Agriculture Reference
In-Depth Information
that the optimum moisture content for E. fetida was above 70% in cow manure. By comparison,
E. andrei cultured in pig manure grew and matured best between 65 and 90% moisture content,
with 85% the optimum (Domnguez and Edwards 1997). According to Reinecke and Venter (1985),
it seems likely that lowering of the growth rate because of low moisture conditions can also retard
sexual development, so earthworms of the same age could develop clitella at different times under
different moisture conditions.
P H
Most species of epigeic earthworms are relatively tolerant to pH, but when given a choice in the
pH gradient, they moved toward the more acid material, with a pH preference of 5.0. However,
earthworms will avoid acid soils of pH less than 4.5, and prolonged exposure to such soils could
have lethal effects (Edwards and Bohlen 1996). Minor increases in acidity caused by addition of
fresh wastes to the vermicomposting bed can be neutralized by the intestinal calcium secretions of
earthworms and excreted ammonia. Lime is commonly added to vermicomposts.
A ERATION
Earthworms have no specialized respiratory organs; they obtain oxygen by diffusion through the
body wall and lose carbon dioxide by diffusion. However, earthworms are very sensitive to anaerobic
conditions, and their respiration rates are depressed in low oxygen concentrations of around 55 to
65%, e.g., at oxygen levels of 0.25 its normal partial pressure (Edwards and Bohlen 1996); feeding
activities might be reduced under these suboptimal conditions. Individuals of E. fetida and other
species have been reported to migrate in large numbers from a water-saturated substrate in which
the oxygen conditions had been depleted or in which carbon dioxide or hydrogen sulfide had
accumulated. However, they can live for long periods in aerated water, such as in trickling filters
in wastewater treatment plants.
A MMONIA
Earthworms are very sensitive to ammonia and cannot survive in organic wastes containing high
levels of this cation (e.g., fresh poultry litter). They also die in organic wastes with large quantities
of inorganic salts. Both ammonia and inorganic salts have very sharp cutoff points between toxic
and nontoxic (i.e., <1 mg/g of ammonia and <0.5% salts) (Edwards 1988). However, organic wastes
containing large amounts of ammonia can become acceptable after its removal by a period of
composting.
Outside the limits of these environmental parameters, both earthworm activity and the rates of
organic waste processing decrease dramatically; for maximum vermicomposting efficiency, wastes
should be preconditioned to make them suitable for vermicomposting.
Earthworm population density is known to affect the rates of earthworm growth and reproduc-
tion. Even when the physical-chemical characteristics of the wastes are ideal for vermicomposting,
problems can develop because of overcrowding. Reinecke and Viljoen (1990), in studies with E.
fetida reared in cow manure, and Domnguez and Edwards (1997), who studied the growth and
reproduction of E. andrei in pig manure, reported that, when grown at different population densities,
the earthworms in the crowded dishes grew more slowly and ended with a lower final body weight,
although the total weight of earthworm biomass produced per unit of waste was greater in the
crowded dishes. Maturation rates were also affected by the population density; earthworms of the
same age developed a clitellum at different times in cultures with different earthworm stocking
rates; usually, it was later in dense populations.
When the environmental conditions are maintained within adequate ranges, a maximum yield
of about 10 dry unit weights of earthworm biomass can be expected from an initial 100 units (dry
weight) of substrate, independent of nitrogen concentration, when a minimum of about 1% or more
 
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