2. Terminology and conceptual background

Back to: Soil health for Victoria's agriculture - context, terminology and concepts

2.1 Soil quality and soil health | 2.2 Ecosystem Services | 2.3 Functions, Processes, Attributes and Indicators | 2.4 Indicator, invex and monitoring

The terms used to describe soils and the environment range from those with precise scientific meaning to those that are more or less symbolic and defy precise definition. Consequently, the meanings for terms that fall more into a symbolic category require some discussion and, where possible, some boundaries with respect to the usage in this report. Without trying to be exhaustive, the following section provides discussion to put the terms into context and clarify their use. Further, more detailed, analysis of their application and relationships can be found in later sections of this report.

2.1 Soil quality and soil health
The principal terms that will be found in the literature and in this report are ‘soil quality’ and ‘soil health’. While there are many definitions in the literature, there is no argument that both terms refer to the capacity of soil to perform the functions that are demanded of it. Hence good or high quality soils and, or, healthy soils are those that perform well and are not incapacitated in any respect. In contrast, poor quality soils and those in ‘poor health’ exhibit various dysfunctional attributes such as erosion, retarded or zero plant growth, and other problems. ‘Poor’ performance of soils is a relative judgement made according to the expectations that we have of soil, thus soil quality and soil health are judged with respect to their “fitness for use”. This concept, ‘fitness for use’, is undoubtedly the simplest representation of the meanings behind soil quality and soil health.

Soil quality, by which differences between soils are recognised, has been used as a concept for a much longer period than soil health. For example, soil quality concepts underlie rationales used in determining land capability (Klingebiel and Montgomery 1961; FAO 1976) and have existed in the literature for two millennia. Virgil (70–19 BC) wrote:

Virgil even went so far as to recommend methods for determining soil quality:

‘Quality’ and ‘Health’ are terms that convey relative meanings, and must therefore be related to some baseline measurement and purpose (or function).
Whereas air and water ‘quality’ can be measured with respect to their absolute purity, and ‘healthy’ air and ‘healthy’ water have meaning with respect to supporting or threatening life, the case for soil is not so simple or clear. There is no such entity as ‘pure soil’ and the problem is compounded by the fact that air and water are constituents of soil. The quality of the air and water in soil has importance for life in and above the soil, so interpreted measurements of these individual constituents, along with many others, are important if we want to rate soil health.

Soil is highly complex and should in many respects be regarded as a living system in much the same way as an ecosystem is understood. There are therefore considerable challenges to the development of integrated assessment tools that are simple to apply and have useful meaning for managing or monitoring soil health. For these reasons and others some authors have refuted the whole notion of soil quality (and soil health) as having any value at all (Sojka and Upchurch 1999).

Because both terms, ‘soil quality’ and ‘soil health’, are in common use, some definitions and distinctions between the terms are adopted here.

Carter et al. (1997) have discussed concepts of soil quality in detail and suggest that the definitions developed by the Soil Science Society of America (1995) and Doran et al. (1996) are comprehensive and well accepted:

and,

The term ‘soil health’ has been used in many instances as synonymous with, and to replace, ‘soil quality’, in particular for dialogue with non–scientists. However, for some, the emphasis on ‘health’ has parallels with living organisms and supports the notion of soil as a living system. Consequently, much of the discussion around soil health has concerned the biological aspects of soil and there has been an increasing focus on organic and biological farming systems as the route to achieving better soil health. Doran et al. (1996) emphasise the living aspects of soil and its sustainability in their proposed definition for soil health:

For the purposes of the Victorian soil health work, we should adopt a balanced approach that acknowledges the importance of biological, chemical and physical aspects of soil as well as the landscape and catchment context and the goals of land managers.

Additional context for the use of the term ‘health’ rather than ‘quality’ is provided by the way in which government has responded to requirements for reporting on the state of the environment. For example the Victorian Government’s report ‘The Health of our Catchments’ (VCMC 2002) provides a snapshot of catchment condition using a range of environmental indicators.

Simplistically, health is a changing state whereas quality more closely relates to the inherent properties of the entity. Just as a cereal variety may intrinsically be good quality, the plants of a particular crop of that variety may exhibit poor health due to a stressful environment and will result in poor grain quality and, potentially, poor animal or human health. The distinction between quality, as inherent, and health, as apparent and dynamic, is a useful one. A further distinction might be made in that soil quality provides a useful paradigm for comparison between different soils, whereas soil health is more concerned with the state of a particular soil at any one time.

We recognise that there is considerable overlap between the terms, but, simply stated, the terms ‘soil quality’ and ‘soil health’ used in this report are distinguished as follows:

Soil quality is the capacity of soils within landscapes to sustain biological productivity, maintain environmental quality, and promote plant and animal health.

Soil health is the condition of the soil in relation to its inherent, or potential capability, to sustain biological productivity, maintain environmental quality, and promote plant and animal health.

