History and progress of vermiculture and vermicomposting

The vermicomposting is the process in which worms are used to convert organic waste into vermicompost, not to be confused with vermiculture or vermicultures, whose objective is to maximize the cultivation of worms without pursuing an optimal biostabilization of the residue used for the feeding of the same; although sometimes both goals are achieved simultaneously (^{Moreno et al., 2014}).

The vermiculture as technology combines the virtues and qualities that makes it potentially viable. The importance of earthworms is not a new phenomenon. The influence of earthworms on agricultural soils was well known in the ancient Greek and Egyptian civilizations that valued the role of earthworms in the soil. The ancient Egyptians were the first to recognize the role that earthworms played in the fertilization of arable land, for which they were considered sacred animals, Aristotle on the other hand defined them as the “intestines of the earth”, but it was Charles Darwin in the nineteenth century who explains the true role of earthworms in the soil, to relate the decomposition of dead plant material (^{Sinha et al., 2010a}; ^{García-Pérez, 2011}; ^{Medany, 2011}; ^{Pathma and Sakthivel, 2012}).

The vermiculture had its began in the mid-twentieth century (^{Sinha et al., 2009}), but the first references the benefit of this activity by removing organic waste was in the 30s to mid 40 s intensive cultivation begins of california red worms in the United States of America to obtain vermicompost. However, knowledge of the vermicomposting process began in the 70s to lay the scientific and technological basis for the development of this system in countries like the United States, Holland, England and Canada (^{Kumar, 2005}; ^{Vargas-Machuca et al., 2008}).

For this decade, vermicomposting is important in Europe, Africa, Asia, Latin America and Australia, where its development is spectacular, with a notable importance of some worm’s production centers (^{Schuldt, 2006}). Companies dedicated to composting in several countries such as England, France, Holland, Germany, USA, Italy, among others, develop a business vision for the vermicompost obtained from various types of organic waste (^{Sinha et al., 2009}; ^{Sinha et al., 2010c}).

Countries like the United States of America own some of the largest vermicomposting plants in the world, producing 3 410 t each year in high tech greenhouses. In the United Kingdom, plants have been created to compost various organic waste, which can generate up to 1 000 t of vermicompost per year. In New Zealand the vermicomposting capacity of some companies is about 5 to 6 thousand tons of green waste a year and they plan to vermicompose approximately 40 000 t of food waste per year. In Australia with the use of solid waste from water treatment plants and water treatment, have a production capacity of vermicompost 600 t of organic fertilizer for sale to local farmers, with this technology will save more than 13 000 m³ space in landfills each year (^{Li et al., 2010}; ^{Sinha et al., 2010a}).

More often developed countries are concerned and recognize that organic waste should be used as resources, rather than being disposed of in landfills, generating environmental problems that are costly to remedy. The recycling of the organic matter can be achieved by alternative treatment methods to obtain a stabilized and marketable product.

Earthworms: key agents in vermicomposting

The earthworms are segregated worms of mainly terrestrial habit, common in humid soils and whose size as adults varies from 1 cm in length by 2 mm in thickness, to more than 1 m and 3 cm in length and thickness respectively (^{Fragoso and Rojas, 2014}). They represent the highest animal biomass in most terrestrial ecosystems, they influence in a very significant way the physical, chemical and biological properties of the soil, and they play an important role in the modification of the soil structure of the soil and in the acceleration of the decomposition organic matter and nutrient recycling (^{Domínguez et al., 2009}). The alteration is reflected in the vertical and horizontal redistribution of the soil organic matter.

This redistribution depends on the ecological groups of earthworms, which according to its location on the ground, feeding behavior and formation of burrows are classified into three categories (^{Lemtiri et al., 2014}): endogenous worms that move within the soil to feed on soil organic matter; while the anecicans take the organic remains from the surface of the soil that they drag inside the galleries, and the epigeous that live permanently associated with accumulations of organic matter on the surface of the soil (^{Kavdir and Ilay, 2011}; ^{Gómez-Brandón et al., 2011b}; ^{Moreno et al., 2014}).

