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Health of our Soils | Index

Chapter 3 - A Geographical Framework for Assessing Soil Quality

K.B. MacDonald, W.R. Fraser, F. Wang, and G.W. Lelyk

Highlights

• A comparison of estimates of potential and actual agricultural land in the Prairie Provinces shows that all good land and much marginal land is already in production.
• A new tool, the index of inherent soil quality (ISQ), was developed to assess inherent soil quality based on broad-scale soil, landscape, and climatic information.
• An index of soil quality susceptibility (SQS) was developed to identify agricultural areas that are at risk of declining soil quality because of various land use and management practices.
• Most agricultural land in the Prairie Provinces is classified as good using the ISQ; limitations to crop production in this region are associated with areas that are dry or saline; areas most vulnerable to declining soil quality are those under intensive summerfallow.
• Considerable area of southern Ontario is at risk of declining soil quality because of intensive cropping, especially of row crops; between 1981 and 1991, the area of Ontario farmland under intense cultivation increased and the area under conservation tillage increased; many Ontario soils have low levels of organic matter.
• Broad-scale geographical assessment of soil quality allows areas to be targeted for further study and produces a baseline against which to make future comparisons; interpretations from these assessments are limited by the level of detail of the databases and the wide span of time between samplings.

A land ethic, then, reflects the existence of an ecological conscience, and this in turn reflects a conviction of individual responsibility for the health of the land. Health is the capacity for self-renewal. Conservation is our effort to understand and preserve this capacity.

Aldo Leopold
Sand County Almanac

Introduction

As a starting point for looking at the health of Canada's agricultural soils, it is helpful to have a general picture of the inherent (natural) quality of these soils, as well as some of the forces that cause this quality to change. This chapter presents a framework for developing just such a picture.

Detailed assessments of soil quality usually involve measuring soil properties and processes at specific field sites. In contrast, a broad assessment examines soil quality generalized over large areas. We used broad-scale information on soil, landscapes, and climate, derived from Soil Landscapes of Canada, to produce a general picture of inherent soil quality. We then superimposed generalized information about farming practices, taken from the Census of Agriculture, to identify areas where soil quality may decline over time under these practices. (Steps in soil quality assessment)

This broad approach to soil quality is useful in targeting areas where soil health is in question and should be looked at more closely. However, interpretation of the data is limited not only by the large land area and type of data used, but also by the 5-year spans between data collections by Statistics Canada in the Census of Agriculture.

Our geographical picture of soil quality focusses on Canada's two largest regions of crop production, the Prairie Provinces and southern Ontario. We estimate:

• potential and actual supply of land for agricultural use (Prairie Provinces)
• inherent soil quality (Prairie Provinces and southern Ontario)
• areas at risk of declining soil quality (Prairie Provinces and southern Ontario)
• trends in land use and management practices that affect soil quality (southern Ontario).

Land for agriculture

Potential land supply

A limited area of Canada has climatic and soil conditions suitable for crop production, particularly of annual crops. These two factors, climate and soil, were used to establish where and how much land potentially suitable for crop production exists in the Prairie Provinces (Table 3-1).

Cropping is considered possible only where there are at least 1050 effective growing degree days (a measure of heat energy available for crop production) and where soil is of adequate quality. In this assessment soil quality was rated using an index of inherent soil quality (ISQ; described later in this section).

Actual land use

Two sources of information were used to estimate actual land use:

1. satellite images obtained in 1989, which can determine land use at a resolution of 1 square kilometre and are limited to general classification of the land use or land cover; and
2. 1991 Census of Agriculture, which provides much more detailed information about land cover, but this information is generalized for areas as large as 10-250 square kilometres.

About one-third of the total land area of the Prairie Provinces meets minimum soil and climatic requirements for agriculture, and about one-quarter has potential for annual cropping (an ISQ rating better than poor; Table 3-1). Potential farmland is larger than actual farmland, as determined by the agriculture census, because it includes some areas occupied by competing uses (such as national and provincial parks, forests, and military reserves), as well as areas of rangeland. Only about one-fifth of the area of the Prairie Provinces is currently cultivated.

Inherent soil quality

We developed an index of inherent soil quality (ISQ), which ranks soils according to four elements that determine their ability to produce crops, as follows:

• soil porosity (providing air and water for biological processes)
• nutrient retention (retaining plant nutrients)
• physical rooting conditions (promoting root growth as a result of certain physical characteristics)
• chemical rooting conditions (promoting root growth as a result of certain chemical characteristics).

