OTHER CLIMATIC CONDITIONS AFFECTING SOIL FORMATION

1. Temperature (ref. Plaster) - Affects speed of chemical reaction in soil - the higher the temperature, the faster the chemical reaction. Thus soils in the tundra develop slowly while those in the tropics develop more rapidly.

2. Temperature also affects organic matter. While warmth promotes the growth of vegetation so more organic matter is added to the soil, it also speeds the decay and loss of organic matter. Organic matter is also affected by other factors such as rainfall. Rainfall causes accumulation of organic matter as well as leaching (movement of substances such as lime, clay, nutrients, etc.) deeper into the soil.

Other Factors of Soil Formation (ref. Plaster & Weil)

3. Biota- plants contribute to soil formation both by breaking down rock and by adding organic matter to the soil. This particular type of plant also influences the characteristics of the soil, that develop. The needles of coniferous trees are more acidic then those of deciduous trees and thus different soil types develop.

Numerous animals play a role in soil development. Large animals such as moles and prairie dogs bore into the lower soil horizons and bring materials to the surface. They build tunnels which are often opened to the surface and encourage movement of water and air into the subsurface layers. Earthworms and other small animals mix soil as they burrow through soil. Earthworms ingest soil particles and organic material and in so doing enhance the production of plant nutrients. They also aerate the soil.

4. Topography refers to the elevation and slope of the land. Steep slopes encourage erosion and allow less rainfall to enter the soil before running off thus preventing soil formation from getting very far ahead of soil destruction

. 5. Time - Soil forming processes take time. A mature, deeply weathered soil may take hundreds of thousands of years to form. The accumulation of clay minerals becomes noticeable after thousands of years.

COMPONENTS

Now that we have established the importance of soil, let’s discuss in more detail the components of soil. It is, indeed, these components that allow it to perform the aforementioned functions.

Soil consists of four components — air, water, inorganic mineral particles and organic particles, and a large variety of plants and animals already mentioned. Air and water are found in pore spaces, while minerals and organic materials comprise the soil solids. While a handful of soil seems to be solid, indeed half is comprised of the poor spaces of air and H₂0. In addition to providing for the circulation of air and H₂0, pore spaces also provide the homes for the microscopic creatures and are the places where roots grow.

Most of the soil’s solid framework is inorganic mineral particles. The mineral particles are classified according to their sizes into sand, silt and clay. Some of the differences between these particles are noted.

Sand and silt are basically broken down rock fragments which consist of quartz, feldspar, mica or other primary minerals which are minerals that have persisted with little change in composition since there were extracted in molten lava. Sand and silt provide a skeleton for the soil. Sand tends to keep the soil loose and counteracts the tendency of clay to make the soil tight and impermeable to H₂0 and plant roots. Silt holds H₂0 which is available for use by plants.

The properties of clay are due to its physical and chemical structure. Clay has a very large surface area, several orders of magnitude larger than that of sand and silt. Clays are also plate-shaped instead of spherical like sand and silt. Because they have internal as well as external surface area, "the surface area of one gram of soil clay can be as large as the surface area of all the walls, floors and ceilings of a house." (Ref Kohnke) This larger surface area permits clay to enter into chemical and physical reactions. Before I discuss these chemical and physical actions of clay, I want to briefly mention humus or the organic component of soil. Although they are unlike in many ways, they are alike in that they both carry a weak electrical charge with the dominant charge being negative. It is this negative electrical charge that is responsible for the chemical properties of the soil that we will discuss.

Before we discuss these chemical properties, let me mention a few other general characteristics of humus. Humus basically consists of organic (carbon-based) substances while clay is inorganic. The organic components of humus include the living organisms, the carbon-containing remains of dead plants and animals, and the new substances that the living organisms of the humus are continuously making from the remains of the old organisms. "More carbon is stored in the world’s soils than in the world’s plant biomes and atmosphere combined" (ref Weil, p. 19). When the living organisms synthesize these new substances which they need for growth, they undergo respiration which results in the release of C0₂. Because of this constant degradation of organic matter and loss of C0₂, repeated additions of new plant and/or animal residues are necessary to maintain soil organic matter. (Ref Weil)

Organic matter serves several vital roles in the soil:

1) it provides a loose granular structure that provides the right conditions for growth of roots and microorganisms;

2) it increases the amount of water that a soil can hold;

3) it is a major soil source of the plant nutrients nitrogen, phosphorus and sulfur--elements which are released as the organic matter decays and become available for plant roots; and

4) organic matter is the food source for carbon and energy for soil organisms.

Now, as mentioned above, clays and organic matter are alike in that they both have a net negative charge and, as a result, are responsible for the chemical reactions of the soil. In describing these chemical reactions, I am going to concentrate on clays, but remember they are also occurring in the organic matter.