Attributes that can be measured to indicate the functional performance or capacity of a soil will be the same whether we are talking about soil quality or soil health. Soil health simply has an added dimension that represents the state of the soil with respect to a scale that would encompass optimal performance (for a stated purpose) as well as degradation.

Soil health is therefore a more practical term that can be used, as an indicator of soil sustainability, when talking to managers of soil. There are widely differing philosophies with respect to what actually comprises a healthy soil and these range from the view that anything ‘artificial’ added to soil compromises soil health, to the view that healthy soils should be biologically sterile (for example for root–borne disease management in intensive vegetable production). However, each of these views is formed by the manager’s goals, their understanding of the soil system, and the management techniques available to them. In a broader context, ‘healthy’ or prospering business is the manager’s goal and soil management (or health) is one aspect of their business. The degree to which a biologically, self–sustaining healthy soil is required as a foundation for a healthy business is currently hotly debated, particularly by the organic and biological farming sector. Economic evaluation of the functions performed by soil and the relative costs to maintain those functions under different management regimes is needed. This also requires additional science in order to understand the interactions between the soil system and the farming system, particularly where the system is chemically, physically and biologically engineered in order to sustain production.

Resilience and resistance are terms found in the literature concerning ecological systems since the 1960s and 70s and more recently in reference to soil quality and soil health (Blum and Santelises 1994). These are terms for which there is also some overlap and ambiguity. For the purposes of the soil health work the definitions provided by Seybold, Herrick and Brejda (1999) are recommended:

Soil resistance is the capacity of the soil system to continue to function without change throughout a disturbance.

Soil resilience is the capacity of a soil to recover its structural and functional integrity after disturbance.

2.2 Ecosystem Services
The language and objectives in natural resource management (NRM) have recently become more holistically focussed than in the past. The notion of ‘ecosystem service’ provision now has currency as the major criterion for assessing the functional worth or capacity of the natural and the managed environment. This is in contrast to the more reactionary alternative approach in NRM that has been more focussed on negative aspects associated with environmental degradation. In the case of soils we can list issues such as erosion, acidification, structure decline, and salinisation as serious degradation issues (which they are). There are many programs and initiatives that have been, and are being, implemented to address these issues. However, the underlying reasons for dealing with them have always been positive, for example, that by addressing these issues, land management will be more productive and sustainable and water quality will be improved. The ecosystem service concept provides an integrated positive driver for NRM just as the soil health concept provides an integrated (albeit symbolic) paradigm for managing the diverse issues that arise in soil management.

The most comprehensive attempt to develop NRM priorities with respect to ecosystem services was carried out by the Goulburn Broken Catchment Management Authority and CSIRO Sustainable Ecosystems who described ecosystem services in the following way:

Soil Health was identified by Binning et al. (2001) as an ecosystem service in its own right. However, soil can be described in the context of other ecosystem services as having a functional role. For example, water filtration and regulation of river flows and groundwater levels are ecosystem services that stem in part from soil and its management. In section 6 the broader relationship between soil health and environmental health is discussed and the soil functions that support ecosystem services are summarised (Table 3).


The sense of the definitions discussed is that soil health is the capacity of the soil to support or supply ecosystem services. Linkages between soil health and ecosystem services can be quantified if the properties and processes controlling soil functions are known and are measurable. Carter et al. (1997) outlined a useful framework to evaluate soil quality that is based on the following sequence: functions, processes, attributes or properties, attribute indicators and methodology (for assessment of soil quality).

Soil Functions
Functions describe what the soil does, or is required to do, for example, support plant growth, support traffic, resist erosion. These functions belong to a suite of ecosystem services directly attributable to soil and they contribute to more global or encompassing ecosystems services such as ‘produce crops’, or ‘provide clean water’. Whilst there are many examples of soil functions given in the literature, these can be broadly classified as:

1.	Supporting life (plant growth, production, biodiversity, health)
2.	Partitioning water (flows and storages: runoff, drainage, water supply to plants, retaining water in dams)
3.	Resisting erosion (maintaining stability, evolving and sustaining plant growth)
4.	Providing physical support (anchorage for plants, wheeled traffic, animal treading, building foundations, roads and dams)
5.	Processing matter (recycling and storing nutrients, absorbing wastes, filtering water, acting as an environmental buffer).

Specific functions should be defined for any particular agro–ecosystem. An example of a soil quality approach for the dairy industry is provided in section 7.2.

Soil Processes
Processes describe what happens in the soil (fluxes of energy and matter) and determine its functional performance. A hydrological example can illustrate this. For example, water moves on and in soil and can be held in soil. This partitioning of water between runoff, infiltration, drainage and retention determines the performance of soil with respect to several functions: supplying water for plant growth, provision of clean water, resisting erosion et cetera.

