Main authors: Else K. Bünemann, Giulia Bongiorno, Zhanguo Bai, Rachel E. Creamer, Gerlinde De Deyn, Ron de Goede, Luuk Fleskens, Violette Geissen, Thom W. Kuyper, Paul Mäder, Mirjam Pulleman, Wijnand Sukkel, Jan Willem van Groenigen and Lijbert Brussaard
Editor: Jane Brandt
Source document: Bünemann, E. K. et al. (2018) Soil quality - A critical review. Soil Biology and Biochemistry, Volume 120, May 2018, pp 105-125

Soil fertility, land quality, soil capability, soil quality and soil health

Various forms of soil assessment are encapsulated in different concepts. Apart from mining minerals, the main interest in soil has traditionally been in its potential for agricultural production. Assessments of the suitability of soil for crop growth may have been made even before the evidence of written records. Documentation can be found in ancient Chinese books such as “Yugong” and “Zhouli”, written during the Xia (2070-1600 BC) and Zhou (1048-256 BC) dynasty, respectively (Harrison et al., 2010), and in the work of Roman authors such as Columella (Warkentin, 1995). Ethnopedology also provides several examples of indigenous soil classifications that focus on indicators that allow judgement of the suitability of particular soils for various crops (e.g., Barrera-Bassols and Zinck, 2003). The suitability of soil for agricultural production is captured in the concept of soil fertility, originating from the German literature on “Bodenfruchtbarkeit” that is predominantly aligned to crop yields (Patzel et al., 2000). Accordingly, the FAO describes soil fertility as “the ability of the soil to supply essential plant nutrients and soil water in adequate amounts and proportions for plant growth and reproduction in the absence of toxic substances which may inhibit plant growth” (»www.fao.org). Mäder et al. (2002) extend that scope in proposing that a fertile soil “provides essential nutrients for crop plant growth, supports a diverse and active biotic community, exhibits a typical soil structure and allows for an undisturbed decomposition”. Nevertheless, the concept of soil fertility is generally operationalized chemically and partly physically in terms of the provision to crops of nutrients and water only.

To address physical and/or biological characteristics of soil, other concepts are more commonly used. One of the earliest is land quality, which integrates characteristics of soil, water, climate, topography and vegetation (Carter et al., 1997; Dumanski and Pieri, 2000) in the context of land evaluation, which aims to assess the use potential of land, based on its attributes (Rossiter, 1996). An early comprehensive elaboration of the concept is the FAO Framework for Land Evaluation (FAO, 1976). Soil survey is part of land quality assessment for land evaluation. It is done once or only repeated over large time intervals, relying heavily on field observations, supplemented with very few measured parameters (Huber et al., 2001). Land evaluation anticipates decisions on the optimal allocation of land for various uses and is, hence, the first step to sustainable land management. In countries with low population densities, the main purpose of land evaluation in the past was to identify fertile land for agricultural production, whereas in more densely populated regions such as Europe it was more targeted at identifying deficient factors in agriculture that could be remedied, in particular by manuring (van Diepen et al., 1991). However, land evaluation has also been used as part of a strategy to assess broader land use options (van Latesteijn, 1995). Similarly, soil capability, i.e. the intrinsic capacity of a soil to contribute to ecosystem services (Bouma et al., 2017), provides a neutral assessment of what soils can do and how their potential can be reached.

Since Mausel (1971) introduced the term soil quality, it has sometimes been used in the context of land quality and land evaluation (e.g. Eswaran et al., 1997). Whereas land quality and land evaluation primarily address the inherent soil properties that do not change easily and are often assessed for the entire profile, soil quality is more focused on the dynamic soil properties that can be strongly influenced by management and are mainly monitored in the surface horizon (0-25 cm) of the soil (Karlen et al., 2003). However, when studying direct impacts of soil quality on water quality it is imperative that inherent soil properties in deeper parts of the soil profile are included in the assessment.

