ABOUT THIS ISSUE
Published by Emporia State University
Prepared and issued by The Division of Biology
Editor: Robert F. Clarke
Editorial Committee: Gilbert A. Leisman, Tom Eddy, Robert J. Boles, John Ransom
Online format by: Terri Weast
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ABOUT THE AUTHOR
Dr. James Mayo is an Associate Professor of Biology at Emporia State University. He is a native Kansan, with a B. S. and M. S. from Utah State University and a Ph. D. from the University of Washington. His
specialties include soil-plant water relations and ecophysiology.
The Land Capability Judging Scorecard (Table 4) is taken from Lind and Harper (1956).
"Thou shalt inherit the holy earth as a faithful
steward, conserving its resources and productivity
from generation to generation. Thou shalt protect
thy fields from soil erosion and thy hills from
overgrazing by thy herds, so that thy descendants
may have abundance forever . If any fail in this
stewardship of this land, his fertile fields shall
become sterile stones and gullies and his descendants
shall decrease and live in poverty or vanish from the
face of the earth. "
--- W. C. Lowdermilk
The eleventh commandment (above) as suggested by W.C. Lowdermilk is an appropriate way to begin an article about soil, since we are so dependent on it for our well being. Long after our oil, gas, and other non-renewable resources have been depleted, we will be dependent upon the soil for our very existence. Civilizations such as the Mayan and Mesopotamian are now believed to have failed because of soil mismanagement (excessive erosion and waterlogging, respectively) (Brown, 1982). Agriculture is the major industry in Kansas and agriculture depends upon the soil, its health and productivity.
Productivity depends upon a soil's ability to support plants. The soil supplies water and nutrients necessary for plant growth and, ultimately, yields of grain, hay, or other crops. Should the soil fail to supply the necessary water and nutrients, yields are reduced accordingly; should the soil be degraded (eg. compaction
or salt build up) yields are accordingly; should the soil be lost through erosion, again yields are reduced accordingly. As the eleventh commandment states, good soil is essential for our existence.
Good soil stewardship with understanding the soil. We must understand: 1) What soil is; 2) soils form and how are they described and classified; 3) Land use capability; 4) Erosion, its cause and cure. This booklet attempts to answer some of these and provide information that will allow you to increase your appreciation
WHAT IS SOIL?
Soil is defined as the complex outer, loose material of the earth's surface. But, it is not simply ground up rock. It is the result of interactions between climate, parent material (eg. rocks such as limestone or sandstone), topography, and organisms (plants, animals, bacteria, fungi) acting overlong periods of time.
Soil may be viewed as a three-phase system consisting of: solids, (minerals, and organics); liquid (water), and gas or soil air. A good agriculture soil consists of 50% solid (45% minerals and 5% organic matter), 20-30% air, and 20-30% water. Soils with less than 20% air or pore space due to compaction or waterlogging will not support good plant growth. The solid-mineral phase is highly important in that the kind of minerals and particle size determine the fertility of water-supplying capacity of the soil. Mineral size is so important that soils are named for the textural classes present in the soil, and these are based on the relative proportions of particle size groups. (Table 1 and Figure 1)
It can easily be seen that as particles get smaller, surface area increases enormously. This is important in that nutrients such as calcium,magnesium, potassium, and iron, as well as water are held in an exchangeable form on the surface of soil particles. Thus, the greater the surface area, the greater the exchangeable nutrients. This kind of mineral is equally important, although beyond the scope of this booklet.
Table 1. Size and Surface Area of Soil Particles used to Classify Soils
|Particle Type||Particle Diameter mm||No. of Particles/g||Surface area cm2/g|
|Very coarse sand||2.00-1.00||90||11|
|Very fine sand||0.10-0.05||722,000||227|
The proportions of particle sizes are used to place a given soil into a textural class from which part of the soil's name is derived (Figure 1). Except for the term loam, the texture class names identify the particle sizes that dominate soil properties. For example, it can be seen that loam soils never had more than 27% clay and may have as little as 7%. Soils can be conveniently grouped as coarse, medium, or fine textured, and the classes normally included under each of these are:
With clay in
The soil name
By following the instructions in the key (Table 2) the textural class of any soil may be determined.