The number of earthworm species described so far is very high, according to ^{Reynolds and Wetzel (2015)}, there are approximately 3 627 species. For vermicomposting, suitable species are those that exhibit suitable characteristics, such as natural ability to colonize organic waste, high rates of consumption, digestion and assimilation of organic matter, tolerance to a wide range of environmental factors, short life cycles, high reproductive rates and resistance to management. Of the known species, only five have been widely used in vermicomposting; Eisenia andrei (Bouche, 1972), Eisenia fetida (Savigny, 1826), Dendrobaena veneta (Savigny, 1826), and to a lesser extent Perionyx excavatus (Perrier, 1872) and Eudrilus eugeniae (Kinberg, 1867; ^{Domínguez and Pérez-Losada, 2010}; ^{Yadav and Garg, 2011}; ^{Domínguez and Edwards, 2011a}).

The earthworms according to their climatic zone are grouped into two categories; they are typical of the temperate zone Eisenia fetida (Savigny 1826), Eisenia andrei (Bouché 1972), Dendrodrilus rubidus (Savigny 1826), Dendrobaena veneta (Rosa 1886), Lumbricus rubellus (Hoffmeister 1843) and Drawida nepalensis (Michaelsen 1907); for the tropics Eudrilus eugeniae (Kinberg, 1867), Perionyx excavatus (Perrier 1872) and Polypheretima elongata (Perrier 1872) (^{Domínguez, 2004}; ^{Domínguez and Edwards, 2011a}; ^{Moreno et al., 2014}). Sand can be found in various habitats, especially the dark and damp (^{Sinha et al., 2010a}). In Table 1, is summarized the main characteristics of their life cycle and temperature and humidity requirements of the main species used in vermicomposting (^{Moreno et al., 2014}).

Table 1 Biological characteristics and environmental conditions of the main species of earthworms temperate used in vermicomposting processes.  

Fuente: ^{Moreno et al. (2014)}.

The earthworms modify the soil structure and activity of microbial communities through the dispersal of inocula with grazing, the reduction of the particle size of the organic material during the intestinal passage, the addition of sugars and other substances, and facilitate the formation of aggregates (^{Aira et al., 2002}; ^{Domínguez, 2004}). During the vermicomposting worms active at both spatial and temporal level, these activities can have a direct effect on the process, as stimulation or reduction of microbial biomass and dispersion and interaction of microorganisms with other biological components of soil. As indirect effects in the relationships with those responsible for the aging and mixing of the materials modified by them with other organic substrates. The worms also produce excreted as urea and ammonium are a source of assimilable nutrient substances by microorganisms (^{Domínguez et al., 2009}; ^{Aira and Domínguez, 2010}).

At present, the scientific community is in search of a technology that is “economically viable” (cheaper, accessible to all nations), “environmentally sustainable” (friendly to the environment, flora, fauna, soil, air and water, with no effect on them) and “socially acceptable” (beneficial to society without any adverse effect on human health). In this sense, the technology of vermiculture combines all these virtues and qualities together. The vermiculture as technology is generating a revolution mainly by the diverse applications that this one has. Earthworms as ecosystem engineers play an important role as “waste manager”, “soil manager and fertility enhancers” and “plant growth promoter”.

However, new findings on its role in “wastewater treatment”, “recovery of contaminated soils”, and more recently for its potential use in modern medicine for the protection of “human health”, as in the reduction of blood pressure, thinning of blood and dissolution of blood clots in patients with stroke and heart, cure for cancer, cure for arthritis and rheumatism, as an anti-inflammatory agent, source of antibiotics and as a source rich “high-quality protein”, have brought a revolution in studies of vermiculture (^{Sinha et al., 2010a}; ^{Sinha et al., 2010b}).

