Soil Types – Classification

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What is the classification of soil? What are the types and properties of soil? Information on soil types.

SOIL CLASSIFICATION

The world has hundreds of thousands of kinds of soil, each with a unique combination of characteristics resulting from a unique combination of the five factors of soil formation. They are limited to no single country. The same kind of soil is found wherever the same combination of the five factors occurs. To these natural combinations must be added those processes of man’s manipulation that drastically change one or more of the critical characteristics, for better or for worse.

Thus to understand the soils, to see the relationships among them, to discover the principles governing their formation and behavior, and to assemble the results of research and experience to pinpoint recommendations to each field and garden, some system of classification is needed. With the named and defined soils shown on maps, each farmer, gardener, and forester can discover the capabilities and management requirements for the soil he has. Without soil maps, the risks of new soil conservation, irrigation, and settlement schemes are so great that failure is almost certain. The soil survey is the great coordinating link between specific fields, gardens, and forests and man’s accumulated knowledge about the adaptability of plants and systems of soil management practices.
Soil Types – Classification
Units of Classification. The lowest unit in soil classification is the soil type. All the soil types that are alike in profile, general shape, and parent material, although differing in texture, are placed in a soil series, named for the place where the soils were first found. Each name denotes a unique combination of many soil characteristics. Thus Miami loam and Miami silt loam are types within the Miami series. Similar series are, in turn, grouped into soil families having generally similar profiles and generally similar kinds of parent materials. Soil families having similar features due to processes of formation are grouped into great soil groups. These are sometimes called genetic soil types or continental soil types, since they are broadly, but not exactly, coincident with climatic types and broad zones of natural vegetation. The great soil groups are the units of kinds of soil for mapping continents and comparing one large region with another. In all, there are around 50 of them. They have been given international names taken from the languages of the countries first defining them carefully as great soil groups.

The great soil groups, are, in turn, grouped into suborders and orders. Thus, because they occupy great zones dominated by one broad association of plants and climate types, the well drained soils with well-developed characteristics are called zonal soils. Those with distinctive characteristics reflecting partly their climatic and biological environment but mainly some dominant local factor, such as wetness or very limy parent material, are called intrazonal groups, since they occur within two or more zones. If the condition responsible for their unique features is widespread, they may cover large areas—for example, the bog soils in big, swampy places. Thirdly, those soils so young that their characteristics are essentially those of the weathered parent material are called azonol soils, since they occur in all zones without respect to the present climate ot vegetation.

In addition, a unit in any of the categories mentioned—soil type, soil series, soil family, or great soil group—may be subdivided according to differences in some characteristic that is not significant to the behavior of the natural soil in its natural environment, but that is significant when the soil is put under use in a cultural environment. Such features include soil slope, depth, effects of erosion, stoniness, and excess salts. Thus there are nearly level, sloping, or rolling phases of Miami silt loam, of the Miami series, of the Miami family, or of the gray-brown podsolic soils—the great soil group to which it belongs. In carrying out detailed soil classification and mapping, some of the recommendations for use are related to a phase of the soil type. Most reliable soil research results are published under the names of the soil classificational units, commonly soil types or their phases.

Zonal Soils. These soils reflect the full effects of climate and living matter on parent rocks without extremes of mechanical or chemical composition, on moderate slopes where there is neither excessive erosion nor waterlogging, and where the landscape has been stable for a long time. There are five primary groups, or suborders, of zonal soils, each dominated by one primary type or system of soil formation: (1) tundra soils; (2) podsolic soils, represented by the podsol and gray-brown podsolic; (3) chernozemic soils, represented by the chernozem; (4) desertic soils typically represented by the desert soils; and (5) latosolic soils of the tropics, represented by the red latosol. Each of these five suborders contains zonal great soil groups that are, in a sense, traditional, since some of their features are due in part to other types of soil formation.

Tundra Soils. These soils have dark brown peaty surfaces and brownish-black subsoils. Below about 12 to 18 inches (30-45 cm) the soil is too cold for plant roots. Characteristically, the surface is bumpy with low mounds. The vegetation is mainly low Arctic shrubs, lichens, and mosses, with some grasses, stunted trees, and herbaceous flowering plants. Many tundra soils, but not all, are underlain by permanently frozen soil material called permafrost, but in regions of warm summers and exceedingly cold winters, permafrost may also underlie other soils. It is the total cold that causes permafrost, and it commonly occurs where the average annual temperature falls below 28° F ( -2.2° C), with variations owing to local relief and cover.

