There are several methods of harvesting vermicompost. Harvesting may be gradual or by bulk at one time. It is normally done during daytime for ease in separating the earthworms.
The vermicompost is ripe and ready for harvest when the raw materials, except for a few, particularly pieces of woody stem, are fully decomposed. At this stage the vermicomposting ingredients would have undergone both thermophilic and mesophilic processes of decomposition.
The height of the pile would have dropped down to about one-third to one-half of that of the original pile; and the pile temperature would be close to ambient temperature. The organic substrates are no longer distinguishable and the vermicompost appears somewhat darkish brown, crumbly, and smells earthy, like that of a freshly excavated fertile soil.
However, it is difficult to ascertain ripe or finished compost by visual inspection and other human senses. To be sure, it has to undergo stability or quality test. By the Jar Test, a compost that is finished and safe to plants as soil amendment would not emit a strong offensive odor.
In this paper, the term “vermicompost” is used rather loosely, without regard as to whether it is fully decomposed and stable as defined above. It includes the earthworm castings, also called vermicast.
Common methods (or techniques or procedure) of harvesting vermicompost or vermicast from a heap or pile are briefly described below. Any method may be adopted exclusively by preference, subject to some limitations. Otherwise, two or more methods may be applied on the same pile. Except for the first method, the rest is intended for harvesting in bulk at one time.
This method is practiced where a gardener wishes to collect small amounts of vermicast just a few days after the compost pile is stocked with composting worms. This becomes a resort when, for example, the gardener finds need of organic soil amendment as ingredient in preparing a fertile potting mix. In this case the pile has not been fully decomposed but the top is covered with a thin layer of vermicast.
The earthworm casts on top of the pile are simply gathered by hand or with a trowel and transferred directly into a container. Otherwise, vermicasts are first moved to the center of the pile to form a single heap and then scooped into a container. Earthworms, if any, are picked individually and returned to the compost pile.
With time, vermicomposts will collect at the bottom of the pile. When in need, these can likewise be collected by hand (you may like to wear gloves).
The vermicompost is first gathered to form a pyramid- or cone-like heap within the composting enclosure provided that the heap is exposed to light. Otherwise, the vermicompost is transferred on to a flat surface elsewhere in open sun over which a plastic sheet or sack or some other substitute is spread.
This method of harvesting vermicompost takes advantage of the earthworms’ sensitivity to light (and heat). If the pyramid is exposed to bright light, the earthworms will tend to move deep into the pyramid.
Vermicomposts from the bottom, sides, and top surface of the heap are then collected by hand or with a trowel. A few minutes are allowed to pass to provide sufficient time for the earthworms to move deeper and another cycle (harvesting-resting) is commenced.
Should there be undecomposed substrates and earthworms, they are collected manually in every cycle.
For faster rate of harvesting vermicompost, the original heap is better divided into several smaller pyramids. Harvesting is rotated among the pyramids so that after harvesting, one is rested and the next cycle goes to another pyramid.
This method of harvesting vermicompost can be applied anytime of the day or even at night. It also has the advantage of ease in separating the vermicompost, undecomposed substrates, and earthworms from each other.
This can be done manually using the same fabricated tool used in screening out rough sand for masonry work. This tool consists of mesh wire nailed on wood.
A small portion of the pile is first transferred into a screener (or sieve or sifter) which is positioned above a container (for example a wheelbarrow) or concrete flooring or flat ground over which a sheet is spread.
The screener is shaken so that fine vermicompost falls toward the ground. Any undecomposed subtrates which are retained in the screener and the earthworms are separated manually.
This method of harvesting vermicompost is based on earthworms’ ability to detect sources of food. They also have the habit of abandoning the pile exhausted of food and moving towards more palatable source.
There are many modifications of the technique, but the basic principle is the same: provide fresh, or otherwise more palatable food to cause the migration of earthworms from the ripe pile to the new food source.
One technique involves constructing a new enclosure adjacent to the one with ripe compost in such a way that the two enclosures share a common boundary. The new enclosure is filled with fresh vermicomposting substrates. When it is ready to be stocked with earthworms, the common boundary, which may consist of hollow blocks, is then removed. This allows the migration of earthworms towards the new food.
Free of earthworms, the vermicompost should be easy to harvest with the use of a shovel. If there are undecomposed substrates, like wood chips and stem segments which are hard to decompose, they are separated manually. However, the migration of earthworms can take several days to complete.
Note: Earthworms can be manually picked from the pile anytime. But try this: Based on their migratory habit as described above, try harvesting earthworms from harvestable vermicompost pile or from one approaching maturity using balled cattle manure or some other food or food mixture. Try reject cabbage also and segments of ripe fruit of jackfruit after the edible parts have been removed. First wrap the earthworm food with a nylon netting with holes sufficient to allow passage of earthworms. Fasten to a plastic twine or small rope or wire. Then insert the balled food into the pile leaving some segment of the twine on top. It is expected that the earthworms will bore into this ball of food.To harvest the earthworms, lift the balled food by the twine. Then crumble or spread the balled food and pick the earthworms.
(Ben G. Bareja Apr. 2, 2013, edited Apr. 28, 2019)