1 - Soil Definition
2 - Soil Creation
3 - Soil Profiles
SOIL FORMATION
Introduction
| Soil is both very easy and very difficult
to 'see'. In one sense it is visible almost everywhere, but simply
looking at the land only shows us the top of the soil. What happens
at depth has a profound impact on the use we - or nature - can make of the
soil. Sometimes this deeper soil is exposed - in road cuts and
building excavations - but usually these display a shattered, and
literally convoluted, view. To really 'see' a soil we need to dig a
pit about a meter square and a couple of meters deep. We do not do
the digging in this class, but rely mainly on the text for information
about soil formation and appearance (My available slides also indicate
that I have not done the required digging either!). The present
discussion of formation is in 3 parts:
1 - Soil Definition 2 - Soil Creation 3 - Soil Profiles |
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1 - Soil Definition
Soil is the result of weathering which we discussed much earlier (see the section "Weathering and mass wasting"). Then we were concerned with the material as it moved away from the place where weathering occurred. In this portion we are concerned with what happens when it remains undisturbed in a single place for a period of time. That place may be the location whether the weathering occurred, the bottom of a slope after mass wasting, or an area where the material was deposited.
The weathered material is slowly converted to 'soil" - but it is never clear what is meant by 'soil' There are definitions which emphasize the use of the material for agricultural purposes, there are geologic definitions which look at it as the 'top' of the solid rock, and their are engineering ones which assess it in terms of an ability to support a structure - or the need to clear it away before building that structure. Certainly it is more than just 'dirt', but we will look at several characteristics before actually defining it right at the end of the section.
Thinking only of the top layer, the soil color can provide a great deal of information, but can be misleading.
Composition:
Four constituents are needed before we have a soil: 1. Inorganic matter (about 45% by volume) the result of the rock weathering 2. Organic matter (about 5%) the result of vegetation decay 3. Soil air (25%) like ordinary air, but CO2 enriched 4. Soil water (the soil solution) (25%) rain water with dissolved organic and inorganic matter) (N.B. Soil water & soil air are 'interchangeable' in the pore spaces of the soil)
(5. Soil organisms - ranging from microbes to rabbits - are also incorporated into the soil and are vital for several processes, but are not usually considered as part of the soil composition) |
A sandy soil - in the Sandhills region of the state |
Soil texture
The inorganic matter commonly occurs in a variety of particle sizes - from boulders to microscopic grains. Three size classes are particularly important:
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- sand (size as in beach sand) - silt (size like talc/talcum powder) - clay (small particles stick together, like modeling clay)
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Organic matter (with various chemical compositions and degrees of breakdown - but usually called 'humus' as a general name) behaves like clay
They are colloids - chemically active, capable of interchanging nutrient ions with the soil water (and the soil water then transports these nutrients into the plant root and eventually into the tissue of the plant
Taken together, these colloids are called the 'clay-humus' complex. Clay can be any color, but is commonly grayish, while humus typically is black, so the complex is usually dark in color.
The soil texture is a fundamental property of a soil, determining the limits of drainage, water availability and fertility. We usually do not see the texture - unless you take a piece of soil and oven dry it to get rid of the organic matter. Then you can examine the individual particles. However, the thumb and finger feel indicated above is usually enough to tell you about the texture.
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Light reddish (and rather sandy) soil revealed on ridge with thin grassy vegetation on upper Piedmont north of Winston-Salem |
The relative amounts of each of these gives the soil texture
sandy soils: readily drained, light and easy to work, warms rapidly in spring, chemically infertile. A extreme sandy soil behaves like beach sand, falling through your fingers when you pick it up. Most sandy soils, however, do have some silt and clay. They tend to have a gritty feel when you take a piece of soil and rub it between thumb and forefinger . clay soils: easily waterlogged, heavy, stays cool, fertile (many nutrients available). Again, in an extreme form, these feel like modeling clay when you rub them between thumb and forefinger. More commonly, you know you have a clay soil when it is possible to extrude a ribbon of clay by squeezing it between the fingers.
loam soils:- mix of sand and clay soil, usually best for agricultural purposes. These have some ribbon-forming properties and tend to feel a bit gritty. They also tend to leave a whitish residue (the silt) on your fingers when you rub this type of soil.
