Soil is the thin layer of very tiny particles overlying the earth's crust.  It is a mixture of solid, liquid and gaseous materials which support plant growth.  Soils provide anchorage to plants and store plant nutrients such as nitrogen, potassium and so on.

SOIL COMPOSITION

A soil consists of five main components:

(a)     Organic matter: Organic matter is the humus obtained when plant and animal matter decomposes. Humus  is the black or dark-brown top layer of the soil. It plays a vital role in maintaining soil fertility.  It forms 5% of the total soil by volume.

(b)     Inorganic matter: This consists of minerals got from the parent rock.  It forms about 45% of the soil materials. Minerals like calcium,  potassium, manganese, iron, aluminum, sodium and phospherous are common in rocks.  These minerals are very important for plant growth.

(c)     Soil air.  Air exists in pre spaces that are not occupied by water.  Air in these spaces is an important component because plant roots and micro- organism such as bacteria make use of it.

(d)     Soil water. Water is perhaps the most important factor influencing plant growth.  Soil water varies from one soil to another depending on whether the soil is saturated with moisture.

(e)     Soil organisms. Very tiny plants and animals live in the soil together as soil organisms. Among these organisms are bacteria.  Bacteria play a vital role in the soil. Its prime function is the conversion of nitrogen from the air into nitrates and ammonium compounds which are used by plant roots in solution. Besides, bacteria play an important role in the decomposition of organic matter into humus.

The formation of soil.

Soil is formed from rocks. Different types of rocks are broken down into tiny particles by a process of weathering(mechanical, chemical and biological).  These processes which aid soil formation are influenced by several factors. The several factors have interacted to form soil.  The main ones  include:

The parent rock: The nature of the parent rock determines the rate of rock breakdown, the colour and texture of a soil, for example areas with volcanic rocks with a low quartz content and a loose structure weather down rapidly.  Besides granitic rocks give rise to sandy soils and rocks containing iron tend to give soil a reddish colour.

Climate: Climate influences the rate and type of weathering , for example chemical weathering is rapid when temperatures and humidity are high.  This leads to deep weathering that produces deep soils.

Living organisms: These organisms add humus to the soil when they die and decay. The roots of plants penetrate cracks in the rocks and help in their weathering. In this way rocks are broken into fine particles that eventually form soil.

Topography/ Relief: Topography affects soil drainage.  Hill tops have well drained and heavily leached soils. Soils on hill-tops particularly the Buganda hilly landscape and the Kenya highlands are reddish, indicating a high degree of leaching.

On the steep slopes, soils are thin and stony because they are heavily eroded and have little organic matter.  Whereas in valley basins, water logged soils occur.

Time: Time is required for all other factor to play their part in soil development.  This varies from place to place. Mature soils may take about 20 to 30 years to form while others take several thousand years.

Man's activities: The influence is either direct or indirect.  Directly man involves in actual breakdown e.g. through mining, quarrying, road construction. Indirectly through his influence on vegetation types and climate etc.

Classification of soils.

Soils can be classified into three major categories:

Zonal soils: These are soils which are fully well- developed in profile and have taken many years to form e.g. latosols (laterites) of the Nyika plateau and Buganda region.

Intrazonal soils: These are soils formed in regions with poor climatic conditions. The soils are, therefore , not well developed.  These soils are particularly found on poorly drained areas.

Azonal soils: These are relatively young soils which have not had enough tome to develop.  These types of soils do not have a clear soil profile. Azonal soil include: soils formed due to glaciation, recent weathered lava, deposited mudflats, soils by waves, alluvial soils and mountain soils(scree soils at the lower slopes.)

Soil profile: A soil profile is a vertical section through the soil showing the various horizontal layers or horizons from the surface down to the underlying rock.  A mature soil has a well developed soil profile with three main horizons, A, B, and C as seen in the diagram below.

