Farm Practice and Soil Health

Summary

Soil health or quality has become of increasing interest, both to farmers concerned with sustainability of production, and to a wider community of environmentalists, water managers and the general public, concerned about water quality, food safety and general environmental degradation. Though originating with the desire to protect soils from specific degradation threats, such as erosion, salinization and pollution by heavy metals and persistent organic pollutants, through the 1980s and 1990s interest in soil quality developed along more holistic lines, viewing soils as important ecosystems in their own right, providing a multitude very valuable ecosystem services. This led to the development of the concept of protecting soil functions, rather than protecting soils from individual threats. This development in thinking about soils is mirrored in the development of soil monitoring and management around the world. Up until the 1990’s, monitoring of soils by national and regional government was focused very much on soil mapping and monitoring levels of pollutants, while farmers focused mainly on levels of major nutrient elements and pH, with the aim of maximising production. In the last decade soil monitoring has broadened in scope, with the acceptance of the idea of soil quality as a concept. New monitoring schemes have been established in many countries,measuring physical and biological indicators of soil quality as well as chemical and geochemical indicators, while national and regional government along with farmers in many cases, have sought to develop simple tools to measure soil quality in the field,encompassing the idea that soil quality for farmers goes beyond simple measures of chemical fertility. This explosion in interest in and concern for soils has not however, always gone hand in hand with a commitment to fundingsoil monitoring and management. As a result, many soil quality monitoring programs set up in the 1990s have either been downgraded or ceased operation all together. In some cases this may have been the result of overambitious programs or an exaggeration of the threats posed to soils or what soil monitoring can deliver. Though in some cases it has been a simply lack of commitment from funding bodies. Likewise, some of the monitoring and management development that was aimed at farmers has been met with less than overwhelming enthusiasm, at least in some cases because of a lack of connection between soil quality as measured, and crop yields, the primary focus of most farmers. Some of the problems that have arisen with the new wave of soil quality monitoring and management can be attributed to the fact that our knowledge of soils and the operation of their functions is far from complete, while some authors still question the usefulness of the soil quality concept as a management tool at all. Nevertheless, the concept of soil quality is now firmly established within the psyche of farmers, land managers and policy makers alike and the need to protect soils and their functions from degradation has never been greater.

1 Introduction

1.1 Development of the soil quality concept.

Interest in soils as a medium for crop growth goes back into ancient history,with Roman texts on agricultural husbandry being copied and recopied right up to the sixteenth and seventeenth centuries (Wild, 1988). Traditional management of soils in temperate agriculture was based on rotations of arable crops with grass leys that improved soil structure and increased fertility, withan important part played by animalmanures and in some cases legumes. After World War II this traditional system declined, with increasing separation of animal husbandry and arable production, leading to the elimination both of grass leys and application of animal manure in many arable systems. Management of soils per se was neglected, leading to increasing concerns about physical deterioration, evident in the Strutt report ‘Modern farming and the soil’ (MAFF, 1971). Although this report dismissed some of the fears about ‘modern’ farming, concern about soil physical condition, particularly evident in soil erosion (Morgan1981) and new concerns over nutrient leaching (Ryden et al 1984) remained through the 1970’s and 1980’s.Parallel concerns were also being voiced overseas, particularly in Canada and the USA (Ketcheson, 1980, Saini and Grant 1980), where soil conservation measures dating back to the ‘dust bowl’ era of the 1930s remained largely confined to reducing erosion. Increasingrecognition about the broader role of soils within the environment and concern over possible deterioration in their ability to fulfil these roles was reflected in the appearance of ‘soil quality’ as an issue rather than soil management being confined to management of specific problems. The concept of soil quality as a definable term within soil science can be traced back to the 1970s. Warkentin and Fletcher (1977) suggested development of a soil quality concept to encompass the fact that:

  1. soil resources are constantly being evaluated for many different uses;
  2. multiple stakeholder groups are concerned about soil resources;
  3. society’s priorities and demands on soil resources are changing;
  4. soil resource and land use decisions are made in a human or institutional context.

This was a remarkably prescient concept in view of more recent developments, yet the idea languished through most of the 1980s,as the primary concern remained with soil protection, particularly from erosion or contamination. By the early 1990s however, the concept had been revived, notably in a report from Canada's Senate Committee on Agriculture, Fisheries and Forestry "Soil at Risk - Canada's Eroding Future", which was a response to increasing concern about wind and water erosion, and salinization on the prairies. Larson and Pierce (1991, 1994) further developed the concept in the scientific literature, but interest in the concept, particularly in the USA, really took off after the publication by the US National Research Council, of “Soil and Water Quality: An Agenda for Agriculture” (NRC 1993), which had itself arisen from concerns about soil erosion contained within the 1985 Food Security Act (PL 99-198; also known as the 1985 farm bill). This report stated that “Protecting soil quality, like protecting air and water quality, should be a fundamental goal of national environmental policy.”It went further, criticising the emphasis on controlling erosion and conserving productivity.

