1. Introduction

• What is soil organic matter?
• Soil quality and the role of SOC
• Do generic critical threshold values exist for SOC?
• Overview of principal functions of SOM in soils

What is soil organic matter?

The term "Soil organic matter" (SOM) has been used in different ways to describe the organic constituents of soil. In this report, SOM will be used as defined by Baldock and Skjemstad (1999) as "all organic materials found in soils irrespective of origin or state of decomposition".

Since SOM consists of C, H, O, N, P and S, it is difficult to actually measure the SOM content and most analytical methods determine the soil organic carbon (SOC) content and estimate SOM through a conversion factor.

The amount of SOC that exists in any given soil is determined by the balance between the rates of organic carbon input (vegetation, roots) and output (CO₂ from microbial decomposition).

However, soil type, climate, management, mineral composition, topography, soil biota (the so-called soil forming factors) and the interactions between each of these are modifying factors that will affect the total amount of SOC in a profile as well as the distribution of SOC contents with depth.

It is important to note that any changes made to the natural status of the soil systems (e.g. conversion to agriculture, deforestation, plantation) will result in different conditions under which SOC enters and exits the system.

Therefore, perturbed systems may still be in the process of attaining a new equilibrium C content and any measurements of SOC have to take into account that the soil is in the process of re-estabilishing equlibrium, which could take >50 years (Baldock and Skjemstad, 1999).

Soil quality and the role of SOC

Since Lal's (1993) initial definition of soil quality as the capacity of soil to produce economic goods and services and to regulate the environment, the term "soil quality" has been refined and expanded by scientists and policy makers to include its importance as an environmental buffer, in protecting watersheds and groundwater from agricultural chemicals and municipal wastes and sequestering carbon that would otherwise contribute to a rise in greenhouse gases and global climate change (Reeves, 1997).

Doran and Parkin (1994) and Doran and Safley (1997) initially distinguished between "soil quality" and "soil health" before inclusively using the term "soil health" and defining it as "the continued capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain biological productivity, promote the quality of air and water environments, and maintain plant, animal and human health".

However, the general perception of a healthy or high-quality soil is one that adequately performs functions, which are important to humans, such as providing a medium for plant growth and biological activity, regulating and partitioning water flow and storage in the environment and serving as an environmental buffer in the formation and destruction of environmentally hazardous compounds.

Considering this wide variety of performance indicators, Karlen et al. (2003) and Norfleet et al. (2003) pointed out that soil quality needs to be assessed with regard to what the soil is used for, as a particular soil may be of high quality for one function and may perform poorly for another.

In particular, the suitability of soil for sustaining plant growth and biological activity is a function of physical (porosity, water holding capacity, structure and tilth) and chemical properties (nutrient supply capability, pH, salt content), many of which are a function of SOM content (Doran and Safley, 1997).

In general, increases in SOM are seen as desirable by many farmers as higher levels are viewed as being directly related to better plant nutrition, ease of cultivation, penetration and seedbed preparation, greater aggregate stability, reduced bulk density, improved water holding capacity, enhanced porosity and earlier warming in spring (Carter and Stewart, 1996; Lal, 2002).

Reeves (1997) noted that "SOC is the most often reported attribute from long-term agricultural studies and is chosen as the most important indicator of soil quality and agronomic sustainability because of its impact on other physical, chemical and biological indicators of soil quality".

However, Janzen et al. (1992) pointed out that the relationship between soil quality indicators (e.g. SOC) and soil functions does not always comply to a simple relationship increasing linearly with magnitude of the indicator and that therefore "bigger is not necessarily better".

Do generic critical threshold values exist for SOC?

SOM concentrations are often cited as major indicators of soil quality. However, only few studies attempt to discuss minimum or maximum threshold values of soil carbon, above or below which the beneficial effect of SOC is diminished.

For example, Janzen et al. (1992) showed from the relationship between SOC in the uppermost 15cm and soil productivity, an upper threshold of SOC existed, beyond which no further increases in productivity were achieved (Figure 1).

