Classifying Prime and Marginal Agricultural Soils and Landscapes: Guidelines for Application of the Canada Land Inventory in Ontario
The intention of this document is the provision of a more comprehensive classification framework for the application of CLI to mineral soils and landscapes in Ontario. The guidelines given here should be used in place of those provided in "Canada Land Inventory, Soil Capability Classification for Agriculture", (ARDA Report No. 2, 1965).
Table of Contents
The Canada Land Inventory for agriculture is an interpretative system for assessing the effects of climate and soil characteristics on the limitations of land for growing common field crops. Common field crops in Ontario include corn, soybeans, small grains, and perennial forages. This system does not classify land for horticultural or other specialty type crops.
The system evaluates three general qualities of mineral soils:
The system classifies mineral soils into seven groups according to their potentials and limitations. The first three classes are considered capable of sustained production of cultivated field crops and are considered prime agricultural land resources. The fourth class is marginal for cultivated field crops. The fifth is capable of hay production and permanent pasture use. The sixth is capable of sustaining unimproved pasture only, and the seventh class has no agricultural capability.
The system emphasizes the potential capability of soils. Therefore, the present land use and management of a given land area may or may not reflect its potential soil capability. For example, a forested area may rate highly under the CLI even though it has not been cleared and developed for agricultural use.
CLI classification uses certain assumptions. These are:
In the CLI system there are seven capability classes. Soils descend in quality from Class 1, which is highest, to Class 7 soils which have no agricultural capability for the common field crops. Class 1 soils have no significant limitations. Class 2 through 7 soils have one or more significant limitations, and each of these are denoted by a capability subclass.
Class 1 - Soils in this class have no significant limitations in use for crops.
Soils in Class 1 are level to nearly level, deep, well to imperfectly drained and have good nutrient and water holding capacity. They can be managed and cropped without difficulty. Under good management they are moderately high to high in productivity for the full range of common field crops
Class 2 - Soils in this class have moderate limitations that reduce the choice of crops, or require moderate conservation practices.
These soils are deep and may not hold moisture and nutrients as well as Class 1 soils. The limitations are moderate and the soils can be managed and cropped with little difficulty. Under good management they are moderately high to high in productivity for a wide range of common field crops.
Class 3 - Soils in this class have moderately severe limitations that reduce the choice of crops or require special conservation practices.
The limitations are more severe than for Class 2 soils. They affect one or more of the following practices: timing and ease of tillage; planting and harvesting; choice of crops; and methods of conservation. Under good management these soils are fair to moderately high in productivity for a wide range of common field crops.
Class 4 - Soils in this class have severe limitations that restrict the choice of crops, or require special conservation practices and very careful management, or both.
The severe limitations seriously affect one or more of the following practices: timing and ease of tillage; planting and harvesting; choice of crops; and methods of conservation. These soils are low to medium in productivity for a narrow to wide range of common field crops, but may have higher productivity for a specially adapted crop.
Class 5 - Soils in this class have very severe limitations that restrict their capability to producing perennial forage crops, and improvement practices are feasible.
The limitations are so severe that the soils are not capable of use for sustained production of annual field crops. The soils are capable of producing native or tame species of perennial forage plants and may be improved through the use of farm machinery. Feasible improvement practices may include clearing of bush, cultivation, seeding, fertilizing or water control.
Class 6 - Soils in this class are unsuited for cultivation, but are capable of use for unimproved permanent pasture.
These soils may provide some sustained grazing for farm animals, but the limitations are so severe that improvement through the use of farm machinery is impractical. The terrain may be unsuitable for the use of farm machinery, or the soils may not respond to improvement, or the grazing season may be very short.
Class 7 - Soils in this class have no capability for arable culture or permanent pasture.
This class includes marsh, rockland and soil on very steep slopes.
Capability Subclasses indicate the kinds of limitations present for agricultural use. Thirteen Subclasses were described in CLI Report No. 2. Eleven of these Subclasses have been adapted to Ontario soils.
Subclass C - Adverse climate: This subclass denotes a significant adverse climate for crop production as compared to the "median" climate which is defined as one with sufficiently high growing-season temperatures to bring common field crops to maturity, and with sufficient precipitation to permit crops to be grown each year on the same land without a serious risk of partial or total crop failures. In Ontario this subclass is applied to land averaging less than 2300 Crop Heat Units.
