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1.0 CHAPTER 1: INTRODUCTION
As a developing country, Guyana is faced with the basic problem of solid waste management. According to Gonzalez (2009) “solid waste” refers to all putrescible and non putrescible solid and semisolid wastes, including but not limited to garbage, rubbish, ashes, industrial wastes, swill, demolition and construction wastes, abandoned vehicles or parts thereof, and discarded commodities. This includes all liquid, solid and semisolid materials which are not the primary products of public, private, industrial, commercial, mining and agricultural operations.
Although waste disposal practices in Guyana have not kept pace with the demands posed by increases in urban population and subsequent increases in waste generation over the past few decades, solid waste management is not at a critical stage (Závodská, 2003). However, it should be noted that the total waste generated in 1995 for Guyana was reported at 42,665 tons and this increased by 5 per cent to 44,831 tons in 1996, a further 2.4 per cent in 1997 and another increase of 2.4 per cent in 1998 (Caribbean Community Secretariat, 2008). Evidently, the most significant increase in the total waste generated in Guyana was from 1999 to 2000 where the waste generation increased from 47,287 tons to 57,256 tons, representing an increase of 21.1 per cent.
According to PAHO/WHO (2004), among the six municipalities of Guyana, the per capita generation of solid waste is greatest in Rose Hall, whereas, the smallest per capita generation corresponds to the municipality of Anna Regina. Moreover, there is inadequate information documented as it pertains to the solid waste composition in Guyana, especially for the municipalities outside of Georgetown.
In Georgetown, the per capita generation of wastes is considered to be the second largest in the country with approximately 180,000 inhabitants generating in excess of 190 tons of waste on a daily basis (Inter-American Development Bank, 1998). According to Závodská (2003) landfilling at Mandella Avenue is considered the only way of disposing municipal solid waste (MSW) in the capital city. Essentially, due to the absence of sufficient finances to develop proper, sanitary landfills, that are lined and have controls for leachate and methane generation, MSW is often dumped into temporary, poorly designed, unlined, unmonitored holes in the ground. As noted by (Závodská, 2003), these makeshift urban landfills are nothing more than shallow excavated trenches, backfilled with solid waste and covered with soil once filled to capacity.
Moreover, due to the lack of sufficient outdoor public waste bins in Georgetown, domestic wastes are often dispersed throughout the city thus making it difficult to take measures against contamination (United Nations, 1997). Additionally, PAHO/WHO (2004) notes that, solid wastes accumulations observed in urban areas’ roads and informal markets tend to increase in the macro and micro rates of vectors as well as bad odours and toxic smokes generated by the in situ burning of these wastes. These informal markets along roadways and open spaces also generate solid wastes (most of them organic in nature) that create problems to the collection system as they do not have a proper storage system. Also, rainwater drainage is often used for disposing solid wastes throughout the city, and accumulated wastes often clog drainage canals rapidly (PAHO/WHO, 2004).
With regards to the composition of solid wastes in Georgetown, it should be noted that waste characterization data reported by (Brown and Vince, 2001) indicate that the organic fraction of the wastes generated exceeds fifty percent (by weight) of the total waste. The organic waste stream is essentially composed of wasteof a biological origin which may include items such as paper and cardboard, food, green and garden waste, animal waste and biosolids and sludges. More so, organic wastes are usually generated as a component of most waste streams and the term is generally not intended to include plastics or rubber even though these polymers are certainly organic in nature. Also, putrescible wastesare a subset of organic wastes with the distinction being that putrescible wastes, for instance food scraps,tend to biodegrade very rapidlywhereas some other organic wastes, for instance paper, tend to require lengthy times or special conditions to biodegrade3.
In addition, with such large quantities of organic wastes being generated, this poses a serious threat to public health. However, the importance of biological processes in the management and recycling of organic wastes has been widely recognized. Vermicomposting, which is essentially one of the most efficient methods for converting solid organic materials into environmentally friendly, useful and valuable products for crop production is gaining recognition around the world, though it is not a popular technology (Edwards, 2004, Aalok et al., 2008).
