**2.2.1 Can soil organic matter be maintained or increased upon compost application?**

Soil organic matter is of essential importance for maintaining soil quality by improving biological, physical and chemical soil conditions (Fig. 1). It consists of a variety of simple and complex carbon compounds. While the stable SOM pool is characterized by hardly decomposable organic compounds with several beneficial, long-term effects for soil amelioration and conservation, the labile pool of SOM provides easily accessible food for soil organisms and nutrients for plant growth (Termorshuizen et al., 2005). Soil fauna, notably earthworms (*Lumbricidae*), in turn, positively influence a wide variety of different physico-chemical properties. These positive effects are especially caused by their feeding behaviour, bioturbation and the production of droppings consisting of organo-mineral complexes. The latter is of main importance for the formation of stable SOM pools in soils.

Even though the importance of SOM for the ecological functionality, efficiency and performance of soils is obvious, its worldwide reduction due to intensive agriculture gives cause for concern (Lal, 2009). According to Termorshuizen et al. (2005), this reduction has multiple reasons, but increased SOM mineralization rates due to intensive soil tillage and use of mineral fertilizers, which has often resulted in decreased application of organic soil conditioners and fertilizers, are among the most important ones.

In order to sustain agricultural productivity in the long term, SOM needs to be maintained by continuous addition of organic residues and amendments. Soil organic matter reproduction rate increases in the order green manure, leaves 15%, slurry, straw, liquid digestate 20 – 30%, fresh compost, stable manure, solid digestate 35 – 45%, mature compost > 50% (Table 1). The main factors for SOM enrichment are quantity, type and humification degree of compost and soil properties such as soil type and clay content. Mature composts increase SOM much better than fresh and immature composts due to their higher level of stable C (Bundesgütegemeinschaft Kompost e. V. [BGK] & Bundesforschungsanstalt für Landwirtschaft [FAL], 2006). There are few trials which show no significant differences in SOM level by the application of diverse C sources (straw, manure, compost). But the

a higher pH value (Smith & Collins, 2007). It can contain considerable amount of plantavailable NO3- while NO4+ content is generally decreasing (Fig. 2). Furthermore, odour potential from compost is significantly reduced (Haug, 1993). But of utmost importance seems the fact that the organic matter has been stabilized, thus containing fairly resistant C compounds (Smith & Collins, 2007). Its application to the land influences several biological, chemical and physical soil properties in a positive and sustainable way which is outlined in

Numerous publications provide evidence on the multiple benefits of compost application to soil. Effects range from soil stabilization and amelioration to phyto-sanitary impacts of mature compost. Feedstock, compost maturity and compost quality can influence intensity and degree of effects on soil physical, chemical and biological properties. Application may trigger short-term improvements such as increasing microbial activity. Long-term effects on soil properties could be achieved by preservation and increase of the stable SOM pool

**2.2.1 Can soil organic matter be maintained or increased upon compost application?**  Soil organic matter is of essential importance for maintaining soil quality by improving biological, physical and chemical soil conditions (Fig. 1). It consists of a variety of simple and complex carbon compounds. While the stable SOM pool is characterized by hardly decomposable organic compounds with several beneficial, long-term effects for soil amelioration and conservation, the labile pool of SOM provides easily accessible food for soil organisms and nutrients for plant growth (Termorshuizen et al., 2005). Soil fauna, notably earthworms (*Lumbricidae*), in turn, positively influence a wide variety of different physico-chemical properties. These positive effects are especially caused by their feeding behaviour, bioturbation and the production of droppings consisting of organo-mineral complexes. The latter is of main importance for the formation of stable SOM pools in soils. Even though the importance of SOM for the ecological functionality, efficiency and performance of soils is obvious, its worldwide reduction due to intensive agriculture gives cause for concern (Lal, 2009). According to Termorshuizen et al. (2005), this reduction has multiple reasons, but increased SOM mineralization rates due to intensive soil tillage and use of mineral fertilizers, which has often resulted in decreased application of organic soil

