GEOGRAPHY NOTES FOR FORM THREE – ALL TOPICS

TOPIC 7 – APPLICATION OF STATISTICS

AMAZING GEOGRAPHICAL PLACES IN
AFRICA – PART 3

7. Ngorongoro Crater

Situated in Tanzania, the Ngorongoro Crater is the
world’s largest inactive, intact, and unfilled volcanic caldera and covers an
area of 100 square miles (260 square kilometers). Although it is called a
crater, it is, more precisely, a caldera formed after a massive volcano
exploded and collapsed two million years ago.

The bowl of the crater is rich with life, and like
many African attractions, the Ngorongoro Crater is home to an abundance of
wildlife, including hippopotamuses, hyenas, leopards, lions, African
buffalo, wildebeests, giraffes, and zebras, to name a few. Of note is the
presence of the critically endangered black rhino.

8. The Namib

This coastal desert covers parts of three Southern
African countries, from north to south: Angola, Namibia (home to
the greatest portion of the desert), and South Africa. Its name, which has
its origin in the Nama language, loosely translates to “an area where there is
nothing.” Yet “nothing” is not an entirely true description of some parts of
the desert. The Namib’s vast expanse across different regions means that
the scenery is diverse and not what one might consider to be typical of a
desert. The desert’s immediate coastal area derives moisture from the
near-constant level of fog, allowing succulent shrubs to thrive there. Farther
inland there are random mountains. Elsewhere there are vast amounts of sand,
dunes, gravel plains, and rock formations, which, depending on the region, are
dotted with bushes, grasses, or trees. The Namib’s varying regions are also
home to a variety of wildlife, including beetles, snakes, birds, antelope, and
elephants.

9. Okavango Delta, Botswana

Deep in the Kalahari Desert of northern Botswana
lies the Okavango Delta, a lush water world in the midst of arid southern
Africa. This unique wetland ecosystem spreads over 15,000 sq km (5,791 sq
miles), creating the largest inland delta in the world.

The Okavango Delta is created as the Okavango River
fans out upon reaching the sands of the Kalahari, creating a maze of reed-lined
waterways and verdant islands. The river is one of the few rivers in the world
that do not flow into a sea or ocean, and one of the largest endorheic deltas
in the world. Instead of emptying into an ocean or sea, the river drains onto
open land. The delta transforms seasonally, flooding in winter then drying in
summer, supporting a diversity of habitats. This range of ecosystems supports
an incredible array of wildlife, from elephants, hippos and big cats to varied
birdlife. The tourist activities include vehicle safaris, bushwalks,
helicopter trips and camping under the delta’s starry night skies.

BENEFITS OF PLANTING TREES – PART 3

9.
Biodiversity conservation

Biodiversity
is vital for all life on Earth, and forests are
vibrant ecosystems that support a wide array of plant and animal
species. Humans use at least 40,000 different species of plants and
animals daily for food, shelter, clothing, and medicinal
needs. Reforestation plays a pivotal role in preserving and
restoring biodiversity. By planting a diverse range of tree species, we
create habitats for various plants, insects, birds, and mammals, fostering a
balanced and resilient ecosystem. Researchers have discovered that up to
2.3 million living species can depend on a single tree!

10.
Cooling Down the Streets

Every
year we listen to the shocking global warming news. For instance, the average
temperature in Los Angeles has risen by 6F in 50 years, and the average global
temperature grew by 1.4 F. This happens as tree coverage declines. Removing
trees and replacing them with heat absorbing asphalt roads and buildings makes
cities much warmer. Trees are cooling cities by up to 10 F by providing shade
and releasing water.

11.
Practical & Commercial Value

Trees
have supported and sustained life throughout our existence. They have a wide
variety of practical and commercial uses. Wood was the very first fuel, and is
still used for cooking and heating by about half of the world’s population.
Trees provide timber for building construction, furniture manufacture, tools,
sporting equipment, and thousands of household items. Wood pulp is used to make
paper.

We
are all aware of apples, oranges and the countless other fruits and nuts
provided by trees, as well as the tasty syrup of North American sugar maples.
But did you know the bark of some trees can be made into cork and is a source
of chemicals and medicines? Quinine and aspirin are both made from bark
extracts. The inner bark of some trees contains latex, the main ingredient of
rubber. How many more uses can you name?

12.
Purifying Air

Have
you ever felt that feeling of cleaner air in the woods or by the seaside? Well,
you were right because it is well known that trees do purify the air. They
absorb pollutant gases such as nitrogen oxides, ozone, ammonia, sulphur
dioxide. Trees also absorb odors and act as a filter as little particulates get
trapped in leaves. A mature acre of trees can yearly provide oxygen for 18
people.

 Soil

Define soil

The term soil is derived from the Latin word “solum” which means ground. Soil is defined as the top layer of the Earth’s surface on which plants grow. Soil is a mixture of minerals, organic matter, gases, water, and countless organisms that together support life on Earth. These components interact slowly yet constantly.

Soil is formed through the process of weathering, which breaks up rocks into small fragments. A large portion of the soil, especially the mineral portion, is formed from the parent rock or parent material. Soil formation, therefore, can be defined as the genesis or evolution of the soil from the parent material. This process is continuous. It takes place through the action of weathering processes on the parent material.

