Keywords

5.1 Introduction

Composting of organic wastes is a bio-oxidative process involving the mineralization and partial humification of the organic matter, leading to a stabilized final product, free from phytotoxicity and pathogens (Zucconi and de Bertoldi 1987; Bernal et al. 2009) . Composting is a controlled aerobic process carried out by successive microbial population combining both mesophilic and thermophilic activities, leading to the production of carbon dioxide, water, minerals and stabilized organic matter (Pereira-Neta 1987) . Composting is one of the most effective means of recycling of organic wastes that can be used as a source of soil amendment and organic matter in agricultural land. The variability of the organic matter undergoing composting makes compost research challenging. In recent years, interest in composting has increased because of the social demand for an environmentally friendly waste treatment technology and for organic agricultural products. Composting is seen as not only an environmentally acceptable method of waste treatment, but also one of the more efficient methods of waste disposal which enables recycling of organic matter (Ko et al. 2008) . The application of composted manure has increased over the years. This practice improves the quality of the crops and preserves the environment (Hoitink 2000; Ko et al. 2008) . However, non-composted manure may have adverse effects on plant growth and/or seed germination (Hoekstra et al. 2002) because of their wide C:N ratio and production of phytotoxic substances such as phenolic and volatile fatty acids during organic matter decomposition (Kirchmann and Widen 1994) . Mesophilic and thermophilic micro-organisms are involved in the composting and their succession is important in the effective management of composting process (Ishii et al. 2000) . Many studies have shown that the composting process eliminates or reduces the risk of spreading of pathogens, parasites and weed seeds associated with direct land application of immature compost and leads to final stabilized product (Larney and Hao 2007; Bernal et al. 2009) . The need to treat and dispose of organic wastes to reduce potential damage has made compost production and its agricultural application an attractive solution. The composting industry is poised for a new era of growth where the product must be evaluated both for safety and quality purposes. Composts prepared from different organic wastes differ in their quality, which further depends upon the composition of raw material and composting technology used for compost production (Ranalli et al. 2001) . Quality of high grade compost generally relates to its fertilizer value. If unstable or immature compost is applied, it can induce anaerobic conditions as the micro-organisms utilize oxygen in the soil pores to break down the material (Mathur et al. 1993) . Another problem associated with immature compost is the phytotoxicity due to the presence of organic acids during early stages of the composting process (Fuchs 2002; Cambardella et al. 2003) . Compost quality is closely related to its stability and maturity. Maturity is associated with plant-growth potential or phytotoxicity (Iannotti et al. 1994) , whereas stability is often related with the compost’s microbial activity. However, both stability and maturity usually go hand in hand, since phytotoxic compounds are produces by the micro-organisms in unstable composts (Zucconi et al. 1985) . Several parameters have been proposed for evaluating compost maturity and stability (Iglesias Jiménez and Pérez García 1992; Wu et al. 2000; Bernal et al. 2009; Raj and Antil 2011; Antil et al. 2012) . However, there is no single method that can be universally applied to all types of compost due to variation of materials and composting technology (He et al. 1995; Benito et al. 2003; Chang and Chen 2010) . Morel et al. (1985) proposed that the maturity of the compost may be assesses by the biological activity of the product, including total microorganisms count, monitoring biochemical parameters of microbial activity and analysis of biodegradable constituents. The changes in microbial properties during the composting process and their association with maturity must be carefully studied. The present chapter reviews the physical [color, odor, temperature, organic matter (OM) loss], chemical [pH, electrical conductivity (EC), water soluble C (WSC), C:N ratio, water soluble organic carbon (Cw):organic N (Norg) ratio, NH4 +-N and NO3 −-N, humic acid (HA): fulvic acid (FA) ratio, humification index (HI) and cation exchange capacity (CEC):total organic carbon (TOC) ratio)] along with biological [seed germination index (GI) and microbiological parameters (bacterial, fungal and actinomycetes counts, O2 and CO2 respiratory, enzyme activities and microbial diversities (coliform, fecal coliforms and fecal enterococci population)] parameters affecting stability and maturity of high quality compost prepared from organic waste materials.