Subclass D - Undesirable soil structure and/or low permeability: This subclass is used for soils which are difficult to till, or which absorb or release water very slowly, or in which the depth of rooting zone is restricted by conditions other than a high water table or consolidated bedrock. In Ontario this subclass is based on the existence of critical clay contents in the upper soil profile.
Subclass E - Erosion: Loss of topsoil and subsoil by erosion has reduced productivity and may in some cases cause difficulties in farming the land e.g. land with gullies.
Subclass F - Low natural fertility: This subclass is made up of soils having low fertility that is either correctable with careful management in the use of fertilizers and soil amendments or is difficult to correct in a feasible way. The limitation may be due to a lack of available plant nutrients, high acidity, low exchange capacity, or presence of toxic compounds.
Subclass I - Inundation by streams or lakes: Flooding by streams and lakes causes crop damage or restricts agricultural use.
Subclass M Moisture deficiency: Soils in this subclass have lower moisture holding capacities and are more prone to droughtiness.
Subclass P - Stoniness: This subclass indicates soils sufficiently stony to hinder tillage, planting, and harvesting operations.
Subclass R - Consolidated bedrock: The occurrence of consolidated bedrock within 100 cm of the surface restricts rooting depth and limits moisture holding capacity. Conversely, in poorly drained soils the presence of the bedrock may, depending on depth, make artificial drainage impossible.
Subclass S - Adverse soil characteristics: This subclass denotes a combination of limitations of equal severity. In Ontario it has often been used to denote a combination of F and M when these are present with a third limitation such as T, E or P.
Subclass T - Topography: This subclass denotes limitations due to slope steepness and length. Such limitations may hinder machinery use, decrease the uniformity of crop growth and maturity, and increase water erosion potential.
Subclass W - Excess water: This subclass indicates the presence of excess soil moisture due to poor or very poor soil drainage. It is distinguished from Subclass I - water inundation which indicates risk of flooding from adjacent lakes or streams.
CLI classification of a soil individual requires the following data:
Soil drainage class: Seven soil drainage classes are used in Ontario and Canada. These are very rapid, rapid, well, moderately well, imperfect, poor, and very poor.
Thickness of unconsolidated soil material over bedrock: Depth to consolidated bedrock is required at sites where bedrock contact occurs at a depth of less than 1 m.
Soil Horizons: The type and thickness of significant soil horizons need to be identified. These include: Depth of topsoil (Ap or Ah horizon); diagnostic and significant B horizons, and depth to and nature of C horizons (parent material).
Texture/Particle Size Distribution: Textural class (mineral material <2mm) for each significant horizon; gravelly modifiers to the textural class are needed where gravel content exceeds 20 % by weight or 15 % by volume.
Soil Reaction: The depth at which free carbonates are detectable with 10% hydrochloric acid is needed in known alkaline soils. High acidity may be inferred in some soils if certain soil horizon characteristics are present such as evidence of podzolization. Soil pH testing, especially of parent materials, is also warranted if high acidity is suspected.
Slope: Slope class based on steepness (%) and length (m).
Surface stoniness: Percent surface coverage of stones >15 cm diameter and boulders >60 cm diameter.
Crop heat units: The average annual crop heat units for an area are determined from the general reference map for Ontario.
Location: The geographic reference point of each soil profile and soil landscape inspection; also location with respect to areas under risk of flooding by streams or lakes.
Collection of the necessary types of data listed under "Information Requirements" and consultation of Tables 1 to 12 which follow, enable the classification of CLI for a soil individual. Classification involves two general steps.
When a soil has a single limitation, the subclass that represents that limitation is shown in the soil capability class (e.g. an excess water limitation is represented by the subclass "W"). As a general rule, only the subclass that represents the most severe limitation is shown, since it is that limitation which has determined the capability class. In cases where two limitations of equal severity exist which have therefore equally determined the capability rating, both are shown in the map symbol. When a soil has more than two limitations of equal severity, however, all of the subclasses that apply to the soil are usually not shown in the class. These conventions are applied in order to simplify soil capability map units and their symbols.
The following list gives conventions that are used when assigning classifications and portraying them on maps.
In considering CLI classification in the context of the whole province of Ontario, climate is the single most important factor. For CLI evaluation in Ontario, climate is characterized by two basic parameters: average annual precipitation, and average annual heat energy during the growing season. Annual precipitation is considered to be non-limiting for the whole province while heat energy and growing season length become generally more limiting with latitude. Subclass C, the climatic limitation, does not generally apply in southwestern, south-central and southeastern Ontario. See Subclass C Adverse Climate.