Data for these four elements came from existing land resource inventories. Each ISQ element was rated at one of four levels (good, good to moderate, moderate to poor, poor). Areas with the most restrictive ISQ rating (poor) are best-suited for perennial crops, such as forages, and for grazing. Areas with ISQ ratings better than poor are suitable for annual crops.

Inherent soil quality ratings can be used to assess the status of soil quality and to make comparisons between regions. Note that, because they are based on information collected over a number of years, these ratings do not provide a current "snap-shot" of soil health but rather give an overall impression of a soil's capacity to produce crops.

Specific ISQ ratings, however, can be used to determine soil health by interpreting the possible effects of degradative processes. For example, the loss of 1 centimetre of soil by erosion is much more serious on a shallow soil with a thin layer of fertile topsoil than on a deep soil with a thick fertile topsoil. In some cases it will be important to maintain the quality of soils with moderate ISQ ratings, because any further decline will make them unsuitable for annual cultivation. In other cases it will be more important to maintain the quality of the best lands, to ensure maximum response to additional inputs, such as fertilizer, to the soil. In this section, the four ISQ elements are rated and mapped both individually and together to assess soil quality in the Prairie Provinces.

Each soil is an individual body of nature, possessing its own character, life history, and powers to support plants and animals.

Hans Jenny
Meeting the Expectations of the Land

Soil porosity

The amount and size of pore space in the soil determines:

• how well the soil drains excess moisture and provides air to plant roots
• how much water is stored and made available to growing plants, depending on annual precipitation.

In each case, the most limiting of these two factors was used to determine the ISQ for this element. Most of the subhumid region in the Prairie Provinces is rated good for this ISQ element (Fig. 3-1A). Areas that have a lower rating for this ISQ element include:

• the Red River Plain area of Manitoba, where soil is unable to supply adequate air to crop roots, particularly in the spring and during wet years
• areas that receive limited precipitation during the growing season, such as southwestern Saskatchewan
• areas dominated by coarse soils, which have limited capacity to store water.

Nutrient retention

Most plant nutrients are supplied from the surface layer of the soil. The capacity of a soil to retain and supply nutrients depends on the storage capacity of the soil (cation exchange capacity). This property is controlled by the amount and type of clay and organic matter in the soil. Figure 3-1B shows the ISQ element rating the relative ability of soil to retain materials in the surface. Much of the cropland in the Prairie Provinces is rated good for this ISQ element. The areas that rate lower have a substantial proportion of soils of coarse texture or low organic matter content, or both, in the surface layer.

This element indicates the extent to which fertilizers and pesticides remain where they are applied and promote crop production. It also indicates how well the soil retains and breaks down waste materials, such as compost and sewage sludge.

Physical rooting conditions

There is a limited range of soil conditions under which crops are most productive. Ideal physical rooting conditions include a soil structure that provides optimum water and air for the plant (not too compacted), and soil firmness sufficient to support the plant and permit management traffic but not great enough to hinder root development and penetration of the soil. Figure 3-1C shows the physical rooting conditions of soil related to the cultivation of annual crops. These conditions are not thought to restrict crop production in the Prairie Provinces. Lower ratings are caused in some areas where the following are found:

• shallow soils, which restrict root growth
• surface crusting (in northern soils)
• pans (dense soil layers, which form a barrier to roots)
• soil deterioration resulting from loss of organic matter.

Chemical rooting conditions

Chemical rooting conditions may be both natural and altered by humans. The range of chemical conditions conducive to the growth of agricultural crops in general includes a pH in the range of 6-8 and relatively low levels of soluble salts. Figure 3-1D shows the locations and levels of natural chemical conditions for a general range of grain and oilseed crops. Restrictive areas in the southern portion of the Prairie Provinces reflect the presence of soils with high salt content. Salts are most restrictive where they are found at the surface and somewhat less where they occur in the deeper soil layers. In the northern regions, low soil pH limits plant growth.

Identifying areas at risk of declining soil health

Soil health is the state or condition of soil properties and processes at a certain time. Soil health can change over time as the inherent properties of the soil, described above, are altered by the natural effects of climate and soil landscape, as well as by human activity (land use and management practices).

Identifying agricultural areas that are at risk of declining soil health should ideally be based on validated computer models that simulate soil degradation using actual field measurements, but these models and the databases (collection of data) needed to run them are not yet available. Instead, we developed an index of soil quality susceptibility (SQS) based on soil-landscape and census information, to locate areas that are at risk of soil degradation. Factors (various landscape conditions, soil characteristics, and land use and management practices) and actual threshold values (points at which a change in soil quality is likely to occur) were selected on the basis of their importance in soil-modifying processes, as well as the information available about them. (Role of databases in geographical assessment of soil quality)

Soil and landscape conditions

Erosion is a process that contributes to the deterioration of soil health through the loss of soil material, both mineral and organic. Fine-textured soils are particularly vulnerable to erosion, especially on sloping land. The factors used to identify areas that are vulnerable to erosion were:

• slope steepness (a slope of more than 9% was chosen as the threshold)
• surface texture of the soil (silty soils were selected as the most vulnerable)
• organic matter levels in the top layer of soil (organic matter levels of less than 2%, based on organic carbon levels of 1% or less).

