They also have wide ranges in such qualities as fertility, tilth, ability to hold available moisture, and susceptibility to erosion.

Well-drained soil types with undulating to rolling topography tend to be similar over broad geographic belts. The similarities may extend across from one great soil group to another. Geographic belts marked by certain combinations of great soil groups can therefore be shown on maps of small scale.

Six broad belts are outlined on our schematic soil map of the world.

One consists of mountains and similar rough landscapes, in which many of the soils are stony, shallow, or both. The local patterns of great soil groups and soil types are especially complex in such areas.

The other five broad belts have simpler patterns, but each includes a number of great soil groups. For that matter, the soil types within a single farm commonly represent two or more great soil groups. The broad regions in the map therefore show major kinds of soils rather than complete patterns. Great soil groups other than the dominant ones exist in every belt. These are also important in many localities. The broad belts themselves have much meaning to farming and forestry.

The tundra region has a cold climate, which restricts biological activity and horizon differentiation. The soils are in cold storage for a large part of each year. The deep regolith is permanently frozen in some parts of the tundra. Well-drained soils of the tundra belt have profiles much like those of podzolic soils, although the horizons usually are thinner and less distinct.

PODZOLIC SOILS dominate a broad belt in the higher latitudes of the northern hemisphere and some smaller areas in the southern half of the world. They include the great soil groups known as Podzols (originally from Russian words meaning "ash beneath," referring to the A₂ horizon), Brown Podzolic soils, Gray-Brown Podzolic soils, and Gray Wooded soils. These groups were formed under forest vegetation in humid, temperate climates.

Podzolic soils commonly have distinct A₂ horizons. Some have B horizons that are accumulations of sesquioxides, humus, or both. Others have B horizons that are mainly accumulations of clay with minor amounts of sesquioxides and humus. Podzolic soils are more strongly weathered and leached than chernozemic or desertic soils but less so than latosolic soils. They are commonly acid, low in bases such as calcium, and low in organic matter. Levels of fertility therefore are moderate to low. Available moisture capacity is variable, depending on depth of soil and textures of horizons. As a group, however, the soils are responsive to scientific management.

LATOSOLIC SOILS dominate equatorial belts of Africa and South America. They are also dominant in southeastern parts of Asia and North America, as well as northeastern Australia and the larger islands of the western Pacific Ocean.

They include the great soil groups known as Laterites, Reddish-Brown Lateritic soils, Yellowish-Brown Lateritic soils, Red-Yellow Podzolic soils, and the several kinds of Latosols.

Red-Yellow Podzolic soils are so named because they have some features in common with each of the broad latosolic and podzolic groups, though they are more closely related to the former. Latosolic soils have been formed under forest and savanna vegetation in tropical and subtropical and humid to fairly dry climates. They do not extend into arid regions but may be found in alternately wet and dry zones where rainfall is low.

Latosolic soils are strongly weathered and leached, usually to great depths. In fact, they are the most strongly weathered soils in the world. Despite the deep, strongly weathered regoliths, most of the soils lack distinct horizons, except for a darkened surface layer or A, horizon. Below this, the profile may remain unchanged for many feet. Red-Yellow Podzolic soils differ from others in having distinct A₂ horizons as well.

Red and yellow profile colors are common to latosolic soils because of the large amounts of iron oxides formed through intense weathering. Supplies of plant nutrients normally are low, but the capacity to fix phosphorus in unavailable forms is high. Most latosolic soils are easily penetrated by water and plant roots and are resistant to erosion. Red-Yellow Podzolic soils are more susceptible to erosion than others, being less permeable on the whole. Ease and depth of penetration by roots is illustrated by reports of tree roots extending down to 60 feet in the regoliths beneath latosolic soils in southeastern Brazil.

High porosities are also evident in the many fine tubular channels in such soils. Available moisture capacities are mostly moderate to high in latosolic soils, although they are low in some. Productivity is normally low when latosolic soils are used without benefit of modern science and industry.

CHERNOZEMIC SOILS have been formed under prairie or grass vegetation in humid to semiarid and temperate to tropical climates. These soils are most extensive in temperate zones, but some areas in the Tropics are also large. They include the great soil groups known as Chernozems, Brunizems or Prairie soils, Reddish Prairie soils, Chestnut soils, and Reddish Chestnut soils in temperate regions. In tropical and subtropical regions, the soils have been known as black cotton soils, Grumusols, regurs, and dark clays.

chernozemic soils normally have dark A, horizons of great thickness, are fertile, and but slightly weathered. The A, horizons are among the most prominent found in soils, whereas the B horizons usually are much less distinct. The A, horizons of chernozemic soils are typically high in organic matter and nitrogen in temperate zones but not in tropical and subtropical zones. The profiles compare in depth to those of podzolic soils. They are not so deep as those of latosolic soils but are deeper than those of desertic soils. Chernozemic soils are less acid, higher in bases, and higher in plant nutrients generally than are podzolic or latosolic soils. They are much higher in organic matter and nitrogen, less alkaline, and lower in bases than desertic soils. Available moisture capacities of the soils are usually moderate to high.

Chernozemic soils of temperate zones are among the naturally most fertile soils in the world. They produce about 90 percent of the grain in commercial trade channels. Within the United States, they form the heart of the Corn Belt and wheat-producing regions. Production varies with seasonal weather, because the soils extend from the margins of humid into semiarid zones.

Chernozemic soils of tropical and subtropical zones commonly have unfavorable physical properties for tillage and plant growth. They are high in clay, plastic, and subject to great shrinking and swelling. Most of them are used for agriculture without benefit of modem technology, and their productivity under simple management is low. Problems in handling the soils are difficult.

DESERTIC SOILS have been formed under mixed shrub and grass vegetation or under shrubs in and climates. The climates range from hot to cold. The soils are prominent in the great deserts of Africa, Asia, and Australia and in the smaller ones of North America and South America.

They include the great soil groups known as Desert soils, Red Desert soils, Sierozems, Brown soils, and Reddish-Brown soils.

Besides the desertic soils themselves, the deserts of the world include large proportions of sandy wastes, rocklands, and very shallow soils with the barest beginnings of horizons. Such lands lack agricultural possibilities.

Desertic soils have been very slightly weathered and leached. The shortage of moisture which restricts weathering and leaching also limits plant growth, leaving the soils low in organic matter and nitrogen. Limited rainfall is also reflected in the shallow profiles normal to the soils.

Their horizons are seldom prominent. Most of them are faint. The A horizon has commonly lost carbonates and perhaps some bases and clay and is lighter in color than the B horizon. The slightly darker B horizon has some accumulation of clay, but it is very low in organic matter. Levels of nutrient elements other than nitrogen are usually moderate to high in the soils. Available moisture capacities are variable, depending on thickness of profile and textures of horizons.

Marked contrast exists in the productivity of desertic soils used in a highly developed agriculture or under nomadic grazing an example of the tremendous impact management may have on productivity.

Beyond the effects of management, however, each soil type or great soil group has an ill-defined range in use and management possibilities under a given agricultural technology. Each also differs in its response to changes in technology. The range in possibilities may be narrow, regardless of technology, as it is for steep slopes of Ashe stony loam in the Smokies. Soils of that kind will produce some forest but are not suitable for pasture or crops. On the other hand, Congaree fine sandy loam has a wide range in use and management possibilities. It is suitable for forest, pasture, and a variety of crops within the present agriculture. It would also be productive under a number of other levels of agricultural technology. Yields obtained from Congaree fine sandy loam in any agriculture depend greatly upon the level of management practiced.