 

 

 

 

 

 

 

 

 

An A horizon contains a layer of humus depending on the area from which the soil profile has been extracted.  The texture of the soil becomes coarser as one moves down from A horizon to C.  A horizon is the most important layer which supports plant growth.  The B horizon is compact and has a high clay content.  The C horizon , is the parent rock which may be partly weathered.

Soil catena: This is the successive arrangement or sequence of arrangement of differing soil types along a slope from the hill summit to the valley bottom.  The hill tops have exposed rocky out crops because of heavy leaching which creates laterite rocks.  On steep slopes with out vegetation, soil erosion is more rapid than on gentle slopes, for this reason steep slopes tend to have a thin layer of soil.  While the valley-bottoms will have deep soils derived from the weathered remains of rock as well as material carried down from above by erosion and mass movement.

 

 

 

Types of soil: (a) Laterite soils

These soils are formed on humid areas of East Africa by the process known as leaching.  This involves the downward movement of water through the soil which results in the removal in solution form the surface layers of soluble mineral and organic compounds, leaving behind the concentration of insoluble iron and aluminum compounds which give rise to a reddish soil known as laterite / murram.

Laterite soils lack acids and mineral nutrients because of being leached and they also lack humus. Therefore they are poor soils.  But they are good for road construction and brick making.

(b)     Sandy soils: These soils are coarse and contain large particles and water easily penetrate them  because they have large pore spaces, because of this they are easily leached and hence they are poor soils.

(c)     Clay soils: Clay soils are fine- grained and contain less or no humus. Sometimes, they are rich in plant nutrients  They retain a lot of water, are acidic and lack enough oxygen which would facilitate the bacteria to decompose the organic matter. This partly explain why humus layer over clay soils may be missing.

(d)     Loam soils: These are the most fertile soils and support the growing of various crops.  Loam soils re a mixture of silt, humus, sand and little clay.

SOIL EROSION

MEANING OF SOIL EROSION.

Soil erosion is the process whereby the topsoil is removed by the forces of nature such as the action of running water, glacier and wind.  The eroded soil is normally transported and deposited in another place.

2.       TYPES OF SOIL EROSION.

There are four main types of soil erosion:

Splash erosion

This is caused by the impact of heavy rain drops which hit the unconsolidated soil particles causing them to be dislodged and deposited else where on the surface.  Splash erosion creates small depressions and can lead to other types of erosion.

Sheet erosion.

This is the type of soil erosion in which loose soil particles are removed evenly by surface run-off over a big area especially after a rain storm.

Rill erosion.

This type of erosion occurs where the rate of rainfall exceeds the rate of infiltration of water.  This process results in the formation of small channels which look  "r" Rill erosion is worse on slopes as it leads to the formation of gullies.

Gully erosion.

Gullies are narrow and deep channels worn in the earth by the action of water.  A sudden heavy down fall of rain may cut deep channels especially if the region consists of soft materials. Gully erosion produces what is known as bad land surfaces.

Causes of soil erosion

Soil erosion can occur naturally but it is mainly caused by human activities.  Therefore soil erosion in East Africa is caused by both physical and human factors.  The following factors can cause soil erosion.

Physical factors

1.       The relief or topography of the area.  Hilly areas are more prone to severe erosion than the undulating or gentle landscapes.. The gradient in these hilly areas offer the required force for the removal of the top soil by running water.

 

 

 

 

 

 

AREAS AFFECTED BY SOIL EROSION IN EAST AFRICA.

 

 

 

 

2.       The climate of the area.  This provides the agents of erosion, for example wind, water and ice. The types of erosion agent varies with climate.  Areas of East Africa receiving high rainfall experience erosion by surface run-off.  Wind erosion is concentrated in the semi-arid areas like Masai land, North East Kenya and North East Uganda. Glacial erosion is confined in the mountain areas of East Africa above 400m a. s. l. , for example Mt. Kenya, Mt. Kilimanjaro and Mt. Rwenzori.

3.       Vegetation is another cause of soil erosion.  Thick vegetation reduces the rate of erosion.Thin vegetation encourages soil erosion because the soil particles are loose and thus easy to erode.