In order to sustain agricultural productivity in the long term, SOM needs to be maintained by continuous addition of organic residues and amendments. Soil organic matter reproduction rate increases in the order green manure, leaves 15%, slurry, straw, liquid digestate 20 – 30%, fresh compost, stable manure, solid digestate 35 – 45%, mature compost > 50% (Table 1). The main factors for SOM enrichment are quantity, type and humification degree of compost and soil properties such as soil type and clay content. Mature composts increase SOM much better than fresh and immature composts due to their higher level of stable C (Bundesgütegemeinschaft Kompost e. V. [BGK] & Bundesforschungsanstalt für Landwirtschaft [FAL], 2006). There are few trials which show no significant differences in SOM level by the application of diverse C sources (straw, manure, compost). But the

Fig. 1 and which will be discussed in the following in more detail.

conditioners and fertilizers, are among the most important ones.

(Amlinger et al., 2007).

**2.2 How does compost influence soil properties and plant growth?** 

majority of studies of different authors have unambiguously proven a better humus reproduction for composted materials (Amlinger et al., 2007).

According to Amlinger et al. (2007), the average SOM demand of agriculturally used soils can be met by applying 7 – 10 Mg (dry matter) compost ha-1 a-1. Therefore, for a long-term increase of SOM, however, more than 10 Mg dry matter compost ha-1 a-1 is required.


Table 1. Organic matter and SOM reproduction of different organic fertilizers when applied at 10 Mg ha-1 a-1 (Data from BGK & FAL, 2006).

#### **2.2.2 How are physical soil properties influenced?**

**Reduction of Bulk Density**: Compost application generally influences soil structure in a beneficial way by lowering soil density due to the admixture of low density OM into the mineral soil fraction. This positive effect has been detected in most cases and it is typically associated with an increase in porosity because of the interactions between organic and inorganic fractions (Amlinger et al., 2007).

**Increase of aggregate stability**: In general, soil structure is defined by size and spatial distributions of particles, aggregates and pores in soils. The volume of solid soil particles and the pore volume influences air balance and root penetration ability. As a general fact the more soil structure is compacted, the more unfavorable are the soil conditions for plant growth. By incorporation of compost into the soil, aggregate stability increases most effectively in clayey and sandy soils. Positive effects can be expected by well humified (promoting micro-aggregates), as well as fresh, low-molecular OM (promoting macroaggregates). Macro-aggregates are mainly stabilized by fungal hyphen, fine roots, root hair and microorganisms with a high portion of easily degradable polysaccharides (Amlinger et al., 2007). Subsequently, the importance of stronger degraded organic compounds for the stabilization of smaller aggregates increases with time of transformation periods ranging from some few to several thousand years (Kong et al., 2005; Lützow et al., 2008; Marschner & Flessa, 2006). In this respect, the aromatic structure of inert organic compounds seems to play an important role in stabilizing micro-aggregates in the range of 2 – 20 µm as well as 20 – 250 µm by polyvalent cation bridges with clay minerals (Tisdall & Oades, 1982). Besides clay minerals and oxides, fine roots, hyphen networks as well as glue-like polysaccharides originated from root and microbial exudates significantly contribute to the formation of micro-aggregates.

Furthermore, aggregate and pore properties of soils are associated with specific "active" surface area influencing several storage and exchange processes in soil. The higher the specific surface area, the more intensive interactions can occur between soil fauna, microorganisms and root hairs under optimum conditions (e.g. sufficient humidity). As a result, a high specific surface area can create the prerequisite for an optimal soil formation (Amlinger et al., 2007).

**Improvement of pore volume and hydraulic conductivity**: Hydraulic conductivity is the percolation rate in soils per area and time unit depending on (i) actual soil moisture tension

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 173

mainly depend on climate, radiation absorption can be influenced by color. Composts are dark-colored resulting in higher light absorption and thus lower albedo (reflection rate of light from a light source). Thus, higher light absorption will warm up soils supplied with compost faster than light-colored soils (Stöppler-Zimmer et al., 1993). This will promote germination of seeds, especially during spring. However, as temperature increases in summer, uncovered dark soils can heat up extremely. As a result, soil can dry out due to a higher evaporation which, in turn, affects plant growth and soil biology negatively. In such case, compost mulching systems offer a good solution because they obtain reduced fluctuations of soil temperature which results from the shading effect of mulch loosely

