A Comparative Study of the Fertilizer-Cum-Pesticide Effect of Vermicomposts Derived from Cowdung and the Toxic Weed Lantana

Abstract

The effect of vermicomposts, derived either from cowdung or the pernicious invasive plant lantana (Lantana camara), has been assessed on the seed germination, plant growth, fruit yield, quality of the produce, and disease resistance of a common vegetable, ladies finger (Abelmoschus esculentus).Seeds of A. esculentus were germinated and grown in soil fertilized with 0, 2.5, 3.75 and 5 t ha⁻¹ of lantana or cowdung vermicompost for 4 months. It was seen that the lantana vermicompost performed at par or better than the cowdung vermicompost in terms of most of the growth and yield parameters observed. Both the vermicomposts encouraged the germination, growth as well as the yield of ladies fingers. The fruits harvested from the vermicompost-treated plots had greater concentrations of minerals, proteins and carbohydrates than the control plants. Vermicomposts also reduced the incidence of pest attacks on the plants. The results confirm that vermicomposting destroys the harmful ingredients of lantana and turns it into as good a biofertilizer, perhaps even better than the vermicompost of cow-dung. The very large quantities of lantana biomass that is generated in the tropical and sub-tropical regions of the world every year, which presently go to waste, now appear capable of becoming a source of organic fertilizer.

1. Introduction

Lantana (L. camara), which is acknowledged as one among the 100 most invasive and colonizing of the world’s weeds [1], has become a major threat to agriculture and forest ecosystems [2,3].It has the ability to grow in widely varying environmental conditions [4,5], often forming large, impenetrable, thickets.Due to its colonizing ability lantana monopolizes the use of light, water, and nutrients in the areas it invades, at the expenses of multi-species vegetation, causing great harm to biodiversity [6]. Being rich in toxic chemicals such astriterpene acids, lantadene A (rehmannic acid) and lantadene B, lantana induces cholestasis, hepatotoxicity and mortality in animals who graze on its foliage [7,8,9]. Lantana is also strongly allelopathic and restricts the growth of surrounding vegetation [10]. Even though efforts have been made since several decades to control lantana by mechanical, chemical, biological or hybrid means [11], no enduring success has been achieved till now and lantana continues to overrun ever new territories. Attempts to use lantana as a feedstock for producing cellulose, ethanol, drugs, or compost could engage only a small fraction of its biomass [12,13,14] with no market penetration so far. Therefore, it is imperative that an economically viable product of large global demand is developed using lantana.

In nature earthworms feed voraciously on the debris of all species of plants, including those known to be toxic to vertebrates.It is believed that these animals carry a class of rare surface-active metabolites in their bodies, which have been termed ‘drilodefensins’ [15]. These compounds cancelledthe inhibitory effects of polyphenols and other toxic chemicals present in plants like lantana on earthworm gut enzymes and enable the earthworms to tolerate high levels of polyphenols if present in their diet.As a result, the earthworms are able to feed on a large variety of phytomass, including streams with high levels of polyphenols.

We have earlier reported [16,17] that even though in nature epigeic and anecic earthworms principally feed upon plant debris—and much less animal dropping in proportion—controlled vermicomposting on large scale has so far been limited to animal manure. We have explained the reasons and have described the concept of high-rate vermicomposting developed by us along with the technological interventions done by us which has made it possible to vermicompost lantana and other weeds on a large scale [16,18].

We have successfully used the epigeic earthworm Eisenia foetida for vermicomposting lantana [17]. Extensive investigations to characterize the lantana vermicompost (LVC) using Fourier transform infrared spectroscopy, thermal gravimetry, differential calorimetric analysis, gas chromatography, and scanning electron micrography (SEM) have revealed intense mineralization of the organic matter, degradation of lignocellulosic materials and polyphenols, reduction of toxic and allelopathic compounds (phenols and sesquiterpene lactones) in the course of lantana’s vermicomposting. SEM has reflected strong disaggregation of the organic matter content in LVC compared to the lantana matrices. Further, in a controlled study, Hussain et al. [19], have observed that LVC enhanced the germination of the seeds, and early growth of the seedlings of ladies finger, green gram (Vigna radiata) and cucumber (Cucumis sativus) when used at appropriate concentrations in soil. However, beyond certain level lantana vermicompost had shown adverse effects. This had raised apprehensions as to whether LVC behaves differently from cow-dung vermicompost (CDVC). It was, therefore, decided to compare the effects of LVC and CDVC under identical conditions. Accordingly, we have carried out this study in which the effect of CDVC has been compared with that of lantana vermicompost on the growth, fruition and quality of the ladies finger produce, in a field-scale study.