The soil texture classes have been aggregated into Soil Texture Groups (Table 1) to enable easier definition of those Subclass Guidelines in which soil texture is relevant. Each Soil Texture Group comprises a fairly exclusive range of soil texture classes, including gravelly modifiers.
*Clay content of sandy clay loam can be as low as 20%.
Subclass D denotes soils which are difficult to till, or which absorb or release water very slowly, or in which the depth of rooting zone is restricted by conditions other than a high water table or consolidated bedrock. In Ontario this Subclass is based on the existence of critical clay contents in the upper soil profile. These soils are generally more susceptible to compaction than are lighter textured soils. Table 2 gives the criteria for applying Subclass D.
Table 2. Determination of Subclass D (undesirable structure and/or low permeability)
Class 2D: The top of a clayey horizon >15 cm thick occurs within 40 cm of the soil surface. Clayey materials in this case must have >35% clay content.
Class 3D: The top of a very fine clayey (clay content >60%) horizon >15 cm thick occurs within 40 cm of the soil surface.
Subclass E is applied to soils which have been badly damaged by erosion. The productivity of such soils is therefore reduced. Organic matter, topsoil and subsoil losses in these soils reduce yields. In extreme situations, where erosion has caused deep gullies, farm machinery use is obstructed.
Subclass F denotes soils having low fertility that is either correctable through fertility management or is difficult to correct in a feasible way. Low fertility may be due to low cation exchange capacity, low pH, presence of elements in toxic concentrations (primarily iron and aluminum), or a combination of these factors.
Table 3. Determination of Subclass E (erosion)
Class 2E: Loss of the original plough layer, incorporation of original B horizon material into the present plough layer, and general organic matter losses have resulted in moderate losses to soil productivity.
Class 3E: Loss of original solum (A and B horizons) has resulted in a plough layer consisting mostly of Loamy or Clayey parent material. Organic matter content of the cultivated surface is less than 2%.
Class 4E: Loss of original solum (A and B horizons) has resulted in a cultivated layer consisting mainly of Sandy parent material with an organic matter content of less than 2%; shallow gullies and occasionally deep gullies which cannot be crossed by machinery may also be present.
Class 5E: The original solum (A and B horizons) has been removed exposing very gravelly material and/or frequent deep gullies are present which cannot be crossed by machinery.
This subclass limitation is applicable to soils subject to periodic flooding by streams and lakes which causes crop damage or restricts agricultural use.
Table 5. Determination of Subclass I (inundation)
Class 3I: Frequent inundation with some crop damage; estimated frequency of flooding is less than once every 5 years (Floodplain); includes higher floodplain-terraces on which cultivated field crops can be grown.
Class 5I: Very frequent inundation with some crop damage; estimated frequency of flooding is at least once every 5 years (Floodplain); includes active floodplain areas on which forage crops can be grown primarily for pasture.
CLass 7I: Land is inundated for most of the growing season; often permanently flooded (Marsh)
Subclass M denotes soils which have low moisture holding capacities and are more prone to droughtiness.
The Stoniness Subclass includes soils sufficiently stony to hinder tillage, planting and harvesting of field crops.
Table 7. Determination of Subclass P (surface stoniness)
Class 2P: Surface stones cause some interference with tillage, planting and harvesting; stones are 15-60 cm in diameter, and occur in a range of 1-20 m apart, and occupy <3% of the surface area. Some stone removal is required to bring the land into production.
Class 3P: Surface stones are a serious handicap to tillage, planting, and harvesting; stones are 15-60 cm in diameter, occur 0.5-1m apart (20-75 stones/100 m2), and occupy 3-15% of the surface area. The occasional boulder >60 cm in diameter may also occur. Considerable stone removal is required to bring the land into production. Some annual removal is also required.
Class 4P: Surface stones and many boulders occupy 3-15% of the surface. Considerable stone and boulder removal is needed to bring the land into tillable production. Considerable annual removal is also required for tillage and planting to take place.
Class 5P: Surface stones 15-60 cm in diameter and/or boulders >60 cm in diameter occupy 15-50% of the surface area (>75 stones and/or boulders/100 m2).
Class 6P: Surface stones 15-60 cm in diameter and/or boulders >60 cm in diameter occupy >50% of the surface area.
This subclass is applied to soils where the depth of the rooting zone is restricted by consolidated bedrock. Consolidated bedrock, if it occurs within 100 cm of the surface, reduces available water holding capacity and rooting depth. Where physical soil data were available, the water retention model of McBride and Mackintosh was used to assist in developing the subclass criteria.
