Compost maturity as expressed by phytotoxicity and Volatile organic acids
VOA in Composts
Maturity is a qualitative trait of compost that may be expressed in a number of ways and is generally held to be relevant to plant performance. Volatile organic acids (VOA) have been cited to be responsible for phytotoxicity (plant growth suppression) when immature compost is used for growing plants (Manios et al., 1989). However, the production of VOA in compost has not been well studied, nor is it known how phytotoxicity attributable to VOA may relate to other factors such as CO2-evolution or salt content (Brinton, 1998).
The presence in compost of volatile organic acids may be recognised by an unpleasant garbage or spoiled-food odour. VOA may be observed in organic materials undergoing rapid decomposition with oxygen deprivation The volatile acids which normally occur in composts are short-chain fatty acids of C2 to C6 in length, with formic acid the shortest and butyric and iso-valeric acid among the largest. It is not well known how the different volatile acids affect plant growth although there is evidence the effects vary with carbon-length and volatility index (Lechner, 1994).
We previously reported a survey of 712 compost samples where 26% had appreciable VOA above 5,000mg/kg at some point during composting while 6% had VOA above 20,000mg/kg. The VOA correlated negatively with age of compost and were highest in the first 20-35 days. VOA levels appeared to be controlled by a complex, dynamic relationship of carbon biodegradability, pH and porosity. The microbial dynamics which are involved are as yet poorly understood, but it is clear that anoxia (no-oxygen) conditions may not be required to produce significant VOA.
Prior work in plant growing media reveal that VOA levels as low as 500mg/kg can exert a phytotoxic influence on plant seedlings (Lynch, 1977). The principal deleterious effects on plants appear to be in the nature of root suppression but nutrient-ion leakage has also been implicated (Lynch, 1977; Lee, 1977). Plants exposed to high VOA do not necessarily die, but may remain static for long periods of time. Thus, rapid identification and response to problem composts is made more difficult, and the resulting economic losses can be very large. Basing plant growth evaluation on maturity standards would be a way to avoid later damage.
VOA in Compost and Resulting Phytotoxicity
The presence in composts of volatile organic acids is certainly well established but poorly understood (Manios, et al. 1989). VOA are metabolic by-products of anaerobic respiration and are breakdown products of grease and fats in raw wastes (Henefeld-Fourrier,1980). The composting process includes many events in which episodic oxygen depletion occurs both at macro- and micro-pore levels. Such oxygen depletion may result in temporary production of copious amounts of short-chain carbon compounds. In a previous paper we discussed odorant and microbial aspects of VOA production (Brinton, 1998).
VOA may also enter compost from source materials. VOA are found in waste water (Kawamura et al., 1985; Anselme, et al., 1985) and may be present in large concentrations in biosolids (Howgrave et al., 1991; Rains et al., 1973) and raw manures (Hoshika, 1982; Zahn et al., 1997). Freshly composted sewage sludge has already been reported to contain high levels of organic acids, primarily acetic acid, that inhibit plant growth (Devleeschauwer et al., 1981). Food scraps from households also contain copious amounts of VOA and one study showed that storage of food waste in plastic bags instead of paper increases VOA production (Detroit, 1996).
Phytotoxicity has been previously associated with immaturity of compost (Ianotti et al., 1994) and depletion of organic acids is correlated with improved plant performance (Herrmann et al., 1993). Many MSW composts have been associated with high levels of phytotoxicity associated with organic acids (de Bertoldi, 1992, 1993). Phytotoxicity of immature composts has been positively correlated with organic acid content (Logsdon, 1989; Manios et al., 1989; Evans and Brinton, 1997).