**Enhancement of nutrient level**: Compost contains significant amounts of valuable plant nutrients including N, P, K, Ca, Mg and S as well as a variety of essential trace elements (Seiberth & Kick, 1969; Bischoff, 1988; Lenzen, 1989; Haug, 1993; Smith & Collins, 2007). Thus, compost can be defined as an organic multi nutrient fertilizer (Hartmann, 2003; Amlinger et al., 2007). Its nutrient content as well as other important chemical properties like C/N ratio, pH and electrical conductivity (EC) depend on the used organic feedstocks and compost processing conditions (Table 2). By an appropriate mixture of these organic input materials humus and nutrient-rich compost substrates can be produced (Table 3) serving as a substitute for commercial mineral fertilizers in agriculture. However, their

However, total nutrient content of compost is not plant-available to the full extent at once. This can be ascribed to the existence and different intensity of various binding forms within the organic matrix which result in a partial immobilization of nutrients (Becker et al., 1995). On the one hand, this condition makes it more difficult to calculate the fertilization effect and to estimate the nutrient balance in advance (Becker et al., 1995). On the other hand, the fertilization effect will last longer due to a slow and gradual release of plant nutrients (Smith & Collins, 2007). Therefore, with compost there is a much better protection from leaching compared to soluble mineral fertilizers. Especially the N fertilization effect of compost is limited due to

diverse beneficial properties for amelioration outreach their nutrient content.

Table 2. Chemical properties of organic feedstock materials.

covering the soil surface.

**2.2.3 How are chemical soil properties influenced?** 

and (ii) number, size and form of soil pores (pore size distribution). Organic matter applied by compost improves water conductivity of soils (Carter et al., 2004) by providing a food source for soil organisms which contribute to the formation of macro-pores, in turn. Additionally, it has a direct structure-stabilizing effect for soil. A resulting increase of hydraulic conductivity is of main importance, especially for clayey soils.

**Increase of field capacity, secondary pore structures and improved water retention**: Field capacity (FC) is defined as the amount of water which a water-saturated soil can retain against gravity after 2 – 3 days. It is mainly influenced by pore volume and pore size distribution because only pores below a pore diameter of 50 µm (corresponding to pF 1.8) can retain water against gravity due to higher capillary force. However, adhesive force of pores with a diameter below 0.2 µm is so high that this water is not available for plants (permanent wilting point or PWP corresponding to pF 4.2). Field capacity and available water holding capacity (AWC, pF 1.8 – 4.2) are generally influenced by the particle size, structure and content of OM. Several studies confirm a significant, positive impact of OM amendment to soils on FC (Evanylo et al., 2008; Tejada et al., 2006; Carter et al., 2004). Amongst others, this effect results from the improved formation of secondary pore structures which can be mainly ascribed to root and animal tubes. This is important for soils with low portions of primary meso-pores. In this respect, compost increases the portion of meso- and macro-pores because of an improved aggregation and stabilization of soil significantly initiated by various soil organisms (Liu et al., 2007). In addition, organic matter (OM) is able to take up 3 to 20 times more water compared to its own weight. Considering these effects, an increase of total organic carbon (TOC) content from 0.5% to 3% resulted in a duplication of AWC (Hudson, 1994).

**Improved air balance**: The portion of air in soils results from the difference between total pore volume and the pores filled with water (Amlinger et al., 2007). Air permeability and air exchange in soils predominantly depends on pores > 50 µm. While sandy soils are characterized by a high portion of primary macro pores resulting in a proper aeration, clayey or compacted soils have few macro pores which may cause lack of oxygen availability. For these latter soils, OM applied by compost has a significant ameliorating effect by improving porous soil structure and its stabilization, stimulating the formation of secondary macro pores especially by roots and animals tubes.