Table 2. A Key to determine soil textured by feel.
Place approximately 1 oz. (or 25g) of soil in the palm of your hand; wet and knead it until it's plastic and putty-like. (i.e., the consistency of molding clay); then:
|Textural Class is|
|A1||Does a ball of the moist soil fall apart when squeezed?|
|A2||The ball of soil remains intact when squeezed... go to B|
|B1||When squeezed between the thumb and forefinger soil does not form a ribbon||loamy sand|
|B2||When squeezed between the thumb and forefinger soil forms a ribbon (choose C1, C2, or C3)|
|C1||Soil forms a weak ribbon less than 1 inch long before breaking (Choose 1, 2, or 3)|
|1. Soil is very gritty||sandy loam|
|2. Soil is very smooth||silt loam|
|3. Soil is neither very gritty nor very smooth||loam|
|C2||Soil forms a ribbon 1-2 inches long before breaking (choose 1, 2, or 3)|
|1. Soil is very gritty||sandy clay loam|
|2. Soil is very smooth||sandy clay loam|
|3. Soil is neither very gritty nor very smooth||clay loam|
|C3||Soil makes a strong ribbon more than 2 inches (choose 1, 2, or 3)|
|1. Soil is very gritty||sandy clay|
|2. Soil is very smooth||silty clay|
|3. Soil is neither very gritty nor very smooth||clay|
Soil particles may be aggregated into larger particles. The degree of aggregation is called soil structure and this plays an important role in determining the proportions of solid, water, and air in a volume of soil. Soils are said to be puddled when structure is destroyed. This reduces water entry; increasing the likelihood of erosion. Minimum tillage, proper tillage timing, and the incorporation of organic matter can improve
soil structure, even though the textural class does not change.
Organic matter, while not present in large amounts (except in organic soils, such as peats) is very important. Organic matter left on the soil surface aids water infiltration and reduces erosion
Organic matter can increase the water-holding capacity of sandy soils (which tend to be droughty), and when incorporated into clay soils it improves structure, thus promoting aeration and drainage. Humus is the result of soil microorganisms acting upon organic matter. It is the product of microbial metabolism and contains a wide variety of complex compounds. Humus contains nitrogen and sulfur, both necessary elements for plants.
In addition to minerals, air, water, and organic matter, soils contain enormous numbers of bacteria, actinomycetes, fungi, algae, protozoa, and a range of larger animals, incuding: insects, earthworms, moles, etc. The kinds and numbers of these organisms reflect, as well as influence, the properties and the health of the soil. Soil ecology is an important area of science contributing much to understanding of soil function.
From the preceding discussion, it is obvious that soil is a complex, dynamic, living substance worthy of study. It is also obvious that all soils are somewhat unique, and managing them is an art as well as a science. Even though soils are to some extent unique, it is possible to describe and classify them. Broadly
speaking, similar climate, parent material, relief, and organisms will in time, produce similar soils.
SOIL AND LAND CLASSIFICATION
A vertical section through a soil in any given field will show distinct horizontal layers (much like a layer cake). Such a section is called a soil profile, and the layers are called horizons (Figure 2).
-- O (Organic Layer)
-- A horizon (Topsoil)
-- B horizon (Subsoil)
-- C horizon (Parent Material)
Fig. 2. Soil Profile
These horizons are diagnostic in that descriptions, including texture, structure, color, acidity, depth , etc., characterize any given soil; and soils with similar horizontal development are given the same name. Generally, three mineral layers make up the soil profile (A, B, C), with an organic horizon (0) often present, particularly in little-disturbed grassland or forest areas. A typical profile may contain: an organic layer (o); and A horizon, from which certain inorganic materials have been leached during formation, particularly clay and/or salts. (This is the horizon of greatest biological activity, containing most of the plant roots, microorganisms and small animals. It is usually darker in color because of organic matter, particularly humus. In many instances the fertile A horizon has been lost by erosion caused by poor farming practices); B horizon which is the subsoil, where materials leached from the A horizon accumulate. (This horizon contains only small amounts of organic matter but often has a salt or clay accumulation layer (eg. clay pan); and the C horizon which is the parent material from which the soil developed. It may consist of fractured bedrock, as is found in limestone areas, or it may be made of material transported by wind (loess) or water (alluvial). It has almost no organic matter, contains little life, but provides the essential mineral particles upon which fertility depends.