2.                 Human factors.

¨     Deforestation through lumbering, settlement, road construction   etc.

¨     Mining or quarrying of open cast nature disrupts the soil structure thus exposing it to erosion.

¨     Overstocking which leads to overgrazing, thus exposing the soil to the agents of soil erosion.

¨     The ploughing of land up and down the slope. This provides man-made channels which can be enlarged into gullies by surface run-off.

¨     The cultivation of the same type of crop on a piece of land year after year, leads to soil depletion if manure or fertilizers are not used.

¨     Bush burning by the shifting cultivators and local herdsmen also encourages the occurance of wind erosions since burning leaves the land bare.

Effects of soil erosion.

¨     Soil erosion leads to the loss of soil fertility by causing the loss of the lightest and smallest soil particles like clay which contain plant nutrients.

¨     The loss of soil fertility by soil erosion results into the decline in crop production (low yields) leading to the occurance of famine in areas concerned.

¨     Soil erosion leads to the silting of rivers and this has the following effects:

¨     Increased sediment load in rivers causes flooding which interrupts communication networks near the valleys.

¨     When silt accumulates behind the reservoirs, it reduces the life of a dam.

¨     Silting pollutes beaches through rivers depositing a lot of silt where they drain into ocean and lakes.

¨     Gullies are common in the wetter areas where gully erosion is prominent.  The formation of gullies creates bad landscape.  Besides deep gullies hinder communication development.

¨     There is loss of soil moisture because erosion exposes the sub- soil whose infiltration rate and water - holding capacity are lower than the top soil.

¨     It leads to the destruction of vegetation cover and in general there is environmental degradation.

 

 

Methods of conservation.

In order to prevent loss of the top soil in East Africa, a number of conservation methods or devices have been adopted.  The following are the most important methods of controlling soil erosion.

Strip cropping

This is the practice of planting different types of crops in alternating parallel strips to ensure that no piece of land  is left bare or exposed to the agents of erosion of erosion at an season.  It will be so because different crops in different strips may ripen at different times  of the year and be harvested at different seasons.  The taller crops in the strips may act as wind breakers.  This can be illustrated in the diagram bellow.

 

 

 

 

 

 

 

 

Contour ploughing

This farming method involves ploughing across the slope (along the contours) instead of up and down. Contour ploughing reduces the risk of rill erosion.

 

Terracing.

This means the cutting of a series of horizontal steps into a hill side to provide cultivable land in an area of steep relief, and to reduce soil erosion. Terracing is practiced in the highland areas of East Africa. This can be illustrated in the diagram below.

 

 

 

 

Maintenance of vegetation cover along steep slopes.

Vegetation cover protects the soil particles from the action of water and strong wind.  Plant roots bind the soil particles together. Planting of trees (afforestation) and grass can stop soil erosion where it has started.

Crop rotation.

This is the means of alternating crops in the fields to avoid growing the same crop in the same field for more than two years in succession.  By rotating different types of crops in successive years, soil fertility can be maintained.

Cover cropping: In some cases, as in plantations, where the gestation period of tree crops is long, cover crops may be interplanted between the young trees to protect the top soil from the full force of the tropical down pour.

Use of fertilizers: The use of fertilizers especially organic manure such as animal dung, compost or decomposed vegetation provide a balanced supply of the major soil minerals and improves the general structure of the soil. (soil structure is the character of a soil shown by the ability of its particles to come together and to hold together to form AGGREGATES).

Fallowing: It is important to allow much- used land to rest or lie fallow, so that  the natural vegetative matter helps to increase the plant nutrients in the soil.  Fallowing also increases the sub soil moisture and improves the general structure of the soil hence reducing soil erosion.  Soil moisture can also be increased through Mulching.

Wind breaks: Trees are planted in rows to reduce the speed of the prevailing wind and prevent it from eroding the soil where it has been exposed.  Wind breaks are common in flat areas where soil is eroded mainly by the wind.

Areas of severe soil erosion in East Africa.