**Reduction of soil erosion and run-off**: Reduced erosion is mainly related to the improved soil structure by the addition of compost which, in turn, is pointed out by better infiltration rate, pore volume and enhanced stability through aggregation (Diacono & Montemurro, 2010). According to Amlinger et al. (2007), experimental trials showed a clear correlation between increases of SOM, reductions of soil density, soil loss and water run-off. The effect of compost on soil erosion has been quantified in detail by Strauss (2003). Five years long compost application resulted in 67% reduced soil erosion, 60% reduced run-off, 8% lower bulk density and 21% higher OM content compared to control plots. Similar results were observed by Hartmann (2003) in a wind tunnel experiment by testing the resilience of compost application against wind erosion for two different soil types: By the incorporation of compost, loss of soil particles from the topsoil was reduced to a maximum of 61 % for a podzol and 71 % for a luvisol.

**Improved heat balance of soils**: Soil temperature influences the reaction rate of chemical, metabolic and biological growth processes of organisms. While temperature fluctuations mainly depend on climate, radiation absorption can be influenced by color. Composts are dark-colored resulting in higher light absorption and thus lower albedo (reflection rate of light from a light source). Thus, higher light absorption will warm up soils supplied with compost faster than light-colored soils (Stöppler-Zimmer et al., 1993). This will promote germination of seeds, especially during spring. However, as temperature increases in summer, uncovered dark soils can heat up extremely. As a result, soil can dry out due to a higher evaporation which, in turn, affects plant growth and soil biology negatively. In such case, compost mulching systems offer a good solution because they obtain reduced fluctuations of soil temperature which results from the shading effect of mulch loosely covering the soil surface.

#### **2.2.3 How are chemical soil properties influenced?**

**Enhancement of nutrient level**: Compost contains significant amounts of valuable plant nutrients including N, P, K, Ca, Mg and S as well as a variety of essential trace elements (Seiberth & Kick, 1969; Bischoff, 1988; Lenzen, 1989; Haug, 1993; Smith & Collins, 2007). Thus, compost can be defined as an organic multi nutrient fertilizer (Hartmann, 2003; Amlinger et al., 2007). Its nutrient content as well as other important chemical properties like C/N ratio, pH and electrical conductivity (EC) depend on the used organic feedstocks and compost processing conditions (Table 2). By an appropriate mixture of these organic input materials humus and nutrient-rich compost substrates can be produced (Table 3) serving as a substitute for commercial mineral fertilizers in agriculture. However, their diverse beneficial properties for amelioration outreach their nutrient content.


172 Management of Organic Waste

and (ii) number, size and form of soil pores (pore size distribution). Organic matter applied by compost improves water conductivity of soils (Carter et al., 2004) by providing a food source for soil organisms which contribute to the formation of macro-pores, in turn. Additionally, it has a direct structure-stabilizing effect for soil. A resulting increase of

**Increase of field capacity, secondary pore structures and improved water retention**: Field capacity (FC) is defined as the amount of water which a water-saturated soil can retain against gravity after 2 – 3 days. It is mainly influenced by pore volume and pore size distribution because only pores below a pore diameter of 50 µm (corresponding to pF 1.8) can retain water against gravity due to higher capillary force. However, adhesive force of pores with a diameter below 0.2 µm is so high that this water is not available for plants (permanent wilting point or PWP corresponding to pF 4.2). Field capacity and available water holding capacity (AWC, pF 1.8 – 4.2) are generally influenced by the particle size, structure and content of OM. Several studies confirm a significant, positive impact of OM amendment to soils on FC (Evanylo et al., 2008; Tejada et al., 2006; Carter et al., 2004). Amongst others, this effect results from the improved formation of secondary pore structures which can be mainly ascribed to root and animal tubes. This is important for soils with low portions of primary meso-pores. In this respect, compost increases the portion of meso- and macro-pores because of an improved aggregation and stabilization of soil significantly initiated by various soil organisms (Liu et al., 2007). In addition, organic matter (OM) is able to take up 3 to 20 times more water compared to its own weight. Considering these effects, an increase of total organic carbon (TOC) content from 0.5% to 3% resulted in a

**Improved air balance**: The portion of air in soils results from the difference between total pore volume and the pores filled with water (Amlinger et al., 2007). Air permeability and air exchange in soils predominantly depends on pores > 50 µm. While sandy soils are characterized by a high portion of primary macro pores resulting in a proper aeration, clayey or compacted soils have few macro pores which may cause lack of oxygen availability. For these latter soils, OM applied by compost has a significant ameliorating effect by improving porous soil structure and its stabilization, stimulating the formation of