Based upon descriptions of soil profiles, depth of horizons, color, texture, structure, acidity, organic matter, etc., soil scientists can name and classify soils into categories with increasing detail into order, suborder, great group, subgroup, family, and series. The USDA's Soil Conservation Service does this according to: Soil Classification, A Comprehensive 7th Approximation. The reader is referred to this basic document for the considerably complex system used by soil scientists to name soils. The soils of Kansas (for the most part) belong to the soil order called Mollisols. These are soils with nearly black organic-rich surface horizons and high in nutrients. They are among the world's most productive soils. The Soil Conservation Service has (or is currently) described the soils in Kansas in great detail, so that it is (or soon will be) possible to obtain a detailed soil series map of every farm and ranch in the state. These soil surveys. done by the SCS on a
county by county basis, may often be obtained from the local SCS office. They contain a wealth of information about local soils, including: aerial photograph soil type maps, use and management guides, engineering properties, soil series classification, profile descriptions, and notes on soil formation. With the soil survey information, it is possible to locate any spot in the county, and look up its soil type and all
of the relevant information. For particular uses, the SCS provides on the spot survey information.
Land is a broader term than soil, which includes not only the soil type, but other physical characteristics, such as location with respect to cities, water supply, existing cover, etc. For example, a grassland may include a variety of soils. The SCS recognizes eight Land Capability Classes (Table 3).
Definitions of LAND CHARACTERISTICS (used to judge land characteristics)
1. Surface Texture - determine Table 2.
2. Movement of Air and Water in the Subsoil (Permeability). Soils may be placed into relative permeability
classes through observation of structure, texture, etc., of a soil pit: Very Slow soils with dense, heavy clay or clay or clay pan subsoil; Slow - crumbly, clayey subsoil; Moderate - highly granular, clay loam subsoil; Rapid - sandy subsoil.
3. Depth of Soil - the depth of soil is determined by the total thickness readily penetrated by roots, i.e., observe the soil pit and measure the depth where most roots occur. Deep Soils - more than 36 inches; Moderately Deep Soils - 20-36 inches; Shallow Soils -10-20 inches; Very Shallow Soils - less than 10
4. Slope - This is the number of feet of fall per 100 feet: Nearly level - less than 1 foot of fall; Gently Sloping - 1-3 feet of fall; Moderately Sloping - 3-5 feet of fall; Strongly Sloping - 6-8 feet of fall; Steep - 8-12 feet of fall; Very Steep - more than 12 feet of fall.
5. Erosion, Wind, and Water -None to Slight - less than 25% of the surface soil lost and no gullies; Moderate 25-75 % of surface soil lost, with or without gullies (but not frequent uncrossable gullies); Severe - 75% or more of the surface soil removed with occasional uncrossable gullies and/or severe
accumulation of soil by wind; Very Severe - 75% of the surface soil removed with frequent uncrossable gullies. (NOTE: uncrossable means uncrossable with farm machinery. Often the A horizon (surface) is completely lost and the B horizon is being farmed.)
6. Surface Drainage -Poor - water is removed so slowly that the soil remains wet for long periods of time (Drowned plants, algal growth on the soil surface, as well as pooled water are indications of poor drainage); Fair - water is removed slowly, surface water is an occasional problem, plants may be stunted but not killed; Good - normal drainage, no obvious problem; Excessive - water is lost exceedingly fast, as in deep sands.