Vermicomposting: a treatment strategy for organic waste

The vermicomposting technology is based on feeding habit detritivore some species of worms, organisms capable of colonizing a wide variety of organic substrates (^{Mamani-Mamani et al., 2012}). The vermicompostaje is a bioprocess that is considered as a viable alternative for the composting of organic waste (^{Hait and Tare, 2011}; ^{Vig et al., 2011}). The earthworms make and stabilize organic waste in a material similar to humus, rich in nutrients, called earthworm humus, worms in it both physically exert action as biochemistry (^{Garg et al., 2012}). The physical action it includes aeration, mixture and grinding organic waste, whereas microbes they are responsible for biochemical degradation and stabilization (^{Aira et al., 2008}). The worm-microbe interaction becomes insoluble organic materials to a soluble form (^{Domínguez and Edwards, 2011b}; ^{Kui et al., 2014}).

The generation of organic solid waste has led to the search for alternatives for its treatment and final disposal. An alternative of transforming organic waste, is the production earthworm humus, proposal ecological the use of chemical fertilizers (^{Cabanillas et al., 2013}). The vermicomposting is called to make the second green revolution for generating organic fertilizer for use in the production of healthy food (^{Sinha et al., 2010b}; ^{Sinha et al., 2010c}). One of the major contributions of vermicompost to the soil is the increase of the population and activities microbial, key factors in rates from nutrient recycling ground, production of materials growth for plants and suppressors infections and plant pests (^{Arancon et al., 2006}), also it promotes the reduction of environmental degradation with increasing use of various agricultural residues (^{Moreno-Reséndez et al., 2014}). The decomposition of organic waste by earthworms is favored by the action of endosymbionts microorganisms of the intestine, they produce extracellular enzymes that degrade cellulose and phenolic compounds, increasing the mineralization of carbon and nitrogen ingested material (^{Aira et al., 2008}; ^{Domínguez et al., 2009}).

Studies have shown that the content of macronutrients and micronutrients in the vermicompost is generally higher than in traditional compost; it contains high levels of the most soluble nutrients such as nitrogen, phosphorus, potassium, calcium and magnesium compared to normal compost, this property improves soil fertility physically, chemically and biologically, resulting in higher crop yields. However, improvements in plant growth and increased yields could not be fully explained by the availability of nutrients. Related studies, suggest that contain vermicompost growth regulatory substances or humic acids which are responsible for plant growth (^{Su et al., 2015}).

Organic solid waste: source of substrates for vermicomposting

There are a number of conventional and unconventional organic waste generated by different agricultural, urban and industrial activities that have been tested and used successfully in the process of vermicomposting (^{Singh et al., 2011}).

The conventional waste

They include traditional manures of animal origin, which are considered as natural organic materials and optimal for feeding and development of various species of earthworms. The manures have been used since ancient times as soil improvers and as stimulants in crop production (^{Nieto-Garibay et al., 2010}). Among the materials that have been used as a substrate for the development of earthworm include manure of cattle, horses, pigs, poultry, rabbits and sheep (^{Gunadi and Edwards, 2003}; Morales et al., 2010). The high availability and nutritional contribution make it an attractive alternative to generate organic fertilizers for soils with nutritional deficiencies (^{Carvajal and Mera, 2010}).

The cattle manure is a nutritionally well-balanced natural food for the worms, which requires no preconditioning although sometimes the presence of seeds requires precomposite (^{Moreno et al., 2014}). There is abundant information on using cattle manure for vermicomposting, as history is the use of leaching and washing vermicompost (Eisenia fetida) bovine manure to meet crop yield using bean. It has been shown that bean plants treated with vermicompost washing conducive taller plants, longer pods, and the highest number of pods per plant and lateral branches (^{Ayyobi et al., 2014}).

The use of bovine manure has also been used to transform waste from the tanning industry with Eudrilus eugeniae fermented with bacteria and subsequent vermicomposting Selenomonas ruminantium mixed with litter. It has been found that the pH and the C:N ratio decrease significantly at the end of the process, with good humification and mineralization polymers (^{Ravindran et al., 2013}). The use of bovine manure in the treatment of slurry of biogas plants and pre-processed vegetable residues and subsequent vermicomposting has been successfully performed. Properties like pH, organic carbon, organic matter and C:N ratio is considerably reduced with respect to the initial raw material, also it has higher concentration of nitrogen, phosphorus and potassium optimal stabilization of the residue (^{Garg and Gupta, 2011}; ^{Yadav et al., 2013}). The vermicomposting based on cow manure generates a stabilized product, with a concentration of macros and important micronutrients (^{Hernández et al., 2010}).

