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Health of our Soils | Index

Chapter 1 - Understanding Soil Health

D.F. Acton and L.J. Gregorich

Highlights

• Soil health is an indicator of environmental health and, like human health, provides an overall picture of the condition of many properties and processes; the terms soil health and soil quality can be used interchangeably.
• Soil health or quality is the soil's fitness to support crop growth without resulting in soil degradation or otherwise harming the environment.
• Soil quality changes slowly because of natural processes, such as weathering, and more rapidly under human activity; land use and farming practices may change soil quality for the better or for the worse.
• Soil health deteriorates mainly through erosion by wind and water, loss of organic matter, breakdown of soil structure, salinization, and chemical contamination.

Introduction

Soils are used for many purposes, such as road and building construction, waste disposal, and crop production. The most practical definitions of soil quality relate to the soil's function, or what it does. The definition of soil health commonly used by agriculturalists has emphasized soil productivity -- a healthy soil produces abundant, high-quality crops. But, over the past 10 years, our thinking about agriculture has shifted. We no longer see it as a closed operation, but rather as part of a much-broader ecological system, which interacts with, and affects other parts of, the system. We need a new definition of soil health that goes beyond productivity and connects with the environment as a whole. To that end, we define soil health for agriculture as the soil's fitness to support crop growth without resulting in soil degradation or otherwise harming the environment.

The terms soil health and soil quality can be used interchangeably. Soil health is the term commonly used by producers and by the popular press. Soil quality is the term favoured by researchers and used most often in scientific writing. Some writers have attempted to join the terms by referring to soil health/quality. We prefer to use both terms to mean the same thing in the remainder of the report.

The thin layer of soil covering the earth's surface represents the difference between survival and extinction for most terrestrial life.

John W. Doran and Timothy B. Parkin
Defining and Assessing Soil Quality

Assessing soil quality

Human health is a composite picture of the condition of the body's various parts and functions. We assess human health by looking at many factors (including physical function, mental capacity, and emotional well-being) and by forming an overall sense of how the body and mind are working. In the same way, soil quality is a composite picture of the state of the soil's many physical, chemical, and biological properties and of the processes that interact to determine this quality. Furthermore, as human health varies from person to person, soil health varies among soil types. Some soils have poor inherent (natural) quality and are not fit for crop production.

Just as we have no single measure of human health, we have no single measure of soil quality. Although soil quality cannot be measured directly, it can be inferred or estimated, both by measuring specific soil properties (such as pH or organic matter content) and by observing soil conditions (such as fertility, structure, and erodibility).

Soil researchers have recently recognized the need for a reliable and systematic way to assess soil quality. One promising possibility is to develop a soil quality index -- a report card that documents a soil's health condition and provides a way to monitor that condition over time and to predict the effects of farming practices. The index would include measurements of certain soil properties, functions, and conditions that provide useful indicators of soil quality (Producers describe soil health). Work is currently under way in several countries to identify suitable soil quality indicators and to develop efficient, reliable methods with which to measure them.

In many ways, soil quality is one measure of ecosystem health. Conserving soil quality means protecting the full range of ecological services high-quality soils provide.

Sandra S. Batie and Craig A. Cox
Soil and Water Quality:
An Agenda for Agriculture

Elements of soil quality

The three main functions of soil are to provide a medium for plant growth, to regulate and partition water flow through the environment, and to serve as an environmental buffer. A soil's chemical, physical, and biological properties work together to make a soil fit to perform these functions.

Plant growth

A good-quality soil is both tillable and fertile. It yields good-quality crops because it:

• provides a suitable medium for seed germination and root growth (including the absence of unsuitable chemical conditions, such as acidity or salinity, that are harmful to plant growth)
• supplies a balance of nutrients to plants
• receives, stores, and releases moisture for plant use
• supports a community of microorganisms that recycle nutrients through decomposition and help plants to resist disease.

Water regulation and partitioning

Water entering the soil as either rain or melting snow has several fates. It can soak into the soil, to be stored or taken up by plants. It can percolate down through the soil and enter the groundwater. If it fails to penetrate the soil, it can move along the soil surface as runoff.

Depending on the amount of precipitation received, a good-quality soil stores enough water to promote optimal crop growth. It allows only a limited amount of water either to run off the soil surface, carrying away soil sediments, or to seep below the root zone into the groundwater.

Environmental buffer

A good-quality soil can accept and hold nutrients and release them as required by plants. To some extent, it can also break down harmful compounds into substances that are nontoxic to plants and animals and do not pollute surface water and groundwater. However, soil has a limited ability to perform this function and should not be expected to repair the damage of chemical contamination caused by human activity.

Changing soil quality

The inherent or natural quality of a soil is determined by the geological materials and soil formation processes (such as chemical and physical weathering) that combine to produce it. The characteristics of a natural soil can be changed by human activities, including land use and farming practices. Decline in inherent soil quality can occur because of erosion, loss of organic matter, compaction, desertification, and other degradative processes. On the other hand, soil quality can be maintained or even improved by regularly adding organic material, using conservation tillage, rotating crops, and growing legumes, among other practices.

Effects of land use and management practices

Agricultural land use refers to the type of farming activity that takes place on an area of land. Examples of agricultural land use include pasture and cultivation of forage crops, cereals, oilseeds, berry fruits, or vegetables. Defining land use also considers whether crops are grown under natural rainfall or irrigation. In general, the more that a specific land use disturbs the land's natural ecology, the greater its effects on soil quality.