Before discussing the net negative charges of clays, lets reiterate why they are important. Plants get nutrients from the soil solution, so obviously nutrients have to be in the soil solution and the properties of clay permit this.

It is the unique structures of clay that permit the critical reactions to occur.

Three elements comprise the basic clay mineral:

A1 ⁺³ - aluminum
Si⁺⁴ - silicon
0^-2 - Oxygen

Clays form silica sheets which consists of silicon bonded to 4 oxygen atoms to form a tetrahedron. These tetrahedra then join together to form a silica sheets.

Fig. 9-5 Plaster
Fig. 9-6 Plaster

Clays also form alumina sheets. The basic unit of the aluminum sheet is of the aluminum atom surrounded by 6 hydroxyl groups (OH^-) to form an octahedron. Octahedra join through the hydroxyl groups to form an alumina sheet.

The silica and alumina sheets can stack together in different ways to form clay minerals.

One type of stacking is to join an alumina sheet to a silica sheet to form a 1:1 layer. As seen in the following figures, there are hydroxyl groups at top and oxygen atoms at bottom. OH groups of 1 layer bond to O atoms of another. H bonds hold the layers together.

Fig 9-6 & 9-7 Plaster

Another way to stack sheets is to sandwich an alumina sheet between 2 silica sheets. The alumina octahedra share O with silica sheets. This is a 2:1 structure. No hydroxyl groups are exposed at the surface and the layers are not cemented by H bonds. These clays are less tightly bonded and can "open up".

Fig. 9-9 Plaster

Clays have a net negative charge for a couple of reasons.

1. Alumina sheets lose their H ions from the OH groups leaving a negatively charged O ion - O^-.

Fig. 9-11 Plaster

Because matter is neutral, the O attracts a positive charge to attain neutrality; new cations like sodium and potassium are attracted to the clay particle.

2. The other reason is isomorphous substitution. In this process, elements that form the backbone of the clay structure such as aluminum and silicon may replace each other. Other elements also may replace aluminum and silicon. Aluminum has a +3 net charge and silicon a +4 charge so if silicon is replaced by aluminum, there will be a deficiency of a 1+ charge; thus, a cation will again be attracted to the clay.

This replacement of one cation for another is called cation exchange. "The ability of a soil to hold nutrients is directly related to the number of cations it can attract to soil colloids." (Ref Plaster). The name given to the total number of exchangeable cations a soil can absorb is called cation exchange capacity. This value is determined by the amount of clay, the type of clay, and the amount of humus.

Different soils have different CECs.

Why is cation exchange important? Because it is the system that enables plant roots to get nutrient ions. The cations (for example calcium) in a mineral fragment are released by weathering into the soil solution where they are attracted to particles of clay, around which they "swarm". The attraction holds them in such a way that they are not readily lost to water moving through the soil. The nutrient ions then enter the roots (through another exchange process).

Fig. 6.5 Harpstead
Fig. 9-16 Plaster

Fig. 7.1 Harpstead
Fig. 1.24 Weil

SOIL HORIZONS

The soil formation processes we have discussed alter rock materials and add organic materials into layers called soil horizons. These horizons vary greatly but a typical profile from a humid forested region may appear as on fig. 2.2 .

MASTER SOIL HORIZONS DESIGNATIONS (from Kohnke)

O -- Surface horizons composed mostly of organic matter
A -- Uppermost horizon of soil composed of mineral material and organic matter
E -- Depending on climatic conditions, soil may or may not have the leached E horizon
which is the zone where leaching or loss of clay and organic matter has occurred
B -- Accumulation of clay and minerals
C -- Partially weathered rock or parent material
R -- Hard rock

Examples of soils throughout the United States (from Harpstead).

1. Arid regions of western United States. Very little organic matter (O horizon) because of sparse rainfall which results in little vegetation.
2. Central States. Thick, rich, dark, O horizon. Lots of rain provides for plentiful vegetation. O horizon extends two or more feet. Most fertile soils in the United States.
3. Region prior to Ohio River. Rainfall and humidity increase and forests appear. Soils have lower content of organic matter and a leached E horizon. Soils are still productive but require man-made intervention because of greater rainfall.
4. East of Ohio River. Soils are geologically older and thus more weathering has occurred. Thick, clay containing subsoils are red due to oxidation of iron.

References

1. Brady, N.C. and Weil, R.R. The Nature and Properties of Soils (12th Ed.) . Upper Saddle River, NJ: Prentice Hall, 1999.
2. Harpstead, M.I., Sauer, T.J., and Bennett, W.F. Soil Science Simplified (3rd Ed.) . Ames, IA: Iowa State University Press, 1997.
3. Kohnke, H. and Franzmeier, D.P. Soil Science Simplified (4th Ed.) . Prospect Heights, IL: Waveland Press, Inc., 1995.
4. Plaster, E.J. Soil Science and Management (3rd Ed.) . Albany, NY: Delmar Publishers, 1997

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