In Figure 3-2, areas where these conditions exist (Prairie Provinces and southern Ontario) are mapped on a background showing inherent soil quality (represented by the most limiting of the four ISQ elements, since a soil is only as good as its greatest weakness allows). Areas that are most susceptible to declining soil quality are those where more than one of the study factors apply, especially where the inherent soil quality is poor. Proportionately less land is at risk of declining soil quality because of soil and landscape features in the Prairie Provinces than in southern Ontario, which has more than 10 times as much land with less than 2% soil organic matter (Table 3-2).

Land use and management practices

Declining soil quality resulting from loss of organic matter is associated with the use of management practices that involve intensive cultivation and exposure of the soil surface, particularly on soils containing low levels of organic matter. We used the following three elements to locate areas where soil quality might decline as a result of land use and management practices:

• cropping intensity (more than 70% of all farmland is cultivated)
• summerfallow intensity (more than 30% of all farmland is in summerfallow; Prairie Provinces only)
• row-cropping intensity (more than 60% of all cropland is row-cropped; southern Ontario only).

In the Prairie Provinces, much of the area where soil and landscape conditions put the soil at risk of declining quality is forest reserve or parkland. Intensive cropping is carried out around Edmonton and north of Calgary in Alberta, in the northeastern part of Saskatchewan's agricultural land, and in southern Manitoba (Fig. 3-3A). These areas generally have soils that rate moderate to high for the ISQ elements capacity to retain nutrients and capacity to supply water and air. Other areas that are vulnerable to declining soil quality are the intensively summerfallowed lands in the arid regions of southern Alberta and southwestern Saskatchewan. These areas often have soils that rate moderate to poor for the ISQ element capacity to supply water and air (Fig. 3-1A). However, because the summerfallow area includes land that is rated good to moderate for this ISQ element, it is possible that soil quality can be improved in these areas by reducing summerfallow. More than 60% of prairie farmers report using conservation tillage on at least 20% of their cultivated land (Table 3-2).

In southern Ontario, intensive cropping is carried out in much of the southwestern region and southeast of Ottawa. The Lake Erie lowlands are especially at risk of declining soil quality because of wide-spread intensive row-cropping (Fig. 3-3B). More than 40% of farmers in southern Ontario report using conservation tillage on at least 20% of their cultivated land (Table 3-2).

. . . agriculture, overall, has been and now is many times more sophisticated a venture than space travel could ever become . . . partly perhaps because it comes out of living nature and is therefore complex, and partly because more human minds have worked on ways to generate an assured food supply than on any other task.

Wes Jackson
Altars of Unhewn Stoned

Trends in land use and management practices

Land use and management practices are factors affecting soil quality that are both under human control and measurable in the short term. An example of a trend in soil quality resulting from changes in farming practices is shown in Figure 3-4. Table 3-2 compares cropping intensity (expressed as the share of farmland occupied by crops) in southern Ontario in 1981 and 1991. There has been a 3% increase in cropping intensity over the 10 years, but no change in the proportion of row crops. By 1991, cropping intensity increased in eastern Ontario and decreased north of Lake Erie, and conservation tillage was more widely used. The areas where both soil-landscape and land-use and management thresholds are exceeded are substantial (Table 3-2). These areas should be targeted for more detailed study to determine the actual extent of soil degradation and to develop appropriate management plans.

Conclusions

Changes in soil quality are natural and occur everywhere. Processes which degrade soil quality may be accelerated by natural conditions of the soil landscape alone or in association with land use and management practices. It is the job of soil researchers to determine how susceptible soils are to degradation and to identify land use and management practices that contribute to or reverse degradation.

This assessment shows the general relationship between soil quality and land use and management practices, but it cannot definitively analyze the effects of farming on soil quality. As research produces a better understanding of soil quality indicators and the processes of soil degradation, both the parameters (factors chosen for study) and the thresholds of these parameters may be changed in future broad-scale assessment of soil quality.

We must remember that knowledge is only in past tense; learning is only in present tense; and prediction is only in future tense. To have sustainable forests [agriculture], we need to be able to know, to learn, and to predict.

Chris Maser
The Redesigned Forest