5.2 Concepts of Composting Process

While composting occurs, efficient composting requires the control of several factors to avoid nuisance problems of such as odour and dust and also for obtaining a quality agricultural product. The controlled conditions are basic for a composting procedure, distinguishing it from aerobic fermentation. Over the last decades, research has been focused on the study of the complex interaction amongst physical, chemical and biological factors that occurs during composting.

The precise details of the biochemical changes taking place during the complex processes of composting are still lacking. The phases which can be distinguished in the composting processes according to temperature are (Fig. 5.1):

Fig. 5.1
figure 1

Patterns of temperature and microbial growth in compost piles

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  1. a.

    Latent phase, which corresponds to the time necessary for the microorganisms to acclimatize and colonize in the new environment in the compost heap.

  2. b.

    Growth phase, which is characterized by the rise of biologically produced temperature to mesophilic level.

  3. c.

    Thermophilic phase, in which the temperature rises to the highest level. This is the phase where waste stabilization and pathogen destruction are most effective.

  4. d.

    Maturation phase, where the temperature decreases to mesophilic and, subsequently, ambient level. A secondary fermentation takes place which is slow and favours humification; that is, the transformation of some complex organics to humic colloids closely associated with minerals and finally to humus. Nitrification reactions, in which ammonia is biologically oxidized to become nitrite (NO2 −) and finally nitrate (NO3 −), also take place (Metcalf and Eddy 1991).

The organic wastes are initially decomposed by the first level consumers such as bacteria, fungi (molds), and actinomycetes. Mesophilic bacteria are the first to appear. Thereafter, as the temperature rises, thermophilic bacteria, which inhabit all parts of the compost heap, appear. Thermophilic fungi usually grow after 5–10 days of composting. If the temperature becomes too high, i.e., greater than 65–70 °C, fungi, actinomycetes, and most bacteria become inactive and only spore forming bacteria can develop. In the final stage, as the temperature declines, members of the actinomycetes become the dominant group which may give the heap surface a white or gray appearance. After these stages the first level consumers become the food of second level consumers, such as mites, beetles, nematodes, protozoa and rotifers. Third level consumers, such as centipedes, rove beetles and ants, prey on the second level consumers.

5.2.1 Factors Affecting the Composting Process

Composting process is affected by several abiotic and biotic factors, which maintain its integrity on optimum and these factors are quite important during process to be maintained. These are discussed followed

5.2.1.1 Temperature

Temperature is the main factor that controls microbial activity during composting. Heating is essential to enable the development of a thermophilic population of micro-organisms which is capable of degrading the more recalcitrant compounds (natural and anthropogenic), and to kill pathogens and weed seeds (Boulter et al. 2000) .

5.2.1.2 pH

A pH of 6.7–9.0 supports good microbial activity during composting. Optimum values are between 5.5 and 8.0 (de Bertoldi et al. 1983; Miller 1992) . Usually pH is not a key factor for composting since most materials are within this pH range. However, this factor is very relevant for controlling N-losses by ammonia volatilization, which can be particularly high at pH  > 7.5.

5.2.1.3 Aeration

Aeration has an indirect effect on temperature by speeding the rate of decomposition and therefore, the rate of heat production. The air requirement depends upon the type of waste (type of material, particle size), the temperature of the compost and the stage of the process. Air supply can be controlled to some extent by the use of a system of aeration. Under natural conditions warm air diffuses from the top of the windrow drawing fresh air into the base and sides (Hellmann et al. 1997) . Aeration is further encouraged by periodic turning of the windrow. Alternatively, air may be actively forced into the pile, usually within a closed or in-vessel system with the aim of maximizing the rate of microbial decomposition. This is relatively costly, but is useful for materials that pose health risks. Forced aeration has also been used successfully on static piles giving a high degree of process control (Sasaay et al. 1997) .

5.2.1.4 Microbial Activity

Microbial activity is influenced strongly by moisture content: activity decreases under dry conditions, and aerobic activity decreases under water-logged conditions due to the resulting decrease in air supply. The recommended optimum water content is 40–60 % on a mass basis (Epstein 1997) . The microorganisms involved in composting develop according to the temperature of the mass, which defines the different steps of the process (Keener et al. 2000) . Bacteria predominate early in composting, fungi are present during all the process but predominate at water levels below 35 % and are not active at temperatures > 60 °C. Actinomycetes predominate during stabilization and curing, and together with fungi are able to degrade resistant polymers.