Introduction

Soil is the loose material that covers the land surfaces of Earth and supports the growth of plants. In general, soil is an unconsolidated, or loose, combination of inorganic and organic materials. The inorganic components of soil are principally the products of rocks and minerals that have been gradually broken down by weather, chemical action, and other natural processes. The organic materials are composed of debris from plants and from the decomposition of the many tiny life forms that inhabit the soil.

Soils vary widely from place to place. Many factors determine the chemical composition and physical structure of the soil at any given location. The different kinds of rocks, minerals, and other geologic materials from which the soil originally formed play a role. The kinds of plants or other vegetation that grow in the soil are also important. Topography-that is, whether the terrain is steep, flat, or some combination-is another factor. In some cases, human activity such as farming or building has caused disruption. Soils also differ in color, texture, chemical makeup, and the kinds of plants they can support.

Soil actually constitutes a living system, combining with air, water, and sunlight to sustain plant life. The essential process of photosynthesis, in which plants convert sunlight into energy, depends on exchanges that take place within the soil. Plants, in turn, serve as a vital part of the food chain for living things, including humans. Without soil there would be no vegetation-no crops for food, no forests, flowers, or grasslands. To a great extent, life on Earth depends on soil.

The study of different soil types and their properties is called soil science or pedology. Soil science plays a key role in agriculture, helping farmers to select and support the crops on their land and to maintain fertile, healthy ground for planting. Understanding soil is also important in engineering and construction. Soil engineers carry out detailed analysis of the soil prior to building roads, houses, industrial and retail complexes, and other structures.

Soil takes a great deal of time to develop-thousands or even millions of years. As such, it is effectively a nonrenewable resource. Yet even now, in many areas of the world, soil is under siege. Deforestation, over-development, and pollution from humanmade chemicals are just a few of the consequences of human activity and carelessness. As the human population grows, its demand for food from crops increases, making soil conservation crucial.

 

COMPOSITION OF SOILS

Soils comprise a mixture of inorganic and organic components: minerals, air, water, and plant and animal material. Mineral and organic particles generally compose roughly 50 percent of a soil's volume. The other 50 percent consists of pores-open areas of various shapes and sizes. Networks of pores hold water within the soil and also provide a means of water transport. Oxygen and other gases move through pore spaces in soil. Pores also serve as passageways for small animals and provide room for the growth of plant roots.

A

Inorganic Material

The mineral component of soil is made up of an arrangement of particles that are less than 2.0 mm (0.08in) in diameter. Soil scientists divide soil particles, also known as soil separates, into three main size groups: sand, silt, and clay. According to the classification scheme used by the United States Department of Agriculture (USDA), the size designations are: sand, 0.05 to 2.00 mm (0.002 to 0.08 in); silt 0.002 to 0.05 mm (0.00008 to 0.002 in); and clay, less than 0.002 mm (0.00008 in). Depending upon the rock materials from which they were derived, these assorted mineral particles ultimately release the chemicals on which plants depend for survival, such as potassium, calcium, magnesium, phosphorus, sulfur, iron, and manganese.

B

Organic Material

Organic materials constitute another essential component of soils. Some of this material comes from the residue of plants-for example, the remains of plant roots deep within the soil, or materials that fall on the ground, such as leaves on a forest floor. These materials become part of a cycle of decomposition and decay, a cycle that provides important nutrients to the soil. In general, soil fertility depends on a high content of organic materials.

Even a small area of soil holds a universe of living things, ranging in size from the fairly large to the microscopic: earthworms, mites, millipedes, centipedes, grubs, termites, lice, springtails, and more. And even a gram of soil might contain as many as a billion microbes-bacteria and fungi too small to be seen with the naked eye. All these living things form a complex chain: Larger creatures eat organic debris and excrete waste into the soil, predators consume living prey, and microbes feed on the bodies of dead animals. Bacteria and fungi, in particular, digest the complex organic compounds that make up living matter and reduce them to simpler compounds that plants can use for food. A typical example of bacterial action is the formation of ammonia from animal and vegetable proteins. Other bacteria oxidize the ammonia to form nitrogen compounds called nitrites, and still other bacteria act on the nitrites to form nitrates, another type of nitrogen compound that can be used by plants. Some types of bacteria are able to fix, or extract, nitrogen directly from the air and make it available in the soil.