“Erosion is not the only……threat to soil quality. Salinization and compaction are important and often irreversible processes of soil degradation. More important, erosion, salinization, compaction, acidification and loss of biological activity interact to accelerate soil degradation.”

The report went on to call for comprehensive policies to address these threats. A number of conferences and symposia were held in the early 1990s concerned with the concept of soil quality and the concept was wholeheartedly embraced in Canada and particularly the USA. The Soil Science Society of America established a special committee charged with developing the concept in 1994 and by the mid 1990s soil quality testing kits, charts and literature had been developed for field use by the farmer, culminating in 1999 in production of the 'Soil Quality Thunderbook' (USDA, 1999), revised in 2004, containing not only instructions for simple soil quality testing, but also comprehensive information on soil. In Canada the federal government set up the Canadian Soil Quality Valuation Program (SQEP) in 1990, which in 1995 produced a wide ranging report “The Health of Our Soils” (Acton and Gregorich, 1995).

In the UK a similar process has taken place, with forward thinking reports published early on in the debate about soil quality followed by long periods of inaction and eventual recognition of the problem. The first report to deal with the problem of soil quality was the Department of the Environments “An assessment of the principles of soil protection in the UK” (DoE 1989), which included recommendations to establish a system of assessment and monitoring of soils and establishment of target values for soil factors. Further reports by variousUK agencies dealing with soils followed,but they coincided with the end of the post World War II period of expanding agricultural production. Large surpluses of many agricultural commodities had led to declining government support for agriculture across Europe and with soil research tied up closely with agriculture a decline in soil research. As a result, even as late as 1996 Sir John Houghton felt it necessary to highlight the neglect of soils at a policy level and called for ‘clear policies to protect this important resource’ in the Royal Commission Report, ‘Sustainable Use of Soil’.Still, even in 2000, when MAFF published ‘Towards Sustainable Agriculture: a pilot set of indicators’ (MAFF 2000), only two soil indicators, soil organic matter (SOM) and heavy metals were included as indicators of soil sustainability.

1.2 What IS soil quality?

One problem with soil quality or soil health (see Box 1) as a concept, which may have contributed to the failure to address it in policy terms, is its broad, ill defined nature. Soil erosion, contamination by heavy metals and even nutrient leaching are relatively simple concepts, they can be relatively easily quantified, their effects on soils and the wider environment can be measured and as such measurable outcomes arising from management actions to mitigate these problems can be determined.Soil quality is not so simple, even to define. Any definition of soil quality must encompass the requirements of all stakeholders. However, needs of different but who are stakeholders are not always easy to identify and may conflict. Focusing on one stakeholder group is therefore likely to constrain the definition. For example, to a farmer soil quality may be defined by a small number of factors, such as crop yield, soil erosion and nutrient concentrations. The Canadian report “The Health of Our Soils” (Acton and Gregorich, 1995) betrays such a focus on agroecosytems with its narrow definition:

“Soil health or quality is the soil's fitness to support crop growth without resulting in soil degradation or otherwise harming the environment.”

For wider policy issues, encompassing many stakeholders, a broader definition is required. The Soil Science Society of America after much discussion about wording came up with such a broad definition:

“the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality and support human health and habitation” (Karlen et al., 1997)

Although the definition of Larson and Pierce (1991)

“the capacity of a soil to function, both within its ecosystem boundaries and with the environment external to that ecosystem”,

which was reduced in 1993 to just

"Simply put: Fitness for use." (Pierce and Larson, 1993).

is attractive for its all encompassing simplicity. Further definitions have been produced by different groups and authors (e.g. Doran and Parkin 1994) though most are generally very similar. However,some authors have sought to divide soil quality into two facets inherent quality and soil condition or healthsometimes referred to as dynamic quality. Inherent quality results from innate properties (qualities) of soils; as determined by the factors of soil formation.These include factors such as texture and are relatively stable in the face of normal management practices. Dynamic quality results from the changing nature (health or condition) of soil properties. These include aspects such as organic matter and are influenced by use and management to a significant degree. Inherent quality is used to evaluate the worth or suitability of a soil for specific uses, such as crop production, forestry or waste disposal. Inherent quality can also be used to compare the abilities of one soil against another. Dynamic quality is the “the ability of the soil to perform according to its potential” and is the focal point for assessing and maintaining soil resources through management. This division of soil quality into inherent and dynamic has gained favour in some quarters (e.g. Karlen et al., 1997, 2001, and is now common in the both the scientific and non scientific literature.