Another residue of animal origin is poultry manure, which contains a high concentration of nitrogen, so it is recommended to mix with organic residues with a high C:N ratio due to its high protein content. However, it is relatively well balanced as a food for worms. The use of poultry manure for vermicomposting is not as widespread as bovine, recent works such as ^{Petmuenwai et al. (2013)}, suggest the use of poultry manure for vermicomposting with different agro-industrial wastes (cassava pulp, peel cassava, peel eucalyptus and oil palm) using Eudrilus eugeniae, it promotes the survival and growth in these worms when used these residues.

The rabbit manure, although having a high nitrogen content, is relatively balanced for use as groundworm feed. Sometimes due to its high ammonium content, it requires a previous washing for composting with worms. This type of waste has been used in combination with other manures, such as horse, goat and cattle, using Eisenia fetida for composting and use in melon cultivation in greenhouse.

This humus promotes greater equatorial and polar diameter, thickness of the pulp, fruit weight and yield, higher content of vermicompost, which happens to be a biofertilizer high quality to enhance the production of this crop (^{Moreno-Reséndez et al., 2014}).

The potential of vermicomposting to process large amounts of manure rabbit with E. fetida under controlled conditions generates good expectation. The decreasing pH, stability in electrical conductivity, decreasing the microbial load in the final product, indicating a higher degree of d stabilizing the final product. These characteristics are important for its safe use as amended organic floor (^{Gómez-Brandón et al., 2013}).

The use of sheep manure for vermicomposting is another very interesting alternative for the generation of biofertilizers. It is a nutritionally balanced food for earthworms, it only requires pre-composting if there are herbal seeds. Some studies report their application with different objectives (^{Moreno-Reséndez et al., 2014}). ^{Coulibaly and Zoro (2010)}, consider the growth and reproduction of Eudrilus eugeniae is favored using sheep manure, chicken, pork and beef, so we recommend these raw materials for large - scale vermicomposting. The use of sheep manure vermicompost mycorrhizal and diazotrophic for growing corn plants bacteria, has shown that this type of humus generated more leaves, wet weight, height and diameter of the stem. The addition of diazotrophic bacteria increases mycorrhizal colonization and with that the phosphorus content but not of nitrogen (^{Gutiérrez-Miceli et al., 2008}).

A product of interest derived from the vermicompost process is extracts or teas. ^{González et al. (2013)}, considered that the extracts present chemical characteristics that depend on their origin and have an effect on the pH, electrical conductivity and concentration of nutrients, for example the extracts of the vermicompost of manure of lamb and bovine mixed with grass presents values high pH and electrical conductivity, as well as macronutrients (except P and Mg) and micronutrients.

Another nutritionally well-balanced substrate for vermicomposting is equine manure. This residue has great potential to generate biofertilizers. In this regard, ^{Moreno-Reséndez et al. (2013)}, mention that the use of manures horse, goat and rabbit vermicomposting promotes the growth and development of seedlings of acacia (Acacia farnesiana), observing higher plant height, fresh weight and dry weight. The cultivated tomato plants have been favored by the use of worm humus from horse manure, goat, alfalfa straw and sand. The benefits generated by the mixtures result in increased plant yields, observing a greater response in terms of number of locules, soluble solids content and tomato fruit size. This type of humus has characteristics, physical and chemical that allow the development of tomatoes (^{Moreno et al., 2008}).

The porcine manure is another type of substrate used for vermicomposting. However, it is used in conjunction with large amounts of structuring organic waste, this residue is also considered a well-balanced food for earthworms. It has been used with good results as a means of germination, cultivation and production of tomato, calendula, pepper and aldiza, it has been proven that it increases the number of shoots, root weight, leaf area and proportion shoot: root of both tomato and marigold, although it has little influence on pepper and aldiza (^{Bachman and Metzger, 2008}).