Most of the soils in the tundra zone are swampy (bog soils) or mountainous (lithosols). Few are useful for farming, except for short-season crops and grasses in favored places where the permafrost is absent or below about 30 inches (75 cm), the soils are well drained or can be made so, and there is a fairly good frost-free season. Because of the stimulating effect of the very long days on some plants, harvests can be had with a short growing season.

Podsolic Soils. The moderately to very strongly acid leached soils of humid, forested regions, with profiles showing tendencies for the movements of iron, humus, or clay, or all three, from the A horizon to the B horizons, are called podsolic. Near the boundaries with other kinds of soils, this movement may be less significant than a more dominant process. Among the several great podsolic soil groups, three are most important:

(1) Podsol: These soils of the cool, humid, forest regions have peaty surface layers, over nearly white acid A« horizons and brown acid B horizons. The name comes from a Russian word for ashlike. Although they are not naturally productive of crops, podsol soils can be made so with careful management, including treatment with lime and fertilizer. Many of the podsols are stony, and in the same region may be large areas of wet soils—bog and half-bog soils.

(2) Gray-brown podsolic: These soils are light colored and leached. They are deeper and better than the podsols. Although not very productive in their natural state, they can be brought to a high level of productivity for a wide range of field and horticultural crops. It was mainly on these soils and their associates that Western civilization developed.

(3) Red-yellow podsolic: These soils of the warm-temperate, humid, forested regions show some influence of the processes dominant among the latosols discussed later. Generally, they are light-colored, acid, and leached. The clays are less active than those of the gray-brown podsolic, and the soils are lower in phosphorus and other plant nutrients. The B horizons tend to be more slowly permeable to water than those of the latosols. Because of this and their low natural fertility, red-yellow podsolic soils are much more subject to erosion.

Scientific agriculture has done much to raise the production potential of these and the gray-brown podsolic soils. By proper fertilization and water control, soils of little or no value in their natural state can be brought to very high levels of production, and electric power for refrigeration makes possible the production and use of livestock products and the wide use of soil-improving grasses and legumes. For example, in the southeastern part of the United States, the once badly eroding soils that had been overused for low-yielding cotton and corn became highly productive again under such improved farming systems or well-managed, rapidly growing forests.

Chernozemic Soils. The outstanding group among these soils is the chernozem, a name derived from the Bussian word for black earth. The surface of black granular soil, 8 to 30 inches (20-75 cm) deep, grades into the lighter-colored parent material through a blocky, transitional B horizon. Near the bottom of the B, or a little below it, is a layer of lime carbonate accumulated by slow leaching out of the upper part of the soil. These soils are nearly neutral, fertile, and highly productive when first cultivated, although after long usage both phosphorus and nitrogen fertilizers are needed for good yields. Water limits production, and droughts are common. Since most of these soils lie deep within the interior of continents, far from the sea, they were little used for crops prior to railroads. Now they furnish a high percentage of the world’s bread grains.

The prairie soils, with black surface soils and clayey B horizon, lie between the chernozem and gray-brown podsolic soils. They are dominant in the famous corn belt of the U. S. Middle West in Iowa, northern and central Illinois, and southern Minnesota.

The reddish-chestnut soils, or southern chernozem, are the chernozem-like soils of warm temperate, and subtropical-subhumid areas. They have some features of the latosols.

The chestnut soils lie on the drier side of the chernozem. They are slightly lower in organic matter and shallower to the lime-rich horizon. These are used for cereals, but droughts are common. Between them and the desert soils are the brown soils, still drier than the chestnut, and used for cereals only in the most favored places.

Desertic Soils. The true desert soils are light-colored, shallow, and low in organic matter because of the very scanty vegetation. Much of the desert landscape is unstable because of the lack of vegetation to hold the soil in place during the hard infrequent rains. The slightly more moist sierozem soils—Russian for gray earth-have a thin but stabilizing vegetation of grass and shrubs, and the warmer, red desert soils have some features of the latosols. All these groups can be used only for extensive grazing, except in small, scattered places where enough extra fresh water accumulates for drought-resistant cereals and beans. Many of them and their alluvial associates are highly developed under irrigation.