As a process for handling organic residuals, vermicomposting represents an alternative approach in waste management, since the material is neither landfilled nor burnt but is considered a resource that may be recycled (Aalok et al., 2008). In this sense, vermicomposting is compatible with sound environmental principles that value conservation of resources and sustainable practices and thus, can be an appropriate alternative for the safe, hygienic and cost effective disposal of the organic fraction of solid wastes (Kaviray and Sharma, 2003). Vermicomposting may be defined as an accelerated process of biooxidation and stabilization of organic wastes that involves interactions between earthworms and microorganisms (Edwards, 2004).
Although the microorganisms are responsible for the biochemical degradation of the organic matter, earthworms are the crucial drivers of the process by fragmenting and conditioning the substrate, increasing the surface area for microbiological activity, and altering its biological activity (Domínguez et al., 2004). In essence, earthworms act as mechanical blenders and by breaking down the organic matter they modify its physical and chemical status, gradually reducing its C:N ratio, increasing the surface area exposed to microorganisms and making it much more favourable for microbial activity and further decomposition. The end product, or vermicompost, is a finely divided peat-like material with high porosity and water holding capacity that contains most nutrients in the form that can be readily taken up by plants. Additionally, these earthworm casts are rich in organic matter and have high rates of mineralization that implicates a greatly enhanced plant availability of nutrients, particularly ammonium and nitrates (Domínguez et al., 2004).
It should be noted that the role of earthworms in the improvement of soil fertility and concentration of pollutants has been known for a long time. However, earthworms were not commercially used for pollution control. In the last two decades, vermicomposting has found commercial applications in pollution management (Agarwal, 2005). This technology essentially involves the application of earthworms for combating the waste disposal problems, for minimizing the pollution effects and to obtain useful products from wastes. It is a small scale, low technology approach and uses locally available labour and raw materials. Furthermore, the transformation of solid wastes into vermicompost can be interpreted as one with a double interest. On the one hand, the wastes are converted into an agriculturally useful organic fertilisers which in turn have the potential to reduce the dependency on nonrenewable chemical fertilisers and pesticides, and, on the other, it controls a pollutant that is a consequence of increasing population, urbanization and intensive agriculture (Kaushik and Garg, 2003).
Moreover, it is imperative to note that plants, like other living things require food for their growth and development. As such, sixteen essential elements are required for plant growth; carbon (C), hydrogen (H), and oxygen (O) are derived from the atmosphere and soil water while the remaining thirteen elements (nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl)) are supplied either from soil minerals and soil organic matter or by organic or inorganic fertilisers (Silva, 2000). While fertilisers may provide the essential nutrients required for plant growth, it is important to note that the rampant use of chemical fertilisers has contributed largely to the deterioration of the environment through the depletion of fossil fuels required for their production, increases in the emissions of carbon dioxide (CO2) and the contamination of water resources (Nagavallemma et al.,2006).
Nevertheless, there now is a growing realization around the world that the adoption of ecological and sustainable farming practices is critical in order to reverse the declining trend in the global productivity and environmental protection (Aveyard 1988, Wani and Lee 1992, Wani et al., 1995). Furthermore, the widespread adoption of vermicomposting in the era of sustainable farming has been proven beneficial by numerous studies. As such, there is evidence that organically based nutrient sources such as vermicompost may provide an alternative to synthetic fertilisers in order to provide nutrition for plants as well as influence their growth and productivity (Edwards, 1998). However, gaps in this field of research do still exist and thus there is continuous need for studies to determine the effects of other vermicomposted organic residues on plant growth. More so, a recent study conducted by Yusuf (2009) who investigated the potential of Eisenia fetida to produce organic fertilisers from three types of manure obtained from local farms, recommended that further research should be done to compare the effects of vermicomposts produced to that of an inorganic fertiliser on plant growth. With recognition of this potential gap in vermicomposting research, the present study therefore aims to compare the effects of vermicompost derived from plant based residues (including vegetable wastes and the peels of fruits) to that of an inorganic fertiliser on the growth of greenhouse Pak Choi (Brassica rapa var chinensis).
1.2 Proposed Title
Vermicomposting: A Sustainable Option for Organic Waste Management in Guyana
1.3 Problem Statement
Organic waste poses a serious environmental problem globally, and Guyana is no exception with the organic fraction of wastes generated on a daily basis being in excess of fifty percent (Brown and Vince, 2001). Management of solid wastes is a major issue in Guyana and waste disposal practices have not kept pace with the increasing demands posed by population growth and waste generation (Závodská, 2003). As a consequence, much of the wastes are often discarded into the environment in an indiscriminate manner, thus making it difficult to take measures against contamination.