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Soil Structure
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The colloids of the clay-humus complex are sticky and cause the various particles of the soil texture to clump together into larger particles. These produce the soil structure. You can see this structure by getting a shovel full of soil and gently shaking it up and down. The particles that fall out - the peds - will show the structure. In North Carolina these peds tend to be vaguely spherical lumps, giving a granular structure when roughly 1" or more in diameter, a crumb structure when smaller. Other shapes are possible. |
The soil structure is a filed term describing the soil in the field. It influences drainage (rounded peds maximize the pore spaces and promote aeration and drainage), rooting depth, and nutrient availability. It can be modified by cultivation, with most agricultural practices aimed at producing crumb or granular structures.
2 - Soil Creation
We can conveniently think of soil creation by starting with a bare rock surface - in fact we often see such bare rocks on the mountains. Weathering breaks down the rock, creating the inorganic material, while simple plants soon colonize the area, adding organic matter. Over time, as weathering progresses and the vegetation gets more complex (as we shall discuss in a later class), the soil gets deeper and deeper. Eventually the soil is likely, in natural circumstances, to reach a kind of equilibrium. The mass wasting from the top (mainly soil creep) and the weathering (rock breakdown) at the bottom are more or less in balance. The land surface is slowly lowered (measuring in geologic time) but the soil is retained.
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Fallen trees often expose the soils - here a rather thin soil, relatively high in organic content close to the tree itself, but in an area where soils are thin and commonly rather poor in nutrients |
In practice, this equilibrium is somewhat like that of the grade of a river - there is never a long static, unchanging period - minor changes are going on all the time. Fire burns the trees and reduces the organic matter, trees keel over and leave depression in the soil, wind blows soil from one place to another. Human activity, whether changing the vegetation or plowing the soil for agricultural purposes, also makes changes in the nature of the soil
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The role of water
Water - mainly rain water - has a variety of roles to play in the soil. First, it is important in the rock weathering. Second, it is a vital contributor to plant survival, and third is has a major role in the creation of the soil profile.
Before we consider this, however, i is worth noting that the link with water indicates that there is a close link between soils and climate. Indeed, for many years it was felt that the once you knew the current climate, you would know about the soils. However, climate changes can occur at a much more rapid rate that that at which soils can change. So we have soils now, in several parts of the world, which reflect past climates - especially those associated with the last Ice Age some 18,000 years ago - rather than the present conditions. For much of North Carolina, however, there has been little significant climate change as far as the soils were concerned, and our soils tend to reflect the current climate.
3 - Soil Profiles
The role of water in the soil is embodied in the development of the soil profile, the series of (usually different colored and textured) layers - called horizons - which we find as we dig into the soil.
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Rain water percolates downward through the soil. As it does so it may physically move some of the clay particles down wards. In areas with abundant rain - such as our state - it is common to have soils without much clay in the top layer (the 'A' horizon just below the layer o organic litter which is the 'O' horizon). Around Chapel Hill it is common to have a loam, or even sandy loam, A horizon about 8 inches deep. Below that is the layer where the clay has been deposited - the 'B' horizon. This is compact and much more difficult to cultivate with a garden spade.
The zones of clay export (the zone of eluviation) and import (zone of illuviation) may not be quite the same as the A and B horizons (because the horizons are defined using color as well as texture), but in most cases we can treat them as being the same.
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The amount of downward clay movement depends on the amount and regularity of the precipitation and on the relationship between precipitation and evapotranspiration. |
This last moves water upwards, so that the rain can only penetrate a certain distance dowanwards before being drawn back up. This determines how far down the clay can go. The water moving back upwards to be evaporated at the surface does not have the force to move clay against gravity - so the clay movement is a one way effect.
The other effect of water is in the movement of nutrients. These are dissolved by water and become part of the soil solution. There is downward movement as the water moves through the profile. Some of this water is intercepted and used by plant roots, some drains out of the bottom of the profile - the resulting loss of nutrients is leaching. Nutrients can also move upwards as water is sucked to the surface for evaporation. However, they cannot be evaporated, and so are left at or near the surface as a layer of nutrients. In fact, they are so concentrated when this happens that they can no longer be thought of as nutrients for plants, but as chemical salts, often of more harm than benefit to plants.
In wet climates the leaching effect dominates, in dry ones the movement of salts to a near surface layer dominates.
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The red soil characteristic of much of North Carolina - particularly the Piedmont. Clay has been removed from the top layers, and the residual red color is a result of the presence of oxides of iron (in one sense the soil can be said to has 'rusted' to this color).
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This is an artificially drained organic soil. Initially it is very fertile, once drained, but in our climate leaching is likely to begin to remove the nutrients rather rapidly. |