5.2.1.5 Moisture Content

Changes in moisture content are related to aeration and temperature; in an aerated static pile system approximately 90 % of the heat loss is due to evaporation of water (Sasaay et al. 1997) . Systems which actively encourage aeration can lead to desiccation and result in a decrease in the rate of decomposition in windrow composting (Itavaara et al. 1997) . The compost must be kept aerobic to avoid the production of odours. Moisture is essential but if the compost is too wet then anaerobic conditions develop. Anaerobic conditions are also undesirable because of the loss of N by denitrification. There may also be build-up of organic acids, such as acetic acid, which can be toxic to plants.

5.2.1.6 C/N Ratio

The C/N is important in determining in general terms the rate of decomposition of organic materials. High C/N ratios make the process very slow as there is an excess of degradable substrate for the microorganisms. But with a low C/N ratio there is an excess of N per degradable C and inorganic N is produced in excess and can be lost by ammonia volatilization or by leaching from the composting mass. Then, low C/N ratios can be corrected by adding a bulking agent to provide degradable organic-C.

5.2.2 Advantages and Disadvantages of Composting

The advantages of composting organic waste materials with direct application are summarized below:

  • Elimination of pathogens and weeds.

  • Microbial stabilization.

  • Reduction of volume and moisture.

  • Removal and control of odours.

  • Ease of storage, transport and use.

  • Production of good quality fertilizers.

However, the disadvantages are:

  • Cost of installation and management.

  • Requirement for a bulking agent.

  • Requirement for large areas for storage and operation.

5.2.3 Benefits of Composting

The benefits of composting can be divided in to two groups: to improve crop productivity and profitability; and to improve soil quality.

Improve crop performance and lower production costs through:

  • Improved yields, product quality and storage life

  • More efficient and reduced use of fertilizers and pesticides, including soil fumigants

  • Better utilization of irrigation

  • Increased crop resistance to pests and diseases.

Improve soil quality through:

  • Better organic matter levels and organic cycles

  • Increased available water to plants

  • Increased nutrient availability and nutrient-holding capacity

  • Improved structure

  • Reduced soil-borne plant pathogens and pests

5.3 Maturity and Stability Assessment for Compost Quality

Composts prepared from different organic waste materials differed in their quality and stability; and quality is closely related to stability and maturity. Hence, it is important to define maturity and stability. Compost maturity is the degree or level of completeness of composting. It is described by several physical, chemical and biological properties and therefore maturity is best assessed by measuring two or more parameters of compost. Compost stability refers to a specific stage or decomposition or state of organic matter during composting, which is related to the type of organic compounds remaining and the resultant biological activity in the material.

Several parameters have been proposed for evaluating compost maturity and stability (Iglesias Jiménez and Pérez García 1992; Bernal et al. 1998, 2009; Wu et al. 2000; Raj and Antil 2011; Raj and Antil 2012a, b; Antil and Raj 2012; Antil et al. 2012) . However, there is no single method that can be universally applied to all types of composts due to variation of materials and composting technology. (He et al. 1995; Itavaara et al. 2002; Benito et al. 2003; Chang and Chen 2010) . The parameters are grouped into maturity and stability shown in Table 5.1.

Table 5.1 Different maturity and stability parameters
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In addition to above parameters, the physical characteristics such as colour, odour, particle size, appearance of larvae of secondary consumers also show the decomposition stage, but give the little information with regard to degree of maturation. The maturity of compost, which may defined as the degree of compost stability in physical, chemical and biological properties is an important factor affecting successful application in agriculture and its impact on environment. The parameters proposed by several workers are presented in Table 5.2. Parameters or criteria proposed or indicated in the literature are grouped into physical parameters, chemical parameters and biological parameters.

Table 5.2 Maturity parameters established for composts of different sources
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5.3.1 Physical Parameters

Physical parameters are frequently used but they give only general information regarding maturity of compost .

