Soils form and progressively develop under the influence of four soil forming factors acting over time: Parent material Relief (topography and drainage) Climate Organisms (vegetation, animals, man)   1. Parent Material - the original material from which soils develop. It is based on type of bedrock and method of deposition. In Manitoba, soils contain some combination of granite, limestone or shale. These rocks break down over time through weathering to form sand (from granite) or clay (from shale). Limestone can break down into sand, silt and clay-sized particles.

Figure 1.1  Distribution of types of bedrock in Manitoba

In southwest and southern Manitoba where temperatures are moderate and fairly large amounts of water are evaporated from the surface, the native vegetation is mainly grass. Most of the biomass from grassland vegetation is found below the surface, resulting in the addition of large amounts of organic matter into the soil, producing black topsoil.

In the cooler, more humid conditions of eastern and central Manitoba, where evaporation is less, the native vegetation is trees. Most of the biomass from forest vegetation is found on the surface, from leaf fall, stem decay and decomposition of mosses. Little organic matter is incorporated into the soil, resulting in grey topsoil.

Human activities such as agriculture have influenced soil formation by modifying large areas of natural vegetation through cultivation. Removing vegetative cover increases water runoff and alters the moisture and temperature status of the soil. Removing excess water through drainage also changes the moisture conditions in the soil. The removal of natural vegetation and mixing of soil layers can adversely alter the properties of the soil. However, through proper management of soil conservation practices, soil erosion, degrading soil quality and loss of natural fertility can be minimized.
 

What are the basic soil properties?

1. Texture
2. Structure
3. Colour
4. Bulk Density
5. Drainage
6. Calcium Carbonate Content

Table 1.4  Textural groups and classes

Texture Group	Texture Class	Texture Class Symbol
Very Coarse	Very coarse sand	VCoS
	Coarse sand	CoS
	Medium sand	S
Coarse	Fine sand	FS
	Loamy coarse sand	LCoS
	Loamy sand	LS
	Loamy fine sand	LFS
Moderately Coarse	Very fine sand	VFS
	Loamy very fine sand	LVFS
	Coarse sandy loam	CoSL
	Sandy loam	SL
	Fine sandy loam	FSL
Medium	Very fine sandy loam	VFSL
	Loam	L
	Silt loam	SiL
	Silt	Si
Moderately Fine	Sandy clay loam	SCL
	Clay loam	CL
	Silty clay loam	SiCL
Fine	Sandy clay	SC
	Silty clay	SiC
	Clay	C
Very Fine	Heavy clay (>60%)	HC

2.  Structure

Soil structure refers to the way in which soil particles cling together to form aggregates. Clay particles tend to cling tightly together and resist separation more than sand particles. As organic matter decomposes to humus, a variety of compounds are released which “glue” soil particles together.

When individual soil particles are aggregated, they form larger, relatively stable primary structures. If the individual aggregates are distinct and clearly separated from one another, the soil is said to have well-developed structure. But if the fine clay and organic particles are dispersed throughout the soil, the result may be a poorly developed structure. If there are no visible aggregates at all, the soil is structureless, described as either single grain (as found in some sands) or massive (as found in some heavy clays).

Types of soil structure include: prismatic, columnar, angular blocky, subangular blocky, platy and granular. Most agricultural soils have either blocky or granular structure. Forest soils usually have a platy structure at or just below the soil surface. Prismatic and columnar structures develop in soils with significant amounts of sodium present in the subsoil.

Structure has a significant effect on soil water properties and the ability of a soil to resist erosion. Good soil structure increases porosity, aeration, drainage and permits easier root penetration, all of which are important on soils with limited internal drainage, such as clays. Conversely, poor soil structure in the topsoil produces hard, massive clods, which makes a poor seedbed for germinating crops. Poor structure in the subsoil results in dense, compact properties which limit root and moisture penetration.

The natural structural properties of surface soil horizons can be changed by tillage, crop rotation, artificial drainage and applications of manure. As a result, it is important to maintain a desirable soil structure to ensure optimum crop production. For example, massive clay soils are difficult to till when dry and are not easily accessible for field equipment when wet. Poorly structured sandy soils are easily pulverized by tillage, making them prone to erosion.

3.  Colour

Soil conditions such as drainage and salinity, and constituents such as organic matter, iron and carbonates, impart characteristic colours to the soil profile. These colours are measurable and are used as part of the soil classification criteria. Light coloured topsoil indicates either low organic matter content or a concentration of carbonates or soluble salts. Dark coloured topsoil, by contrast, indicates high organic matter content. Subsoil colour is an indicator of drainage that is often more reliable than the actual moisture conditions at the time a soil is examined. Bright colours, such as light brown, yellow or reddish subsoil, is characteristic of a well-drained profile. Dull grey, bluish-green or rust colours indicate a poorly drained profile.

