Handbook of Vermicomposting by E.SREENIVASAN - HTML preview

CHAPTER 2

EARTHWORMS

The earthworm is a segmented invertebrate. Its body holds its tube-shape because it is full of a liquid called coelomic fluid found between the body wall and the alimentary canal. Earthworm has a long, cylindrical body with a pointed head (Padashetty,S.&Jadesh,M.,2014)(Fig.2). In some species the posterior end is slightly flattened, while in others the body is cylindrical throughout. Rings that surround the moist, soft body allow the earthworm to twist and turn, especially since it has no backbone. With no true legs, bristles (setae) on the body move back and forth, allowing the earthworm to crawl. Earthworm breathes through its body surface. Food is ingested through the mouth into a bag like structure referred to as crop. In some species a distinct crop is absent. Later the food passes through the gizzard, where ingested stones grind it up. After passing through the intestine for digestion, what’s left is eliminated as castings.

Fig.2: Earthworms

Distribution

Earthworms are found all over the world, except in areas under constant snow and ice, mountain ranges, deserts and areas almost entirely lacking in soil and vegetation.  Species which are widely distributed are called peregrine, whereas others termed as endemic do not spread successfully to other areas. (Ansari, A.A. and Ismail,S.A.,2012)

Factors affecting distribution

The distribution of earthworms in soil is affected by physical and chemical characters of the soil, such as temperature, pH, moisture, organic matter and soil texture. (Govindan,1998)

Temperature

The activity, metabolism, growth, respiration and reproduction of earthworms are all influenced greatly by temperature.

pH

pH is a vital factor that determines the distribution of earthworms as they are sensitive to the hydrogen ion concentration. pH and factors related to pH influence the distribution and abundance of earthworms in soil. Several workers have stated that most species of earthworms prefer soils with a neutral pH. There is a significant positive correlation between pH and the seasonal abundance of juveniles and young adults.

Moisture

Prevention of water loss is a major factor in earthworm survival as water constitutes 75-90% of the body weight of earthworms. However, they have considerable ability to survive adverse moisture conditions, either by moving to a region with more moisture or by means of aestivation. Availability of soil moisture determines earthworm activity as earthworm species have different moisture requirements in different regions of the world. Soil moisture also influences the number and biomass of earthworms.

Organic matter

The distribution of earthworms is greatly influenced by the distribution of organic matter. Soils that are poor in organic matter do not usually support large numbers of earthworms. Several workers have reported a strong positive correlation between earthworm number and biomass and the organic matter content of the soil (Ismail,1997).

Soil texture

Soil texture influences earthworm populations due to its effect on other properties, such as soil moisture relationships, nutrient status and cation exchange capacity, all of which have important influences on earthworm populations.

Classification

Earthworm belongs to Annelida phylum and to Oligochaeta class that comprises more than 1800 species; most of the species belong to Lumbricidae family, including the genera: Dendrobaena, Eisenia and Lumbricus. The classification of one of the species’ of this family, E. fetida which is known as red worm, brandling worm, red wiggler worm is shown in Table.2.

                       Table.2: Taxonomic classification of Eisenia fetida

Based on their feeding habits, earthworms are classified into detritivores and geophagous. Detritivores feed near the soil surface. They feed mainly on the plant litter and other plant debris in the soil. These worms comprise the epigeic and the anecic forms. Geophagous worms, feeding deeper beneath the surface ingest large quantities of organically rich soil. These are generally called as humus feeders and comprise of endogeic earthworms.

The morpho-ecological groupings relate to several factors including general size, shape and pigmentation, burrow construction, position in the soil profile, source of food and reproductive potential. The three groups of earthworms are:

1. Litter dwelling earthworms (Epigeic species)

There are several deeply pigmented or red species that normally live in the rotting litter or organic matter on the surface of soils. They grow and reproduce very prolifically compared with true soil dwelling earthworms.

 Some of the species commonly used in vermicomposting are Dendrobaena veneta (blue nosed worm), Eisenia fetida (tiger or brandling worm), and Eisenia andrei (red tiger worm) and Eudrilus eugeniae.

2. Topsoil dwelling earthworms (Endogeic species)

Just below the surface live another group of small earthworms, in the first few centimetres of topsoil. They improve soil structure in the root zone of plants and recycle dead organic matter. One notable species is Allolobophora chlorotica (green worm).