In landfills, organic wastes decompose anaerobically in order to produce biogas (predominantly methane, a significant greenhouse gas) as well as leachate which contains nutrients and soluble organic materials (Waste 2020, 2001). Furthermore, the leachate has the potential to pollute groundwater and may also release and mobilise heavy metals from landfills. Some organic wastes such as sludges and biosolids may also contain heavy metals or nutrient pollutants and uncontrolled disposal of these substances may lead to site contamination or water pollution4.
Nevertheless, the biological process of vermicomposting presents a viable opportunity to decompose and convert the organic fraction of solid wastes into agriculturally useful organic fertilisers using locally available species of earthworms. Moreover, there is accumulating evidence which indicates that vermicompost may provide the essential nutrients required for plant growth. The widespread adoption of this technology can serve a double function; first in terms of minimizing the amount of organic wastes required for disposal and secondly by recycling these wastes into a valuable product that can be utilized for crop production, it may reduce the dependency on chemical fertilisers.
1.4 Purpose of Study
This research intends to compare the effects of vermicompost derived from plant based residues (including vegetable wastes and the peels of fruits) to that of an inorganic fertiliser on the growth of greenhouse Pak Choi (Brassica rapa var chinensis)
1.5 Significance of the Research
Given the fact that solid waste management is a serious environmental issue in Guyana and that the organic fraction of the total waste that is generated is in excess of fifty percent, it is important to note that vermicomposting can play a major role by recycling such wastes into environmentally friendly, useful and valuable organic fertilisers which can enhance crop production. Additionally, research has shown that vermicompost plays a significant role in improving the growth and yield of various crops. As such, this research will not only serve to supplement prior studies done where Eisenia fetida was used to convert organic residues such as vegetable wastes and manure into organic fertilisers, but it will also compare the effects of the vermicompst produced from plant based residues to that of an inorganic fertiliser on the growth of greenhouse Pak Choi (Brassica rapa var chinensis). Moreover, this research will potentially give weight to the argument of vermicomposting as a sustainable technology for recycling organic wastes which can in turn improve the management of organic solid waste in Guyana. Furthermore, this study may also support and provide a framework for future experimental studies in Guyana using the vermicomposting process, and a combination of these results could be used to promote and encourage the widespread use of organic fertilisers.
1.6 Research Questions
This research seeks to provide answers to the following questions:
1. What are the chemical properties (NPK) of the vermicompost samples, soil and inorganic fertiliser?
2. Is plant growth affected by the quantity of vermicompost applied?
3. What is the mineral nutrient content in Pak Choi plant tissues using the various fertiliser treatments?
4. Which fertiliser option results in best overall plant growth?
1.7 Research Objectives
Specific objectives could be derived from the research questions and they are as follow:
1. To determine the chemical properties (NPK) of the vermicompost samples, soil, and inorganic fertiliser;
2. To determine the effects of different quantities of vermicompost derived from plant based residues on the growth of greenhouse Pak Choi (Brassica rapa var chinensis);
3. To determine the mineral nutrient content in Pak Choi plant tissues;
4. To determine which fertiliser option results in best overall plant growth.
1.8 Scope of Research
Chapter 1: Background of solid waste management in Guyana, overview of the application of vermicomposting in organic waste management, background on fertilisers (inorganic and organic), the proposed research title, statement of the problem, purpose of the study, significance of the study and finally research questions and objectives
Chapter 2: This chapter outlines the literature review which explores, based on specific themes, similar areas of studies that lead up to the current research.
Chapter 3: This chapter describes the proposed methodology
Chapter 4: This chapter outlines how the analysis of the data collected and findings will be done.
Chapter 5: Concludes the study
2.0 CHAPTER 2: LITERATURE REVIEW
2.1 Organic and Inorganic Fertilisers
Growth is defined as the increment in dry mass, volume, length, or area that results from the division, expansion, and differentiation of cells (Lambers, 2008). However, in relation to plants, growth refers to the process by which a plant increases in the number and size of leaves and stems (Rayburn, 1993). Plants, like other living things require energy for proper development and as such, sixteen essential elements are required for plant growth. Each is equally important to the plant; yet, each is required in vastly different amounts. Carbon (C), hydrogen (H), and oxygen (O) are derived from the atmosphere and soil water while, the remaining thirteen elements are supplied either from soil minerals and soil organic matter or by organic or inorganic fertilisers (Silva, 2000). A fertiliser as defined broadly by Nielsson (1968) is any material, organic or inorganic, natural or synthetic, that is placed on or incorporated into the soil to supply plants with one or more of the chemical elements necessary for normal growth.