Management practices are methods that a farmer uses to tend the land, cultivate a crop, or care for livestock. On pasture lands, these practices include animal stocking, rotational grazing, weed control, and protecting vegetation along water courses. On cultivated lands, management practices include crop selection and rotation, tillage methods, residue management, traffic management, use of fertilizers and other nutritional amendments, pest control, and water management.

Crops that provide high-density and continuous ground cover offer greater protection against erosion than row-cropping systems or systems that include extensive use of cultivated fallow. Minimal tillage for weed control or seedbed preparation alters soil structure less and maintains crop residues better than more-intensive tillage. Systems that return plant nutrients at the rate of their removal by crops help maintain soil tilth and cover, which protect against erosion. Reduced use of pesticides on erodible soils or use of pesticides along with effective measures for erosion control reduces the risk of contaminating surface water. Systems that reduce the use of pesticides on highly permeable soils reduce the risk of contaminating groundwater. (Soil Quality Indicators)

Processes that reduce soil quality

Wind and water erosion, loss of organic matter, breakdown of soil structure, salinization, and chemical contamination -- all processes that affect soil quality (Fig. 1-1) -- are accelerated by inappropriate land use and management practices. These processes reduce the soil's ability to grow crops and to maintain a healthy environment.

Erosion

Erosion is a process that removes and redistributes soil. Although some erosion takes place gradually, most results from extreme weather events, such as a windstorm or a heavy rainfall.

Both wind and water erosion remove topsoil, which is the soil layer best fitted to support life. Loss of all or part of this surface layer impairs the soil's ability to produce a crop by reducing its fertility and its ability to accept and store water and air. The materials removed may be redeposited in some nearby leeward or lowlying area with little or no apparent consequence to the environment at that locale. In some areas, however, the materials are transported to streams, rivers, and lakes -- even to oceans -- which may considerably affect the water quality in these areas.

Each further loss of topsoil compounds the effects of erosion, so the soil increasingly loses its ability to produce crops and to regulate and partition the flow of water in the environment. As soil fertility declines, lost nutrients are often replaced by applying fertilizer, and the chance of nutrient loading in the runoff during subsequent erosion events increases.

Protecting the soil against erosion usually involves keeping it covered with crops or crop residues. Using methods such as conservation tillage and residue management, green manuring, continuous cropping, and winter cover-cropping helps to maintain soil cover.

Loss of organic matter

Loss of soil organic matter is usually related to the loss of topsoil through erosion. Organic matter is also lost by microbial oxidation, in which soil microorganisms use organic matter in the soil as a food source during their normal metabolism. Management practices that add little organic matter to the soil or increase the rates of organic matter decomposition (such as summerfallowing and excess tillage) lead to reduced levels of organic matter in the soil.

Because organic matter is rich in nitrogen, phosphorus, and other nutrients, loss of soil organic matter reduces a soil's fertility and its capacity to produce crops. Organic matter holds more water per unit weight than mineral matter and is needed for a well-aggregated soil structure. Its loss also reduces the soil's capacity to accept, store, and release water for plant growth.

Changes in soil structure

Changes in soil structure affect soil quality in many ways. Pore space in the soil is important as a pathway for water entering the soil, a storehouse and provider of soil water and air to the plant, and a conduit for water leaving the soil.

High-quality soils have many pores of various sizes and shapes, with varying continuity. Farming practices that increase the rate of erosion, break down soil aggregates, or reduce soil pore space (compact the soil) are the most common means of modifying soil structure. A breakdown of soil structure reduces the soil's capacity to produce crops, which in turn affects its capacity to regulate and partition water flow through the environment.

Salinization

Soil salinity (an excess of salts in the soil) reduces the soil's capacity to produce crops by restricting the amount of water the plant can withdraw from the soil. Crops respond to increasing salinity in much the same way as they do to increasing drought stress -- even though water is present in the soil, the plant responds as though the soil were dry or nearly dry.

Soil salinity is mainly controlled by geological and climatic factors. However, any change in the hydrologic (water) cycle, such as results from draining or flooding low-lying areas, changing the shape of the land's surface, or increasing or reducing vegetative growth, can affect soil salinity. (Agri-environmental indicator project)

Topsoil doesn't need to be bulldozed, paved over, or washed away to lose its productivity. If it is mismanaged, or polluted, it can lose many of the natural soil properties that directly contribute to plant growth or the soil's capability to respond to agricultural management.

R. Neil Sampson
Farmland or Wasteland: A Time to Choose

Agrochemical contamination

Agricultural land can become contaminated in many ways, including atmospheric deposition of industrial wastes and direct application of agricultural chemicals, municipal waste, or irrigation water containing salts or chemicals. Here, we consider only soil contamination that results from adding agrochemicals (agricultural chemicals). These chemicals are used in crop production to improve nutrient levels in the soil (fertilizers) and to reduce damage of the crop by pests (pesticides). Unused chemicals may remain in the soil as soil contaminants or become water contaminants by entering surface waters through runoff or groundwater through leaching. Management practices that involve either adding fertilizer (particularly nitrogen) beyond the requirements of the crop or heavily applying certain pesticides on highly permeable soils have the greatest potential to exceed the soil's capacity to act as an environmental buffer.

Environmental health

Soil health is a key component of environmental health. The findings on soil quality presented in the following chapters are a valuable addition to the information needed to assess the environmental effects of agriculture and to monitor the progress toward sustainable agriculture in Canada.