The threshold value for SOC for these dryland sites in Alberta, Canada, was at 2% SOC, which is in accordance with the observations by Howard and Howard (1990), who estimated that the threshold value for most soils was at 2% SOC (equivalent to 3.4% SOM), below which most soils are prone to structural destabilisation and crop yields are reduced.

Figure 1: Relationship between organic C concentration in the surface 0-15cm of soil and soil productivity as determined by total dry matter yield at dryland sites in Alberta, Canada (redrawn from Janzen et al., 1992).

However, Doran and Safley (1997) argued that different soil types are likely to have different threshold values. For example, threshold values established for highly weathered Ultisols in the southeastern US indicate that surface SOC levels of 1.2% are sufficient to attain maximum productivity.

By comparison, the same value for Mollisols under grasslands in the Great Plains would be regarded as an indicator for degraded conditions, limiting soil productivity. Baldock and Skjemstad (1999) showed that different soil types not only have different total SOC contents but that the distribution of SOC with depth varies according to soil type.

Similarly, Körschens et al. (1998) found that soils with different clay contents reach different SOC equilibria. In a 90 year field trial, they found that sandy soils containing 3% clay equilibrated at 0.7% SOC and soils with 21% clay reached 2.0% SOC; however, the mineralisable carbon content for both soil types was 0.4-0.5%.

Baldock and Skjemstad (1999) proposed contents of SOC which are considered to be low, medium and high for various climatic and management combinations and soil types. The influence of climate and management on SOC levels was evident and demonstrated that attributes such as "low" or "high" can only be used in a relative sense.

They further pointed out that the amount of C required to perform a specific function is likely to be different as, for example, the amount required to ensure an adequate nutrient supply is likely to differ from the amount required to ensure structural stability. In conclusion, it is apparent from the studies discussed here, that soil type and climatic setting can affect the individual SOC threshold values.

To effectively increase SOC, the rate of input must exceed the rate of loss from decomposition and leaching processes. In most agricultural cases, this is achieved by stubble retention, rotating crops with pasture, or the addition of organic residues such as animal manure, litter or sewage sludge.

For example, Johnston (1991) showed that SOC of a sandy soil could be increased from 0.7 to 0.9% over 6 years by return of crop residues, which was associated with a consistent increase in arable crop and sugar beet yields. Subsequent annual applications of farmyard manure (FYM) increased SOC from 1% to 3.4% whereas long-term application of fertiliser N had no measurable effect on SOC levels.

Similarly, Paustian et al. (1992) showed in a 30-year-long Swedish field trial that biannual additions of various organic carbon residues (straw, sawdust, green manure, and FYM) had positive effects on soil C levels (Figure 2). The highest accumulations occurred with sawdust plus N and manure amendments. It was suggested that the quality of the amendments was related to these trends as lignin contents were high for sawdust and FYM (30%) and low for straw (15%).

However, Paustian et al.'s (1992) study also showed that green manure had only 6% lignin but had higher C accumulation compared with straw. In turn, this was related to higher crop productivity and returned inputs due to the higher N content supplied by green manure.

Figure 2: Effect of amendment carbon input rate and type on soil C accumulation (0-20cm) in a 30 year old Swedish field experiment (redrawn from Paustian et al., 1992).

When FYM additions ceased, SOC content immediately started to decline; however, even 104 years after the last addition, the plot contained more SOC than the control plot. The rapid decline together with the levelling off at levels higher than the control plot was attributed to the initial rapid loss of labile carbohydrate material and the increased level of long-term stabilised humic material.

Figure 3: Changes in SOC content on the Hoosefield continuous barley experiment with no fertiliser applied (control), annual application of NPK fertiliser, annual application of FYM (35 t ha^-1 ) and FYM applied from 1852-1871 (modified from Haynes and Naidu, 1998).

The importance of examining threshold values at which organic carbon becomes effective and asserts a positive influence on soil properties should not be underestimated, as detrimental effects can occur if too much carbon is added to the soil. Therefore, although carbon increase is usually helpful to improve soil functions (especially in Australian soils, which are poor in carbon), more is not always better.