**Reduction of soil erosion and run-off**: Reduced erosion is mainly related to the improved soil structure by the addition of compost which, in turn, is pointed out by better infiltration rate, pore volume and enhanced stability through aggregation (Diacono & Montemurro, 2010). According to Amlinger et al. (2007), experimental trials showed a clear correlation between increases of SOM, reductions of soil density, soil loss and water run-off. The effect of compost on soil erosion has been quantified in detail by Strauss (2003). Five years long compost application resulted in 67% reduced soil erosion, 60% reduced run-off, 8% lower bulk density and 21% higher OM content compared to control plots. Similar results were observed by Hartmann (2003) in a wind tunnel experiment by testing the resilience of compost application against wind erosion for two different soil types: By the incorporation of compost, loss of soil particles from the topsoil was reduced to a maximum of 61 % for a

**Improved heat balance of soils**: Soil temperature influences the reaction rate of chemical, metabolic and biological growth processes of organisms. While temperature fluctuations

hydraulic conductivity is of main importance, especially for clayey soils.

duplication of AWC (Hudson, 1994).

podzol and 71 % for a luvisol.

secondary macro pores especially by roots and animals tubes.

Table 2. Chemical properties of organic feedstock materials.

However, total nutrient content of compost is not plant-available to the full extent at once. This can be ascribed to the existence and different intensity of various binding forms within the organic matrix which result in a partial immobilization of nutrients (Becker et al., 1995). On the one hand, this condition makes it more difficult to calculate the fertilization effect and to estimate the nutrient balance in advance (Becker et al., 1995). On the other hand, the fertilization effect will last longer due to a slow and gradual release of plant nutrients (Smith & Collins, 2007). Therefore, with compost there is a much better protection from leaching compared to soluble mineral fertilizers. Especially the N fertilization effect of compost is limited due to


Table 3. Chemical properties of compost products.

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 175

low mineralization rates and microbial immobilization (Kehres, 1992, Vogtmann et al., 1991). In the first year after compost application, only 10 – 20% of the total N content will be mineralized according to Becker et al. (1995). Bidlingmaier et al. (2000) reported N mineralization rate of 10% in the first year and 40% in total in the long term. In contrast to the low N availability, a higher fertilization effect for P (50%), K (100%) and Mg could be observed (Bidlingmaier et al., 2000). With respect to micro nutrients, an increased plant

**Increase of cation exchange capacity (CEC)**: The CEC is one of the most important indicators for evaluating soil fertility (Fig. 1), more specifically for nutrient retention and thus it prevents cations from leaching into the groundwater. Kögel-Knabner et al. (1996), Kahle & Belau (1998) and Ouedraogo et al. (2001) proved that compost amendment resulted in an increase of CEC due to input of stabilized OM being rich in functional groups into soil. According to Amlinger et al. (2007), SOM contributes about 20 – 70% to the CEC of many soils. In absolute terms, CEC of OM varies from 300 to 1,400 cmolc kg-1 being much higher