2. Materials and Methods

2.1. Soil and the Vermicomposts

The studies were conducted in a field situated within the boundary of Pondicherry University, India. The study area lies on the eastern coast of the peninsular South India, at 11°56′ N, and 79°53′ E. The studies were performed in the months spanning February–May which is the season known to be the most suited for the cultivation of ladies finger in the place where the authors work. The soil used in the study was obtained from within the Pondicherry University campus; its characteristics are presented in

Table 1. Composition of the soil and the vermicomposts deployed in the present study.

Aspect	Lantana VC	Cow Dung VC	Soil
Total nitrogen, g/Kg	19.6 ± 2	23 ± 2.7	0.69 ± 0.05
Available phosphorus, g/Kg	7.5 ± 0.8	5.3 ± 0.4	0.26 ± 0.04
Available Potassium, g/Kg	18.5 ± 1.8	14.8 ± 2.1	0.81 ± 0.08
Total organic carbon, g/Kg	283 ± 18	258 ± 26	8.79 ± 0.63
C/N	14.4 ± 0.6	11.2 ± 0.7	13.9 ± 0.9
Particle density, g/cm3	1.4 ± 0.3	1.5 ± 0.1	2.7 ± 0.04
Bulk density, g/cm3	0.35 ± 0.02	0.4 ± 0.01	1.4 ± 0.03
Water holding capacity, %	252 ± 17	235 ± 22	35 ± 3
Porosity, %	72 ± 1.9	66 ± 1.9	49 ± 1
Electrical conductivity, mmhos cm−1	10.1 ± 0.24	11.7 ± 0.9	0.16 ± 0.02
pH	6.4 ± 0.17	7.2 ± 0.2	6.35 ± 0.15

. Cowdung was procured from the local farmers in the vicinity of Pondicherry University campus and lantana was harvested from its stands in and around the campus. Vermicomposts from both were generated using the concept of high-rate vermicomposting and the FLUVTS machine as elaborated earlier [20] for obtaining vermicompost from paper waste. In both cases the earthworm dropping, which are easily distinguishable and separable from the parent substrate, were identified as the vermicompost.

2.2. Design of Experiments

Plants were grown outdoors in 50-litre LDPE (low-density polyethylene) containers filled with soil. The design of experiments consisted of the use of controls without any amendment and of vermicomposts at three levels: 2.5, 3.75, and 5 t ha⁻¹ [21]. In each of the selected treatment, a total number of 175 seeds were sown in 35 bags. The Kulemagali vendai variety of ladies finger, which is locally available, was used. The number of seeds germinated over an 8-day period were counted to obtain germination success in terms of percentage of seeds germinated. On day nine, four seedlings from each bag were removed so as to keep a single healthy plant in each bag. Plants were allowed to grow up to 100 days. Throughout this period the bags were periodically irrigated with tap water.

2.3. Sampling and Analysis

After 100 days, the plant samples consisting of five randomly-selected whole ladies finger plants per set, were harvested for the assessment of morphological growth which was recorded in terms of mean plant height, number of leaves and branches, stem diameter, and above-ground biomass. The plant’s roots were rinsed with water to clear off the adhering soils, before further analysis. Dry weight of the plants was determined by oven drying their known quantities at 105 °C to constant weights. The yield of ladies finger on the basis of pods per plant, and the length (cm), diameter (mm), and weight (g) of the pods per plant was recorded on alternate days. The chlorophyll content of the vegetable’s leaves was determined on the basis of the procedure detailed by Moran and Porath [22] and Wellburn [23]. The vegetable’s pods were analyzed for their content of protein, carbohydrate and ash by the Kjeldahl, Anthrone and dry ashing methods, respectively (Nielson, 2010). The total solids of the pods were determined by heating their measured quantities at 105 °C to constant weights, as per the procedure of Nielson [24].

To measure the pH and the electrical conductivity (EC) of the vermicast and the soil, their 1:2 (w/v) suspensions were prepared in water using Digison^TM digital pH meter 7007 and ET^TM611E EC meter, respectively. The bulk density, particle density, and total porosity of the soil and vermicast samples were measured following the procedure reported by Carter and Gregorich [25]. The two matrice’s capacity to hold water was determined by measuring their gravimetric water content following and saturation of samples and draining of the excess water [26].

Total organic carbon was estimated using the modified dichromate redox method for respective weeds and their vermicast as described by Heanes [27]. Determination of total nitrogen was done with the modified Kjeldahl method [28] employing a KelPlus^TM instrument. Extractable/available potassium and phosphorus were determined employing Elico^TM CL378 flame photometer and ammonium molybdate-ascorbic acid method, respectively, after the samples were extracted with Mehlich 3 solution [25].

