By Dr William Stiles: IBERS, Aberystwyth University

Organic or green waste generation (materials including food, plant or animal wastes, such as manure) is a major problem for agricultural businesses, which can be difficult and costly to manage effectively. Vermicomposting (sometimes referred to as a wormery) is an effective and rapid method for organic waste management, which can simultaneously recycle valuable resources such as nutrients.

The vermicomposting process is similar to traditional composting, in that organic waste is broken down through biological decomposition in an aerobic environment to produce stabilised organic fertilizer. Unlike composting however, vermicomposting includes action by earthworms as well as microorganisms, which acts to biodegrade organic waste at a faster rate.

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Vermicomposting produces a fertiliser and soil conditioner which has a high porosity, water holding capacity, and nutrient status, but has a low C:N ratio. It has great potential as an organic fertiliser with studies demonstrating comparable effectiveness to mineral fertilisers and improved physical characteristics in soil amended with vermicompost. In addition to this product, as a secondary effect, this process also generates high biomass of earthworms, which can also have a financial value and therefore benefit farm business production.

The actions of earthworms in the vermicomposting process are both physical and biochemical. Earthworms break up the organic material through physical action, which also increases turnover and aeration, increasing the activity rates of other microbial organisms. Through active digestion the organic material is also transformed by enzymatic processes, which acts to recycle important nutrients into forms which are readily available for plant uptake.

Effect on plant growth:

The nutrient status of the material produced by vermicomposting will depend on the initial raw material. However, due to the influence of earthworm activity, nutrient levels are typically higher in vermicompost than traditional compost. Vermicompost can also improve the physical structure of soil, positively influencing characteristics such as porosity, infiltration, drainage and aeration, which can increase the potential for root growth.

In a study considering the reasons for additional plant growth associated with vermicompost application, plants were observed to have increased growth after vermicompost application regardless of the extra nutrient availability. In other words, other factors, or characteristics, associated with the vermicompost were deemed to have strong beneficial effects from the point of view of plant productivity, in addition to that which would be expected from extra nutrients alone. This suggests the earthworm activities may increase biologically active substances in the vermicompost, such as humic acids. Earthworms may also regulate the availability of trace elements or heavy metals through absorption and bioaccumulation in earthworm tissues. This may be harmful to the earthworm itself, but will reduce the potential for harm to plants by limiting availability.

However, high concentrations or applications rate of vermicompost can have a negative effect on plant growth or may suppress seed germination. This is expected to be the primarily due to high soluble salt content, but may also be due to organic substances in the vermicompost which contribute to phytotoxicity. Therefore, it is important to consider application rate of this material.

Factors to consider:

In addition to the caution suggested with regard to soluble salt content and rate of application, there is some concern over the safety of the resultant vermicompost, as this technique does not involve a thermophilic stage of decomposition (where heat is produced as a by-product of the microbial breakdown of organic material), as is the case with typical composting. This may result in the survival of pathogens, which may be highly undesirable. A limited amount of studies has shown the process of earthworm digestion to be sufficient to reduce pathogens, yet this remains under investigated and is potentially contentious. 

Vermicomposting can also result in significant emissions of greenhouse gases, which can reduce some of the environmental benefits of the process. In addition, losses of carbon and nitrogen during the composting process can reduce the agronomic values of the resultant compost. Losses to the atmosphere are potentially unavoidable with this method (at least, without significant infrastructure to capture gaseous release), however when compared to a typical composting process, vermicomposting has been shown to release a lesser amount of carbon. Studies have been undertaken to establish management to mitigate this effect (i.e. aeration, turning, adding livestock manure, and others), but the results remain controversial and unclear, with studies often reporting conflicting results.  

Establishment of vermicomposting system

Certain environmental factors need to be considered for the effective management of a vermicomposting system. Earthworms will need adequate moisture content to maintain activity rates, as these organisms breathe through their skin, which should be maintained in the region of 60 – 80%. Earthworm activities are also significantly affected by temperature; should temperatures drop below 10oC reproduction and metabolic rates will decline. The system should also be aerated, as these organisms require oxygen, which is typically achieved through manual turning. However, this should be undertaken sensitively as earthworms are highly photosensitive and prolonged exposure to light will have adverse effects.

Selection of the right variety of earthworm is of particular importance. Earthworms used in the vermicomposting process must possess the following characteristics: 1) high rates of organic matter consumption, digestion and assimilation; 2) high tolerance of environmental stress; 3) high reproductive rate; and 4) rapid growth and maturation rate of hatchlings. Of the three earthworm forms (epigeic, endogeic and anecic), epigeic earthworms (those that live in the surface layer of the soil) are the most suitable earthworms for vermicomposting process as they occupy the organic soil horizons and feed primarily on decaying organic matter. 

The nature of the organic waste applied also needs to be considered. The C:N ratio of feed material will affect earthworms when this is either too high or too low, by influencing growth and reproduction rates. Feed material pH is also important; whilst earthworms can process most organic materials, certain substances, such as those with pH <5, may require pre-treatment or bulking with other organic materials thus consideration of input material is essential.

Summary

The vermicomposting system offers a method for nutrient recycling, which can simultaneously reduce the burden of organic agricultural waste whilst producing value added products such as fertiliser, soil conditioner, and earthworm biomass.

Vermicomposting is a faster system for breaking down organic waste than traditional composting approaches, allowing effective management of large organic waste burdens. However, questions remain regarding the safety of this approach due to the potential for pathogen survival, and the effectiveness of vermicompost as a fertiliser or soil conditioner due to high soluble salt contents when applied in high concentrations. Further research is need to fully investigate these effects.