
DO + TM	7.5 (0.54) abα	8.16 (0.40) cdα	8.16 (0.98) cα	1.96 ns
DO + PT	8.0 (0.89) bcα	8.33 (0.51) dα	7.33 (1.03) abcα	2.19 ns
TM + PT	7.66 (0.51) bα	7.0 (0.63) bcα	7.0 (0.63) aα	2.50 ns
DO + TM + PT	6.66(1.03) aα	6.33 (0.81) aα	6.66 (1.03) aα	0.24 ns
F	3.7 ns	9.08 ***	2.02 ns

Note: Daniellia oliveri; TM: Terminalia macroptera; PT: Piliostigma thonningii. The different numbers assigned to the different Arabic letters in the columns and Greek letters in the rows indicate that the values are significantly different. * = p < 0.05; *** = p < 0.001; ns = non-significant.

and Figure 10). However, these differences were significant only between treatments at the 20 g dose, whereas between doses the differences were significant only for D. oliveri litter powder. For the other treatments, no significant differences were observed. Likewise, the interaction between dose and treatment was non-significant.

The number of leaves ranged from 6.33 to 8.83 leaves per plant depending on treatment and dose. Treatments amended with D. oliveri powder alone or in combination with P. thonningii generally recorded the highest leaf numbers, while the mixture containing all three litter powders (DO + TM + PT) consistently produced the lowest values.

Only amendment with a mixture of D. oliveri and P. thonningii litter powders improved the number of leaves on okra plants compared with the control. In contrast, the mixture of the three types of litter powder consistently reduced the number of leaves, which may indicate an inhibitory effect on the vegetative development of the plants.

These differences may be associated with the mineralization rate and the varying availability of nutrients resulting from the decomposition of the litter powders. Some combinations may enhance the availability of nitrogen and other nutrients involved in leaf apparatus formation, whereas others may generate temporary nutritional imbalances.

These results are similar to those in other reports [30], who showed that leaf powders from Annona senegalensis, Terminalia glaucescens and Tithonia diversifolia improved the number of leaves in maize. Also, they agree with reports of inhibitory effects of certain tropical leaf litters on crop growth [31].

Collar diameter varied according to treatment and dose (

Table 9. Variations in collar diameter of okra plants at 90 days at different doses and treatments.

	Doses (g)
Litter	10	20	30	F
T	4.25 c	4.25 b	4.25 c
DO	3.93 (0.83) c	3.75 (0.59) bc	3.48 (0.58) ab	0.67 ns
TM	3.68 (0.57) bc	3.73 (0.58) bc	4.05 (0.08) c	1.04 ns
PT	3.95 (0.38) c	3.5 (0.80) ab	3.75 (0.60) bc	0.79 ns
DO + TM	3.76 (0.65) bc	4.1 (0.14) c	4.2 (0.36) c	1.60 ns
DO + PT	4.1 (0.25) c	4.03 (0.34) bc	3.83 (0.47) bc	0.86 ns
TM + PT	3.03 (0.08) bc	3.13 (0.48) a	3.4 (0.54) ab	1.19 ns
DO + TM + PT	2.78 (0.84) a	3.0 (0.30) a	3.20 (0.41) a	0.37 ns
F	3.56 **	5.28 **	4.5 **

Note: Daniellia oliveri; TM: Terminalia macroptera; PT: Piliostigma thonningii. The different numbers assigned to the different Arabic letters in the columns and Greek letters in the rows indicate that the values are significantly different. ** = p < 0.01; ns = non-significant.

). Significant differences were observed between treatments at each dose level (10, 20 and 30 g), whereas differences between doses within the same treatment were non-significant. The interaction between dose and treatment was therefore limited.

Collar diameter values ranged from 2.78 to 4.25 cm depending on treatment and dose. The control maintained relatively stable values across all doses, while treatments amended with mixtures containing D. oliveri and T. macroptera generally produced the highest collar diameter values. In contrast, the mixture containing all three litter powders (DO + TM + PT) consistently recorded the lowest stem diameter values (Figure 11).

For the 20 g dose, the treatment combining D. oliveri and T. macroptera litter powders showed the greatest collar diameter (4.1 cm), approaching the control values. This result suggests that certain combinations of litter powder may support stem development more effectively through a more balanced release of nutrients.

In contrast, the combination of the three species (DO + TM + PT) consistently reduced the collar diameter at all applied rates. This effect may be associated with the occurrence of antagonistic interactions between the compounds resulting from the decomposition of litter powders or with reduced availability of certain elements essential for vegetative growth.

Although several treatments produced numerical variations in collar diameter, most values remained close to the control, indicating that stem thickening in okra was less responsive to litter powder amendments than plant height. These results partially agree with reports of positive effects of agroforestry litter powders on tomato stem diameter, confirming that the response may vary depending on crop species and litter composition [30].