5.3.1.1 Colour and Odour

During composting of organic wastes , a gradual darkening or melanisation of the material takes place . The final product, after a sufficiently long period of maturation, is dark brown or almost black colour (Iglesias Jiménez and Pérez García 1992). Sugahara et al. (1979) proposed a simple technique to determine the maturity of compost by measuring the degree of darkness of composting material. It is also possible to monitor it visually the gradual process of compost darkening. In general unpleasant odour emission takes place during first thermophilic phase and then starts decreasing, with the maturity of compost. At the end of composting process, when optimal maturation is achieved, the unpleasant odour should be absent in a compost heap even on its turning of the material (Haug 1980; Vander Hoeck and Oosthoeck 1985) . Although colour and odour are the simplest criteria to evaluate the maturity and stability of the compost but for confirmation some physical, chemical and biological parameters may also studied .

5.3.1.2 Temperature

Temperature evolution is an indication of microbial activity during the composting process. The temperature in the compost heap increased to thermophilic range (60–70 °C) during the first few days and then decreased gradually to a constant temperature and finally reached to ambient level (Golueke 1981; Satisha and Devarajan 2007; Raj and Antil 2011, 2012b) . Stickelberger (1975) stated that compost is matured enough when its temperature remains more or less constant and does not vary with the turning over the material. Tiquia et al. (1996) also considered the temperature an important and simple maturity parameters and found positive correlation with other parameters during composting of spent pig manure saw dust litter. However, Iglesias Jiménez and Pérez García (1992) reported that exclusive determination of temperature in pilling composting plant cannot be considered in all cases the conclusive criterion to estimate the stability of compost. The rise in temperature to the thermophilic range is very necessary for weed seed and pathogen destruction in the final compost and regular monitoring of temperature in the composting pile is required to ensure the proper decomposition of organic material.

5.3.1.3 Weight Loss/Organic Matter Loss

The weight loss determination is the simplest procedure to measure the mineralization rate of organic matter (OM) during composting . The cumulative loss of organic matter increased with composting time in all types of composts and also measured in the form of weight loss. The weight loss/OM loss was found highly correlated with C:N ratio and other maturity parameters (Chefetz et al. 1996; Raj and Antil 2012b) . However, thorough examination of maturity parameters involving a long term study of a single compost pile may find some correlation between them and curing time of compost (Adani et al. 1995; Wu et al. 2000) .

5.3.2 Chemical Parameters

Chemical methods are widely used to assess the compost maturity. These parameters are more reliable than the physical parameters.

5.3.2.1 pH

The pH is a good indication showing the development in different stages of composting . The pH dropped slightly at the beginning of the composting process due to production of organic acids. Soon after, on utilization of these acids as substrates by other aerobic microbes, the pH increased, during the cooling and maturation stages, following lowering in pH and reached a value close to neutral (Satisha and Devarajan 2007; Ko et al. 2008) . This trend of pH could be used to monitor the stabilization and maturation of compost (Gray et al. 1971; Wu et al. 2000; Raj and Antil 2012b) .

5.3.2.2 EC

The EC is a measure of dissolved salts in the compost . This measure is significant because it reflects the salinity of the compost, and overly saline compost is likely harmful to plants. The sum of soluble salts in the water extracts is increased with the maturation of compost because of release of organic acids and soluble salts during organic matter decomposition indicating the stability of compost (Avnimelech et al. 1996; Wu et al. 2000) .

5.3.2.3 C:N Ratio (Solid Phase)

This is the criteria traditionally used to determine maturity of compost . The relevance of C:N ratio relies on the fact that a decrease in ratio implies in the degree of humification of organic matter. As the decomposition progressed due to losses of carbon mainly as carbon dioxide, the carbon content of the compostable material decreased with time and N content per unit material increased which resulted in the decrease of C:N ratio. The researchers reported that a C:N ratio below 20 was assumed to be indicative of maturity compost (Golueke 1981) , a ratio of 15 or less is preferable (Morel et al. 1985; Bernal et al. 2009) . On the other hand, some researchers (Chen and Inber 1993; Sellami et al. 2008) reported that C:N ratio alone is not a sufficient criteria to determine the compost maturity and it is very necessary to associate it with some other chemical and microbial parameters to establish compost maturity (Goyal et al. 2005) .