As the concept of soil quality has developed to encompass more than merely agronomic aspects, there has been a move away from protection of soils from defined threats, such as erosion and contamination, to protection of soil functions. This so called ‘functional approach’ allows multiple interests to be accommodated within soils policy (Karlen et al 1997), though this is not a trivial exercise. For example, water infiltration and storage is a function of soils that will impact on a number of factors, including erosion, surface and ground water quality and crop growth. Each will impact different stakeholders in different ways, but their issues all relate to the ability of the soil to absorb and store water, a functional attribute. However, the functions required from soils will vary from one stakeholder to another and are not fully agreed upon.Often the functions that receive the greatest weight are those related to crop production. Perhaps the most all encompassing definition of soil function came from Blum (1993).

  • a medium for biomass production: food crops, animal production, timber and wood products;
  • an interface between the atmosphere and water resources in terms of the soils' ability to buffer and filter;
  • a bio-geochemical reactor or regulator to facilitate the re-cycling of (largely organic) wastes and to de-toxify organic chemicals;
  • supporting the built environment;
  • a reservoir and source of biodiversity;

Managing soils to protect their functions is,despite its problems, now widely accepted (Doran et al 1996, Karlen et al 1997 Kirchmann and Andersson 2001) and has generally replaced the soil protection approach, although this is still an important part of soil policy and many schemes remain in use, such as The Sludge (Use in Agriculture) (Amendment) Regulations 1990 (DEFRA 1990). The functional approach is accepted at policy level as evidenced in the EU Thematic Strategy for Soil Protection “Soil is essentially a non-renewable resource and a very dynamic system which performs many functions and delivers services vital to human activities and ecosystems survival.” (Directive proposal (COM(2006) 232) (EU 2006)).Of course the more functions that are included the more complex any soil quality assessment becomes as do the management responses to such assessments.

1.3 Criticism of the concept

Although the concept of soil quality is now common currency within both scientific and policy spheres, it is by no means universally accepted as a good thing. Criticism has been particularly strong in North America, where the concept has been most strongly embraced and policy decisions based upon it implemented. Most notable criticism comes from Sojka and Upchurch (1999), Sojka et al (2003) and a group of twelve distinguished authors, mostly in the USA (Letey et al 2003). Though others have also been critical of the wide acceptance of the concept before it has been demonstrated to be useful or even measurable (Wander and Drinkwater 2000), with Sanchez et al (2003) making a particularly strong attack, calling the soil quality concept “politically and socially attractive but scientifically misleading”. The criticisms focus on a number of areas, some of which are of most relevanceto the development of the concept in the USA, some of which are relevant to all systems or potential systems of soil assessment and management based on the soil quality concept.

The main criticisms include the failure of soil quality assessment to adequately encompass all the competing requirements for high quality, simultaneously (Sojka et al 2003). For example, for crop production a high quality soil requires high nutrient status, but this may conflict with water quality issues. Although Karlen et al. (2001) attempt to defend the soil quality concept, stating that “Inherent differences among soils and differences in land use are two main reasons that stakeholders must specify the type of soil functions that are critical for the assessment” they fail to acknowledge that soils may have to fulfil very different functions simultaneously. They go on to state that “there can be no single value or expression that describes soil quality for all uses”, the conclusion must be that there must be different assessments for each use on each soil and then a value judgement to decide which function takes priority, the complexity involved in achieving this is obvious. Another criticism is that soil quality always encompasses biological quality, which is often based around biodiversity. However, methodology for measuring biological factors, never mind interpreting them, is very poorly developed beyond simple assessments such as earthworm numbers.

In the USA, development of the soil quality concept along quite narrow lines; soil quality test kits, scorecards and leaflets to rate soil quality in agricultural systems, has led to criticisms including too narrow an emphasis, particularly as development of these assessment systems has occurred on a narrow range of soil types and so may not be at all appropriate for the full (large) range of soils and climates occurring in the USA. Soil quality tests used in the USAoften rely on highly dynamic soil processes, such as soil respiration and highly subjective aspects such as smell, the appropriateness of which is in question (Sojka and Upchurch 1999). Furthermore, while traditional soil tests for nutrients have relatively well researched quantifiable cost benefit data for crop production, there is no such information available for many soil quality attributes. Soil quality indices such as those espoused by Doran and Parkin (1996) are easy to devise with but very difficult to interpret.