Ultimately, the decay of plant and animal material results in the formation of a dark-colored organic matter known as humus. Humus, unlike plant residues, is generally resistant to further decomposition.

C

Water

Soil scientists also characterize soils according to how effectively they retain and transport water. Once water enters the soil from rain or irrigation, gravity comes into play, causing water to trickle downward. Water is also taken up in great quantities by the roots of plants: Plants use anywhere from 200 to 1,000 kg (440 to 2,200 lb) of water in the formation of 1 kg (2.2 lb) of dry matter. Soils differ in their capacity to retain moisture against the pull exerted by gravity and by plant roots. Coarse soils, such as those consisting of mostly of sand, tend to hold less water than do soils with finer textures, such as those with a greater proportion of clays.

Water also moves through soil pores independently of gravity. This movement can occur via capillary action, in which water molecules move because they are more attracted to the pore walls than to one another. Such movement tends to occur from wetter to drier areas of the soil. The movement from soil to plant roots can also depend on how tightly water molecules are bound to soil particles. The attraction of water molecules to each other is an example of cohesion. The attraction of water molecules to other materials, such as soil or plant roots, is a type of adhesion. These effects, which determine the so-called matric potential of the soil, depend largely on the size and arrangement of the soil particles. Another factor that can affect water movement is referred to as the osmotic potential. The osmotic potential hinges on the amount of dissolved salts in the soil. Soils high in soluble salt tend to reduce uptake of water by plant roots and seeds. The sum of the matric and osmotic potentials is called the total water potential.

In soil, water carries out the essential function of bringing mineral nutrients to plants. But the balance between water and air in the soil can be delicate. An overabundance of water will saturate the soil and fill pore spaces needed for the transport of oxygen. The resulting oxygen deficiency can kill plants. Fertile soils permit an exchange between plants and the atmosphere, as oxygen diffuses into the soil and is used by roots for respiration. In turn, the resulting carbon dioxide diffuses through pore spaces and returns to the atmosphere. This exchange is most efficient in soils with a high degree of porosity. For farmers, gardeners, landscapers, and others with a professional interest in soil health, the process of aeration-making holes in the soil surface to permit the exchange of air-is a crucial activity. The burrowing of earthworms and other soil inhabitants provides a natural and beneficial form of aeration.

III

SOIL FORMATION

Soil formation is an ongoing process that proceeds through the combined effects of five soil-forming factors: parent material, climate, living organisms, topography, and time. Each combination of the five factors produces a unique type of soil that can be identified by its characteristic layers, called horizons. Soil formation is also known as pedogenesis (from the Greek words pedon, for "ground," and genesis, meaning "birth" or "origin").

 

A.                       Parent Material

A

The first step in pedogenesis is the formation of parent material from which the soil itself forms. Roughly 99 percent of the world's soils derive from mineral-based parent materials that are the result of weathering, the physical disintegration and chemical decomposition of exposed bedrock. The small percentage of remaining soils derives from organic parent materials, which are the product of environments where organic matter accumulates faster than it decomposes. This accumulation can occur in marshes, bogs, and wetlands.

Bedrock itself does not directly give rise to soil. Rather, the gradual weathering of bedrock, through physical and chemical processes, produces a layer of rock debris called regolith. Further weathering of this debris, leading to increasingly smaller and finer particles, ultimately results in the creation of soil.

In some instances, the weathering of bedrock creates parent materials that remain in one place. In other cases, rock materials are transported far from their source-blown by wind, carried by moving water, and borne inside glaciers.

