CLASSIFIED SOILS IN THE TROPICS

Classified Soils in the Tropics According to the USDA Soil Taxonomy, Oxisols are the most abundant soils in the humid and perhumid tropics covering about 35 percent of the land area. Ultisols are the second most abundant, covering an estimated 28 percent of the region. About half of the Ultisols and 60 percent of the Oxisols are located in humid and perhumid tropical Africa and Asia. In tropical Africa, they are abundant in the eastern Congo basin bordering the lake region; in the forested zones of Sierra Leone; in Ivory Coast; in parts of Liberia; and in the forested coastal strip from Ivory Coast to Cameroon.


The Alfisols, which have high to moderate fertility, cover a smaller area of the humid tropics. In west Africa they are found in Ivory Coast, Ghana, Togo, Benin, Nigeria and Cameroon. They are, however, the most abundant soils in Africa's subhumid and semi-arid zones, covering about one third of these regions. The Alfisols are widely distributed in the subhumid and semi-arid tropical regions of Africa, including large areas in western, eastern, central, and southeastern Africa.

Table. Geographical distribution of soils in the humid and semi-arid tropics
(millions of hectares).

 

Soil order	Tropical Africa	Tropical Asia	Tropical America	Total	Percent
Humid Tropics1)
Oxisols	179	14	332	525	35
Ultisols	69	131	213	413	28
Alfisols	21	15	18	54	4
Others	176	219	103	498	33
Total	445	379	666	1490	100
Semi-arid Tropics2)
Alfisols	466	121	107	694	33
Ultisols	24	20	8	52	1
Others	972	178	198	1348	66
Total	1462	319	313	2094	100

¹⁾ Data from NAP (1982).
²⁾ Data adapted from Kampen and Burford (1980). Part of the subhumid tropics is included.

How soils are classified | SOIL CLASSIFICATION

How soils are classified. Pedologists classify soils according to the characteristics of a polypedon. The Soil Survey Staff of the United States Department of Agriculture uses a system that consists of 10 orders (groups) of soils. They are (1) alfisols, (2) aridisols, (3) entisols, (4) histosols, (5) inceptisols, (6) mollisols, (7) oxisols, (8) spodosols, (9) ultisols, and (10) vertisols.

1. Alfisols develop under forests and grasslands in humid climates. Some agricultural soils are alfisols.
2. Aridisols occur in dry regions and contain small amounts of organic matter. Desert soils are aridisols.
3. Entisols show little development. They resemble the parent material and occur in many climates.
4. Histosols are organic soils. They form in water-saturated environments, including swamps and bogs.
5. Inceptisols are only slightly developed. They are more common in subhumid and humid climates, but also occur in most other kinds of climates.
6. Mollisols develop in prairie regions. They have thick, organically rich topsoils.
7. Oxisols are the most chemically weathered soils. They have a reddish color and occur in tropical regions.
8. Spodosols contain iron, aluminum, and organic matter in their B horizons. They form in humid climates.
9. Ultisols occur in warm, humid climates. They are moist, well-developed, acid soils.
10. Vertisols form in subhumid and arid warm climates. They develop wide, deep cracks during dry seasons.

CHARACTERISTICS OF SOIL

The method and rate of soil formation differs throughout a body of soil. As a result, the soil develops layers. These layers are called soil horizons. Soil horizons may be thick or thin, and they may resemble or differ from the surrounding horizons. The boundaries between the layers can be distinct or barely noticeable.

Most soils include three major horizons. The upper two, called the A and B horizons, are the most highly developed layers. The A horizon is also known as topsoil. The lowest horizon, called the C horizon or the subsoil, is exposed to little weathering. Its composition resembles that of the parent material. Pedologists describe soils by the characteristics of the soil horizons, including (1) color, (2) texture, (3) structure, and (4) chemical conditions.

Color. Soils range in color from yellow and red to dark brown and black. The color of a soil helps pedologists estimate the amounts of air, water, organic matter, and certain elements in the soil. For example, a red color may indicate that iron compounds are present in the soil.

Texture of a soil depends on the size of its mineral particles. Sands are the largest particles. The individual grains can be seen and felt. Silts are just large enough to be seen, and clays are microscopic. Pedologists divide soils into textural classes according to the amounts of sand, silt, and clay in a soil. For example, the mineral portions of soils classified as loam contain from 7 to 27 per cent clay and less than 52 per cent sand. In silty clay, more than 40 per cent of the mineral particles are clay, and more than 40 per cent are silt. Texture helps determine how thoroughly water drains from a soil. Sands promote drainage better than clays.

Structure. When soil particles aggregate, they form clumps of soil that are called peds. Most peds range from less than 1/2 to 6 inches (1.3 to 15 centimeters) in diameter. Their shape and arrangement determine a soil's structure. The ability of peds and soil particles to stick together and hold their shape is called consistence.

Most soils contain two or more kinds of structures. Some soils have no definite structure. In some such soils, the peds lack a definite shape or arrangement. In others, the particles do not aggregate.

There are three main kinds of soil structures: (1) platelike, (2) prismlike, and (3) blocklike. Platelike peds are thin, horizontal plates that occur in any horizon. Prismlike peds are column-shaped subsoil structures. Blocklike peds look like blocks with flat or curved sides. Large, flat-sided, blocklike peds commonly occur in subsoils. Small, rounded, blocklike peds make up most topsoils. They contain more organic matter and hold water and nutrients better than do larger peds.

Chemical conditions. Soils can be acid, alkaline, or neutral. The amounts of acid and alkali in a soil influence the biological and chemical processes that take place there. Highly acid or alkaline soils can harm many plants. Neutral soils support most of the biological and chemical processes, including the process by which green plants obtain many nutrients. This process is called cation exchange. Many nutrients and other elements dissolve in the soil solution, forming positively charged particles called cations. The negatively charged clay and humus attract some cations and prevent them from being leached (washed away) from the topsoil by drainage waters. The solution that remains in the soil contains other cations. Nutrient cations on the clay and humus and those in the soil solution change places with nonnutrient cations that are on roots. The roots can then absorb the nutrients.

