Maturity of Composting Products

The Composting Council of Canada ( defines the compost maturity as following (the guidelines for that are shown in Table 12.3): Compost maturity was chosen as one of the parameters for determining the grade of compost in Canada because it is so important to product performance. Immature compost may stunt, damage, or even kill plants, rather than enhancing their growth. Maturity is not related to quality, but to what stage in the composting process the material has progressed. Mature compost is material in which biological activity has been slowed. All of the easily degraded molecules have been broken down, leaving the complex organic material behind. It is difficult to identify the original feedstock materials. A fine texture, dark color, and a rich earthy smell often characterize mature composts. As organic material composts, large complex molecules are broken down in a series of steps. The final products are simple, stable molecules, which make up the humus-like matrix of nutrients and organic matter that we

Table 12.3 Guidelines for compost maturity (in: The Composting Council of Canada, http://

Required tests of compost maturity


1. Two of the three following tests:

(a) Carbon/nitrogen ratio (C/N) less than or equal to 25.

(b) Oxygen uptake less than 150 mg O2/kg organic matter/h

(c) Germination of cress or radish seeds in compost equal to more than 90% that of control sample, and plant growth rate in soil/compost mix not less than 50% that of control sample.

2. Compost must be cured for a minimum of 21 days, and must not reheat upon standing to greater than 20°C above ambient temperature.

3. Compost must be cured a minimum of 21 days and organic matter must be reduced by at least 60% by weight.

4. Compost must be cured for a 6-month period

(a) As carbon is broken down through composting the C/N ratio drops. (C/N ratio starts ideally at 30, but can be higher)

(b) Microbes require oxygen, so a drop in the O2 required signals a slowing of microbial activity.

(c) Cress (Lepidium sativum) and radish (Raphanus sativus) are small seeds, quick to germinate and sensitive to phytotoxic (plant damaging) substances like the organic acids temporarily present in immature composts.

Microbial activity produces heat. When pile is no longer heating up, the level of microbial activity has dropped.

As composting progresses, water vapor and carbon dioxide are given off, resulting in a lighter, denser product.

In the absence of other tests, 6 months under proper conditions to promote effective composting is considered sufficient to achieve maturity.

call compost. While this finished product is highly beneficial to plants, some of the intermediate stages may temporarily produce compounds, such as organic acids, that can be harmful to plant growth. This is why even compost made of high-quality materials, that is applied too soon to lawns or gardens may appear to "burn" leaves, stunt growth, or even kill sensitive plant species. Immature composts continue to break down once they are incorporated into the soil. This can affect plant health by consuming or tying up two resources that growing roots need. The high level of microbial activity in unfinished compost requires a large intake of oxygen, and the microbes may pull this from the surrounding soil, essentially suffocating the roots. The high C/N ratio of immature compost also means that, as the carbon compounds continue to break down, microorganisms will draw on soil nitrogen to assist in the process, leaving the root zone temporarily nitrogen poor. It is therefore crucial that responsible compost producers should ensure that their compost has time to fully mature before selling it to most customers, as compost that is still "hot" when applied can do serious damage to both customers' plantings and your reputation.

Maturity is an important parameter for the nutrient management of compost: N availability in compost is closely related to the maturity reached during composting (Bernal et al. 1998; Griffin and Hutchinson 2007). Larsen et al. (2007) found that anaerobically digested and composted municipal solid waste immobilized N in the initial stage of decomposition. The N immobilization in the composted municipal solid waste indicates that it was not matured and still was metabolically active. The rate of mineral N release after the initial immobilization was similar between the anaerobically digested and composted municipal solid waste composts (MSWC). Amlinger et al. (2003) reported also similar N effects of compost amendments: an N release ranging between 5% and 15% in the first year.

The composition and the maturity of composts influence the potential for plant disease suppression, and in addition the management of the composting process, in particular the oxygen supply, seems to be the most important factor affecting compost quality (Fuchs and Larbi 2004).The three most important factors to control during fermentation are: (i) the moisture of the material, (ii) the air composition, and (iii) the temperature. At the end of the maturation, the following quality parameters have to be analyzed: (1) pH, (2) salt content, (3) ammonium, (4) nitrite, and (5) nitrate content (Fuchs and Larbi 2004). Plant test can be also very important for the quality analysis of composts, because (i) plants react to compost quality as a whole, and so hidden problems can appear, and (ii) plants react during the entire test period (about 2 weeks), thus the evolution of some parameters can be observed. This is particularly important for nitrogen availability (Fuchs and Bieri 2000). Wang et al. (2007) studied the effect of oxygen on the compost maturity and found that when C/N ratio, NO content and composting temperature were used as indexes to evaluate compost maturity, the compost matured in 35 days in the micro-aerobic treatment, compared with 45-50 days in the aerobic treatment. Biosolids compost is a good organic amendment, but immature compost can exhibit phytotoxic behavior, which can be attributed to different toxic substances (Zubillaga and Lavado 2006). The authors studied the phytotoxic behavior and found that phytotoxic effects on germinating ryegrass were mainly related to extract pH and electrical conductivity. Potentially toxic elements, volatile organic acids, phenolic compounds, and ammonia were not related to germination. Tiquia (2005) found significant correlations between the humification parameters and the microbial properties of the manure of compost. Spaccini and Piccolo (2007) studied the changes in the molecular composition of composted organic biomasses and found that the largest decrease in the molecular components occurred when compost was stabilized from 60 to 90 days, whereas its composition did not significantly vary after stabilization at 150 days. The stability and maturity of compost are highly related to the nature and content of water-soluble organic matter: fractionation of the water-extractable organic carbon showed that the ratio of hydrophobic to hydrophilic carbon increased to values greater than unity for stabilized compost (Said-Pullicino et al. 2007). The water-soluble fractions (carbon, nitrogen, carbohydrates, and phenols) show major fluctuations during the first month of composting (Castaldi et al. 2007). McEachin et al. (2007) found that measurements of mineralizable carbon and mineralization rate of composts in soil, and electrical conductivity and mineralization rate of soil and compost mixtures can be used as indicators of compost maturity. Kovács et al. (2007) analyzed the applicatibility of self-heating, oxygen consumption, and carbon dioxide production as measures of stability during the composting of a mixture of communal sewage sludge and wood chippings and green waste and concluded that for both types of compost the stabilization process can be described by means of the actual O2 consumption and CO2 production rates on the 2nd day. Compost quality, particularly maturity is linked to the composition of the microbial community structure (Steger et al. 2007).

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