B.                       Climate

Climate directly affects soil formation. Water, ice, wind, heat, and cold cause physical weathering by loosening and breaking up rocks. Water in rock crevices expands when it freezes, causing the rocks to crack. Rocks are worn down by water and wind and ground to bits by the slow movement of glaciers. Climate also determines the speed at which parent materials undergo chemical weathering, a process in which existing minerals are broken down into new mineral components. Chemical weathering is fastest in hot, moist climates and slowest in cold, dry climates.

Climate also influences the developing soil by determining the types of plant growth that occur. Low rainfall or recurring drought often discourage the growth of trees but allow the growth of grass. Soils that develop in cool rainy areas suited to pines and other needle-leaf trees are low in humus.

C.                       Living Organisms

As the parent material accumulates, living things gradually gain a foothold in it. The arrival of living organisms marks the beginning of the formation of true soil. Mosses, lichens, and lower plant forms appear first. As they die, their remains add to the developing soil until a thin layer of humus is built up. Animals' waste materials add nutrients that are used by plants. Higher forms of plants are eventually able to establish themselves as more and more humus accumulates. The presence of humus in the upper layers of a soil is important because humus contains large amounts of the elements needed by plants.

Living organisms also contribute to the development of soils in other ways. Plants build soils by catching dust from volcanoes and deserts, and plants' growing roots break up rocks and stir the developing soil. Animals also mix soils by tunneling in them.

 D.                       Topography

Topography, or relief, is another important factor in soil formation. The degree of slope on which a soil forms helps to determine how much rainfall will run off the surface and how much will be retained by the soil. Relief may also affect the average temperature of a soil, depending on whether or not the slope faces the sun most of the day.

E.                        Time

The amount of time a soil requires to develop varies widely according to the action of the other soil-forming factors. Young soils may develop in a few days from the alluvium (sediments left by floods) or from the ash from volcanic eruptions. Other soils may take hundreds of thousands of years to form. In some areas, the soils may be more than a million years old.

G.                      Horizons

Most soils, as they develop, become arranged in a series of layers, known as horizons. These horizons, starting at the soil surface and proceeding deeper into the ground, reflect different properties and different degrees of weathering.

Soil scientists have designated several main types of horizons. The surface horizon is usually referred to as the O layer; it consists of loose organic matter such as fallen leaves and other biomass. Below that is the A horizon, containing a mixture of inorganic mineral materials and organic matter. Next is the E horizon, a layer from which clay, iron, and aluminum oxides have been lost by a process known as leaching (when water carries materials in solution down from one soil level to another). Removal of materials in this manner is known as eluviation, the process that gives the E horizon its name. Below E horizon is the B horizon, in which most of the iron, clays, and other leached materials have accumulated. The influx of such materials is called illuviation. Under that layer is the C horizon, consisting of partially weather bedrock, and last, the R horizon of hard bedrock.

Along with these primary designations, soil scientists use many subordinate names to describe the transitional areas between the main horizons, such as Bt horizon or BX2 horizon.

Soil scientists refer to this arrangement of layers atop one another as a soil profile. Soil profiles change constantly but usually very slowly. Under normal conditions, soil at the surface is slowly eroded but is constantly replaced by new soil that is created from the parent material in the C horizon.

H.                      SOIL CHARACTERISTICS

Scientists can learn a lot about a soil's composition and origin by examining various features of the soil. Color, texture, aggregation, porosity, ion content, and pH are all important soil characteristics.

I.   Color

Soils come in a wide range of colors-shades of brown, red, orange, yellow, gray, and even blue or green. Color alone does not affect a soil, but it is often a reliable indicator of other soil properties. In the surface soil horizons, a dark color usually indicates the presence of organic matter. Soils with significant organic material content appear dark brown or black. The most common soil hues are in the red-to-yellow range, getting their color from iron oxide minerals coating soil particles. Red iron oxides dominate highly weathered soils. Soils frequently saturated by water appear gray, blue, or green because the minerals that give them the red and yellow colors have been leached away.

