Compost Maturity and Stability
By E. Dring
During a composting process, organic matter biodegrades into a humic material. Maturity is a measurement of the humus matter and its end-of-life suitability. Microorganism activity is an indicator of biological stability and stable compost is an indication of maturity (Butler et al., 2001).
Observations - when compost matures it becomes darker and the initial unpleasant smells disappear (Antil et al., 2014). An unpleasant smell can also be an effect of anaerobic composting (Sullivan and Miller, 2001). The colour change is down to an increase in humus matter. Colour can be assessed using CIELAB colour variables, which look at lightness, red to greenness and yellow to blueness. Compost stability can be indicated by the stabilisation of colour variables (Khan et al., 2009; Rashwan et al., 2020).
Volatile Solids - a loss on ignition test is used to calculate the volatile solids of compost, which approximates organic matter content (loss on ignition is basically put the sample in a furnace). The volatile solid content will decrease during aerobic composting because organic carbon is broken down by microorganisms. A 50% decrease in volatile solids is commonly expected (Sullivan and Miller, 2001). However, materials are consumed at different rates so this cannot be the only indicator of maturity (Finstein et al., 1986).
Carbon to Nitrogen Ratio - various chemical properties indicate maturity. It can be expected that mature compost will have a neutral pH value and a higher electrical conductivity compared to organic matter (Antil et al., 2014). At the start of the composting process, organic carbon is broken down by microorganisms, but only when water is available during the process. The degradation of protein releases ammonia, in turn, increasing the alkalinity of the compost (Venelampi et al., 2010). Microorganism activity increases the temperature of the compost. This leads to the suppression of nitrification (Sullivan and Miller, 2001).
Nitrification is the oxidation of ammonia into nitrite, which is then nitrified by bacteria to form nitrates (Figure 1). Various processes cycle nitrogen through the biosphere, atmosphere and geosphere (Figure 2). Maturity is indicated when the concentration of dissolved organic carbon decreases, due to a depletion of nutrients (Wu et al., 2000; WRAP 2015). When carbon concentrations decrease, respiration rates of the material are lower, reducing the release of carbon dioxide. More available oxygen increases nitrification and reduces denitrification. Therefore, increasing the availability of nitrogen for plants.
Figure 1 - The Nitrogen cycle.
A disproportionate quantity of nitrogen should not be immobilised in mature compost. Yellow leaves are an indication that a plant is uptaking too little nitrates (Sullivan and Miller, 2001). Other factors can also cause nutrient deficiency, such as compost with extreme moisture conditions and pH values.
Concentrations of carbon and nitrogen are strongly associated with the raw material inputs of the compost (Sullivan and Miller, 2001). It has been found that by the initial addition of dissolved organic carbon, such as glucose and sucrose, could reduce the ammonia emissions from the organic matter and elevate maturation (Meng et al., 2018). Raw materials which are more alkaline have a greater potential of releasing ammonia. The carbon to nitrogen ratio of stable compost will often fall from 30 to below 20: 1. If organic matter is acidic, the loss of carbon and nitrogen is proportionate. Therefore, the C: N ratio of the mature compost is similar to that of its immature state (Sullivan and Miller, 2001).
Figure 2 - Nitrogen flux processes.
Spectroscopy: A spectroscopy analysis can be used to test maturity. For example, Fourier-transform infrared spectroscopy can observe the carbon present in organic matter (Kumar et al., 2013). Spectroscopy is a rapid test, however, the equipment used is expensive (WRAP, 2015).
The initial carbon: nitrogen ratio of a compost pile can play an important role in the product’s maturity. The compost’s stability, on the other hand, is observed to be mostly affected by the aeration rate (Guo et al., 2012).
Oxygen Uptake Rate: This method looks at the reduction of oxygen present in a flask that contains compost. Very stable compost will have an oxygen uptake rate of ≤10 mmol O2 per kg of organic matter per hour. At the start of the composting process, the specific oxygen uptake rate initially decreases quickly, before decreasing slowly (WRAP, 2015).
Solvita Test: This test is performed at room temperature, a colorimetric gel is situated in the compost for 4 hours (Venelampi et al., 2010). The colour of the gel suggests the concentration of carbon dioxide and ammonia emitted from the compost. A soil moisture content of 60 to 80 % is recommended to avoid invalid results (Brinton, 2000). The test only requires a short time period which is advantageous, and the results are semi-quantitative (Sullivan and Miller, 2001). Compost for use as a growing medium should only respire 10 mg CO2 per gram of organic matter, per day (WRAP, 2015).
Microorganisms: Immature compost has a high enzyme activity (WRAP, 2015). Biological activity stabilises when a compost becomes mature (Antil et al., 2014). Though, many factors can inhibit the functions of microorganisms, such as compost dryness and lack of oxygen (Venelampi et al., 2010).
Self-Heating: A Dewar test provides an indication of compost maturity by measuring the temperature at which a compost pile self-heats. The compost is placed in a Dewar vessel, and the temperature is measured frequently. A peak in temperature is recorded and used to evaluate the compost's maturity. Stabilised compost will not rise above 20°C (Brinton et al., 1995). The Rottegrad test is an adapted maturity classification, categorised by self-heating temperatures of compost. Fresh compost suitable for arable land application has a Rottegrad II or III. Mature compost suitable for horticulture has a Rottegrad IV or V (Table 1; WRAP, 2015). For a pilot-scale disintegration test (i.e., ISO 16929) to be valid, compost must reach this maturity. Several factors can make Rottegrad results vulnerable to error, such as the amount of compost and its insulation (Venelampi et al., 2010).
Table 1 - Rottegrad scale and self-heating temperature of compost in relation to its maturity (Brinton et al., 1995; WRAP, 2015)
Plant Germination: If compost is immature, it might contain phytotoxic compounds. These toxins can negatively affect root development (Brinton, 2000). Phytotoxicity can be increased in anaerobic composting, this is due to the production of volatile fatty acids (Venelampi et al., 2010). When oxygen is available, these compounds degrade as compost matures. Seed germination can be a valuable indicator of toxicity (Luo et al., 2018).
There are many factors that can alter the mature state of compost; therefore, a number of properties should be analysed to get accurate results of maturity and stability. When compost matures it gets darker and then the colours stabilise. Odours produced are less potent and often earthy. Volatile solids are lost throughout the composting process. Typically, the carbon to nitrogen ratio of the compost will fall; however, this is dependent on the organic matters initial C: N ratio and pH value. Normally, a compost’s pH value would neutralise. When compost stabilises, microorganism activity is reduced. Therefore, oxygen uptake is reduced along with the rate of CO2 respired. With less enzyme activity, compost has a lower self-heating temperature. It is important to understand the maturity of compost because it affects its suitability for different applications.
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