Table 8. Determination of Subclass R (shallowness to bedrock)
Class 3R: Consolidated bedrock occurs at a depth of 50-100 cm from the surface causing moderately severe restriction of moisture holding capacity and/or rooting depth.
Class 4R: Consolidated bedrock occurs at a depth of 20-50 cm from the surface causing severe restriction of moisture holding capacity and/or rooting depth.
Class 5R: Consolidated bedrock occurs at a depth of 10 to 20 cm from the surface causing very severe restrictions for tillage, rooting depth and moisture holding capacity. Improvements such as tree removal, shallow tillage, and the seeding down and fertilizing of perennial forages for hay and grazing may be feasible.
Class 6R: Consolidated bedrock occurs at a depth of 10-20 cm from the surface but improvements as in 5R are unfeasible. Open meadows may support grazing.
Class 7R: Consolidated bedrock occurs at < 10cm from the surface.
This subclass denotes a combination of limitations of equal severity. In Ontario it has often been used to denote a combination of fertility (F) and moisture (M) when these are present with a third limitation such as topography (T) or stoniness (P).
The steepness of the surface slope and the pattern or frequency of slopes in different directions are considered topographic limitations if they: 1) increase the cost of farming the land over that of level or less sloping land; 2) decrease the uniformity of growth and maturity of crops; and 3) increase the potential of water and tillage erosion.
S = Simple Slopes >50 m in length
The presence of excess soil moisture, other than that brought about by inundation, is a limitation to field crop agriculture. Excess water may result from inadequate soil drainage, a high water table, seepage or runoff from surrounding areas. This limitation only applies to soils classified as poorly or very poorly drained.
This subclass denotes significant adverse climate for crop production as compared to the "median" climate which is defined as one with sufficiently high growing-season temperatures to bring common field crops to maturity, and with sufficient precipitation to permit crops to be grown each year on the same land without a serious risk of partial or total crop failures.
The climate subclass (Table 12) is based upon the average Crop Heat Unit (CHU) value for the area of consideration. The average CHU value is given by the CHU map in Figure 1.
If the C limitation applies in a given area, it is only cited in the classification notation of those soils present which have no other capability limitations, soils that would be class 1 if not limited by climate. Soils of the same area with other limitations are also downgraded by the same relative degree as the best soils but the C limitation is not cited in the classification notation. For example, a soil with a moderate wetness limitation (2W) is designated "3W" (but not "3CW") if it occurs in an area of 1900 to 2300 CHU.
Figure 1: Average accumulated Crop Heat Units (CHU) available for warm-season crops in Ontario (Brown and Bootsma, 1993)
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Ap horizon: The surface horizon of a mineral soil where organic matter has accumulated, and then been disturbed by clearing and cultivation; "plough layer".
B horizon: A weathered soil horizon of a mineral soil beneath the A horizon and which is characterized by one or more of the following:
Bt horizon: B horizon characterized by enrichment of silicate clay as indicated by a higher clay content than the eluviated soil layer above it.
Calcareous soil: a soil containing sufficient calcium carbonate and magnesium carbonate to effervesce carbon dioxide when treated with 0.1 N hydrochloric acid.
Cation Exchange Capacity: The total amount of exchangeable cations that a soil can absorb.
Ck horizon: a C horizon comprising relatively unweathered parent material containing calcium carbonate and magnesium carbonate that is easily detectable with 0.1 N hydrochloric acid.
Coarse fragments: mineral particles which are greater than 2.0 mm in diameter. Includes fine gravel, cobbles, stones and boulders.
Crop Heat Units: values which provide an index of the average total climatic heat energy available for the growth and maturity of warm season crops.
Eolian: sediments transported and deposited by wind.
Erosion: the wearing away of the land surface by running water, wind, ice, or other geological agents, such as gravitational creep.
Field capacity: The amount of water remaining in a soil that has been saturated and then allowed to freely drain.
Floodplain: nearly level land situated on either or both sides of a channel that is subject to overflow flooding; flood prone alluvial areas adjoining inland or coastal waters; floodprone areas of islands.
Gully: a channel which has been caused by erosion due to the convergent flow of heavy water runoff and which interferes with machinery operation.
Igneous rock: rock formed by the cooling and solidification of magma. In the Canadian Shield such rock is usually high in silica, resistant to weathering and acidic.
Marsh: continuously inundated wetland areas supporting hydrophytic vegetation; can include shallow standing water (<0.5 m depth).