**Increase of pH value, liming effect and improved buffering capacity**: Soil pH is is an indicator for soil acidity or soil alkalinity and is defined as the negative logarithm of hydrogen ions activity in a soil suspension. It is important for crop cultivation because many plants and soil organisms have a preference for slight alkaline or acidic conditions and thus it influences their vitality. In addition, pH affects availability of nutrients in the soil. Compost application has a liming effect due to its richness in alkaline cations such as Ca, Mg and K which were liberated from OM due to mineralization. Consequently, regularly applied compost material maintains or enhances soil pH (Kögel-Knabner et al., 1996; Diez & Krauss, 1997; Kahle & Belau, 1998; Stamatiadis et al., 1999; Ouedraogo et al., 2001). Only in some few cases a pH decrease was observed after compost application (Zinati et al., 2001). **Reduction and immobilization of pesticides and persistent organic pollutants (POPs)**: A contamination of soils or composts with pesticides and POPs can occur in consequence of environmental pollution, conventional farming practice by using chemicals and pesticides and by incorporation of contaminated materials into compost or soil. Therefore, unpolluted feedstocks should be generally preferred for composting in order to avoid critical concentrations of pollutants. However, pesticides and POPs can be degraded or immobilized during compost processing or by the properties of the final compost product. Based on temperature and oxidative microbial and biochemical processes, composting contributes to an effective reduction of organic pollutants. For instance, polychlorinated biphenyls (PCB) were degraded up to 45% during composting (Amlinger et al., 2007). Linear alkylbenzene sulphonates (LAS), Nonylphenols (NPE) and Di (2-ethylhexyl) phthalate (DEHP) which are mainly found in sewage sludge, are degraded almost completely under oxidative conditions (Amlinger et al., 2007). Furthermore, the degradation rate of halogenated organic compounds and pesticides is much higher than in soils, especially during the thermophilic stage (Amlinger et al., 2007). Mineralization rate of pollutants is reported to be more effective in compost soil mixtures if mature compost is applied. This concerns especially the degradation of polycyclic aromatic hydrocarbons (PAH) and other hydrocarbons (Amlinger et al., 2007). Due to the high level of humified OM, particularly mature composts contribute to sorption and immobilization of POPs resulting in a lower

uptake of Cu, Mn and Zn was reported (Amlinger et al., 2007).

than CEC of any inorganic material.

availability of POPs and reduced toxicity.

Table 3. Chemical properties of compost products.

low mineralization rates and microbial immobilization (Kehres, 1992, Vogtmann et al., 1991). In the first year after compost application, only 10 – 20% of the total N content will be mineralized according to Becker et al. (1995). Bidlingmaier et al. (2000) reported N mineralization rate of 10% in the first year and 40% in total in the long term. In contrast to the low N availability, a higher fertilization effect for P (50%), K (100%) and Mg could be observed (Bidlingmaier et al., 2000). With respect to micro nutrients, an increased plant uptake of Cu, Mn and Zn was reported (Amlinger et al., 2007).

**Increase of cation exchange capacity (CEC)**: The CEC is one of the most important indicators for evaluating soil fertility (Fig. 1), more specifically for nutrient retention and thus it prevents cations from leaching into the groundwater. Kögel-Knabner et al. (1996), Kahle & Belau (1998) and Ouedraogo et al. (2001) proved that compost amendment resulted in an increase of CEC due to input of stabilized OM being rich in functional groups into soil. According to Amlinger et al. (2007), SOM contributes about 20 – 70% to the CEC of many soils. In absolute terms, CEC of OM varies from 300 to 1,400 cmolc kg-1 being much higher than CEC of any inorganic material.

**Increase of pH value, liming effect and improved buffering capacity**: Soil pH is is an indicator for soil acidity or soil alkalinity and is defined as the negative logarithm of hydrogen ions activity in a soil suspension. It is important for crop cultivation because many plants and soil organisms have a preference for slight alkaline or acidic conditions and thus it influences their vitality. In addition, pH affects availability of nutrients in the soil. Compost application has a liming effect due to its richness in alkaline cations such as Ca, Mg and K which were liberated from OM due to mineralization. Consequently, regularly applied compost material maintains or enhances soil pH (Kögel-Knabner et al., 1996; Diez & Krauss, 1997; Kahle & Belau, 1998; Stamatiadis et al., 1999; Ouedraogo et al., 2001). Only in some few cases a pH decrease was observed after compost application (Zinati et al., 2001).

**Reduction and immobilization of pesticides and persistent organic pollutants (POPs)**: A contamination of soils or composts with pesticides and POPs can occur in consequence of environmental pollution, conventional farming practice by using chemicals and pesticides and by incorporation of contaminated materials into compost or soil. Therefore, unpolluted feedstocks should be generally preferred for composting in order to avoid critical concentrations of pollutants. However, pesticides and POPs can be degraded or immobilized during compost processing or by the properties of the final compost product. Based on temperature and oxidative microbial and biochemical processes, composting contributes to an effective reduction of organic pollutants. For instance, polychlorinated biphenyls (PCB) were degraded up to 45% during composting (Amlinger et al., 2007). Linear alkylbenzene sulphonates (LAS), Nonylphenols (NPE) and Di (2-ethylhexyl) phthalate (DEHP) which are mainly found in sewage sludge, are degraded almost completely under oxidative conditions (Amlinger et al., 2007). Furthermore, the degradation rate of halogenated organic compounds and pesticides is much higher than in soils, especially during the thermophilic stage (Amlinger et al., 2007). Mineralization rate of pollutants is reported to be more effective in compost soil mixtures if mature compost is applied. This concerns especially the degradation of polycyclic aromatic hydrocarbons (PAH) and other hydrocarbons (Amlinger et al., 2007). Due to the high level of humified OM, particularly mature composts contribute to sorption and immobilization of POPs resulting in a lower availability of POPs and reduced toxicity.

