Plants Response to the Application of Vermicompost: A Review

Organic fertilizer has long been recognized in agriculture for maintaining soil fertility and used for maximizing plant growth and yield of plants. Vermicompost: organic fertilizer is now becoming an interesting approach and proven to be effective means of improving soil structure, enhancing soil fertility and increase crop yield. Plants growth is highly stimulated by vermicompost; because it contains the nutrients and other essential nutrients like phosphorus and potassium.Vermicompost can increase growth, flowering and yields of vegetable and ornamental crops, even in a small rate. The effects of vermicompost on plants are not solely attributed to the quality of mineral nutrition is provided but also to its other growth regulating components such as plant growth hormones and humic acids.

nitrates, phosphates, and exchangeable calcium and soluble potassium in plant-available forms are also present in vermicompost (Orozco et al. 1996). In addition to this the author indicated that vermicompost increased N availability, C, P, K, Ca and Mg availability in the casts are found (Orozco et al. 1996).

Vermicomposting process
Vermicomposting is a decomposition process involving the joint action of earthworms and microorganisms (Aira et al. 2000). Microorganisms, particularly earthworms are used to enhance the process of waste conversion and produce a better product (Adhikary, 2012).

Nutrients in vermicompost
Vermicompost is an excellent soil additive made up of digested compost. Worm castings are much higher in nutrients and microbial life and therefore, are considered as a higher value product (Adhikary, 2012). Worm castings contain up to 5 times the plant available nutrients found in average potting soil mixes. Chemical analysis of the castings was conducted (Ruz, J. et al, 1992) and found that it contains 5 times the available nitrogen, 7 times the available potash and 1.5 times more calcium than that found in 15 cm of good top soil. In addition, the nutrient life is up to 6 times more in comparison to the other types of potting mixes. It is reported that phosphorous while passage through gut of worms is converted to the plant available form (Reinecke, A., et al. 1992.). Phosphorous is usually considered as a limiting element for plant growth. Therefore, any process that significantly increases phosphorous availability through plants and organic matter will be very important for agriculture. The average potting soil mixes that is found in the market are usually sterile and do not have a microbial population. The combination of nutrients and microbial

Effect of vermicompost on plants
Plants growth is highly stimulated by vermicompost; because it contains the nutrients and other essential nutrients like phosphorus and potassium (Fernández, L. et al., 2010). A number of field experiments have reported positive effects of even low application rates of vermicompost to crops (Norman and Clive, 2005). In tomato and okra vermicompost has increased the yield effectively (Suthar, S., 2010). Similarly, Sallaku et al., 2009 showed relative growth rate of cucumber (Cucumis sativus) seedlings was found to be significantly higher due to the application of vermicompost. Many studies also showed that combined application of vermicompost with the recommended inorganic fertilizer can increased yield in most crops. Tomato showed increment in yield with combined applications of vermicompost and 50% of the recommended inorganic fertilizers (Kolte et al., 1999). Arancon et .al. (2002) reported significantly increased growth and yields of field tomatoes and peppers when vermicompost applied to field plots at rates of 20 t/ha and 10 t/ha and at rates of 10 t/ha and 5 t/ha compared with those receiving equivalent amounts of inorganic fertilizer. Ushakumari et al. (1999) reported that the yields of okra increased with the application vermicompost at the rate of 12 t/ha together with 100% or 75% of the recommended fertilizer. Similarly, Athani et al. (1999) demonstrate that vermicompost applied at rates of 2 kg/plant together with 75% of the recommended rate of inorganic fertilizers, promoted shoot production of bananas. A lower application rate of 2t/ha vermicompost plus recommended amounts of inorganic fertilizers, increased tomato yields to a level similar to those of tomatoes in soils treated with 4 t/ha vermicompost and 50% of the recommended rates of inorganic fertilizers . Potatoes produced the greatest marketable yields after amending the soils with 75% of the recommended inorganic fertilizers and 2.5 t/ha vermicompost (Mrinal et al., 1998). Sunflowers gave most after soil treatments with 50% of the recommended application rates of inorganic fertilizer and 5t/ha or 10t/ha of vermicompost (Devi et al., 1998). Peas increased yields and production after amending soils with 100% of the recommended application rate of inorganic fertilizers, in combination with vermicompost produced from farm manures applied at rates of 10t/ha to soils (Ramachandra et al., 1998). Zende et al. (1998) reported increased yields of sugarcane after amending soils with vermicompost at rates of 5t/ha together with 100% of the recommended application rate of inorganic fertilizers.
Studies reported better yield and growth in wheat crops applied with vermicompost. Yield of wheat was improved by more than 40% by the application of vermicompost fertilizer (Palanisamy, S., 1996). Studies made on the agronomic impacts of vermicompost on rice crops (Oryza sativa) reported greater population of nitrogen fixers, actinomycetes and mycorrhizal fungi inducing better nutrient uptake by crops and better growth (Kale, R.D., et al. 1992).