5.3.2.4 C:N Ratio (Water Extract)

Composting is a biochemical transformation of organic matter by microorganisms whose metabolism occurs in water soluble phase . Therefore, a study of the changes occurring in the soluble organic matter can be useful for the assessing maturity of compost. A Cw:Norg ratio of 5–6 was established by Chanyasak and Kubota (1981) as an essential indicator of compost maturity. However, this ratio is sometimes difficult to evaluate since the concentration of organic-N in water extract of mature samples is usually very low. For this reason, Hue and Liu (1995) suggested using the Cw:Norg ratio as a suitable parameter for assessing compost maturity, proposing a value of < 0.70 as a new index of compost stability. However, Bernal et al. (1998) found that some immature compost reached this value at thermophilic stage, thus they proposed a limit < 0.55 of Cw:Norg to describe well matured and stabilized composts. Raj and Antil (2011) also confirmed this value as maturity index of composts prepared from farm and agro-industrial waste materials .

5.3.2.5 Water Soluble Carbon (WSC)

The WSC represents the most easily biodegradable C fraction during the composting process because it consists of sugars, organic acids, amino acids and phenols, apart from the soluble fraction of fulvic acids (Garcia et al. 1991) . The concentration of WSC declined with composting time in all types of composts. The biodegradable C fractions were consumed first by microbes, which leads the decomposition of complex organic compounds resulting the release of CO2 at the end and some of them polymerizes with nitrogenous compound producing humic substances. Eggen and Vethe (2001) and Garcia et al. (1991) established the 0.5 % value of WSC as a maximum content above which compost could be considered as mature. Other values of this parameter were those suggested by Hue and Liu (1995) (WSC < 1 %) and by Bernal et al. (1998) (WSC < 1.7 %).

5.3.2.6 NH4 + and NO3-N Concentration

Compost maturity can also be defined in terms of nitrification. When the NH4 +-N concentration decreases and NO3-N concentration increases in the composting material is considered ready to be used as compost (Finstein and Miller 1985) . A high level of NH4 + concentration indicates an un-stabilized material. The absence or decrease in NH4 +-N is an indication of both good quality and completion of maturation process (Tiquia et al. 1997) . Zucconi and de Bertoldi (1987) suggested the maximum limit of NH4 +-N content to be below 0.04 % for mature compost. However, Raj and Antil (2011, 2012b) did not found this limit as an indication of maturity of composts prepared from different organic wastes . They suggested that decrease in NH4 +-N concentration and increase in NO3-N concentration indicates the maturity of compost. Bernal et al. (1998) suggested that the ratio of NH4 +: NO3-N < 0.16 is an indication of maturity of composts prepared from wide range of organic wastes .

5.3.2.7 CEC (Ash Free Material Basis)

The CEC of the composts increased with the advancement of composting time due to decomposition of organic matter . The humification process produces functional groups influenced by the increased oxidation of organic matter, which leads to rise in CEC. Consequently, measurement of CEC should be considered useful for estimating the degree of maturity (Harada and Inoko 1980) . They found a highly significant negative correlation between the CEC and C:N ratio and proposed a value  > 60 cmol (P+)/kg (on ash free material basis) to determine the degree of maturity of composts. Later on several workers validated this criterion for assessment of maturity of different composts (Estrada et al. 1987; Iglesias Jiménez and Pérez García 1992; Namkoong et al. 1999) . However, Bernal et al. (1998) could not found CEC > 60 cmol (P+)/kg (ash free basis) a valid criteria for maturity of composts prepared from the combination of agricultural and sewage sludge waste. This was also supported by Raj and Antil (2011, 2012b) for composts prepared from farm and agro-industrial wastes. Earlier, Roig et al. (1988) reported that correlation for CEC/TOC versus Ct/Nt (t = total), seems to be very useful for estimating the degree of maturity of cattle, sheep, chicken and rabbit manure. They suggested that a value of CEC/TOC over 1.7 reflect a good humification level of these waste manures. Iglesias Jiménez and Pérez García (1992) reported that numerical value of CEC is influenced by nature of the original material and suggested that CEC/TOC (total organic carbon) ratio can be used as an index value for assessment of maturity of composts as also evidenced by Raj and Antil (2011) who found this criteria highly correlated with other parameters and suggested CEC/TOC > 1.7 as maturity index .