Synergisms between Compost and Biochar for Sustainable Soil Amelioration 177




In general, compost creates a favourable environment for plant and root growth especially by

2. decreasing soil erodibility due to the formation of stable aggregates. Consequently, plant roots are less exposed to direct damage caused by eroded topsoil and water can better infiltrate into soil. Furthermore, air exchange is less interfered by the compaction of subsurface soil or by the formation of a soil crust, which tends to "seal" the surface

(Buchmann, 1972; Richter, 1979; Krieter, 1980; Fox, 1986; Löbbert & Reloe, 1991); 3. intensifying essential interactions between root hairs, soil fauna and microorganisms

4. improving percolation. On the one hand, this prevents waterlogging which can result in a decay of plant roots due to anaerobic soil conditions. On the other hand, loss of

5. enhancing water storage capacity and improving water retention which helps plants

6. providing valuable macro- and micro-nutrients in the long term (Gottschall, 1984) due to slow mineralization rates, better nutrient adsorption as well as enhanced storage

7. improving buffering capacity which helps to maintain uniform reactions and conditions

8. promoting the degradation, reduction or immobilization of harmful substances like pesticides, POPs, heavy metals and phyto-toxic compounds which can interfere plant

9. providing microbial symbionts and beneficial soil organisms a habitat which, in turn,

10. protecting plants from pathogens and diseases due to antiphytopathogenic potential of compost (Hoitink, 1980; Nelson & Hoitink, 1983; Hoitink & Fahy, 1986; Blume, 1989;

better overcome critical climate conditions like droughts (Hartmann, 2003);

due to an enhancement of specific surface area (Amlinger et al., 2007);

like compost product which is subjected to only slow decomposition rates.

leaching.

decomposed, transformed or immobilized.

nutrients is reduced by decreased run-off;

capacity which prevents from leaching;

Hadar et al., 1992; Bidlingmaier et al., 2000);

has a positive influence on vitality and growth of plants;

for better plant growth;

life and health;

harm plant life or human health (Amlinger et al., 2007).

**2.2.5 How are plant growth, plant health and crop quality influenced?** 

1. promoting a porous soil structure for optimized root penetration;

easily degradable organic substances are decomposed, further decomposition and transformation of the remaining by-products occur, finally resulting in a stable humus-

**Immobilization of heavy metals**: Similar to pesticides and POPs, there are several sources for heavy metal input. To a limited extent, heavy metals or trace elements serve as plant nutrients while their accumulation can cause toxicity. In this respect, OM applied by compost is able to adsorb heavy metals and reduce their solubility resulting in immobilization. Apart from some non-crystalline minerals with very high surface areas SOM has probably the greatest capacity to bond most heavy metals (Amlinger et al., 2007). The sorption strength of heavy metals to SOM generally decrease in the following order: Cr(III) > Pb(II) > Cu(II) > Ag (I) > Cd (II) = Co(II) = Li(II). On the other hand, significant correlations between the solubility of Cd, Cu, Zn, Pb and Ni and SOM content have been reported by Holmgren et al. (1993) for a range of soils from the USA. Organic matter applied by compost even effectively prevents mobilization of heavy metals for a long time after the cessation of compost addition (Leita et al., 2003). In order to guarantee the lowest possible pollutant input over time, raw materials for composting have to be separated and preferably unpolluted feedstocks should be used. However, the prevention of environmental pollution and thus the contamination with heavy metals is basically a general matter for the society and for politics. As long as there is an emission of pollutants by industry and society immission will occur including the pollution of valuable organic feedstocks.

