Annex VI - Vermicomposting

The use of worms for digesting animal and vegetable wastes is an expanding industry for producing potting media. Worms are able to degrade waste products rapidly and efficiently. They produce a fine, peat-like material with good structure, porosity, aeration, drainage, and moisture holding capacity, which contains inorganic nutrients.

All species of worms require conditions to be aerobic. Optimal temperature for activity is 25-30 °C (Edwards and Burrows, 1988). In general worms are pH tolerant, but they do tend to thrive in more acid conditions. They are also sensitive to ammonia and inorganic salts so some wastes (such as pig and poultry manure) must be first composted slightly and washed. Worms can be bred simply in windrow systems, heaps, or boxes and work best if small quantities of wastes are frequently added to the system in successive layers. Once degradation has been completed worms can be harvested for transfer to other wastes or for use as feed supplements for fish, poultry, and pigs (BOSTID, 1981). Additionally, partial sterilization of the finished product may be desired to kill residual worms, cocoons, insects, and reduce the presence of pathogens (Edwards end Burrows, 1988).

Two species of worms most suited to tropical climates are Eudrilus eugeniae and Perionyx excavatus. Both species grow extremely rapidly, reproduce prolifically, but are extremely temperature sensitive (Edwards and Burrows, 1988). P. excavatus is easily handled and harvested where the larger sized E. eugeniae is not.

Overall, nitrate quantities are greater in worm-digested wastes than in the composted wastes where the nitrogen is in the form of ammonia. Inclusion of vermicompost in potting mixes may reduce the need for additional potassium and sulfur and eliminate addition of trace elements and phosphorus (Handreck, 1986). Basal dressings of slow release fertilizers may be necessary and nutrient variability between batches of vermicompost necessitate calculation of fertilizer applications.

Industrial wastes such as spent mushroom compost, processed potato wastes, paper pulp solids, and brewery wastes are excellent substrates for worm composting and need no modification or dewatering prior to worm inoculation (Edwards, 1988). Pig manure solids are the most productive waste for growing worms. Cattle manures must be separated prior to worm digestion, but do not need prior composting, nor do horse manures. Other substrates which have been studied for vermicompost production include: sheep manure; mixtures of carpet underfelt, lawn clippings, cardboard, goat manure, and domestic wastes, kitchen scraps; and mixtures of cardboard, wheat, maize, lucerne and linseed meals, rice pollard, and oat hulls (Handreck, 1986).

In S. Africa, several tree species were successfully grown in mixtures of composted pine bark and worm digested abattoir wastes suggesting that vermicompost has potential use in commercial tree nurseries (Donald and Visser, 1989) In this study, addition of unleached vermicompost to a composted pine bark-based growing media was not conducive to survival and good growth for Acacia mearnsii and Pinus patula, but a fertilized 1:1 mixture was sufficient to raise Eucalyptus grandis seedlings