5.3.2.8 Humification Parameters

The humified fraction of the soil organic matter is the most important one responsible for organic fertility functions in the soil (Bernal et al. 2009) . So the evaluation of the humification degree of the OM during composting is a criterion for compost maturity. During composting, humic substances (alkali-extractable organic-C, CEX) are produced and humic acid like organic-C (CHA) increases, while fulvic acid like organic-C (CFA) and water extractable organic-C decreases due to microbial degradation (Singh et al. 1987; Singh and Amberger 1990; Raj and Antil 2012b) . Some indices used for evaluation of the humification level in the material during composting include (Roletto et al. 1985; Senesi 1989)

  • Humification ratio (HR): CEX/Corg × 100

  • Humification index (HI): CHA/Corg × 100

  • Percent humic acids (PHA): CHA/CEX × 100

  • Humic acid to fulvic acid ratio: CHA/CFA

Since maturation also implies the formation of some humic like substances, the degree of OM humification is generally accepted as a criterion of maturity and above parameters can be used as maturity index (Bernal et al. 2009) . Saviozzi et al. (1988) proposed HA:FA ratio to judge the matutity of compost. Iglesias Jiménez and Pérez García (1992) adopted a ratio of 1.9 to determine the maturity of composts prepared from city waste and sewage sludge compost. Raj and Antil (2012b) proposed HI > 30 as a maturity index for the composts prepared from agro-industrial and farm wastes composts. Numerous chemical, physico-chemical and spectroscopic methods such as: elemental and functional groups composition, ratio of absorbance measured at 465 and 665 nm (E4/E6), molecular weight distribution, electrophoresis and electrofocusing, pyrolysis-gas chromatography–mass spectrometry (GC-MS), infrared and Fourier transformed–infrared (FT-IR) spectroscopy, electron spin resonance (ESR) spectroscopy and fluorescence spectroscopy (Moral et al. 2009) . Amongst these methods, advanced techniques such as NMR, FT-IR and pyrolysis have been employed to achieve a better understanding of the structural changes of the OM during composting and hence to evaluate composting efficiency and compost maturity (Chen 2003) . However, these humification parameters depend on the type of waste used for composting and their same value cannot be used for all type of composts .

5.3.3 Biological Parameters

Biological methods are widely used to assess the compost maturity. These parameters are comparatively more reliable than the other parameters.

5.3.3.1 Germination Test

The application of immature compost to arable soil may inhibit seed germination and or reduce the root length of seedlings due to the creation of reducing conditions in the soil and the presence of phytotoxic compounds in the compost. Based on this fact the germination test has been developed to determine the degree of maturity, GI was calculated using the following expression (Zucconi et al. 1981) :

$$ \text{GI}=\text{Percent seed germination}\times \text{mean root length} $$

They used cress seeds (Lepidium sativum) because of their quick response for determining the presence of phytotoxic substances in the compost. The proposed value of GI above 50 % is an index to judge maturity of compost. Some workers also studied the plant growth test (assessment of top growth and sometimes root biomass) and found that growth of plants inhibited by immature compost (Inbar et al. 1993; Iannotti et al. 1994; Chefetz et al. 1996) . Zucconi and de Bertoldi (1987) discussed the differences between the seed germination and growth tests. Germination tests provide an instant picture of phytotoxicity, whereas growing test will be affected by continuing changes in the stability or maturity of the compost tested: there may be damaging effects on growth in the earlier stages, but beneficial effects later on, with different conclusions depending up on time of assessment. According to Zucconi et al. (1981) , a GI value of 80 % has been used as an indicator of disappearance of phytotoxicity in composts. Tiquia et al. (1996) used this value not only as an indication of disappearance of phytotoxicity, but also an indication of maturity of compost. However, Antil and Raj (2012) reported a lower value of GI (> 70 %) as a maturity index of composts prepared from wide range of organic wastes . Ko et al. (2008) reported that the compost having GI value > 110 % was considered mature compost because the GI values reported by previous researchers were not a suitable threshold value for determining the maturity of composted animal manure. However, the results obtained using GI should be interpreted with caution, because the GI was affected by the type of seed used and applied extraction rates (Bernal et al. 1998; Wu et al. 2000; Tang et al. 2006) .

