Green Manure

Green manure is incorporation of fresh plant biomass into the soil to improve nutritional level concerning organic matter and nitrogen, to reduce soil erosion, and it may also serve as sources of allelochemicals for suppression of weed species. Although green manures usually favor beneficial microorganisms, there can be a short-term increase in plant pathogens such as Rhizoctonia solani Kühn (Weinhold 1977). However some compounds as isothiocyanates can have inhibitive effects to soil-borne fungal pathogens (Sarwar et al. 1998) or reduce the occurrence of some pests. Rapeseed, sudan grass, and white mustard green manures decreased the number of Columbia root-knot nematode (Meloidogyne chitwoodi Golden, O'Bannon, Santo, and Finley) by up to 90% (Mojtahedi et al. 1993a, b).

Legume species and some cruciferous plants suppress weeds and simultaneously improve soil conditions (increasing of organic matter and nitrogen in the soil). Crops suitable as green manures include, e.g., Brassica spp., cowpeas (Vigna spp.), soya beans, annual sweetclover (Melilotus spp.), sesbania, guar (Cyamopsis tet-ragonoloba (L.) Taub.), crotalaria (Crotalaria spp.), velvetbeans (Mucuna spp.), clover (Trifolium spp.), jack-beans (Canavalia spp.), Ipomoea spp. (Batish et al. 2001), and buckwheat (Fagopyrum spp.) due to quick development and others. The genus Crotalaria (C. juncea, C. spectabilis, C. pallida Aiton), is one of the important green manure legumes spread out over several tropical and subtropical regions. These species are grown as plant antagonistics to parasitic nematodes due to the content of pyrolozidine alkaloid and they release also other substances which can be phytotoxic to some weeds too (Daimon 2006).

Mustard species as green manure crops reduced total weed biomass in soya bean by 40% four weeks after emergence and 49% six weeks after emergence (Krishnan et al. 1998). Petersen et al. (2001) noted that isothiocyanates released from winter turnip rape Brassica rapa L. var rapa ssp. oleifera (DC.) Metzg. biomass strongly suppressed germination of five common weed species - spiny sowthistle (Sonchus asper (L.) Hill), scentless mayweed (Matricaria inodora L.), smooth pigweed (Amaranthus hybridus L.), barnyard grass (E. crusgalli (L.) P. Beauv.) and blackgrass (Alopecurus myosuroides Huds.). The mechanism that suppresses germination of weeds is called biofumigation. Biofumigation is the name for one type of allelopathy that includes effects of the chemicals produced by Brassica green manure (Kirkegaard and Sarwar 1998). Cruciferous plants contain compounds called glucosinolates in their plant tissues. When the plant cells are damaged, glu-cosinolates can be hydrolyzed by myrosinase enzyme and transformed into different bioactive compounds acting as naturally produced pesticides (Gimsing et al. 2007). Same compounds are released from soil-incorporated Brassica tissues (Morra and Kirkegaard 2002). The biofumigant properties of cruciferous plants are just ascribed to highly toxic isothiocyanates. From the major volatiles released from chopped plants, allyl-isothiocyanates and methyl-isothiocyanates were the most inhibitive compounds, completely inhibiting the germination of all species at concentration of 1 mg kg-1 (Vaughn and Boydston 1997).

Glucosinolate concentrations differ in dependence on environmental conditions, species, age of the plant, health, and plant part (Brown and Morra 1996; Kirkegaard and Sarwar 1998; Sarwar et al. 1998). Roots produce different glucosinolates than shoots. The glucosinolates which are hydrolyzed to isothiocyanates were found primarily in the taproot and larger lateral roots (>2 mm), whereas younger fine roots (<2 mm diameter) had higher levels of indolyl glucosinolates (Kirkegaard and Sarwar 1999). The concentration of both glucosinolates and isothiocynates in soil was highest immediately (30 min) after incorporation of Australian canola (Brassica napus annua L.) and Indian mustard (B. juncea L.) and they could be detected for up to 8 and 12 days, respectively. The non-isothiocyanated liberating glucosinolates were found at lower concentrations but tended to persist longer in the soil (Gimsing and Kirkegaard 2006).

Selection of species that release high levels of allyl-isothiocyanates or benzyl-isothiocynates would be optimal for allelopathic activity (Vaughn and Boydston 1997). High yielding and agronomically adapted varieties of B. napus and B. juncea could be selected for higher levels of root glucosinolates as a source of natural biofumigants without compromising seed quality (Kirkegaard and Sarwar 1999). According to Petersen et al. (2001), turnip rape (Brassica rapa L.) produce about 0.5 g m-2 of isothiocyanates (about 20% of the concentration in the commercial fumigant), when 600 g m-2 (dw) total biomass were incorporated into the soil. Hence, using mustards as green manure could be a promising method to suppress weed germination.

Crops seeded too soon after the incorporation of a cruciferous crop can also be damaged. Brassica species may injure potato, spearmint, and cucumber if green manure was incorporated only a few days before planting, because the most severe inhibition by plant residues occurs at the early stages of residue decomposition. However, there is species selectivity. For example, green pea was injured by rape-seed but not by white mustard (Al-Khatib et al. 1997). According to Jaakkola (2005), white mustard appears to be more toxic to spinach and pea than to scentless mayweed (Tripleurospermum inodorum (L.) Sch. Bip) and other annual weeds. It could be caused by incorporation of manure infested with weeds. Krishnan et al. (1998) observed soya bean biomass and yield reduction by the incorporation of green manure crops containing weeds.

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