Typically, the concept of soil quality is considered to transcend the productivity of soils (Larson and Pierce, 1991; Parr et al., 1992) to explicitly include the interactions between humans and soil, and to encompass ecosystem sustainability as the basis for the benefits that humans derive from soils as well as the intrinsic values of soil as being irreplaceable and unique (Carter et al., 1997). The term soil quality in this broader sense was already used by Warkentin and Fletcher (1977). Recently, soil quality assessment is increasingly incorporated in land evaluation, as land evaluation procedures are now used in many different ways and for a range of purposes, including sustainable land management (Hurni et al., 2015), environmental risk assessments, monitoring of environmental change (Sonneveld et al., 2010) and land restoration (Schwilch et al., 2012). In the land-potential knowledge system LandPKS, general management options are based on long-term land potential (depending on climate, topography and inherent soil properties) and can be modified according to weather conditions and dynamic soil properties (Herrick et al., 2016). The integration of soil quality and land evaluation goes as far as developing soil natural capital accounting systems, stressing the importance of soils for human wellbeing (Robinson et al., 2017).

In a program to assess and monitor soil quality in Canada (Acton and Gregorich, 1995), the term soil quality was used interchangeably with soil health and, in spite of the wider context in which it was presented, defined primarily from an agricultural perspective as “the soil's fitness to support crop growth without becoming degraded or otherwise harming the environment”. The term soil health originates from the observation that soil quality influences the health of animals and humans via the quality of crops (e.g. Warkentin, 1995). Indeed, linkages to plant health are common, as in the case of disease-suppressive soils (Almario et al., 2014). Soil health has also been illustrated via the analogy to the health of an organism or a community (Doran and Parkin, 1994; Larson and Pierce, 1991).

The debate about soil quality vs. soil health arose quickly after the concept of soil quality was criticized in the 1990s. In contrast to soil quality, soil health would “capture the ecological attributes of the soil which have implications beyond its quality or capacity to produce a particular crop. These attributes are chiefly those associated with the soil biota; its biodiversity, its food web structure, its activity and the range of functions it performs” (Pankhurst et al., 1997). These authors further consider “that the term soil health encompasses the living and dynamic nature of soil, and that this differentiates it from soil quality”. They therefore “adopt the view that although the concepts of soil quality and soil health overlap to a major degree and that in many instances the two terms are used synonymously (....), soil quality focuses more on the soil’s capacity to meet defined human needs such as the growth of a particular crop, whilst soil health focuses more on the soil’s continued capacity to sustain plant growth and maintain its functions”. Meanwhile, the debate subsided and partly changed focus. For example, Moebius-Clune et al. (2016) consider that soil quality includes both inherent and dynamic soil properties, and that soil health is equivalent to dynamic soil quality. The differential usage may also link to the observation of Romig et al. (1996), that, whereas soil quality is the preferred term of researchers, soil health is often preferred by farmers.

The differences between land quality and soil quality observed by Karlen et al. (2003) and between soil quality and soil health observed by Pankhurst et al. (1997) and Moebius-Clune et al. (2016) can be summarized in a transition in focus from land quality to soil quality and soil health going from inherent to dynamic soil properties. The website of the »Natural Resources Conservation Service, USA  states that “soil health, also referred to as soil quality, is defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”.

Soil quality vs soil health

We conclude that the distinction between soil quality and soil health developed from a matter of principle to a matter of preference and we therefore consider the terms equivalent.

We further express this by explicitly including the soil biota/biodiversity and related soil functions and soil-based ecosystem services in the figures in this section of iSQAPERiS.

Like in land quality assessment and land evaluation, approaches to soil quality and soil health go beyond the reductionist approach of measuring (indicators of) soil properties and processes. Although such measurements remain important from a practical perspective (Kibblewhite et al., 2008a), the concepts of soil quality and soil health also include the capacity for emergent system properties such as the self-organization of soils, e.g. feedbacks between soil organisms and soil structure (Lavelle et al., 2006), and the adaptability to changing conditions.

Linking soil quality to soil functions and ecosystem services

Ecosystem services are defined as “the benefits which humans derive from ecosystems” (Costanza et al., 1997). With the early concept developed by Doran and Safely (1997), soil quality was addressing not only one ecosystem service such as provision of food, but also trying to represent and balance the multi-functionality of soil. This has recently been further embedded in the development of “functional land management”, which assesses both the benefits and trade-offs of a multifunctional system for managing soil-based ecosystem services in agriculture (Schulte et al., 2014) and a wider range of land uses (Coyle et al., 2016).