Latosolic Soils. The latosolic soils include many groups of reddish leached soils of the tropics that were formerly called laterite. The clays are mainly kaolin, iron oxide, and alumina. Even though rich in clay, most of the soils are non-sticky and permeable. Under conditions of alternate waterlogging and free drainage, the clay material becomes soft and doughy when exposed to wetting and drying, and the material hardens to a slag-like rock material that is very resistant to weathering. Where the effects are weak, the result is concretions rather than a solid mass. The term laterite is now reserved for all of these materials, both the hardened forms and those that harden on exposure. The ground-water laterite soils have a layer of such material more or less hardened as an essential characteristic, and laterite is an erratic constituent of most latosols as a relict of a previous environment under a different climate, a different relief, or both.

Because of the great age of the landscapes in the tropics, the great variations in climate and elevation, the intensive weathering, and the shifting of climate, vegetation zones, and geological erosion cycles, there are more local kinds of soil among the latosols and their azonal and intrazonal associates than in all the other soil regions of the world combined. Among the many groups, perhaps the red latosol of the tropical rain forest may be taken as the main group. They have a very low level of nutrients that is continually renewed by the luxuriant vegetation, but once that vegetation is removed, crops may be grown without fertilizer for only a very few years. Then the forest must return to rejuvenate the structure and fertility of the soil.

Young latosolic soils from nutrient-rich volcanic ash or lava, and the associated alluvial soils, can be used continuously for crops, but only a few such soils exist in the tropics that have within themselves the dynamic to remain productive. Agriculture on such soils consists chiefly of small farms with some kind of shifting cultivation for food crops, sometimes supplemented by small amounts of such industrial crops as cacao, oil palm, or coffee; and large plantations producing rubber, oil palm, bananas, tea, coffee, sisal, pineapple, and other export crops.

Intrazonal Soils. Any of the intrazonal soils most commonly found with one group of zonal soils may extend over into the region of another group. Thus within the region of podsolic soils are bog soils and half-bog soils, with layers of peat underlain by waterlogged soil and the wet wiesenboden, or meadow soils. Many of these can be used if drained. In fact, the drained wiesenboden, or humic gray soils, are among the most productive of temperate climates. Black soils, called rendzina, have surface horizons resembling chernozem but are underlain by chalk, marl, or soft limestone at shallow depths. Also mixed with gray-brown podsolic soils are rich, brown, forested soils, less acid and less clayey in the B horizon, that are called brown forest soils. The partly zonal and partly intrazonal terra rossa soils of Mediterranean climates are red, clayey, neutral soils from hard limestone. Soils dominated by extreme hardpans or claypans are usually separated from their zonal associates as planosols. Then too, soils somewhat resembling the characteristic podsol occur all the way from the Arctic to the equator on imperfectly drained sands with fluctuating water tables. These are known as ground-water podsols.

Associated especially with the chernozemic and desertic soils are soils strongly affected by salts, either now or at some time in the past. These include the saline, or solonchalk, soils; the alkali soils, or solonetz; and the leached or degraded alkali soils, or soloth.

Among the latosolic soils are all the groups suggested above, the groundwater laterite soils, and the important tropical black clays, or black cotton soils, that are poor in organic matter and rich in magnesium. They crack deeply in the dry season, and as the rains come the surface mulch is washed into the cracks. When wet, the cracks close and the soil between them is squeezed up into low mounds. In this way the soils are subject to a repeated, churning that produces a series of low swales and mounds.

Azonal Soils. The most important of the azonal soils is the group of alluvial soils from which people receive a significant part of their food supply within each soil region. As soon as soils from alluvial material have been in place long enough to have the characteristics of any genetic group, such as podsol, red latosol, or chernozem, they cease to be classed as alluvial soils. Greater in extent are the young skeletal soils, so thin over rock that they lack characteristics to identify them with one of the genetic groups. Those on hard rocks are called lithosols and the ones on soft rocks regosols.

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