What is Vermicast ?
Vermicast is produced by the feeding action of earthworms. Earthworms ingest organic matter, fragmenting and grinding it into a finely divided peat like material with high porosity, aeration, drainage and water holding capacity. This process enhances microbial activity and accelerates the rate of decomposition. This leads to a humification effect where unstable organic matter or decomposing plant and animal matter is oxidized and stabilized (1).
Humus forms the dark brown or black mass of the upper soil and is important for storing and releasing plant nutrients. The process is similar to composting except it is non thermophilic or a cold process utilising naturally occurring soil dwelling microorganisms. Vermicast has a large surface area and a high cation exchange capacity providing strong absorbability and retention of nutrients (2).
As a fertiliser, vermicast contains nutrients in a form that are readily taken up by plants, such as nitrates, exchangeable phosphorous, soluble potassium, calcium and magnesium (1). A typical breakdown of vermicast is as follows:
Microbially, vermicast contains a far more diverse microbial population than other composts (3). Micro organisms play an important part in soil fertility, they not only mineralize complex substances into plant available nutrients but bacteria in the earth worms' digestive system also synthesize a whole series of biologically active substances including plant growth regulators.
Earth worms promote the production of plant hormones, auxins, gibberellins and cytokinins from organic waste dramatically Auxins are responsible for cell elongation, cytokinins for promoting cell division and gibberellins for stem elongation. These hormones are dose significant and play a fundamental role in plant metabolism. They can influence plant growth and development as well as crop quality significantly when present at very low concentrations (4).
Table 2) Biological make up of a typical sample of Revital vermicast
How does it work?
Until recently scientific documentation of the responses of plants to the application of vermicast to soil or container media has been poor, even though anecdotal evidence has been abundant.
Over the past five years the soil ecology laboratory at the Ohio State University has developed a comprehensive research program in vermicomposting. They have found consistently the addition of relatively small amounts of worm castings to standard horticultural container mixes, and even commercially prepared premium quality container media has resulted in dramatic improvements to plant growth.
A consistent and interesting trend for trials with plants grown in container media is for the best responses to occur when worm castings constitute only 10 - 20% of the volume of the mix and with increasing proportions of castings not always improving plant growth as well. Pasture trials have shown the same effects with as little as 20 - 40 grams of vermicast per square metre making significant improvements to grass growth (5).
Broad acre application of 200kg /ha of vermicast onto lucerne gave a 15% increase in yield (6). The low % of castings required for a dramatic response indicate that the dramatic responses are more than simply a function of supplying plant nutrients and that other related growth stimulants are involved (3).
It has now been found that vermicast increases microbial populations, and adds biologically active metabolites such as plant growth regulators (aukins, cytokinnins and gibberellins ) (7). The addition of vermicast has consistently improved seed germination, enhanced seedling growth and development and increased plant productivity much more than would be possible from the mere conversion of mineral nutrients into more plant available forms.
Vermicast has also been shown to reduce the incidence of plant diseases (8). Research indicates that microbial activity and the make up of the microbial communities existing in vermicast play an important role in plant growth responses. The reason being that different microbes act on different substances from simple sugars to complex substances. If any are missing it will inhibit growth. The diversity of microbes in vermicast allows all substrates to be broken down.
The role of earthworm derived humic acids has also received much attention recently. Humic acids are produced by the breakdown of organic matter by microorganisms . They are generally negatively charged so attracting positive ions, for example calcium. Humic substances promote the conversion of a number of elements into forms available to plants, of particular importance is phosphate. Phosphate reacts with other minerals in the soil, (particularly iron and aluminium) and becomes locked or unavailable to plants. Humic acids help substitute iron and aluminium with other elements e.g calcium making phosphate plant available.
Enhancing this is mycorrhizal fungi. Vermicast contains large concentrations of mycorrhizal fungi spores (9). Enzymes associated with this fungi are able to unlock soil phosphate making it plant available. Mycorrhizal fungi work mutualistically with plant roots extending the roots network throughout the soil. Mycorrhizal fungi supply the roots with important nutrients such as phosphorous and the plant supplies the fungi with carbon. This association is very important for plants such as clover.
Vermicast is humus rich. The break down of organic material by earth worms accelerates the humification of organic matter. The humic and fulvic acids produced in this process have been proven to stimulate plant growth beyond that produced by solely the mineral nutrient found in vermicast. Humic acids are large complex molecules. Partial oxidation of humic acids allow bonding sites for plant nutrients including calcium and magnesium and other humic like materials produced in the faeces of earthworms which exhibit auxin, gibberellin and cytokinin like activities (8).
Studies of the positive effects of these humic substances on plant growth when full requirements for mineral nutrition are met resulted consistently in positive effects on growth independent of nutrition (8). Humic acids have been reported to enhance mineral uptake by plants by increasing the permeability of membranes of the root cells (8). Vermicast humic acids appear to have greater effects upon the root growth of the plants than on the above ground parts of the plant. Stimulation of root growth, increased proliferation of root hairs, and enhancement of root initiation by humic acids have been reported commonly by several other researchers (8).
The best plant growth responses are when vermicast and traditional fertilisers are used together.
References
(1) Edwards & Burrows, 1998
(2) Atiyeh, R., Arancon, N., Edwards, C., Metzger, J., 2001
(3) Subler, S., Edwards,C., 1998
(4) Atiyeh, R., Edwards, C., Subler, S., Metzger, J., 2000
(5) Foster, 1999
(6) Murphy, 2001
(7) Tomati, Galli and Doube, 1997
(8) Atiyeh, R., Lee, S., Edwards, C., Arancon, N., Metzger, J.. 2002
(9) Gange, A., 1993
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