5.3.3.2 Oxygen and CO2 Respirometry

The aerobic respiration rate was selected as the most suitable parameters to assess the aerobic biological activity and hence stability . The respiration can be measured in several ways: carbon dioxide evolution, oxygen consumption and self-heating, which are indicative of the amount of degradable OM still present and which are related inversely to stabilization (Zucconi and de Bertoldi 1987) . Immature compost has a strong demand for oxygen and high CO2 production rates, due to high microbial activity as a consequence of the abundance of easily biodegradable compounds in the raw material. For this reason, O2 consumption and CO2 production are indicative of compost stability and maturity (Iannotti et al. 1994; Hue and Liu 1995; Barrena-Gómez et al. 2006) . Hue and Liu (1995) set the limit of the CO2 production rate for compost maturity at < 120 mg CO2 kg−1h−1. Wang et al. (2004) used a respiration rate of < 1 mg CO2-C g−1 dwd−1 to define a highly stabilized compost. Cooperband et al. (2003) suggested a NO3-N/CO2-C g−1 ratio > 8 per day as an index for compost maturity. The relationship between CO2 respiration and phytotoxicity of immature compost was studied by García-Gómez et al. (2003) and found that the CO2-C evolved correlated with plant growth and immature compost caused N-immobilisation in the soil, leading to plant N-deficiency .

5.3.3.3 Microbial Population/Count

The decomposition of organic matter is a microbial process and directly related with total microbial count and their activity during composting of organic wastes (Morel et al. 1985) . This method is based on the initial hypothesis that the microbial population stabilized on the maturity of compost. Some important studies in the evolution and quantification of different group of microorganisms involved in the process of composting have been carried (Albonetti and Massari 1979; De Bertoldi and Zucconi 1980) . The microbial population increased at thermophilic stage and then decreased at maturation phase and become constant at the end of composting indicating the stable nature of composting material (Petkova and Kostov 1996; Tiquia et al. 1996) . A decrease in bacterial and fungal counts, and increase in actinomycetes counts and stable at the end of composting appear as useful indicator for establishing biological stabilization and optimum degree of compost maturity (Antil and Raj 2012) . A gummy whitish appearance in composting materials indicates the presence of actinomycetes. For compost hygenization, the average number of total coliform, faecal coliforms and faecal enterococci densities should be less than < 500 MPN/g (Vuorinen and Saharinen 1997; Dahshan et al. 2013) .

5.3.3.4 Enzyme Activities

Various hydrolytic enzymes (cellulase, xylanase and protease) are believed to control the rate at which different substrates are degraded. Enzymes are the main mediators of various degradative processes (Tiquia et al. 1996) . Maximum activities of cellulase, xylanase and protease observed between 30 and 60 days of composting can be taken as index of compost maturity (Goyal et al. 2005) .

5.4 Compost Quality Standards

Different official and private organisations (European commission 2001; TMECC 2002; BOE 2005; BSI 2005; Ge et al. 2006) proposed many compost quality standards by taking into consideration compost properties viz. foreign matter (inert contamination), potentially toxic elements (organic contamination and heavy metals), sanitisation (pathogens and phytopathogens), maturity and stability, weed seeds, water, OM and nutrient content (Bernal et al. 2009) . At present, there is a need for standardization of such criteria at the international level.

5.5 Conclusion

The composting of organic waste materials has been demonstrated to be an effective method of producing end-products which are stabilized, ensuring their maximum benefit for agriculture. Composts are valuable products which can be used as sources of soil amendment and organic matter in agricultural land. Composts prepared from different organic wastes differ in their quality and stability. Several indices based on physical, chemical and biological parameters have been used for compost by different authors. Despite all the proposed methods to establish the degree of maturity and stability of composts, no single method can be universally applied to all composts due to wide variations in composition of raw material used for composting and composting technology. Hence, compost maturity should be assessed by measuring two or more parameters. However, it is necessary to standard the criteria used by official institutes from different countries.