Physico-chemical changes in soils in response to vermicompost applications
The improvements in growth and yields of crops grown in potting media in greenhouses or in field soils that had been substituted or amended with vermicompost could be attributed to several factors. Vermicompost contribute to improvements in physico-chemical and biological characteristics of the planting media or field soils that favored better plant growth. However, the changes in pH reported contrasted with those from the work of Tyler et al. (1993) who reported increases in substrate pH in response to increasing concentrations of composted turkey litter added to a plant container medium. Electrical conductivity in pig manure vermicompost increased linearly as a result of increasing salt concentration (Atiyeh et al., 2001a, b).
Amounts of soil N, P and K increased significantly after incorporating vermicompost into soils (Venkatesh et al., 1998, Sreenivas et al., 2000. In soils planted with strawberries, the amounts of total extractable N, microbial biomass N and dissolved organic N were statistically similar, between all treatments at the end of the growth cycle of strawberries but there were more orthophosphates in those soils that received vermicompost treatments than in soils treated with inorganic fertilizers. Maheswarappa et al. (1999) reported increased amounts organic carbon, improvements in pH, decreased bulk density, improved soil porosities and water-holding capacities, increased microbial populations and dehydrogenase activity of soils in response to vermicompost treatments.

Plant growth regulator production in vermicompost
There is a very substantial body of evidence demonstrating that microorganisms, including bacteria, fungi, yeasts, actinomycetes and algae, are capable of producing plant growth hormones and plant growth regulators (PGRs) such as auxins, gibberellins, cytokinins, ethylene and abscisic acid in appreciable quantities (Arshad and Frankenberger, 1993). There have been many studies of the production of plant growth-regulating substances by mixed microbial populations in soil, but there are relatively few investigations into their availability to plants, and persistence and fate in soils or documenting reliably their effects on plant growth (Arshad and Frankenberger, 1993). Norman and Clive (2005) reviewed that PGRs can be taken up by plants from soil in sufficient quantities to influence plant growth. It was shown that auxins produced by Azospirillum brasilense could affect the growth of graminaceous plants (Kucey, 1988). Gibberellins can also influence plant growth and development (Arshad and Frankenberger, 1993). Increased vigor of seedlings has been attributed to microbial production of cytokinins by Arthrobacter and Bacillus spp in soils (Jagnow, 1987). Since the process of vermicomposting increases microbial diversity and activity, it is possible that vermicompost could be a definitive source of plant growth regulators produced by interactions between microorganisms and earthworms, which could contribute significantly to enhancement of plant growth, flowering and yields. The presence of plant growth regulating substances in the tissues of Aporrectodea caliginosa, Lumbricus rubellus and Eisenia fetida was confirmed by Nielson (1965) who isolated indole substances from earthworms and reported increases in growth of peas due to the earthworm extracts.
Vermicompost originating from animal manure, sewage sludges or paper-mill sludges have all been reported to contain large amounts of humic substances (Senesi et al., 1992;Garcia et al., 1995;Masciandaro et al., 1997;Elvira et al., 1998). Studies of the effects of humic substances on plant growth, under conditions of adequate mineral nutrition, have consistently produced in positive growth effects (Chen and Aviad, 1990). Shakir and Mikhal (2004) reported that vermicompost can suppress the incidence of plant pathogens such as Pythium, Rhizoctonia and Verticillium significantly, by general or specific suppression mechanisms. They also demonstrated that vermicompost applied to soils have considerable influence on the trophic structure of nematode populations, significantly suppressing plant parasitic species populations. Greenhouse experiments have shown that low substitutions of vermicompost into soil-less plant growth media can decrease the amounts of feeding and damage by sucking pests such as aphids and mealy bugs and chewing pests such as caterpillars.