During the experiments some of the vegetable plants were found to have been infested with leaf miners and leaf spot diseases. These infestations were caused by plant pests Liriomyza spp. and fungus Alternaria alternate, respectively [29,30]. In case of severity, the leaf spot disease generates concentric dark brown spots on the leaves, eventually causing the death of the leaves [31]. Some of the ladies finger pods were found to be infested with fruit borer Eariasvittella. The extent of infestation was calculated as percentage of the weight of the effected fruits with reference to the total fruit weight in each treatment.

2.4. Assessment of Levels of Significance

The effect of the vermicompost treatments was compared with the controls using statistical test of one-way analysis of variance (ANOVA). The overall effect of LVC and CDVC on all the morphological and biochemical aspects of ladies finger was compared by a two-way ANOVA. Comparisons were made as types of vermicomposts (VC), concentration of vermicomposts (N) and their interactions.

3. Results and Discussion

3.1. Seed Germination

The findings are summarized in Figure 1. Vermicompost treatments significantly enhanced the seed germination compared with the controls (Figure 1a), however no statistically significant variation was seen between the effects of the cowdung and the lantana vermicompost treatments. The highest germination success (95%) was seen in 5 t ha⁻¹ lantana vermicompost (LVC) treatment. The next best success (94%) occurred in the 3.75 t ha⁻¹ cowdung vermicompost (CDVC) treatment. Even though seed germination is primarily an internally regulated mechanism which is governed by the genotype of the plant, several environmental factors and fertilization regimes can also alter the germination success [18]. Several of the studies have suggested that besides the plant hormones and phenolic compounds, increased nitrate and ammonium concentrations in the vermicompost play a strong role in seed germination [32,33].

3.2. Plant Growth

Ladies finger plants grown in VC amended soils have shown enhanced growth in terms of all the variables recorded (Figure 1b–g). Within the range of vermicompost concentrations explored by us, the trend of positive effect was: greater the vermicompost application more the benefit. Apart from the number of leaves in CDVC, all trends had the pattern 5 t > 3.75 t > 2.5 t ha⁻¹ > control. Except for the length of the roots, the growth of ladies finger went up profusely even when the concentration of both the vermicomposts was increased only marginally (from zero to 2.5 t ha⁻¹). Similar observations were recorded for flowering, where higher LVC treatments yielded a greater number of flowers and induced earlier flowering relative to the controls and the lower LVC treatments. In case of CDVC, the 3.75 t ha⁻¹ treatment performed better than other treatments (

Table 2. Flowering and yield of A. esculentus plants grown in soil fertilized with different levels of lantana and cowdung vermicomposts. The numbers which do not differ significantly from controls (p < 0.05) carry an asterisk. Single, double, and triple stars indicate the significance levels at p < 0.5, <0.01 and <0.001, respectively.

Parameters Observed	Type of VC	Vermicompost Concentrations (t/ha)				ANOVA
		0	2.5	3.75	5	Type of Vermicompost (VC)	Concentration of Vermicompost (N)	VC*N
Days to flower	LCVC	52.7 ± 4.85	43.2 ± 2.30	39.0 ± 3.37	37.3 ± 2.41	NS	***	NS
	CDVC		43.3 ± 2.75	38.6 ± 2.84	39.3 ± 2.79
No. of flowers	LCVC	2.9 ± 0.32	9.0 ± 1.05	16.3 ± 1.16	18.0 ± 2.16	***	***	***
	CDVC		8.3 ± 0.95	12.8 ± 1.32	10.1 ± 0.88
No. of pods	LCVC	1.7 ± 0.48	6.2 ± 0.63	13.7 ± 1.06	16.2 ± 2.10	***	***	***
	CDVC		6.5 ± 0.53	10.8 ± 1.03	8.6 ± 0.52
Length of pods (cm)	LCVC	7.1 ± 0.50	10.9 ± 1.11	11.6 ± 0.94	13.1 ± 1.34	NS	**	*
	CDVC		11.1 ± 0.98	11.7 ± 0.69	11.5 ± 1.10
Diameter of pods (mm)	LCVC	11.4 ± 0.70	15.4 ± 1.04	16.0 ± 0.96	16.3 ± 0.72	NS	*	NS
	CDVC		15.6 ± 0.77	16.7 ± 1.00	15.9 ± 1.21
Weight of pods/plant (g)	LCVC	5.4 ± 0.50	91.9 ± 9.30	143.8 ± 8.47	170.5 ± 16.2	***	***	***
	CDVC		61.8 ± 6.20	101.6 ± 8.98	85.7 ± 8.72
Yield t/ha	LCVC	0.5 ± 0.05	9.0 ± 0.91	14.1 ± 0.83	16.8 ± 1.60	***	***	***
	CDVC		6.1 ± 0.61	10.0 ± 0.88	8.4 ± 0.86
Percentage infected fruits	LCVC	39.2 ± 12.39	9.3 ± 3.79	9.1 ± 5.42	8.0 ± 4.73	NS	*	NS
	CDVC		13.4 ± 6.46	7.6 ± 2.77	7.6 ± 3.63