HOW SOIL IS FORMED

Soil begins to form when environmental forces break down rocks and similar materials that lie on or near the earth's surface. Pedologists call the resulting matter parent material. As soil develops through the centuries, organic material collects, and the soil resembles the parent material less and less. Glaciers, rivers, wind, and other environmental forces may move parent material and soil from one area to another.

Soils are constantly being formed and destroyed. Some processes, such as wind and water erosion, may quickly destroy soils that took thousands of years to form.

Soil formation differs according to the effects of various environmental factors. These factors include (1) kinds of parent material, (2) climate, (3) land surface features, (4) plants and animals, and (5) time.

Kinds of parent material. The type of parent material helps determine the kinds of mineral particles in a soil. A process called weathering breaks down parent material into mineral particles. There are two kinds of weathering, physical disintegration and chemical decomposition. Physical disintegration is caused by ice, rain, and other forces. They wear down rocks into smaller particles that have the same composition as the parent material. Sand and silt result from physical disintegration.

Chemical decomposition mainly affects rocks that are easily weathered. In this kind of weathering, the rock's chemical structure breaks down, as when water dissolves certain minerals in a rock. Chemical decomposition results in elements and in chemical compounds and elements that differ from the parent material. Some of these substances dissolve in the soil solution and become available as plant nutrients. Others recombine and form clay particles or other new minerals.

The mineral content of parent material also affects the kinds of plants that grow in a soil. For example, some plants, including azaleas and rhododendrons, grow best in acid soils that contain large amounts of iron.

Effects of climate. Climate affects the amount of biological and chemical activity in a soil, including the kinds and rates of weathering. For example, physical disintegration is the main form of weathering in cool, dry climates. Higher temperatures and humidity encourage chemical decomposition as well as disintegration. In addition, decaying and most other soil activities require warm, moist conditions. These activities slow down or even stop in cold weather. Therefore, soils in cool, dry climates tend to be shallower and less developed than those in warm, humid regions.

Effects of land surface features also influence the amount of soil development in an area. For example, water running off the land erodes the soil and exposes new rock to weathering. Also, soils on slopes erode more rapidly than those on flat areas. They generally have less time to form and therefore develop less than do soils on flat terrains.

Effects of plants and animals. Soil organisms and organic material help soil develop, and they also protect it from erosion. The death and decay of plants and animals add organic material to the soil. This organic material helps the soil support new organisms. Soils that have a cover of vegetation and contain large amounts of organic material are not easily eroded.

Effects of time. Soils that are exposed to intense soil formation processes for long periods of time become deep and well developed. Soils that erode quickly or have been protected from such processes for a long time are much less developed.

COMPOSITION OF SOILS

The mineral and organic particles in soil are called soil particles. Water and air occupy the spaces between the particles. Plants and animals live in these pore spaces. Plant roots also grow through the pore spaces.

Minerals supply nutrients to green plants. Particles called sands, silts, and clays make up most of the mineral content of soils.

Sands and silts are particles of such minerals as quartz and feldspars. Clays consist of illite, kaolin, micas, vermiculite, and other minerals. Trace amounts of many minerals add nutrients, including calcium, phosphorus, and potassium, to the soil. Most soils are called mineral soils because more than 80 per cent of their soil particles are minerals.

Plant and animal matter consists of organic material in various stages of decay. Many organisms also live in the soil. These soil organisms include plant roots, microbes, and such animals as worms, insects, and small mammals. Bacteria, fungi, and other microbes decompose (break down) dead plants and animals. Many soil organisms help mineral and organic particles aggregate (come together) and form clumps of soil. Roots, burrowing animals, and natural weathering break apart large clumps of soil.

Decaying organic material releases nutrients into the soil. In addition, some organic material combines with mineral particles. Other decaying material forms organic soil particles called humus. Most humus is black or dark brown, and it holds large amounts of water. Only 6 to 12 per cent of the volume of particles in most mineral soils is organic. However, these small quantities greatly increase a soil's ability to support plant life. In some soils, called organic soils, more than 20 per cent of the soil particles are organic.

Water that enters the soil dissolves minerals and nutrients and forms a soil solution. Much of the solution drains away, but some remains in the pore spaces. Green plants obtain water and some nutrients by absorbing soil solution through their roots.

Air replaces the water that drains from the larger pore spaces. Soil organisms live best in soils that contain almost equal amounts of air and water.

Do You Know, WHAT IS SOIL?

Do You Know, What is Soil ? Soil is an important natural resource that covers much of the earth's land surface. Most life on earth depends upon the soil as a direct or indirect source of food. Plants are rooted in the soil and obtain nutrients (nourishing substances) from it. Animals get nutrients from plants or from animals that eat plants. Certain microbes in the soil cause dead organisms to decay, which helps return nutrients to the soil. In addition, many kinds of animals find shelter in the soil.

Soil contains mineral and organic particles, other plant and animal matter, and air and water. The contents of soil change constantly. There are many kinds of soils, and each has certain characteristics, including color and composition. The kind of soil in an area helps determine how well crops grow there. Soil forms slowly and is destroyed easily, and so it must be conserved so it can continue to support life.

Soil scientists, called pedologists, use the term polypedons for the bodies of individual kinds of soil in a geographic area. Polypedons can be indefinitely large, but some have a surface area of only about 10.8 square feet (1 square meter). Some polypedons measure less than 5 inches (13 centimeters) deep. Others are more than 4 feet (1.2 meters) deep.