## **2.2.4 How are biological soil properties influenced?**

One of the most important effects of compost use is the promotion of soil biology. In this respect, the following three aspects seem essential: (i) Food supply for soil heterotrophic organisms by adding degradable carbon compounds with OM (Blume, 1989); (ii) optimization of habitat and niche properties in soil, e.g. water and air balances, increase of specific surfaces, retreat areas etc.; (iii) introducing compost biota into soil as an inoculant (Amlinger et al., 2007; Sahin, 1989; Werner et al., 1988). Compost has a stimulation effect on both the microbial community in the compost substrate as well as the soil-born microbiota of soils (Table 4). Two fractions of OM are responsible for the level of microbial activity in general: (i) Easily degradable organic compounds (labile OM pool) may increase microbial activity and biomass temporarily while (ii) a persisting increase of microbial biomass depends on a constant enhancement of stable OM which is particularly promoted by mature compost addition.


Table 4. Soil bacterial and fungal biomass in soils and compost (United States Environmental Protection Agency - Solid Waste and Emergency Response, 1998).

Microorganisms perform several ecological and environmental functions. With regard to compost, the following microbial effects seem of main importance:


**Immobilization of heavy metals**: Similar to pesticides and POPs, there are several sources for heavy metal input. To a limited extent, heavy metals or trace elements serve as plant nutrients while their accumulation can cause toxicity. In this respect, OM applied by compost is able to adsorb heavy metals and reduce their solubility resulting in immobilization. Apart from some non-crystalline minerals with very high surface areas SOM has probably the greatest capacity to bond most heavy metals (Amlinger et al., 2007). The sorption strength of heavy metals to SOM generally decrease in the following order: Cr(III) > Pb(II) > Cu(II) > Ag (I) > Cd (II) = Co(II) = Li(II). On the other hand, significant correlations between the solubility of Cd, Cu, Zn, Pb and Ni and SOM content have been reported by Holmgren et al. (1993) for a range of soils from the USA. Organic matter applied by compost even effectively prevents mobilization of heavy metals for a long time after the cessation of compost addition (Leita et al., 2003). In order to guarantee the lowest possible pollutant input over time, raw materials for composting have to be separated and preferably unpolluted feedstocks should be used. However, the prevention of environmental pollution and thus the contamination with heavy metals is basically a general matter for the society and for politics. As long as there is an emission of pollutants by industry and society

immission will occur including the pollution of valuable organic feedstocks.

One of the most important effects of compost use is the promotion of soil biology. In this respect, the following three aspects seem essential: (i) Food supply for soil heterotrophic organisms by adding degradable carbon compounds with OM (Blume, 1989); (ii) optimization of habitat and niche properties in soil, e.g. water and air balances, increase of specific surfaces, retreat areas etc.; (iii) introducing compost biota into soil as an inoculant (Amlinger et al., 2007; Sahin, 1989; Werner et al., 1988). Compost has a stimulation effect on both the microbial community in the compost substrate as well as the soil-born microbiota of soils (Table 4). Two fractions of OM are responsible for the level of microbial activity in general: (i) Easily degradable organic compounds (labile OM pool) may increase microbial activity and biomass temporarily while (ii) a persisting increase of microbial biomass depends on a constant enhancement of stable OM which is particularly promoted by mature

**Material Bacteria Fungi**

Microorganisms perform several ecological and environmental functions. With regard to


Pesticide containing soil 19 6 Reclaimed soil after surface mining 19-70 8-97 Fertile soil 6-46 9-46 Mature green waste compost 417 155

Table 4. Soil bacterial and fungal biomass in soils and compost (United States Environmental Protection Agency - Solid Waste and Emergency Response, 1998).

compost, the following microbial effects seem of main importance:

**[106 g-1**

 **dm] [10<sup>3</sup>**

 **g-1 dm]**

**2.2.4 How are biological soil properties influenced?** 

compost addition.

easily degradable organic substances are decomposed, further decomposition and transformation of the remaining by-products occur, finally resulting in a stable humuslike compost product which is subjected to only slow decomposition rates.