).

In comparison to CDVC, the shoot length and the plant biomass were significantly higher in the ladies finger plant grown in LVC amended soil; however there was no statistically significant difference vis a vis shoot diameter and the number of branches. As elucidated by Hussain and Abbasi [18], vermicompost amendment in soil enhances the available nutrient content of the soil, besides making the soil porosity, density, and water holding capacity more plant-friendly. In addition, soils amended with vermicomposts were seen to be rich in fulvic and humic acids, and plant hormones [34], which apparently boost the growth of plants compared to the controls. The results of the present investigation show that in some aspects LVC has outperformed CDVC while in some other aspects no significant difference was seen between the two. This makes it evident that lantana loses its toxic and allelopathic constituents during its vermicomposting and the resultant vermicompost, has positive influence on the growth of ladies finger. Equally significant is the finding that the positive influence matches—at times even surpasses—that of CDVC.

3.3. Yield and Biochemical Aspects

Vermicompost treatments are seen to have significantly enhanced the yield of the ladies finger pods as reflected in the average numbers and weights of pods per plant, and the average length and diameter of the pods (Table 2). In comparison to the CDVC, LVC had significantly higher number of pods per plant. It also led to pods of higher average weight. However, no significant difference was seen in case of length and diameter of the pods. Vermicompost treatments had also significantly increased the concentrations of chlorophyll and carotenoids in the ladies finger leaves, and the total solids and ash content of its fruits in comparison to the control plots (Figure 2a–d). No statistically significant difference, however, was seen between the LVC and the CDVC in terms of influence on chlorophyll, carotenoids, total solids, protein and carbohydrates content (Figure 2e–f). These gains, like the plant growth parameters, can perhaps be attributed to the increased plant available nutrients in soil fortified with vermicomposts, compared to the controls. This is consistent with similar effect reported when manure−based vermicomposts were deployed [35,36]. Overall, LVC appears to be as beneficial for the cultivation of ladies finger as CDVC.

3.4. Disease Incidence

Both the vermicomposts were able to induce disease resistance in the test plants (Figure 1h, Table 2). In terms of reducing the incidence of disease, LVC has performed marginally better than CDVC; however, the difference was not statistically significant. The fractions of infected fruits was lesser in CDVC treatments of 3.75 and 5 t ha⁻¹ than in the corresponding LVC applications. However, again, the difference was not statistically significant. In a recently published review, Hussain and Abbasi [18] have documented a number of scientific studies reporting the positive role of manure-based vermicomposts in reducing pests and disease in several botanical species. The present work shows that LVC also possesses a similar virtue.

Previous reports on pathogen-protecting attribute of manure–based vermicomposts reveal that better nutrient availability, and presence of antimicrobial compounds such as flavonoids, phenols and humic acids in the vermicomposts, are the likely factors that may have imbibed the vermicomposts with the ability to resist pathogens [37]. Evidently these beneficial attributes are also present in LVC.

4. Summary and Conclusions

A comparative study on the effects of vermicomposts derived from lantana (LVC) and cowdung (CDVC) was carried out in terms of success in seed germination, seedling growth, yield of fruits, fruit quality and plant pathology of ladies finger (Abelmoschusesculentus). Contrary to the apprehensions that lantana being a toxic and allelopathic weed, its vermicompost may be unfriendly to other species of plants and the soil, LVC manifested no such negative attribute. Rather, LVC, like CDVC, enhanced the fraction of seeds that germinated, promoted the growth of the ladies finger plants, increased the fruit yield, improved the chlorophyll and carotenoid levels, and induced resistance against pests and disease, in comparison to the controls. In most of the aspects LVC had an equally beneficial, if not better, effect than CDVC. The findings add credence to the possibility that the lantana phytomass—of which enormous quantities are generated every year in the tropical and sub-tropical world—can serve as feedstock for producing much-in-demand organic fertilizer in the form of LVC.