Metamorphic rock: rock derived from pre-existing rocks, but differing from them in physical, chemical, and mineralogical properties as a result of natural geological processes, principally heat and pressure, originating within the earth.
Mineral soil: a soil consisting predominantly of, and having its properties determined predominantly by mineral matter (sand, silt, clay, coarse fragments). Horizons must contain less than 30 % organic matter by dry weight, with the exception of the surface layer which may exceed 30 %, if it is less than 40 cm thick and comprised of mixed peat, or less than 60 cm thick and comprised of fibric peat.
Organic soil: a soil composed primarily of decomposed or partly decomposed plant and animal materials. As such it contains more than 30 % organic matter by dry weight. The total depth of the organic material must exceed 40 cm if the soil is moderately to well decomposed (mesic or humic) , or 60 cm if the material is poorly decomposed (fibric).
Pedology: the aspects of soil science dealing with the genesis, distribution, mapping, taxonomy and interpretation of soils.
pH: a measure of the acidity or hydrogen ion activity of a soil. It is used as an indicator of the acidity or alkalinity of soils. Ontario soils range from extremely acid to moderately alkaline (pH range 4.0 to 8.0).
Porosity: the percent volume of the total bulk soil that is not occupied by solid particles.
Rill: a small intermittent water course only a few centimetres wide. Rills do not interfere with tillage or other machinery.
Sedimentary rock: rock formed by the lithification of mechanical, chemical, or organic sediments. The principal sedimentary rocks are sandstones, shales, limestones, and conglomerates.
Soil classification: the systematic arrangement of soils into categories on the basis of their characteristics. Broad groupings are made on the basis of general characteristics, and subdivisions on the basis of more detailed differences in specific properties.
Soil drainage: as a description of the natural condition of the soil, soil drainage refers to the frequency and duration of periods during which the soil is free of saturation. In a well drained soil, excess water (gravitational water) is removed readily but not rapidly. In a poorly drained soil, the root zone is saturated for long periods unless it is artificially drained. In a very rapidly drained soil, water is removed very rapidly in relation to supply and most crop plants suffer from lack of water.
Soil fertility: the status of a soil in the relation to the amount and availability to plants of elements necessary for plant growth.
Soil horizon: a distinct layer of soil, approximately parallel to the ground surface, within a soil profile. It differs from adjacent genetically related layers in properties such as colour, structure, texture, and consistence, and chemical, biological, and mineralogical composition. In soil surveys, the soil horizons comprising soil profiles are usually logged to depth of at least one metre. Generally, a soil horizon should more than 5 cm thick to be considered as significant to soil profile characterization.
Soil Map Unit: A kind of soil, or a combination of kinds of soils that can be shown as a delineation or polygon on a soil map.
Soil morphology: the physical constitution of a soil exhibited by the kinds, thicknesses and arrangements of the soil horizons comprising the soil profile.
Soil organic matter: the organic fraction of the soil which includes plant and animal residues
at various stages of decomposition. Expressed as a percentage of dry soil weight.
Soil Pedon: The basic soil profile sampling, description and classification unit at a representative site in a soil landscape. A soil pedon can be classified by soil series plus any applicable phases.
Soil permeability: the ease with which gases or liquids penetrate or pass through the soil.
Soil profile: a vertical section of the soil through all its horizons and extending in to the parent material.
Soil Reaction: The degree of acidity or alkalinity of a soil soil usually expressed as a pH value.
Soil structure: the aggregation and arrangement of primary soil particles (sand, silt, clay, organic matter) into discrete soil peds or aggregates.
Soil texture: the relative proportions by weight of the various particle size fractions in the mineral fraction of the soil (sand, silt, clay), as described by the soil texture classes. The sand fraction is further subdivided into coarse sand, sand, fine sand and very fine sand, based on the proportions of various sand sizes within the sand fraction. Therefor, loamy sand textures can be more precisely described as loamy coarse sand, loamy sand, loamy fine sand and loamy very fine sand, as the case may be. Likewise, the sandy loam textures can be classified as coarse sandy loam, sandy loam, fine sandy loam, and very fine sandy loam. When the gravel percentage present is between 20 and 50 percent by weight (equates to approximately 15 to 35% gravel by volume) the texture class is gravelly (e.g. gravelly sandy loam, gravelly loam). When the gravel fraction exceeds 50 % (>35% volume), a "very gravelly" descriptor is used (e.g. very gravelly sandy loam, very gravelly sand).
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