Among scientists, the concept of ecosystem services is often used in connection with the concept of soil functions. ‘Function’ is, however, variably used as a synonym for

  1. process,
  2. functioning,
  3. role and
  4. service

(Baveye et al., 2016; Glenk et al., 2012). Therefore, Schwilch et al. (2016) advise against using the term, but Baveye et al. (2016) note that function “in a narrow and well-defined context (...) has been used in connection with soils for over 50 years, and has served as a conceptual foundation for an appreciable body of research and significant policy making, at least in Europe” (e.g., the Soil Thematic Strategy of the European Commission, 2006). Therefore, we concur with Baveye et al. (2016) that “it makes sense to try to retain both “function” and “service” terminologies, as long as they can be articulated (...) with respect to soil properties and processes”. In their seminal paper reconstructing how the notion that nature meets, or gets in the way, of the needs of people has pervaded concepts and theory in ecology vs. soil science, Baveye et al. (2016) argue that mainstream ecology, by its emphasis on organisms, tended to neglect the soil, in particular the non-living soil, whereas mainstream soil science tended to avoid the term ecosystem, emphasizing the importance of soil properties and processes in landscape terms.

Soil function

In accordance with Glenk et al. (2012), we define soil functions as (bundles of) soil processes that underpin the delivery of ecosystem services.

This definition will suffice for all practical purposes related to manageable soil functions, which can be used to address the gap between “what is” and “what can be”, based on soil capability , i.e. “what soils can do” (Bouma et al., 2017), which is, in the context of this review, what living soils can do. Complementary to this bottom-up approach, soil functions can be used in a top-down approach when identifying the gap between what is currently measured in soil assessment schemes and what should be measured in view of assessing the soil functions that are impacted by, or to be managed in view of current and upcoming policies (van Leeuwen et al., 2017), possibly through the use of environmental accounting systems increasingly adopted by policymakers, such as the soil natural capital accounting system proposed by Robinson et al. (2017).

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Figure 2

Just as ecosystem services are influenced by (bundles of) soil processes, the latter are in turn affected by soil threats. The EU Soil Thematic Strategy identified the main threats to soil quality in Europe as soil erosion, organic matter decline, contamination, sealing, compaction, soil biodiversity loss, salinization, flooding and landslides (European Commission, 2002; Montanarella, 2002). Soil threats have been emphasized in order to inform risk assessment exercises indicating (geographical) areas where soil functioning is potentially hampered (van Beek et al., 2010). Different schemes linking soil-based ecosystem services and soil functions have been developed (Haygarth and Ritz, 2009; Kibblewhite et al., 2008a; Tóth et al., 2013), but none of them includes soil threats. The scheme presented by Kibblewhite et al. (2008a) and modified by Brussaard (2012) was developed as a conceptual basis for the iSQAPER project, including soil threats as affecting the various soil functions and associated ecosystem services (Figure 2). The soil functions in Figure 2 equate almost entirely to the “intermediate services” defined by Bennett et al. (2010), which are similar to the soil processes presented by Schwilch et al. (2016). The ecosystem services in this scheme can be seen as a soil-related sub-set of the ecosystem services mentioned in the Common International Classification of Ecosystem Services (»CICES), currently elaborated in the »Mapping and Assessment of Soil Ecosystems and their Services (MAES-Soil) Pilot project.

It has been argued that soil quality can indeed only be assessed in relation to one or several soil functions, ecosystem services or soil threats (e.g. Baveye et al., 2016; Bouma, 2014; Sojka and Upchurch, 1999; Volchko et al., 2013). Therefore, clear definitions of these terms as well as firmly established associations with soil quality indicators are the basis of any functional soil quality concept.

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Figure 3

 

As soil quality plays a role in decision-making in the face of soil threats, the DPSIR (driver–pressure–state–impact–response) framework (European Environmen tAgency, 1998) has frequently been adopted for use in EU policy to support decision-making and as a means to bridge the science-policy gap (Tscherning et al., 2012).

DPSIR

Applying the DPSIR framework to soil, “drivers” are pedoclimatic conditions and land use policies, while “pressures” are land use and management and the associated soil threats.

Pressures and drivers and their variabilities and interactions determine the “state” of the soil, with subsequent “impact” on soil and ecosystem functioning, and the “response” in terms of the delivery of ecosystem goods and services (Figure 3). Subsequent adaptive management may be re-active to observed deterioration of soil functioning or pro-active to reach transitions to newly desired soil functioning. To assess any changes in the status of soil quality, assessment tools are needed, and these are the subject of »Approaches to soil quality assessment and »Soil quality indicators.


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