3. Deep burrowing earthworms (Anecic species)

Anecic species live deeper down in the soil profile in permanent vertical burrows that can be up to two metres long. They help create topsoil by dragging dead organic material from the soil surface down into their burrows, ingesting it along with soil and then egesting the mixture back on the surface as nutrient-rich earthworm casts. Species in this category are highly valued and have been successfully bred for land restoration projects. One beneficial species is Lumbricus terrestris (the lob worm).

The Anecic types burrow deep in the soil but come to the surface at night to forage for freshly decaying residues.

Basic Requirements

Earthworms need these basic things for vermicomposting:

1. Bedding: Bedding is any material that provides the worms with a relatively stable habitat. This habitat must have the following characteristics: High absorbency- Worms breathe through their skins and therefore must have a moist environment in which to live. If a worm’s skin dries out, it dies. The bedding must be able to absorb and retain water fairly well if the worms are to thrive. Good bulking potential- If the material is too dense to begin with, or packs too tightly, then the flow of air is reduced or eliminated. Worms require oxygen to live, just as we do. Different materials affect the overall porosity of the bedding through a variety of factors, including the range of particle size and shape, the texture, and the strength and rigidity of its structure. Low protein and/or nitrogen content (high C:N ratio)- Although the worms do consume their bedding as it breaks down, it is very important that this be a slow process. High protein/nitrogen levels can result in rapid degradation and its associated heating, creating inhospitable, often fatal, conditions. Heating can occur safely in the food layers of the vermiculture or vermicomposting system, but not in the bedding.

2. Housing: Sheltered culturing of worms is recommended to protect the worms from excessive sunlight and rain. A low cost unit can be arranged in vacant cowsheds, poultry sheds, basements and back yards.

3. Containers: Bricks or cement tanks are to be constructed separated in half by a dividing wall.

4. Environmental conditions: The environmental conditions are vital and may affect the breeding, cocoon production and hatching of young earthworms.

a. Temperature

In vermicomposting, temperatures are kept generally kept below 35oC. Most worm species used in vermicomposting require moderate temperatures from (10-35oC).While tolerances and preferences vary from species to species, temperature requirements are generally similar. In general, earthworms tolerate cold and moist conditions far better than they can hot and dry conditions.

b. Moisture

Earthworm requires plenty of moisture for their growth and survival. They need moisture in the range 60–75 %. The soil should not be too wet else it may create an anaerobic condition and drive the earthworms from the bed (Ronald and Donald, 1977). It is very important to moisten the dry bedding material before putting them in the bin, so that the over all moisture level is well balanced.

c. pH

Earthworms are pH sensitive and generally most of them survive at pH ranging from 4.5 to 9. The alteration of pH in the worm bed is due to the fragmentation of the organic matter under a series of chemical reactions. The soil pH is a major factor limiting the abundance and distribution of earthworms.

Nutrition

• Earthworms obtain their nutrition from microorganisms, especially fungi and also nematodes
• The grinding action of earthworm’s gizzard increases surface area of the organic matter and promotes microbial activity in organic wastes as they pass through earthworm guts
• Earthworm feeding favors aerobic microorganisms at the expense of anaerobic microbes
• Vermicomposts are very much more microbially-active than the parent organic wastes with diverse microbial communities

Species

The selection of species of earthworms for vermicomposting should focus on species where, consumption of organic biomass, rapid growth and reproduction is within short time span (Gunasegaran and Desai,1999).Some of the characteristics the earthworms for attaining the objectives of vermicomposting are detailed below:

1. Worms should be capable of inhabiting a wide range of organic materials

2. It should have high fecundity rate with short incubation period.

3. The period of interval from hatching to maturity should be very short.

4. It should have less vermistabilization (period of inactivity after initial inoculation to organic wastes).

5. Wide adaptability (tolerance) to environmental factors (capability to live in varying temperature and moisture conditions);

6. High growth rate, low incubation period, high reproduction and cocoon production rate;

7. High consumption, digestion and assimilation rate for organic matter decomposition;

8. Easy to culture.

The vermicompost produced using different species of earthworms show variation in nutrient composition. So, the selection of the suitable species for particular vermicomposting application is important. It is well established that epigeic species of earthworms are used widely for the purpose of vermicomposting of different organic wastes (Ismail,2005).A list of common earthworm species suitable for vermicomposting has been presented in Table.3

Table.3: List of some earthworm species suitable for vermicomposting

               (Source: Chattopadhyay, G.N., 2012)

Some of these species identified as most suitable for breaking down solid wastes are: Eisenia fetida (and the closely-related E. andrei), Eudrilus eugeniae and Perionyx excavatus.(Fig.3) These species are prolific breeders,maintaining a high reproduction rate under favorable, moisture and food availability. They show high metabolic activity and hence are particularly useful for vermicomposting.(Radhakrishnan,B. and Muraleedharan,N.,2010) Other species can also be used but these species are the commonest.