Table 3. Land Capability Classes
|Class||Description, Limitations, and Use|
|I||nearly level, well drained, few limitations, can be cropped intensively|
|II||gently sloped, subject to moderate erosion, may require terraces, strip cropping, etc.|
|III||moderate slope, subject to severe erosion, suitable for cultivation only with intensive treatment|
|IV||strongly sloping or gullied; severe limits to cropping, requires careful management; only moderately productive|
|V||level, poorly drained or stoney; extreme limits for crops, useful for pasture, woodland, orgwildlife without special treatment|
|VI||strongly sloping to steep, shallow, or stoney. Only useful for pasture with moderate conservation treatment|
|VII||steep, eroded, or very shallow soil; requires intensive conservation measures for use as pasture|
|VIII||steep, rough, broken, barren, or permanently swamps; not suitable for cultivation. grazing, or forestry|
Table 4. Table Land Judging Score Card after Lind & Harper (1956)
Using Table 4, you can evaluate any given field or pasture and place it into a Land Capability Class using Table 3. While judging a piece of land, make notes as to possible treatments or changes in management that would improve any of the land class factors, eg. if the permeability of the subsoil is poor, what can be done to improve it, or what changes in management such as conversion to pasture would be desirable. Some of the possibilities are: switching from row crops to strip cropping; leaving crop residues (rather than burning or moleboard plowing); control of herbaceous weeds, brush, or trees; converting to pasture from crops; terracing or contour farming; gully control; grassed waterways. It is helpful to make notes concerning these kinds of management possibilities as the land is judged. The SCS soil surveys also contain much land capability information.
The Council on Environmental Quality states that Kansas cropland is losing 3.78 tons of soil per acre per year by water erosion and 2.9 tons per acre per year by wind erosion. Under normal conditions only 1.5 tons of soil per acre per year are formed; thus losses in excess of formation must be considered serious. We are , on the average, losing our topsoil at a greater rate than it is being formed. We are, by analogy with a bank account, spending our capital rather than living off the interest. If continued, we are guilty of breaking the XIth Commandment.
Before erosion can be stopped, it must be recognized. Gulley erosion is obvious to all, and thus easily recognized. Other kinds of erosion may be less easily identified; but can be quite serious, although not so spectacular. The following are also evidences of soil movement and loss: 1) buried vegetation at the low end of the field , i.e., small individual fans; 2) Plants or small stones perched on pedestals (Fig. 3); 3)
small rills in a field; 4) erosion pavement (i.e., the concentration of gravel and rock at the soil surface due to the washing or blowing away of the soil); 5) wind scoured depressions or wind buried plants.
All of these are signs of soil movement and represent loss that may be serious.
Fig. 3. Pedestalling indicates soil loss (from Ellison).
To increase your appreciation and understanding of soil, all or part of the following activities are suggested:
1. Go to a known (perhaps favorite) spot on the aerial soil map, i.e., learn to locate places on aerial photos.
2. Dig a soil pit or, better yet, find a road cut and examine the profile. Using Table 2, determine the textural class of the horizons.
3. Using Table 3 and 4, determine land capability.
4. Make observations on erosion.
5. Obtain the SCS Soil Survey for your county. Locate yourself on the aerial soil maps. Look up the soil series and read about the profile. See how many different kinds of information may be found concerning that particular soil.
If you do all, or part, of these activities listed above, you will find that you have a much better understanding of Kansas' basic and most important resource - soil.
Bidwell, O.W. 1956. Major Soils of Kansas. Kansas Ag. Expt. Sta. Circular 336. Manhattan, Kansas.
Brown, Lester R. 1982. R&D for a Sustainable Society. American Scientist, 70:14·17.
Council on Environmental Quality. 1981. Environmental Trends U.S. Gov't Printing Office.
Ellison, Lincoln, A.R. Croft, and R.W. Bailey. 1951. Indicators of Condition and Trend on High Mountain Watersheds. USDA Handbook #19.
Foth, H.D., L.V. Withee, B.S. Jacobs, and S.J. Thien. 1980. Laboratory Manual for Introductory Soil Science, Wm.C. Brown, Publishers.
Hausenbuiller, R.I. 1978. Soil Science. Wm.C. Brown, Publishers.
Lind, R.C., and H.B . Harper. 1956. Land Judging in Kansas. Kansas Ag. Expt. Sta. Circular 236. Manhattan, Kansas.
Lowdermilk, W.C. 1961. In Our Renewable Wild Lands A Challenge. Walter P. Cottam, University of Utah Press, Salt Lake City. p . 139.
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