The Numerous Benefits of Castings in Soil and Plant Health and Growth

The Numerous Benefits of Castings (Vermicast)

As vegetative matter takes up minerals and nutrients from the soil, water and air, those nutrients are stored.  During the decomposition process those nutrients are released back in the same natural balance as was required for vigorous, healthy growth.  Without restoring these nutrients back to the ground, the soil becomes depleted.  With ordinary soils, fertilizers, and composts, plants and vegetative matter must wait for the nutrients to break down further, and then seek out these nutrients.  But with vermicast they have already been broken down to more bioavailable forms, and are readily available when needed.  Furthermore, in vermicast there is no excess of nitrates and phosphates, which are water soluble and which, when applied in much higher concentrations in manufactured fertilizers, dissolve in run off to pollute our land and waterways.

Beside the greater bioavailability of nutrients, another tremendous value of vermicast lies in the plant growth stimulants, the cationic exchange rate (aka cationic exchange capacity, CEC) and the soil benevolent biota (beneficial microorganisms).  

Biota:

The biota introduced to the soil in vermicast works away out of sight, releasing the minerals already there and trapping free nitrogen from the atmosphere.

Cationic Exchange Rate or Capacity:

An important and often unrecognized feature of vermicast is its cationic exchange rate. This is the rate at which the cationic soil trace elements can attach themselves to vermicast. Everything in nature has an electrical charge. Some charges are positive, cations, and some are negative, anions. Organic vegetative matter is anionic and, because vermicast is highly vegetative matter, it is strongly anionic. Most trace elements are cationic.  In simple terms this means that trace elements are attracted to vermicast and readily bond to it in the same way that opposite poles of a magnet attract each other. Plants have a stronger pull than the vermicast and can therefore draw the trace elements away from the vermicast and into their roots.

Conclusions From Studies and Research on Castings (Vermicast)

Earthworms increase the amount of mineralized nitrogen from organic matter in soil. The microbial composition changes qualitatively and quantitatively during passage through the earthworm intestine (Pedersen and Hendriksen, 1993).

Earthworms not only disperse microorganisms important in food production but also associated with mycorrhizae and other root symbionts, biocontrol agents and microbial antagonists of plant pathogens as well as microorganisms that act as pests (Edwards and Bohlen, 1996).

Several researchers have demonstrated the ability of earthworms to promote the dispersal of beneficial soil microorganisms through castings, including pseudomonads, rhizobia and mycorrhizal fungi (Edwards and Bohlen, 1996; Buckalew et al., 1982; Doube et al., 1994a; Doube et al., 1994b;Madsen and Alexander, 1982; Reddell and Spain, 1991; Rouelle, 1983; Stephens et al., 1994).

During vermicomposting process when organic matter passes through the worm’s gut, it undergoes physico-chemical and biochemical changes by the combined effect of earthworm and microbial activities.  Vermicasts are coated with mucopolysaccharides and enriched with nutrients. The cellulolytic, nitrifying and nitrogen fixing microbes are found established in the worm cast (Kale et al., 1988).  

Vermicasts are excellent media for harbouring N-fixing bacteria (Bhole, 1992).  

Earthworms directly cycle the nitrogen by excretion in the casts, urine and mucoprotein and through the turnover of earthworm tissues (Lee, 1985).  

Earthworms have multiple, interactive effects on rates and patterns of nitrogen mineralization and immobilization in natural and managed ecosystems (Edwards and Lofty, 1977; Lee, 1983; Lavelle and Martin, 1992; Blair et al., 1995b).

Earthworm casts are enriched in terms of available nutrients and microbial numbers and biomass, relative to the surrounding soil (Shaw and Pawluk, 1986; Lavelle and Martin, 1992).  

Earthworms reject significant amounts of nutrients in their casts. In part these
losses result from the intense microbial activity in their gut, and from their own metabolic activity. Eg. The elimination of N due to fast turnover of this element in microbial biomass. A significant proportion of C assimilated by earthworms is secreted as intestinal and cutaneous mucus with greater C:N ratios than those of the resource used (Lavelle et al., 1983; Cortez and Bouche, 1987).

  

Joshi and Kelkar (1952) reported that earthworm casts contained greater percentage of finer fractions like silt and clay than in the surrounding soils. This change in mechanical composition of soil was probably due to the grinding action of earthworm gizzard. The chemical analysis of vermicasts revealed that they were richer in soluble salts, neutral or alkaline in reaction and had higher percentage of exchangeable Na, K and Mg but a lower exchangeable Ca than in corresponding soil.

Earthworm casts also contained greater amounts of Nitrogen (N), Phosphorous (P) and Potassium (K). The vermicasts contained higher amounts of nitrate nitrogen and possessed a greater nitrifying power than the corresponding soils.  Vermicompost also contained Mg, Ca, Fe, B,Mo and Zn in addition to some of the plant growth promoters and beneficial microflora.  Several valuable compounds were also produced through the earthworm – microfloral interaction, which included vitamins such as B12 and plant growth hormones such as gibberellins.  Barois et al., (1987) observed an activation of N mineralization, with the casts having 270 percent more ammonia than the bulk soil.  Within a year of application of vermiculture technology to the saline soil, 37 percent more N, 67 percent more P2O5 and 10 percent more K2O were recorded as compared to chemical fertilizer (Phule, 1993).