For example, too much carbon can result in surface crusting, increased detachment by raindrops and decreased hydraulic conductivity (Haynes and Naidu, 1998). One reason for structural breakdown is a high content of monovalent cations, which can occur if too much animal waste is added.

Similarly, high additions of NH₄⁺ fertiliser may accumulate and both high organic and N additions could cause not only environmental problems but would contribute to increased dispersive effects (summarised in Haynes and Naidu, 1998). As a rule of thumb, waste applications of over 100 t ha^-1 are considered a possible hazard (Haynes and Naidu, 1998). Water-repellencey is another possible consequence of too much organic matter application (Olsen et al., 1970).

It is important to note, however, that alkyl carbon is a major contributor to water-repellent attributes and it is therefore possible that water repellent soils do not contain particularly high amounts of organic matter but are rather dominated by alkyl carbon (Shepherd et al., 2001).

Overview of principal functions of SOM in soils

The functions of SOM can be broadly classified into three groups: biological, physical and chemical (Figure 4). These groups are not static entities and dynamic interactions occur between these three major components.

Figure 4: Functions ascribed to SOM. Note that interactions occur between the different soil functions (modified from Baldock and Skjemstad, 1999).

It is these interactions among the soil functions, the different requirements for optimal SOM levels for each function and the individual soil mineralogical characteristics that preclude a generic number for optimal SOM levels. Furthermore, SOM is a highly heterogenous substance and varies in its chemical and physical properties, depending on the soil forming factors listed previously.

SOC requirements are likely to differ according to function and soil type. Figure 5 illustrates how soil type (represented by clay content) relates to requirements of SOC to perform specific functions.

For example, for CEC SOC is of greater importance in sandy compared with clayey soils. SOC is required in larger amounts in sandy soils because most clayey soils can provide a substantial proportion of CEC through charge derived from clay minerals. For biological (energy for biological processes and provision of nutrients) and thermal properties, SOC is required irrespective of clay content. Baldock and Skjemstad (1999) and Skjemstad (2002) noted that total SOC may not be a good indicator for assessing how well a particular soil function is likely to perform; mainly because the different pools, which make up the bulk SOC, vary considerably in their physical and chemical properties.

By comparison, POC is most important in providing energy for biological processes and humus is an important source of essential soil nutrients. Soil thermal properties (i.e. the ability to warm up quickly in cold climates) are a function of colour, and the inert carbon pool, which consists of highly aromatic structures such as charcoal, plays the most important role here.

Figure 5: The optimal expression of each SOM function requires different proportions of soil organic carbon pools (soluble, particulate, humus and inert). To which degree SOM can influence a particular function may also vary by soil type (represented by clay content).

Glossary from NRCS:

Soil Organic Matter (SOM) :     The total organic matter in the soil. It can be divided into three general pools: living biomass of microorganisms, fresh and partially decomposed residues (the active fraction), and the well-decomposed and highly stable organic material. Surface litter is generally not included as part of soil organic matter.
Organic Matter, Active Fraction:     The highly dynamic or labile portion of soil organic matter that is readily available to soil organisms. May also include the living biomass. Particulate organic matter (POM) and light fraction (LF) are measurable indicators of the active fraction. POM particles are larger than other SOM and can be separated from soil by sieving. LF particles are lighter than other SOM and can be separated from soil by centrifugation.
Organic Matter, Stabilized Organic Matter:     The pool of soil organic matter that is resistant to biological degradation because it is either physically or chemically inaccessible to microbial activity. These compounds are created through a combination of biological activity and chemical reactions in the soil. Humus is usually a synonym for stabilized organic matter, but is sometimes used to refer to all soil organic matter.
Humus or humified organic matter:     Complex organic compounds that remain after many organisms have used and transformed the original material. Humus is not readily decomposed because it is either physically protected inside of aggregates or chemically too complex to be used by most organisms. Humus is important in binding tiny soil aggregates, and improves water and nutrient holding capacity.

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