1 Eisenia fetida

Eisenia fetida, popularly known as red wriggler, red worm, tiger worm etc is perhaps the most widely used earthworm for vermicomposting. Mature individuals can attain up to 1.5 g body weight. Each mature worm on average produces one cocoon every third day and from each cocoon emerge from 1 to 3 individuals on hatching within 23 days. Average life of this worm is 1–2 years.

2 Eudrilus eugeniae

Eudrilus eugeniae commonly known as Night Crawler is a native of Equatorial West Africa. It is a fast growing species capable of accumulating body mass at the rate of 12 mg per day. A mature worm can attain body weight up to 4.3 g/individual. Maturity is attained over a period of 40 days, and, a week later, individuals commence cocoon production (on average one cocoon day−1). Its life span has been estimated from 1 to 3 years. The temperature tolerance of the species is lesser than that of E. fetida. This species is widely used as vermicomposting worm in tropical and sub-tropical regions.

3 Perionyx excavatus

This species is highly adaptable and can tolerate a wide range of moisture and composition of the waste materials. Average growth rate of Perionyx excavatus is 3.5 mg per day and body weight (maximum) 600 mg. Maturity is attained within 21–22 days and reproduction commences by 24th day, with 1 to 3 hatchlings per cocoon. This species is considered by many as best suited for vermicomposting in tropical climates.

The survival, growth, mortality, and reproduction of these species have been studied in detail in several laboratories, in a range of organic wastes, including pig, cattle, duck, turkey, poultry, potato, brewery, paper, and activated sewage sludge. All of the species tested could grow and survive in a wide range of different organic wastes, but some were much more prolific, others grew more rapidly, and yet others attained a large biomass quickly. Most organic wastes can be broken down by these worms, but some organic wastes (e.g. wood wastes) have to be pretreated in various ways to make them acceptable to the earthworms.

Fig.3: Common species of Earthworms for vermicomposting

             Eisenia  sp.                     Eudrilus sp.                  Perionyx sp.

Microbiology

Microorganisms live in the alimentary canal of earthworms in a complex, mutually beneficial inter-relationship (Ansari,A.A and Ismail,S.A., 2012). Recently some of these microbes were isolated and identified in the gut of Eudrilus eugeniae by Prabha,M.L.,et al (2014).The various types of bacteria isolated and identified  were Proteus mirabilis, Staphylococcus aureus, E.coli and Klebsiella sp. and the fungi identified were Aspergillus flavus, A. niger, A. ternus, Alternaria sp. and Pencillium sp. Although these microorganisms are the same as those in the soils, the microbial population in earthworm casts is found to be much higher as compared with the surrounding soil. Earthworm casts usually have a greater population of fungi, actinomycetes and bacteria and higher enzyme activity than the surrounding soil. Microbial activity in earthworm casts may have an important effect on soil crumb structure by increasing the stability of the worm-cast-soil relative to that of the surrounding soil. Earthworms are very important in inoculating soils with microorganisms. Many microorganisms in the soil are in a dormant stage with low metabolic activity, awaiting suitable conditions like the earthworm gut or mucus to become active. Earthworms have been shown to increase the overall microbial respiration in soil, thereby enhancing microbial degradation of organic matter.

Microbial action

Earthworms make vermicompost by feeding on the waste. The other organisms which accompany them also assist in the complex process of breaking down the matter. The overall mechanism behind this is given below:

1. The organic matter, fungi, protozoa, algae, nematodes and bacteria ingested by earthworm is passed through its digestive tract. The majority of the bacteria and organic matter pass through undigested as ‘casting’ with metabolite wastes such as ammonium, urea and proteins.

2. During this, the worm also secretes mucus, containing polysaccharides, proteins and other nitrogenous compounds. Through feeding and excreting, worm creates a number of “burrows” in the material which helps in aeration.