4.  Bulk Density

Bulk density is the apparent density of a soil, measured by determining the oven-dry mass of soil per unit volume. The volume of soil is determined using sampling cores and is measured before soil is oven-dried to avoid any changes in volume due to drying. Bulk density is usually expressed in g/cm³ or Mg/m³.

Bulk density tends to be higher in sandy soils than in clays. A typical clay soil has a bulk density around 1.1 g/cm³; a sandy soil’s bulk density is approximately 1.3 g/cm³; compacted soils may have a bulk density as high as 1.8 g/cm³.

5.  Drainage

Soil drainage is the speed and extent of water removal from the soil by runoff (surface drainage) and downward flow through the soil profile (internal drainage). It also refers to the frequency and duration when the soil is not saturated.

Figure 1.6  Soil drainage classes on four sandy soils A.  Shilox (rapid) B.  Stockton (well) C.  Long Plain (imperfect) D.  Lelant (poor)

Drainage classes: rapid/excessive - water is removed rapidly in relation to supply – very coarse textured soils in higher landscape positions have rapid internal drainage well (and moderately well) - water is removed readily to in relation to supply - development of a B horizon is evidence of well to moderately well internal drainage imperfect - water is removed somewhat slowly in relation to supply to keep the soil wet for a significant part of the growing season – a B horizon may not be present; an AC horizon and the possible presence of some mottles (gleying) at depth are indicators of imperfect drainage poor (and very poor) - water is removed so slowly that the soil remains wet or the water table is near the soil surface for a large part of the time - extensive mottling, peat buildup and blue-grey colours indicative of saturated conditions are prevalent Mottles - rust-coloured spots in the subsoil formed from alternating wetting and drying conditions.   Gleying – a soil-forming process which occurs under poor drainage conditions, resulting in the production of grey colours and mottles. In general, drainage is primarily influenced by soil texture and relief. Coarse-textured, porous soils allow excess water to pass through the soil whereas finer-textured, compact clay materials tend to restrict water movement. Nevertheless, texture and drainage are independent factors, with relief having a greater influence on the drainage class of a soil than its texture. For example, sands in low-lying areas with high water tables are poorly drained, and clays in relatively higher portions of the landscape can be well-drained.

6.  Calcium Carbonate Content

Calcium carbonates (and, to a lesser extent, magnesium carbonates) are common to most agricultural soils in Manitoba. They are derived mostly from fragments of limestone rocks. Over time, carbonates dissolve and move in the soil water.

The calcareous nature of Manitoba soils is basically what maintains their neutral to high pH. Adequate levels of calcium and magnesium, both essential nutrients for plant growth, are usually present in calcareous soils. Since most of the agricultural soils in Manitoba are calcareous, the addition of lime to raise the pH is not a required practice.

Soil surveyors use dilute hydrochloric acid (HCl) to check for the presence of carbonates. Calcium and magnesium carbonates react with HCl to produce carbon dioxide (CO₂) which can be identified by bubbling and fizzing in the area where the HCl was applied. The greater the carbonate content of the soil, the more aggressive the reaction is with HCl.

The depth at which dilute HCl reacts with calcium carbonate (CaCO₃) gives an indication of internal soil drainage and soil development. Over time, soils with good internal drainage have had significant amounts water infiltrate and percolate through the soil. Provided they have not been affected by wind, water, or tillage erosion, they will be free of CaCO₃ in the surface layer and the subsoil layer below the surface horizon. In these soils, dilute HCl will not fizz until it comes into contact with the CaCO₃ below the subsoil layer. With the exception of leached micro depressions, less infiltration and percolation of water in imperfectly drained soils is reflected in the presence of CaCO₃ at the surface or in the subsoil layer below the surface layer. Very low infiltration and percolation of water in poorly drained soils (with the exception of leached depressions) usually results in calcareous (CaCO₃) surface layers. Therefore, dilute HCl will fizz nearer to or at the surface in imperfectly and poorly drained soils.

Calcium carbonate content is expressed as the “calcium carbonate equivalent,” and can range from 0% in extremely leached soil profiles to over 40% in the high lime tills found in the Interlake region of Manitoba.

How do we organize and classify soils?

Soil surveyors are able to distinguish differences in soil properties and group soils according to their mode of formation. This is done by digging holes and inspecting the layers, as well as examining the surrounding landscape features.

