Ministry of Environment


2.2 Soil And Soil Processes

L.M. Lavkulich and K.W.G. Valentine

Soil is the naturally occurring unconsolidated mineral or organic material at the surface of the earth which is capable of supporting plant growth. The particular type of soil at any position on the earth is a result of the geologic or parent material, climate, biota, topography and time. These factors of soil formation do not act singly but together in various ways to produce a particular soil. The soil must not be regarded as a passive, inert body on the earth's surface. It is a continually changing system within the total environment.

Soils possess properties that have been inherited from the parent material, for example the amount of stones, mineralogy and thickness. They also have properties and features that are the result of soil processes, including such things as the kind and amount of organic matter, the formation of horizons (layers), new minerals and changes in porosity. For instance the features of a valley bottom soil will be largely inherited from the conditions of sediment accumulation in that part of the valley, whereas the humus content of the topsoil will be the result of the amount of organic litter added to the surface and the balance of the constructive and destructive processes acting on this litter.

Soils have both internal and external properties. The internal properties are often shown as various layers or horizons that are exposed in a vertical cross-section of the soil body. External properties include such attributes as slope and aspect. Soils often grade imperceptibly from one kind to another and occur as a relatively continuous blanket on the land surface of the earth. This continuous blanket is divided into soil classes defined upon the basis of soil properties, many of which reflect processes of soil genesis. Local changes in slope, elevation or aspect are dominant factors in determining changes in soil properties. It is on the basis of the soil's internal and external properties that soil resource inventories are prepared.

Soil Components

Soils are multiphase systems, composed of a mixture of numerous kinds and sizes of mineral grains, organic fragments, air and water. The proportions of these components vary from point to point on the surface of the earth, and from time to time, depending upon the factors of soil formation and the influence of man's activity.

Soil air plays an important role in plant growth and the activity of soil organisms. Its main constituents, as in the atmosphere, are nitrogen, oxygen and carbon dioxide. Various organisms use the oxygen and give off carbon dioxide. The supply of oxygen has to be constantly replenished and this is the importance of adequate pore spaces or aeration within the soil. For adequate aeration approximately 10% by volume of the soil should be occupied by air.

The importance of soil water lies in its roles in plant growth and in the chemical and physical reactions that occur in soils. Water provides the medium by which plants take up their nutrients in solution through their root systems; it may also be considered a plant nutrient itself. Thus water is crucial to plant growth. Molecules of water also play a central role in the chemical weathering processes of hydration, hydrolysis, oxidation, reduction and solution. In physical weathering soil particles and rock fragments are disrupted by the expansion of ice in cycles of freezing and thawing and during wetting and drying. Wetting and drying cycles play an important role in the production of soil structure and in the physical translocation of material such as clay within a soil.

Air and water in the soil have a reciprocal relationship since both compete for the same pore spaces. For example, after a rain or if the soil is poorly drained, the pores are filled with water and air is excluded. Conversely, as water moves out of a moist soil, the pore space is filled with air. Thus the relationship between air and water in soils is continually changing.

The mineral portion of soils is composed of a wide variety of inorganic compounds. The minerals are commonly referred to as primary or secondary. Primary minerals originate from the geologic substrata. They include quartz, feldspar, mica, amphibole and pyroxene. They respond to the environmental factors and in the presence of water and air are altered or weathered. As a result of chemical weathering processes the primary minerals become secondary minerals such as illite, vermiculite, chlorite, montmorillonite and kaolinite (often called the clay minerals) and the hydrous oxides of iron and aluminum. Particle size of the mineral fraction varies widely, from stones (over 250 mm in diameter), through cobbles (250-75 mm), gravel (75-2 mm), sand (2-0.05 mm), silt (0.05-0.002 mm) to clay (less than 0.002 mm). All combinations of these particle sizes can occur in soils, and the expression of the amounts of the various size fractions is called soil texture. Although coarse fragments and sand sized particles play but a small role in chemical processes occurring in soils, they are very important in soil drainage and in the engineering uses of soils. Silts and clays play the most important role in water and nutrient retention as well as the swelling and shrinking properties of soils.

The organic portion of soils results from the accumulation of animal and plant residues added to the mineral soil. It is this organic portion that differentiates soil from geological material occurring below the earth's surface which otherwise may have many of the properties of a soil. Organic compounds undergo decomposition by soil flora and fauna. This process produces humus, the most active and important form of organic matter for crop growth and soil formation. Organic matter is an extremely important constituent of soils, as it is the source of many nutrients necessary for plant growth, it enhances the soil's water holding capacity and its presence favours good soil structure especially in the topsoil.