Soil Attributes
Attributes are the measurable properties of the soil system or components that support or regulate processes in the soil and hence the functions of the soil. A useful distinction can be made between properties that are fixed, inherent attributes of a particular soil (e.g. texture) and properties that are changeable or dynamic (e.g. soil structure). Management of soil health is concerned with manipulating the dynamic soil properties in the context of fixed soil attributes. Attributes may be directly measured physical, chemical or biological parameters, for example, clay percentage, exchangeable sodium, number of earthworms. Attributes may also be measurable properties that are affected by other attributes. For example, saturated hydraulic conductivity (regulating the passage of water through soil) is a measurable attribute that would be affected by clay percentage (through its influence on texture and structure), by exchangeable sodium (through its affect on sodicity and soil structure), and by earthworms (through the macropores created by their burrowing activities). Attributes may also be, in some instances, estimates of properties that are based on other measured or directly observed properties, for example, hydraulic conductivity can be estimated from soil texture and descriptions of soil structure. Such estimates are referred to in the literature as Pedotransfer Functions and range from the conceptually simple to the mathematically complex.

The key to managing soil health lies in our ability to understand, measure, model and predict components in the simple hierarchical framework:

ECOSYSTEM SERVICES ≤ SOIL FUNCTIONS < SOIL PROCESSES < SOIL ATTRIBUTES

There is a deceptive simplicity to this framework which belies the natural complexity of the system.

Interactions between components and processes mean that the realities behind the framework are rarely linear or simple. Soil differences and inherent variability at pedon³, paddock, landscape and regional scales compound the problems of measurement (Wilding and Drees 1983). Sensitivity of soil properties and functions to seasonal changes (e.g. shrinking and swelling of soil as it dries or wets) and to seasonal differences (e.g. wind erosion during droughts) exacerbate the task of assessing soil health with respect to management alone and independently of other external influences.

2.4 Indicator, index and monitoring
An indicator is a measurable parameter of a system that can be used to represent the condition of the system or its ability to perform system functions. An indicator could therefore be a measure of the functional performance itself (e.g. crop production and plant health), a measure of or surrogate for a process affecting function (e.g. seasonal water use consumption and conversion to dry matter), or a time sensitive attribute of the soil (e.g. depth of water extraction, amount of stored water, pH). A good indicator is sensitive to change, easily measured, has a clearly defined and repeatable methodology, is easily interpreted (not subject to system ‘noise’) and ideally, is reversible (sensitive to improvement as well as decay). Thus pH is an example of a good soil indicator as it is sensitive to change, can be measured consistently and easily and can be related to the soil’s capacity to support plant growth.

Indicators for soil quality or soil health cover a range of physical, chemical and biological soil properties (Brussard et al. 2002; Doran, Molina and Harris 1994; Gregorich and Carter 1997; Hamblin 1998, 1999; Pankhurst 1999; MacEwan and Carter 1996; Walker and Reuter 1996). The debate has been broad but reasonable agreement appears to exist about the suite of indicators, a minimum dataset, and methods for measurement. However, baseline data are lacking and, although there is recognition of the importance of preferred indicators, there is little knowledge of thresholds or rates of change, particularly with respect to biological indicators of soil health. There is therefore a need for research in some disciplines to determine the relationships between indicator values and performance of soil functions.

An index is usually a value on a relative scale that has no meaning per se but can be used to judge system conditions comparatively in space or time. For example, students are graded by summing marks achieved in assignments and exams but the grade awarded to a student does not give an indication of what precisely the student knows or does not know. It merely indicates their overall performance relative to other students and to a pass level.

In recent years there have been many attempts to derive an index for soil health or soil quality (e.g. Andrews and Carroll 2001; Andrews et al. 2002; Barbiroli et al. 2004; Hatcher 2002; Jaenicke and Lengnick 1999; Wienhold et al. 2004; Xu et al. 2006). Combining measures into a single index that can be used as a long term monitoring aid is an attractive proposition and has been achieved to a degree in other fields. The index of stream condition (ISC), which aggregates a number of individual indicators of stream condition, is a good Victorian example (Catchment and Water Division, 2001). Such an index would appear to work well where individual indicators are related but is difficult to interpret when they are not. If the former is the case, an improvement in score always correlates with improvement in overall condition, if the latter is the case, a score improvement could be achieved if a single factor increases sufficiently to outweigh decreases in other factor conditions. Single combination indices are therefore dogged by problems associated with methods of combination of parameters (e.g. summative, divisive) and weights applied to individual factors. Such is the case with the search for a soil index.

Monitoring is the periodic repetition of measurements made on a site or population in order to track any changes that occur in condition of the system being monitored. Monitoring is necessary to provide evidence of the impacts of management and other environmental factors on soil and land condition. Soils are extremely complex systems, variable in space and time, sensitive to management (land use and associated practices) and to weather conditions. McKenzie, Henderson and McDonald (2002) have comprehensively summarised the issues associated with soil monitoring, given sound recommendations with respect to implementation in Australia and stressed the need to define the purpose for monitoring in contrasting two purposes:

1.	reducing risk in decision making
2.	improving process understanding.

Monitoring must be given context. Selection of monitoring parameters (indicators) and sites requires recognition of spatial relevance (land and land use that each monitoring site represents) and understanding of processes that influence soil change at this site (including rate, sensitivity, reversibility).