It is important to note that the quality of plant products can be considerably affected by plant nutrition. Moreover, the question is often asked whether there is any major difference in plant quality between plants supplied with organic or inorganic fertilisers. As such, (Mengel et al., 2001) notes that in organic fertilizers such as farmyard manure, slurries and green manure, most plant nutrients, including potassium, magnesium and phosphate, are present in an inorganic form. Other nutrients, specifically, nitrogen and sulphur, are converted to inorganic forms by soil microorganisms before the absorption by plant roots takes place. Thus, although plants may be supplied with organic fertilizers, they nevertheless take up inorganic nutrients derived from these organic materials (Mengel et al., 2001). Inorganic and organic fertilizers do, however, differ in the availability of the plant nutrients they contain. Nutrients in inorganic fertilizers are directly available to plant roots, whereas the nutrients of organic materials and especially organic nitrogen are of low availability.
It should also be noted that chemical fertilisers are either sterile or have insignificant microbiological activity. They are primarily composed of water-soluble chemical salts and as such organic material rarely forms part of chemical fertilisers. Once these salts have been depleted from a chemical fertiliser, re-application is required in order to maintain the nutrient levels. However, in the case of vermicompost, due to the presence of nitrifying and nitrogen fixing bacteria in the compost, nitrogen can be easily fixed from the atmosphere and converted to plant soluble nitrates. Evidently, the process continues as long as there is sufficient organic matter (which is present in vermicompost) and therefore, re-application is not required at the same rate as chemical fertilisers5.
Another important distinction that must be highlighted is the fact that microbiologically active vermicompost is capable of regenerating the nutrients from the atmosphere, organic matter and water and thus replaces those lost from chemical fertilisers by leaching, plant uptake and chemical reactions5. With respect to moisture holding capacity and improvement of soil structure, chemical fertilisers have an insignificant effect, since they primarily consist of water-soluble salts. Vermicompost, on the other hand, due to the aggregate nature of the worm castings, has appreciable water holding capacity and its use leads to improved soil structure5.
Moreover, the disadvantage of chemical fertilisers comes in mistakenly thinking that it will substitute for all the benefits of organic materials (Nebel et al., 2001). In the absence of sufficient detritus, soil organisms starve, humus content declines, and all the desirable properties of the soil decline as the top soil mineralizes. With the soil’s loss of nutrient holding capacity, applied inorganic fertiliser is prone to simply leach into waterways. Nebel et al., (2001) also emphasizes the point that the case is not one in which chemical fertilisers do not have a role to play in enhancing crop production but rather, a keen understanding of the different roles played by organic materials and inorganic nutrients is required and that each type is used as necessary. This is important to consider since the exclusive use of organic material may provide insufficient amounts of one or more nutrients required to support plant growth.
Vermicomposting is a simple biotechnological process of composting, in which certain species of earthworms are used to enhance the process of waste conversion and produce a better end product (Nagavallemma KP et al., 2006). According to Kumar (2005), the product is the result of organic wastes consumed by earthworms, digested and excreted in the form of granules. More so, vermicomposts are finely divided peat-like materials with high porosity, aeration, drainage, and water-holding capacity (Edwards & Burrows, 1985). They have a vast surface area, providing strong absorbability and retention of nutrients (Shi-wei & Fu-zhen, 1991) and they contain adequate quantities of NPK and several micronutrients essential for plant growth.