3. Some bacteria require oxygen (aerobic) whereas some prefer its absence

(anaerobic). Anaerobic bacteria are responsible for the stench from stagnant drains, landfill sites, etc. With the aerobic conditions in vermicompost, aerobic microbial growth increases. Accompanying this microbial growth is the breakdown of organic nitrogen compounds to ammonia and ammonium. The sweet smelling aerobic process overcomes the ugly smell of anaerobes. That is why worm compost piles (properly maintained) smell so nice.

4. The whole process consumes organic matter and creates a ruffled surface in the burrow walls resulting in favourable environment for obligate aerobes (such as Pseudomonas spp., Zoogloea spp., Micrococcus spp., etc). The continued growth of the microbiological population continues to increase the rate of decomposition of the material.

5. Air flows through the material minimize the formation of sulfide and ammonia gases, odours that are typically present in anaerobic conditions. Objectionable odours disappear quickly, due to microorganisms associated with the vermicast.

Reproduction

Mature worms have a prominent band around their body, which is called as the clitellum. This is usually visible around 8-12 weeks of age. During copulation, the worms will join together at the clitellum (sometimes for quite a long period of time(Fig.4). Reproductive material is exchanged. When the worms separate, a ring of mucus material forms at the clitellum of each worm. This process is known as copulation. Sperm from the other worm is stored in sacs. As the mucus slides over the worm, it encases the sperm and eggs inside. After slipping free from the worm, both ends seal, forming a lemon-shape cocoon approximately 3.2 mm long (Fig.5).Two or more baby worms will hatch from one end of the cocoon in approximately 3 weeks. Baby worms are whitish to almost transparent and are 12 to 25 mm long.

Red worms take 4 to 6 weeks to become sexually mature.

Fig. 4: Earthworms:Copulation

Fig.5: Earthworms: Cocoons

Calculating Rates: Epigeic worms reproduce very rapidly and the worm populations double every 60 to 90 days, if the following conditions are provided:

1) Adequate food (continuous supply of nutritious food);

2) Well aerated bedding with moisture content between 70 and 90%;

3) Temperatures maintained between 15 and 30ºC;

4) Initial stocking densities greater than 2.5 kg/m2 but not more than 5 kg/m2

The term ‘stocking density’ refers to the weight (initial) of worm biomass per unit area of bedding. For instance, when we start with 5 kg of worms in a bin with a surface area of 2 m2, the initial stocking density would be 2.5 kg/m2. The onset of rapid reproduction will be delayed at very low densities, and may even stop it completely. It is clear that worms need a certain density in order to get a chance of running into each other and reproducing frequently. At lower densities, they just don’t find each other as often as the typical worm grower would like. On the other hand, densities higher than 5 kg/m2 begin to slow there productive urge, as competition for food and space increase. While it is possible to get worm densities up to as much as 20kg/m2, the most common densities for vermicomposting are between 5 and 10 kg/m2.Worm growers tend to stock at 5 kg/m2 and “split the beds” when the density has doubled, assuming that the optimum densities for reproduction have by that point been surpassed. If the above guidelines are followed, a grower can expect a doubling in worm biomass about every 60 days. Theoretically, this means that an initial stock of 10 kg of worms can become 640 kg after one year and about 40 tonnes after two years.

The main barriers to achieving optimum rates of reproduction are:

Lack of knowledge and experience Growing worms is part science, part “green thumb”. You need the knowledge, but you also need to do it to learn how to do it well.

Lack of dedicated resources Increasing worm populations requires paying attention to what is happening and responding accordingly. This takes time and effort. If the beds or windrows are neglected, the worms will likely survive, but the population will not increase at an optimum rate.

Life cycle and population

Earthworms are hermaphrodite, which means each worm is both male and female (Diaz Cosin, D.J., et al, 2011). However, each worm must still mate with another worm of its species in order to reproduce. During mating, any two adult worms can join together to fertilize each other's eggs. Fertilized egg contains in a mucous tube secreted by the clitellum that slips over its head and then into the soil through its mouth as an egg-case or cocoon. These cocoons are about the size of a match stick head and change color as the baby worms develop, starting out as pale yellow and when the hatchlings are ready to emerge, cocoons are reddish-brown.

Number of cocoons and hatchling period varies for each species and depend upon the environmental conditions (Table.4). The lifespan of the earthworm in the wild is not certain, but researchers estimate a normal lifespan of about 3 years. The earthworm population is self controlled and limited by available food, space, and environmental conditions.

Table.4: Comparison of Lifecycle and Growth of Different Earthworm Species