Soil Processes

There are four basic processes that occur in the formation of soils, namely additions, losses, translocations and transformations. The two driving forces for these processes are climate (temperature and precipitation) and organisms, (plants and animals). Parent material is usually a rather passive factor in affecting soil processes because parent materials are inherited from the geologic world. Topography (or relief) is also rather passive in affecting soil processes, mainly by modifying the climatic influences of temperature and precipitation.

The various types of additions, losses, translocations and transformations that occur within the soil may be seen in Figure 2.2.1. Most additions occur at the surface. The most obvious ones include solar energy, water controlled by climate, and organic material derived principally from the vegetation.

Losses occur both from the surface and from the deep subsoil. For instance, water is lost by evapotranspiration and carbon dioxide by diffusion at the surface and, on a more catastrophic level, large masses of soil can be stripped by erosion. Materials suspended or dissolved in water are the main forms of losses from the subsoil.

Translocation refers to the physical movement of material within soil. The material can be in the solid, liquid or gaseous form, the movement can be in any direction from and to any horizon. For instance clay, organic matter and iron and aluminum hydrous oxides are commonly moved from the surface horizon to a subsurface horizon. Conversely, in very dry climates salts are moved upwards in solution by capillarity, and in very cold climates solid mineral fragments are moved upwards by frost action.

Additions, losses and translocations all involve movement. Figure 2.2.1 shows this movement in two dimensions. Since a soil is three dimensional and most soils occur on slopes, it must be remembered that these movements can occur laterally as well as vertically. Water can carry clay in suspension laterally through a subsurface horizon, or topsoil can move slowly en mass from a surface horizon upslope to form a new surface downslope.

Transformations involve the change of some soil constituent without any physical displacement. Chemical and physical weathering and the decomposition of organic matter are included here.

All these processes occur to a greater or lesser extent in all soils. The properties that characterize one soil are the result of a particular balance among all the processes. Other soils will be different because they have been formed by groups of processes having different balances.

Soil Forming Factors

Soils are a product of the environment. Often this is illustrated in the form of a mathematical expression, namely:

soil = f (parent material, climate, biota, topography, time)

Although the above expression shows soils to be the function of a number of individual factors, the factors are not mutually exclusive but interdependent. For example, the kind of vegetation found at any one location on the earth's surface is dependent on climate, parent material, topography, time and, in fact, soil. It is obvious that numerous combinations of the factors are possible. This leads to many different kinds of soils, each representing a certain combination of the factors of soil formation. Parent material, topography and time are sometimes referred to as the passive factors of soil formation, as they do not form soils without the active factors of climate and biota.

Parent material may be of two general kinds, organic or mineral. Organic parent materials are formed by biological action where plant and animal tissue is produced faster than it is decomposed. This occurs in cold and wet regions where biological decomposition is very slow. Soil inherits many properties from the parent material from which it forms, for example, the kinds of minerals, particle size and the chemical elements. Thus, parent materials are the building blocks upon which the other factors of soil formation manifest their effects.

Climate is an active factor of soil formation because it governs the amount, distribution and kind of precipitation as well as the amount and distribution of solar energy available at any point on the earth's surface. Biological, physical and chemical processes take place in water, thus the amount of water available leaches the soil, allows nutrients to be obtained by plants and governs the kind of vegetative cover. The form of precipitation, whether rain or snow, is also important in relation to processes which take place in soils. Solar energy, usually expressed as temperature, is an important part of climate because it controls the form of water falling onto the soil surface as well as in the soil. Also as temperature, it increases the rate of reactions, such as chemical reactions, evapotranspiration and biological processes. Wide fluctuations in temperature, especially in the presence of water cause shrinking and swelling, frost action and general weathering in soils. It can be seen that both water and temperature complement each other in changing parent material and affecting the biota on any soil landscape.

