According to Edwards and Arancon (2006), vermicomposts are organic materials, broken down by interactions between earthworms and microorganisms to produce fully-stabilized organic soil amendments with low C:N ratios. They also have a huge and diverse microbial and enzymatic activity, fine particulate structure, good moisture-holding capacity, and contain nutrients such as N, K, P, Ca and Mg in forms readily taken up by plants. They contain both plant growth hormones and humic acids which can act as plant growth regulators.
Vermicompost is the primary product in vermicomposting, with the earthworms as the necessary second product. It is a special type of compost produced through the action of earthworms, microorganisms, and other organisms which participate in composting. It is therefore a mixture of vermicast, the degraded organic matter which pass throught the guts of earthworms, and the compost produced by other organisms.
Worm castings or vermicast contain organic particles in fine sizes. These are transformed chemically and physically as the organic material passes through the digestive tract of the earthworms. Nutrients are transformed into forms that are more readily available to plants such as nitrate and ammonium ions (NO3-, NH4+), available phosphorus, and soluble potassium, calcium and magnesium.
However, the extent of degradation of the original organic materials is not complete. Most worms are only capable of digesting simple organic compounds like sugars. A few species can digest cellulose but not lignin, the highly complex compound that joins together cellulose fibers in wood. But the microorganisms in the worm’s intestine, through a symbiotic relation in which the worm’s mucus serve as substrate, help in decomposing complex organic compounds into simpler substances that are in turn digested by the worm (Trautmann and Krasny 1997).
Some of the earthworm’s mucus is excreted and mixed with the vermicasts, favoring microbial activity. The high content of ammonia and organic matter in partial stage of decomposition in vermicasts further provide nutrients which promote microbial growth and high rate of decomposition (Trautmann and Krasny 1997).
Properties and Tests to Evaluate Mature Vermicompost
A mature vermicompost is one which has undergone the final phase of “curing” under a mesophilic process through the action of moderate temperature-requiring microorganisms. It has a minimal microbial activity due to the depletion of degradable organic matter that serves as food. It has a pH about 7.0, finely granulated and friable, colored dark and smells like that of fertile soil. It contains both essential macronutrients and micronutrients as well as soil-dwelling organisms including microorganisms and insects.
However, its nutrient and biotic compositions vary depending on such factors as the nutrient composition of the original organic materials, other materials in the compost bed, environmental conditions within the compost bed, type of composting or degree of care to exclude other organisms, and many more.
For example, vermicompost produced from pure grass has lesser amounts of NPK and other nutrients as compared to those from grass mixed with manure or Gleciridia (Guerrero 2009). The mature compost may also contain substances that are inhibitory to plant growth. These may include heavy metals, salts, or other toxic compounds that are present in the original compost ingredients (Trautmann and Krasny 1997).
Earthworms may ingest pieces of sand and even fragments of glass to serve as grits to assist in digestion and these will be excreted with the casts. Outdoor compost piles are invaded by various invertebrates, insects and arachnids. In the absence of thermophilic process, microbial pathogens, insect pests and weed seeds will not be killed.
Immature compost may inhibit plant growth due to presence of methane, ammonia, acetic acid, or other substances. Using immature compost may also impair plant growth because the microorganisms which continue the process of decomposition compete with the plants for nitrate and oxygen.
To ensure that it is mature or finished, several simple tests are available. The protocol is provided by Trautmann and Krasny (1997). Compost stability can be evaluated through the Jar Test, Self-Heating Test, and Respiration Test, while compost quality can be tested through Phytotoxicity Bioassay.
The Jar Test is quite easy. A compost sample is moistened and placed in a jar or plastic bag, sealed, and kept under ordinary room conditions. After a week, the jar or bag is opened. If the compost is mature, the odor that will be expelled is earthy, otherwise it will be putrid or offensive.
ARANCON NQ, EDWARDS CA. 2006. Effects of vermicompost on plant growth. In: Guerrero R.D. III, Guerrero-del Castillo MRA (eds.). Vermi Technologies for Developing Countries. Proceedings of the International Symposium-Workshop on Vermi Technologies for Developing Countries. Nov. 16-18, 2005, Los Baños, Laguna, Philippines. Philippine Fisheries Association, Inc. p 32-65.
EDWARDS CA, ARANCON NQ. 2006. 2006. The science of vermiculture: the use of earthworms in organic waste manangement. In: Guerrero R.D. III, Guerrero-del Castillo MRA (eds.). Vermi Technologies for Developing Countries. Proceedings of the International Symposium-Workshop on Vermi Technologies for Developing Countries. Nov. 16-18, 2005, Los Baños, Laguna, Philippines. Philippine Fisheries Association, Inc. p. 1-30.
GUERRERO RD III. 2009. Vermicompost and Vermimeal Production. MARID Agribusiness Technology Guide. 22 p.
TRAUTMANN NM, KRASNY ME. 1997. Composting in the classroom: scientific inquiry for high school students. Retrieved May 29, 2011 from http://cwmi.css.cornell.edu/compostingintheclassroom.pdf.
(Ben G. Bareja, September 2011)