Moreover, vermicomposting differs from composting in several ways. It is a mesophilic process, utilizing microorganisms and earthworms that are active at 10-32°C (not ambient temperature but temperature within the pile of moist organic material). The process is faster than composting; because the material passes through the earthworm’s gut, a significant but not yet fully understood transformation takes place, whereby the resulting earthworm castings (worm manure) are rich in microbial activity and plant growth regulators, and fortified with pest repellence attributes as well (Gandhi et al., 1997). These metabolites (i.e. growth regulators and polysaccharides) are strongly responsible for the fertilising value of casts. The polysaccharides present in the casts act as a cementing substance, contribute to soil structure by ensuring a better aeration, water retention, drainage and aerobic condition which are useful for root development and nutrient availability (Antonello, 2007). There is evidence that casts are able to influence plant metabolism, rooting initiation and development in controlled environments (Edwards et al., 1980; Tomati et al., 1990) as well as stimulate plant growth in open fields.
In addition, Sultan (1997) indicates that vermicompost has a special place in agriculture because of its presence of readily available plant nutrients, growth enhancing substances, and a number of beneficial microorganisms like nitrogen fixing, phosphorous solubilising and cellulose decomposing organisms. Moreover, Sultan (1997) suggests that vermicomposting has the potential to recycle organic wastes for which no proper mechanisms are available, or that which the conventional techniques such as incineration may be hazardous. It should be noted that, by recycling organic wastes in agriculture brings in the much needed organic and mineral matter to the soil (Nag, 2008). Since most recyclable wastes are organic, they directly add organic matter and the plant nutrients. When the organic input plays a vital role in improving the physical and biological properties of soil, the nutrient input improves its fertility, thus, providing a favourable environment for plant growth. As such, organic wastes recycling leads to an improvement in overall soil fertility and productivity.
2.2.1 The Role of Earthworms in Vermicomposting
Earthworms are segmented and bilaterally symmetrical worms, with an external gland (clitellum) producing an egg case (cocoon), a sensory lobe in front of the mouth (prostomium), with the anus at the posterior end of the animal body, no limbs but possessing a small number of bristles (chaetae) on each segment (Dominguez and Edwards, 2004). Furthermore, earthworms constitute more than 80 percent of soil invertebrates’ biomass in many ecosystems. Pandey et al., (2008) notes that, about 10-15 percent net primary production is channelized through earthworms. Essentially, the earthworm acts as an aerator, crusher, mixer, grinder, chemical degrader and biostimulator. This in itself describes the earthworm’s role in decomposition. Earthworms are known to help the soil in respiration, nutrition, excretion, stabilization etc. In addition, these organisms help to regulate soil temperature and thus stimulate useful activities of aerobic microorganisms (Pandey et al., 2008).
It should be noted that the food after passing through the alimentary canal of the earthworm, emerges as a compact concentrated mass termed vermicastings. The earthworm’s casts contain more microorganisms, organic matter and inorganic minerals in a form that be used by plants. Vermicastings contain excreta, earthworm cocoons and undigested food making them excellent as organic manure. It is porous and has moisture absorbing capacity. It is also rich in vitamins, antibiotics and enzymes; upases, cellulases and chitinases. These enzymes continue the disintegration of organic matter after excretion from the worm as casts and, these casts are also rich in nitrates, phosphates and potash.
It is important to note that the ability of some species of earthworms to consume and breakdown a wide range of organic residues such as sewage sludge, animal wastes, crop residues and industrial refuse is well known (Edwards et al., 1985; Kaushik and Garg, 2003). Moreover, different species of earthworms have quite distinct life histories and occupy different ecological niches. However, research indicates that the epigaeic species are expected to be the most suitable for vermiculture and vermicomposting (Dominguez and Edwards, 2004). Epigaeic species tend to live above the mineral soil surface typically in the litter layers and plant debris and feed on them. They are phytophagous and most of the species have an insignificant role in humus formation. Nevertheless, they are noted for having high reproductive and cocoon production rates with a relatively short lifespan (Pandey et al., 2008). Moreover, they have high metabolic activities and hence, are particularly useful for vermicomposting. Examples of epigaeic species include; Eisenia fetida, Eisenia andrei, Eudrilus eugniae, Perionyx excavatus and Drawida medesta.
Additionally, it is important to note that Eisenia fetida and Eisenia andrei are closely related species which are commonly used for management of organic wastes by vermicomposting. They are peregrine and ubiquitous with a worldwide distribution and many organic wastes become naturally colonised by them. Another reason why these two species are prefered in vermicomposting relates to the fact that they both have good temperature tolerance and can live in organic wastes with a range of moisture contents (Dominguez and Edwards, 2004).