Biota is another active factor in soil formation. The kind and amount of plants and animals that exist bring organic matter into the soil system as well as nutrient elements. This has a great effect on the kind of soil that will form. For example forest vegetation tends to accumulate organic matter on top of the mineral soil where it decomposes and adds organic acids to the soil below. This aids in weathering mineral soil particles. Soils under grassland, however, accumulate organic matter within the mineral soil as a result of the extensive root system which dies back periodically. Animals play an important part in soils; in some cases animals ingest soil particles and mix the mineral and organic portions, e.g. earthworms. Other animals tunnel through soils mixing the various components. Similarly, microorganisms have great effects on soils in decomposing organic matter, fixing nitrogen from the atmosphere, and making other soil nutrients available for plant growth. Biota, in conjunction with climate, modify parent material to produce soil.

Topography is often considered a passive factor modifying the effects of climate. Everyone is aware of the effect that aspect, or orientation of the soil surface to the sun, has on the type of vegetation. South-facing slopes tend to be drier than north-facing slopes as the result of higher temperatures. Similarly, as one goes from the valley bottom to the tops of mountains or plateaus the climate becomes cooler. Topography also redistributes the water reaching the soil surface. Runoff from uplands creates wetter conditions on the lowlands, in some cases saline sloughs or organic soils. Thus as a redistributor of the climate features, topography affects soil processes, soil distribution and the type of vegetation at the site.

Finally it must also be remembered that soils develop over long periods and the balance between processes is liable to change in that time. For instance, chemical weathering is likely to be a very important form of transformation when a river sediment is first exposed. But as vegetation slowly becomes established the addition and decomposition of organic matter will become more important and chemical weathering less so.

Soil Horizons

The effect of the processes described acting within the soil is to form different horizontal layers or horizons down a vertical section. Soil horizons run roughly parallel to the surface of the earth and may be composed of mineral or organic material. They differ from adjacent horizons in such properties as colour, structure, texture and consistence. A soil profile is a vertical section down through these horizons to the parent material. Soil horizons are labelled with capital letters to denote the major horizon followed by small letters to denote its particular characteristics. A diagrammatic representation of a mineral soil profile with the designations used in Canada for some horizons is shown in Figure 2.2.2.

In mineral soils there are generally three major horizons; A, B and C. The A horizon is uppermost, where the maximum accumulation of organic matter in situ occurs and where maximum removal of materials occurs by solution, suspension or erosion. It is also where the largest transfers of energy take place, both solar and biological. This is truly where the action is! The B horizon is where the materials accumulate that were freed in the upper portion of the soil body. There is a close relationship between the A and B horizons. Translocations as well as many biological and chemical reactions take place between them. The B horizon, however, tends to be more stable than the A for short term differences. The C horizon is often termed the parent material. The effects of soil processes have not manifested themselves appreciably and thus the material is little modified.

Small letters are used to modify the major mineral soil horizon letters A, B and C. These small letters denote the predominant properties of the major horizon. Thus h indicates a horizon enriched with organic matter and t indicates a horizon where clay has accumulated. Small letters can be combined; for example, a horizon labeled Bhf indicates that it is a layer within the soil where organic matter, iron and aluminum have accumulated.

Major organic soil horizons are found in two distinctive kinds of environments, well aerated or poorly aerated. In well aerated environments leaves, needles, twigs and branches fall to the earth, accumulate and begin to decompose on top of the mineral soil. The degree of decomposition is important in understanding soil processes, and it is the criterion for distinguishing three horizons L, F and H. L has the least decomposition and H the most with F denoting moderate decomposition. In poorly aerated environments (wet areas) the organic materials also accumulate but their character is different because of the saturated condition. In these areas the horizons are denoted O. They will be composed mainly of mosses, rushes and woody material.

Small letters are used in organic soils to denote the degree of decomposition of the organic material. They are used only with O for soils of wet environments. For instance Of would be a horizon with little decomposition and Oh would be one of advanced decomposition. Moderate decomposition is indicated by m.

A full description of major and minor horizon and layer designations may be found in the Canadian System of Soil Classification (see Further Reading).

Further Reading

  • Canada Soil Survey Committee, 1978. The Canadian System of Soil Classification, Agriculture Canada, Queen's Printer, Ottawa.

    This publication describes the most recent revision of the system of classifying soils used in Canada.

  • Millar, C.E., L.M. Turk. and H.D. Foth, 1965. Fundamentals of Soil Science, 4th ed., Wiley, New York. 491 pp.

    An introductory soil science text, oriented mainly towards agronomy.

  • Simonson, Roy W., 1959. Outline of a generalized theory of soil genesis. Soil Sci. Soc. Amer. Proc., 23: 152-156.

    The paper upon which the section on soil processes is largely based.