Okra plants grown with intact litter amendments recorded a higher average height (18.04 cm) compared with those treated with litter powders (14.36 cm). Similarly, the average number of leaves was higher under intact litter treatments (8.06 leaves) than under litter powder treatments (7.63 leaves). In contrast, collar diameter values remained relatively similar between intact litter (3.96 cm) and litter powder treatments (3.76 cm), with no significant statistical difference.

Table 10. Average variation in growth parameters between intact litter and litter powders.

Parameters	Intact Litter	Litter Powders	t Student
Height (cm)	18.04 (5.15)	14.36 (4.38)	3.99493 ***
Number of leaves	8.06 (1.20)	7.63 (0.93)	2.051 *
Diameter at collar (cm)	3.96 (0.48)	3.76 (0.58)	1.90828 ns

Note: The different numbers assigned to the different Arabic letters on the line indicate that the values are significantly different. * = p < 0.05; *** = p < 0.001; ns = non-significant.

and Figure 12 present the average variation in okra growth parameters under intact litter and litter powder treatments. Significant differences were observed between the two amendment types for plant height and number of leaves, whereas collar diameter was not significantly affected (Table 10). These results indicate that intact litter amendments promoted vegetative growth more effectively than litter powders.

Intact litter may contribute to moisture retention and to the improvement of the physical properties of the substrate, which can favor the vegetative development of okra plants. In the case of litter powders, the faster mineralization process may lead to nutrient losses or temporary imbalances in the availability of mineral elements for plants.

The absence of significant differences in collar diameter suggests that stem thickening was less sensitive to the form of litter applied than other growth parameters such as plant height and leaf production.

4. Discussion

The main factors influencing litter decomposition are litter quality, physico-chemical environment, decomposer organisms, and plant species composition and diversity [2]. The physical quality and chemical composition of leaves vary considerably among plant species and strongly influence ecosystem properties and functioning [32,33]. Good ecosystem functioning is generally associated with the biochemical and physical quality of leaf litter [34]. In the present study, all litter treatments produced germination rates higher than 50%, while the higher germination rates, better than control, were observed with D. oliveri, T. macroptera and P. thonningii litter. These factors may explain the significant differences observed between treatments in the present study, particularly the superior germination rates recorded with D. oliveri, T. macroptera and P. thonningi compared with the control.

Leaves generally have lower nutrient concentrations [35], but higher concentrations of lignin and tannins [36]. In addition, both intra- and interspecific differences in litter significantly affect nitrogen inputs and losses [7,37]. Within ecosystems, variations in litter quality influence decomposition and mineralization rates [35,38]. According to the results, D. oliveri produced the greatest improvement in okra height at 10 g and 20 g, whereas P. thonningii showed the highest values at 30 g. This may partly explain why D. oliveri alone or in combination with other species generally produced higher okra height and leaf number values compared with the other treatments.

Differences in litter quality between species may result from variations in nutrient content, biochemical compounds and their ratios, as well as genotypic differences among species [36,39]. Litter rich in lignin decomposes more slowly than litter containing higher amounts of starch [2]. Because litter is generally rich in cellulose and lignin, its decomposition often requires specialized microorganisms and is therefore relatively slow [2,33,34]. Decomposition rates are negatively correlated with the initial litter C/N and lignin/N ratios, whereas they are positively correlated with initial nitrogen content [40,41]. Intact litter produced significantly higher germination rates than litter powders (p < 0,001). These observations may justify the higher germination rates and greater plant height observed with intact litter compared with litter powders, since intact litter may ensure a slower and more progressive nutrient release.

The degree of litter decomposition is also strongly influenced by soil conditions and microbial communities. Forest organic topsoil favors decomposition through higher microbial activity and suitable microclimatic conditions [6,42]. Soil pH, temperature and NH4−-N concentration are also important determinants of litter decomposition rates.

Temperature, humidity and other microclimatic factors further affect decomposition processes. Several studies have shown that litter decomposition may slow during the rainy season [43]. Moreover, litter generally decomposes faster outside its site of origin when environmental conditions are more favorable [3,32]. For example, hardwood litter decomposes more rapidly in deciduous forests than in coniferous forests [44]. Similarly, decomposition rates may decrease with increasing altitude [45,46,47,48].