 

J. Texture

A soil's texture depends on its content of the three main mineral components of the soil: sand, silt, and clay. Texture is the relative percentage of each particle size in a soil. Texture differences can affect many other physical and chemical properties and are therefore important in measures such as soil productivity. Soils with predominantly large particles tend to drain quickly and have lower fertility. Very fine-textured soils may be poorly drained, tend to become waterlogged, and are therefore not well-suited for agriculture. Soils with a medium texture and a relatively even proportion of all particle sizes are most versatile. A combination of 10 to 20 percent clay, along with sand and silt in roughly equal amounts, and a good quantity of organic materials, is considered an ideal mixture for productive soil.

 

K.                       Aggregation

Individual soil particles tend to be bound together into larger units referred to as aggregates or soil peds. Aggregation occurs as a result of complex chemical forces acting on small soil components or when organisms and organic matter in soil act as glue binding particles together.

Soil aggregates form soil structure, defined by the shape, size, and strength of the aggregates. There are three main soil shapes: platelike, in which the aggregates are flat and mostly horizontal; prismlike, meaning greater in vertical than in horizontal dimension; and blocklike, roughly equal in horizontal and vertical dimensions and either angular or rounded. Soil peds range in size from very fine-less than 1 mm (0.04 in)-to very coarse-greater than 10 mm (0.4 in). The measure of strength or grade refers to the stability of the structural unit and is ranked as weak, moderate, or strong. Very young or sandy soils may have no discernible structure.

L.                       Porosity

The part of the soil that is not solid is made up of pores of various sizes and shapes-sometimes small and separate, sometimes consisting of continuous tubes. Soil scientists refer to the size, number, and arrangement of these pores as the soil's porosity. Porosity greatly affects water movement and gas exchange. Well-aggregated soils have numerous pores, which are important for organisms that live in the soil and require water and oxygen to survive. The transport of nutrients and contaminants will also be affected by soil structure and porosity.

 

M.                     Ion Content

Soils also have key chemical characteristics. The surfaces of certain soil particles, particularly the clays, hold groupings of atoms known as ions. These ions carry a negative charge. Like magnets, these negative ions (called anions) attract positive ions (called cations). Cations, including those from calcium, magnesium, and potassium, then become attached to the soil particles, in a process known as cation exchange. The chemical reactions in cation exchange make it possible for calcium and the other elements to be changed into water-soluble forms that plants can use for food. Therefore, a soil's cation exchange capacity is an important measure of its fertility.

 

pH

Another important chemical measure is soil pH, which refers to the soil's acidity or alkalinity. This property hinges on the concentration of hydrogen ions in solution. A greater concentration of hydrogen results in a lower pH, meaning greater acidity. Scientists consider pure water, with a pH of 7, neutral. The pH of a soil will often determine whether certain plants can be grown successfully. Blueberry plants, for example, require acidic soils with a pH of roughly 4 to 4.5. Alfalfa and many grasses, on the other hand, require a neutral or slightly alkaline soil. In agriculture, farmers add limestone to acid soils to neutralize them.

 

N.                       SOIL CLASSIFICATION

As yet there is no worldwide, unified classification scheme for soil. Since the birth of the modern discipline of soil science roughly 100 years ago, scientists in different countries have used many systems to organize the various types of soils into groups. For much of the 20th century in the United States, for example, soil scientists at the USDA used a classification scheme patterned after an earlier Russian method. This system recognized some three dozen Great Soil Groups.

In 1975 a new classification scheme known as soil taxonomy was published in the United States and is now used by the USDA. Unlike earlier systems, which organized soils according to various soil formation factors, the new system emphasizes characteristics that can be precisely measured, including diagnostic horizons (which give clues to soil formation), soil moisture, and soil temperature. In a manner similar to the kingdom, phylum, class, order, family, genus, species system used to classify living things, the USDA soil taxonomy employs six categories. From the general to the more specific, its categories are order, suborder, great group, subgroup, family, and series. This system has classified more than 17,000 types of soil in the United States.