Kale (1991) has attributed the improved growth in pastures and in other crops like rye and barley to the chemical exudates of the worms and microbes in association with them.

Tomati et al., (1983) related the beneficial influence of worm cast to the biological factors of natural plant growth hormones like gibberellins, cytokinins and auxins released due to metabolic activity of the microbes harboured in the cast.  It has also been indicated that the chemical exudates of worms and those of microbes in the cast, influence the rooting or shoots of layers. 

In a field trial Kale and Bano (1986) observed that the seedling growth of rice in nursery increased significantly due to vermicompost application, and transplanting of seedlings could be made one or two days earlier than the usual practice. After transplanting the growth of seedlings in main field was more favourably influenced by worm cast than the chemical fertilizer. This was attributed to higher availability of nitrogen for plant growth. The improved growth was also attributed to the release of plant growth promoting compounds from worm cast, which in their opinion could easily replace the chemical fertilizers at nursery level.

Atiyeh et al. (2000) found that compost was higher in ammonium, while vermicompost tended to be higher in nitrates, which is the more plant-available form of nitrogen.  Similarly, work at NSAC by Hammermeister et al. (2004) indicated that “Vermicomposted manure has higher N availability than conventionally composted manure on a weight basis”. The latter study also showed that the supply rate of several nutrients, including P, K, S and Mg, were increased by vermicomposting as compared with conventional composting.  These results are typical of what other researchers have found (e.g., Short et al., 1999; Saradha, 1997, Sudha and Kapoor, 2000). It appears that the process of vermicomposting tends to result in higher levels of plant-availability of most nutrients than does the conventional composting process.

The literature has less information on this subject than on nutrient availability, yet it is widely believed that vermicompost greatly exceeds conventional compost with respect to levels of beneficial microbial activity.  Much of the work on this subject has been done at Ohio State University, led by Dr. Clive Edwards (Subler et al., 1998). In an interview (Edwards, 1999), he stated that vermicompost may be as much as 1000 times as microbially active as conventional compost, although that figure is not always achieved.  Moreover, he went on to say that “…these are microbes which are much better at transforming nutrients into forms readily taken up by plants than you find in compost – because we’re talking about thermophillic microbes in compost – so that the microbial
spectrum is quite different and also much more beneficial in a vermicompost.  I mean, I will stick by what I have said a number of times that a vermicompost is much, much preferable to a compost if you’re going in for a plant-growth medium.”

Many researchers have found that vermicast stimulates further plant growth even when the plants are already receiving optimal nutrition.  Atiyeh at al (2002) conducted an extensive review of the literature with regard to this phenomenon. The authors stated that: “These investigations have demonstrated consistently that vermicomposted organic wastes have beneficial effects on plant growth independent of nutritional transformations and availability. Whether they are used as soil additives or as components of horticultural soil less media, vermicomposts have 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.”  

Moreover, the authors go on to state a finding that others have also reported (e.g., Arancon, 2004), that  maximum benefit from vermicompost is obtained when it constitutes between 10 and 40% of the growing medium.  It appears that levels of vermicompost higher than 40% do not increase benefit and may even result in decreased growth or yield.  Atiyeh et al further speculate that the growth responses observed may be due to hormone-like activity associated with the high levels of humic acids and humates in vermicomposts: “…there seems a strong possibility that …plant-growth regulators which are relatively transient may become adsorbed on to humates and act in conjunction with them to influence plant growth”.  This important concept, that vermicompost includes plant-growth regulators which increase growth and yield, has been cited and is being further investigated by several researchers (Canellas et al, 2002).

There has been considerable anecdotal evidence in recent years regarding the ability of vermicompost to protect plants against various diseases.  The theory behind this claim is that the high levels of beneficial microorganisms in vermicompost protect plants by out-competing pathogens for available resources (starving them, so to speak), while also blocking their access to plant roots by occupying all the available sites.  This analysis is based on the concept of the “soil foodweb”, a soil-ecology-based approach pioneered by Dr. Elaine Ingham of Corvallis, Oregon (see her website at http://www.soilfoodweb.com for more details). Work on this attribute of vermicompost is still in its infancy, but research by both Dr. Ingham’s labs and the Ohio State Soil Ecology Laboratory are very promising.  With regard to the latter institution, Edwards and Arancon (2004) report that “…we have researched the effects of relatively small applications of commercially-produced vermicomposts, on attacks by Pythium on cucumbers, Rhizoctonia on radishes in the greenhouse, and by Verticillium on strawberries and Phomopsis and Sphaerotheca fulginae on grapes in the field. In all of these experiments, the vermicompost applications suppressed the incidence of the disease significantly.”

The authors go on to say that the pathogen suppression disappeared when the vermicompost was sterilized, indicating that the mechanism involved was microbial antagonism.  In recent research, Edwards and Arancon (2004) report statistically significant decreases in arthropod (aphid, mealy bug, spider mite) populations, and subsequent reductions in plant damage, in tomato, pepper, and cabbage trials with 20% and 40% vermicompost additions to Metro Mix 360 (the control).  They also found statistically significant suppression of plant-parasitic nematodes in field trials with peppers, tomatoes, strawberries, and grapes. Much more research is required, however, before vermicompost can be considered as an alternative to pesticides or alternative, non-toxic methods of pest control.