The origin of the materials and the soil properties are examined in each layer of soil. Each layer, or horizon, of soil is classified according to properties and designations highlighted in Table 1.5. The sequence of horizons makes up the soil profile. A is the topsoil horizon, B is the middle or subsoil horizon and C is the designation of the parent material. The A and B horizons make up the solum. Each horizon is further described using the lower-case suffixes in Table 1.5.

Soil Horizon - a layer of soil running approximately parallel to the land surface and differing from vertically adjacent layers in terms of physical, chemical and biological properties such as colour, structure, texture, pH, etc.

Repeating or alternating layers of different colours, textures, etc. in the soil profile are referred to as a stratified profile. This is referred to as a cumulic profile in soil survey reports.

Table 1.5  Soil horizon designations
 

Organic Horizons - contain more than 30% organic matter by weight
O	an organic horizon developed mainly from mosses, rushes and woody materials
Of	fibric horizon (least decomposed)
Om	mesic horizon (intermediate decomposition)
Oh	humic horizon (most highly decomposed)
LFH	organic horizons developed from leaves, twigs and woody materials
Mineral Horizons - contain less than 30% organic matter by weight
A - surface horizon (topsoil)		Leaching (removal) of materials in solution and suspension Maximum accumulation of organic matter
B - middle horizon (subsoil)		Enrichment in clay, iron, aluminum, organic matter, sodium Change in colour or structure from horizons above or below
C - parent material		Unaffected by soil forming processes except for gleying and the accumulation of carbonates and soluble salts
AB, BC, and AC		transitional horizons

Lower case suffixes used to further describe mineral horizons
h	horizon enriched with organic matter (eg. Ah, Ahe, Bh, Bhf)
e	eluviated (leached) horizon of clay, iron, aluminum, organic matter (eg. Ae, Ahe)
p	plow layer; disturbance by man's activities, such as cultivation (Ap)
b	buried horizon (Ab)
m	modified by hydrolysis, oxidation or solution to give a change in colour or structure (Bm, Bmk)
t	horizon enriched with clay at least 5 cm (2 in.) thick (Bt, Btg, Bnt)
n	high Na (sodium) horizon - ration of exchangeable Ca to Na is 10 or less Prismatic or columnar structure that is hard to very hard when dry (Bn, Bnt)
g	grey colours or mottles, indicative of permanent or periodic intense reduction (wet conditions) (Bg, Bgj, Ckg, Ckgj)
f	enrichment with non-crystalline Fe and Al combined with organic matter (Bf, Bfh)
j	weak (juvenile) expression of soil processes (Btj, Ckgj)
k	presence of carbonates, visible by effervescence when dilute HCl is added (Bmk, Ck)
ca	layer of carbonate accumulation that exceeds the amount present in the parent material (Cca)
s	soluble salts present (Cks)
z	frozen horizon (permafrost)

Soils in Canada are classified using The Canadian System of Soil Classification, by Agriculture and Agri-Food Canada. This classification system is similar to the hierarchical classification system used to classify the plant and animal kingdoms. The system goes from very broad to very detailed classifications:

1. Order           
2. Great Group
3. Subgroup
4. Association
5. Series
6. Phase
    

Table 1.6  Classification criteria of soils vs. automobiles

Classification Catergory	Soils	Automobiles
I.  Order	Chernozemic	General Motors
II.  Great Group	Black	Car
III.  Subgroup	Orthic Black	Chevrolet
IV.  Association	Fine loamy, mixed, cool, subhumid	4-door Sedan
V.  Series	Newdale	Impala
VI.  Phase	NDL/xcxs	loaded, good condition

 

I.  Soil Orders - based on properties that reflect the effects of the dominant soil-forming processes.
Chernozemic –	most grassland, agricultural soils in Manitoba (high organic matter in A horizon)
Gleysolic –	poorly drained soils (saturated, reduced, mottles)
Luvisolic –	forest soils (Ae and Bt horizons)
Regosolic –	young soils along rivers, slopes, sand dune areas (weak horizon development)
Solonetzic –	sodium-affected soils (sodium in B horizon)
Vertisolic –	heavy clay soils with high shrink-swell potential (cracks and shear planes)
Brunisolic –	catch-all category (weak B horizon)
Cryosolic –	frozen soils
Podzolic –	B horizon with Fe, Al, organic matter
Organic –	more than 30% organic matter by weight

 

Figure 1.7  Relative abundance of soil orders found in agro-Manitoba	Figure 1.8  Mineral soil orders found in agro-Manitoba: 1. Chernozem 2. Luvisol 3. Gleysol 4. Regosol