Okra plants grown with intact litter amendments recorded a higher average height (18.04 cm) compared with those treated with litter powders (14.36 cm). Similarly, the average number of leaves was higher under intact litter treatments (8.06 leaves) than under litter powder treatments (7.63 leaves). Therefore, better vegetative performance was observed under intact litter treatments, where okra plants recorded higher average height and higher average number of leaves compared with litter powder treatments. The decomposition of one litter species may also be influenced by neighboring species. Compared with monocultures, mixed-species litter often decomposes more rapidly, indicating positive non-additive interactions among litter types. In monocultures, highly lignified tissues may hinder decomposition because of their structural stability [35]. Increasing tree species richness generally enhances microbial diversity, which may accelerate decomposition rates [14]. When litter species differ in nutrient concentrations, high-diversity mixtures often decompose more rapidly [48,49]. The results showed that mixtures of litter powders frequently produced higher germination rates than single-species powders.

This may explain why mixtures of litter powders frequently produced higher germination rates than single-species powders in the present study. Certain combinations, particularly those including D. oliveri, also improved okra height, collar diameter and number of leaves. These results suggest possible complementary interactions among litter types during decomposition and nutrient release.

The effect of litter mixtures on decomposition depends strongly on litter quality and species composition [7]. A litter mixture may alter the decomposition of individual litter types. Synergistic effects are more likely when litter species have similar structures, whereas mixtures with contrasting leaf textures may not exhibit such interactions [7,45]. Litter mixtures can therefore produce additive or non-additive effects, including synergistic or antagonistic interactions, compared with monocultures. In contrast, the mixture composed of the three species (DO + TM + PT) consistently showed the lowest plant height values. On the other hand, this may explain why the mixture composed of the three species (DO + TM + PT) consistently produced the lowest values for plant height, leaf number and collar diameter in several treatments. The combination of the three litter types may be associated with antagonistic interactions during decomposition or temporary nutrient immobilization, although these mechanisms were not directly evaluated in the present study.

However, to better understand the impact on growth, further studies under field conditions, including biomass and yield assessments, would be valuable to validate and extend the findings obtained in the present study.

Leaf litter quality is known to vary considerably among plant species depending on nutrient concentration, lignin content and leaf structure [35,50]. Physical and chemical characteristics of litter strongly influence decomposition processes and nutrient mineralization. Species producing litter with lower lignin content and better nutrient availability generally decompose more rapidly and release mineral elements more easily into the soil [35]. This may explain the superior performance of D. oliveri observed in the present study, particularly at moderate doses where plant height and leaf production were generally higher.

However, an important limitation of the present study should be acknowledged when comparing intact litter and litter powders. Although both amendment types were applied at equivalent masses, intact leaves likely provided greater soil surface coverage and a different spatial distribution compared with powdered litter. Consequently, intact litter may have acted not only as a nutrient source but also as a mulch layer, contributing to improved soil moisture conservation, reduced evaporation, moderation of soil temperature fluctuations, and maintenance of favorable conditions for soil microbial activity. Therefore, the superior vegetative performance observed under intact litter treatments may not be explained exclusively by differences in decomposition rate and nutrient release, but also by these physical effects. In perspective, future studies should consider standardizing litter applications based not only on mass but also on soil surface coverage to better distinguish between physical and biochemical mechanisms. Furthermore, recalcitrant litter may decompose more rapidly in mixtures because nutrients released from high-quality litter can stimulate microbial colonization and decomposition of poorer-quality litter [51]. The present study demonstrated clear differences among litter species and amendment forms. However, additional analyses of litter quality parameters, such as C/N ratio, lignin, tannins, phenolic compounds, and nutrient concentrations, could address the limitations of the study. From this perspective, integrating litter quality characterization with decomposition dynamics, nutrient availability and microbial activity measurements would contribute to a better understanding of the mechanisms underlying the effects of litter amendments on okra growth.

5. Conclusions

Intact and powdered leaves litter from Daniellia oliveri, Terminalia macroptera and Piliostigma thonningii showed no toxic effects on okra seed germination at the tested doses (10, 20 and 30 g). Germination and growth parameters varied according to litter type, mixture and application dose. For intact litter, germination reached 100% with D. oliveri and P. thonningii at 10 g and with T. macroptera at 20 g. For litter powder treatments, several mixtures achieved 100% germination at 16 DAS, including D. oliveri + T. macroptera at 30 g, D. oliveri + P. thonningii at 20 g, T. macroptera + P. thonningii at 30 g, and the mixture of the three species at 10 and 30 g.

Growth parameters, including plant height, number of leaves and collar diameter, also varied among treatments. According to the results, treatments containing D. oliveri improved vegetative growth compared with the control, whereas the mixture containing the three litter powders generally reduced plant performance.

These results suggest that forestry leaf litter may represent a useful organic amendment for improving okra germination and vegetative development, depending on litter composition and application dose.

Further studies are needed to characterize the chemical composition of the litter materials and to evaluate their effects on biomass production and fruit yield under different cultivation conditions.