2.2.2 Recent studies on Vermicomposting
The use of vermicompost, as a source of organic manure in supplementing chemical fertilizers is becoming popular day by day (Kumar, 2005). As such, there is accumulating scientific evidence that vermicompost can influence the growth and productivity of plants significantly (Edwards, 1998). The beneficial effects of vermicompost on plants may be due to their physical and chemical properties such as particle size, porosity, water holding capacity, air capacity, electrical conductivity and pH which are even more important than the concentration of nutrients (Gouin, 1998). Moreover, a number of greenhouse and field studies have examined the responses of plants to the use or substitution of vermicompost to soil or greenhouse container media (Chan & Griffiths 1988; Edwards & Burrows 1988; Wilson & Carlile 1989; Mba 1996; Buckerfield & Webster 1998). Most of these studies have confirmed that vermicomposts, whether used as soil additives or as components of horticultural media, improved seed germination and enhanced the rates of seedling growth and development.
According to a study by Edwards and Burrows (1988), cabbages grown in compressed blocks from pig waste vermicompost in a greenhouse and subsequently transplanted to the field were larger and more mature at harvest as compared to those grown in a commercial blocking material. Moreover, in a field experiment in which cassava peel mixed with guava leaves and vermicomposts produced from poultry droppings were applied to field crops, Mba (1983) reported higher shoot biomass and increased seed yields of cowpea. Venkatesh et al. (1997) also reported that yields of Thompson Seedless grapes were significantly higher when vermicompost was applied and Kumar (2005) has reported significantly higher yields when vermicompost was applied to chillies, watermelons and paddies as compared to farm yard manure. It should also be noted that organic vermicompost could help to produce additional yields of crops to an extent of 30% more yield than normal yields as indicated by (Venkataratnam, 1994). Furthermore, Atiyeh et al., (2000) have reported the differences in the effects of vermicomposts and composts on marigold and tomato plants. As such, plants were less responsive to the composts than vermicomposts. This difference in growth may be due to the fundamental differences between the composting and vermicomposting processes which use different microbial communities, with composting tending to result in the release of mineral nitrogen in the ammonium form, where as vermicomposting releases most the nitrogen in the nitrate form, the form readily available for plant uptake.
Another study conducted by (Arouiee, 2009) which investigated the effects of different levels of vermicompost on seed germination parameters and the growth of greenhouse tomato (Lycopersicun esculentum) concluded that there were significant differences between treatments. The highest seed germination rate was in 25% vermicompost. Tomato seedlings growing in 100% vermicompost had the lowest amount of chlorophyll, the lowest leaf diameter, lowest dry weight and were the shortest seedlings between all treatments. Furthermore, the application of 50% vermicompost increased the inter-node number, root dry weight and nitrogen content of tomato seedlings significantly as compared to the control plants. Also, the incorporation of 25% vermicompost increased significantly the shoot dry weight and leaf area of tomato seedlings compared to the control.
Similarly, an experiment conducted to determine the effects of vermicompost on the growth, yield and fruit quality of tomato (Lycopersicum esculentum var. Super Beta) in a field condition by (Azarmi et al., 2008) revealed that the addition of vermicompost at a rate of 15 t ha-1 significantly increased growth and yield compared to the control. Vermicompost applied at this rate also increased electrical conductivity of fruit juice and percentage of fruit dry matter up to 30 and 24%, respectively. The content of K, P, Fe and Zn in the plant tissue increased 55, 73, 32 and 36% compared to untreated plots respectively. Moreover, the result of this experiment showed that the addition of vermicompost had significant positive effects on growth, yield and elemental content of plants as compared to the control.
2.2.3 Vermicompost and Inorganic Fertilisers
It is important to note that intensive cropping systems with fertilizer responsive crops that rely on high inputs of inorganic fertilizers often lead to unsustainability in production. However, complete dependence on organic sources of nutrients may also be in adequate to attain the most productivity. Furthermore, few studies have compared the effects of vermicompost to that of inorganic fertilisers on the growth of plants. The results of these studies indicate that the combined application of organic and inorganic fertilisers helps to increase crop productivity and quality and thus maintain soil fertility.
As such, (Ushakumari et al., 1999) have proven that amending soils with vermicomposts applied at 12 t ha-1 in combination with 100 or 75% of
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