The top level of the system consists of 12 orders: alfisols, andisols, aridisols, entisols, gelisols, histosols, inceptisols, mollisols, oxisols, spodosols, ultisols, and vertisols. Each term employs a Latin or Greek word root to describe a range of soil characteristics. Mollisols, for example (from the Latin mollis, for "soft") are soils with thick, dark surface horizons that have a high proportion of organic matter. Such soils can be found in the midwestern United States stretching up into Canada and in portions of northwestern North America. Regions in New England and the eastern portion of Canada, meanwhile, contain spodosols (from the Greek spodos, meaning "wood ash"), which are characterized by a light-colored, grayish topsoil and subsoil accumulation of aluminum, organic matter, and iron. Soil scientists classify soils in many of the southern United States as ultisols (from the Latin for "last"), heavily weathered soils with high concentrations of aluminum. In the southwest, meanwhile, aridisols (from the Latin aridus, for "dry"), featuring little organic matter, are found, as their name implies, in arid lands with little plant growth.

The suborder and great group names of the soil taxonomy provide increasing levels of detail. The suborder aqualf, for example, combines aqu from the Latin aqua, for "water," and alf from alfisol to describe wet soils. Using assorted roots and combining them in different ways, scientists describe soils in a highly specialized and specific language. Aeric fragiaqualfs, for example, are wet, well-developed soils with aerated surface layers and restrictive subsoils.

 

O.                      SOIL USE

For most of human history, soil has not been treated as the valuable and essentially nonrenewable resource that it is. Erosion has devastated soils worldwide as a result of overuse and misuse. In recent years, however, farmers and agricultural experts have become increasingly concerned with soil management.

P.                        Erosion

Erosion is the wearing away of material on the surface of the land by wind, water, or gravity. In nature, erosion occurs very slowly, as natural weathering and geologic processes remove rock, parent material, or soil from the land surface. Human activity, on the other hand, greatly increases the rate of erosion. In the United States, the farming of crops accounts for the loss of over 3 billion metric tons of soil each year.

In a cultivated field from which crops have been harvested, the soil is often left bare, without protection from the elements, particularly water. Raindrops smash into the soil, dislodging soil particles. Water then carries these particles away. This movement may take the form of broad overland flows known as sheet erosion. More often, the eroding soil is concentrated into small channels, or rills, producing so-called rill erosion. Gravity intensifies water erosion. Landslides, in which large masses of water-loosened soil slide down an incline, are a particularly extreme example.

Wind erosion occurs where soils are dry, bare, and exposed to winds. Very small soil particles can be suspended in the air and carried away with the wind. Larger particles bounce along the ground in a process called saltation.

 

Q.                      Soil Management

To prevent exposure of bare soil, farmers can use techniques such as leaving crop residue in the soil after harvesting or planting temporary growths, such as grasses, to protect the soil from rain between crop-growing seasons. Farmers can also control water runoff by planting crops along the slope of a hill (on the contour) instead of in rows that go up and down.

Soil faces many threats throughout the world. Deforestation, overgrazing by livestock, and agricultural practices that fail to conserve soil are three main causes of accelerated soil loss. Other acts of human carelessness also damage soil. These include pollution from agricultural pesticides, chemical spills, liquid and solid wastes, and acidification from the fall of acid rain. Loss of green spaces, such as grassland and forested areas, in favor of impermeable surfaces, such as pavement, buildings, and developed land, reduces the amount of soil and increases pressure on what soil remains. Soil is also compacted by heavy machinery and off-road vehicles. Compaction rearranges soil particles, increasing the density of the soil and reducing porosity. Crusts form on compacted soils, preventing water movement into the soil and increasing runoff and erosion.

With the world's population now numbering upwards of 6 billion people-a figure that may rise to 10 billion or more within three decades-humans will depend more than ever on soil for the growth of food crops. Yet the rapidly increasing population, the intensity of agriculture, and the replacement of soil with concrete and buildings all reduce the capacity of the soil to fulfill this need.

As a result of an increased awareness of soil's importance, many changes are being made to protect soil. Recent interest in soil conservation holds the promise that humanity will take better care of this precious resource.