Table 1.7  Comparison of four mineral soils in Manitoba
 

Factor	Chernozem	Luvisol	Gleysol	Regosol
Native vegetation	Grassland	Forest	Moisture-loving grasses	Limited vegetative growth
Moisture regime	Normal	Normal	Wet	Variable to dry
Formative processes	Vegetation puts bulk of biomass production below ground	Vegetation puts bulk of biomass production above ground	Moist or saturated conditions affect decomposition process	Relatively young soils not fully stabilized by vegetation
Distinguishing features	Thick topsoil horizon (Ah)	Strongly leached horizon (Ae)	Dull, blue-grey colours and mottles (Bg or Cg)	Little soil profile development due to droughtness, erosion, or deposition
Typical landscape position	Midslope	Upper slopes	Depressions	Upper slopes

 

II. Great Group – broad separations of soil zones based on climate and native vegetation patterns. The five soil zones recognized across the prairies are: Brown, Dark Brown, Black, Dark Grey Chernozems; and Grey Luvisol (Figure 1.9). Climate and vegetation have determined the organic matter levels in the topsoil over time, resulting in darker colours with increasing organic matter content in cooler, wetter regions.
 

Figure 1.9  Soil zones of the Canadian prairies (courtesy PFRA). Scale is 1 inch = 230 miles (1:14,572,800)

 

III. Subgroup – subdivisions of each great group. For the Chernozem great group, the subgroups are:

• orthic:  typical A, B, C profile
• rego:  no B horizon
• calcareous:  carbonates (k)
• eluviated:  Ae/Bt horizons present
• solonetzic:  Bn, Bnt horizons present
• gleyed:  presence of mottles, or gleying (g), in B and/or C horizon
• vertic:  horizon disruption or mixing caused by shrinking and swelling.

 

IV. Soil Association (or Catena) - a sequence or family of related soils located in the same climatic zone formed from similar parent material under different landscape positions resulting in different profile characteristics. These soils are adjacent to one another from hilltop to depression. Variation in soil horizons from hilltop to depression is caused by the amount of water available at each point along the slope as a function of infiltration, runoff, run-on and proximity to the water table. Each soil type located along the slope is a soil series (e.g. The Newdale association includes six soil series: Newdale, Rufford, Varcoe, Angusville, Penrith and Drokan).

 

Figure 1.10  Soils of the Newdale Association

V. Soil Series - an individual soil type, with a particular kind and arrangement of soil horizons developed on a particular type of parent material and located in a particular soil zone. The properties of a particular soil series are determined by moisture influences and landscape position. As a result, an individual soil series can usually be found in a specific part of a given field. 

A soil series name is often derived from a town or landmark in or near the area where the series was first recognized (e.g. Newdale soil series).

 

VI. Soil Phases -  variations of a soil series because of factors such as erosion, topography (slope), stones, salinity, improved drainage and peaty layers. This type of information is only found in detailed soil survey reports.

1. Degree of Erosion:
     x = non-eroded or minimal
     1 = slightly eroded (25-75% of A horizon removed)
     2 = moderately eroded (>75% of A and part of B horizon removed)
     3 = severely eroded (all of A and B horizons removed)
     o = overblown (subsoil deposited over topsoil)
    
2. Slope Class:
     x = 0 - 0.5% (level)
     b = 0.5 - 2% (nearly level)
     c = 2 - 5% (very gently sloping)
     d = 5 - 9% (gently sloping)
     e = 9 - 15% (moderately sloping)
     f = 15 - 30% (strongly sloping)
     g = 30 - 45% (very strongly sloping)
     h = 45 - 70% (extremely sloping)
    
3. Stoniness:
     x = nonstony (<0.01% of surface covered)
     1 = slightly stony (0.01 - 0.1%)
     2 = moderately stony (0.1 - 3%)
     3 = very stony (3-15%)
     4 = exceedingly stony (15 - 50%)
     5 = excessively stony (>50%)
    
4. Degree of Salinity:
     x = non-saline (0-4 dS/m)*
     s = weakly saline (4-8 dS/m)
     t = moderately saline (8-15 dS/m)
     u = strongly saline (>15 dS/m)

*Sensitive crops may exhibit negative effects of salinity at levels <4 dS/m - this is a general salinity rating for traditional annual crops (wheat, canola) which are not significantly affected by soil salinity levels below 4 dS/m. 

Other rating systems (refer to Manual for Describing Soils in the Field) evaluate salinity with greater detail using the following classes:

1. Nonsaline (0-2 dS/m)
2. Slightly saline (2-4 dS/m)
3. Weakly saline (4-8 dS/m)
4. Moderately saline (8-15 dS/m)
5. Strongly saline (>15 dS/m)

 

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