Soil formation (pedogenesis) is principally initiated by the weathering of the parent rocks. Weathering can be chemical, biological or mechanical. But the type of soil and rate of soil formation depend on a number of interacting factors (interplay of factors) in a particular environment, hence soil is the product of its own environment. As weathering takes place, the parent rock is broken down into smaller units which mix with organic matter, water, air and living organisms to make up the soil.

Factors for Soil Formation

Describe factors for soil formation

Soil continuously changes. The changes are generally slow but in certain circumstances, especially where human activities are involved, the changes can be rapid. The study of soil involves understanding the factors responsible for its formation. These factors are parent materials, climate, living organisms, relief (topography) and time.

1. Parent material

This is the most important factor in soil formation since it determines the type of soil formed, soil colour, soil depth, the rate of soil formation, soil structure, soil texture, porosity and mineralogical composition or its fertility. It also influences soil maturity, such that if the parent rock is hard, it takes a long time for soil to mature while the rate of maturity is fast where the parent rock is soft. The fast maturity of soils, formed from soft rock is due to the fast rate of weathering process. Mature soils are deep and productive while immature soils are shallow and less productive.

2. Climate

Climate is the principal factor governing the rate and type of soil formation as well as the main agent determining the distribution of vegetation. Dead vegetations decay to form humus as one of the components of the soil.The important variables under climate include temperature, precipitation and wind.

Temperature affects the rate of decomposition of organic matter. It contributes to the rate of soil and soil profile developments through weathering. Where there is high temperature, soil development tends to be fast due to the fast rate of weathering, and where temperature is low, there is also a low rate of soil development due to the low rate of weathering process.

The main effect of temperature on soil is to influence the rate of reactions; for every 10°C rise in temperature, the speed of a chemical reaction increases by a factor of 2 or 3 (twice or thrice). Temperature, therefore, influences the speed of disintegration and decomposition of the parent materials and its consolidation to form the soil.

Precipitation also affects soil profile development. In some areas soil is eroded, leading to soil profile destruction while in areas deposition leads to positive soil development due to accumulation of weathered materials and organic matter. Rainfall adds moisture, which facilitate both chemical and mechanical weathering and hence soil profile development.

The water in soils includes all forms of water that enter the soil system and is derived mainly from precipitation as rain. The water entering soils contains appreciable amounts of dissolved carbon dioxide, forming a weak carbonic acid. This dilute, weak acid solution is more reactive than pure water. It thus reacts with unconsolidated minerals and organic matter, breaking them down into mineral (clay, sand) and organic debris (humus) respectively.

Wind has both positive and negative impacts on soil profile development. It can erode the soil through deflation leading to soil degradation or it can deposit some materials at the edge of the desert to form the soil called loess.

3. Organisms

The organisms influencing the development of soils range from microscopic bacteria to large mammals including man. In fact, nearly every organism which lives on the surface of the Earth or in the soil affects the development of soils in one way or another.

Vegetation influences both chemical and mechanical weathering leading to the development of the soil profile. Also vegetation contributes to soil fertility by adding humus in the soil after dying and decomposing. Some plant roots (legumes) have nodules with bacteria that fix nitrogen into the soil. Plants roots modify the soil by increasing porosity, improving the soil depth and aeration.

Higher plants (particularly grasses) extend their roots into the soil and act as binders. So they prevent soil erosion. The roots also assist in binding together small groups of particles hence developing a crumby or granular structure. Large roots are agents of physical weathering as they open and widen cracks in rocks and stones. When plants die they contribute organic matter to the soil, which acts as a binder of the soil particles. Higher plants intercept rain and they shelter the soil from the impact of raindrops. They also shade the soil and hence reduce evaporation.

Microorganisms play a vital role in decomposition of the organic matter to form humus. When plants die, leaves are dropped onto the soil surface where microorganisms act on them and decay plant tissue. The organic matter is used as an energy source for microorganisms, increasing their population in the soil. These organisms utilize easily digestible materials (like simple sugars and carbohydrates) found in the plant material, leaving more resistant materials (such as fats and waxes) behind. The material left behind is not easily decomposed; it comprises the humus found in soil. Humus acts as a gluing agent, essentially holding primary soil particles (sand, silt, clay) together to form secondary aggregates or ‘peds’. These organisms and the humus they help create aid in the soil formation.

Mammals such as moles, ground squirrels and mice burrow deeply into the soil and cause considerable mixing up of the soil, often by bringing up subsoil to the surface, and creating burrows through which the top soil can fall and accumulate within the subsoil. In doing so, they facilitate weathering process by loosening the soil particles.

Man also adds humus to the soil which contributes to soil profile development. Activities of man are too many and too diverse. Main roles include:

cultivation of soils for production of food and tree crops, which in many cases has negative effects causing impoverishment of the soil and erosion; and

indiscrimate grazing, casual burning, cutting of trees, manure and fertilizer use, all of which alter the soil characteristics.

4. Relief (Topography)

This refers to the outline of the Earth’s surface. All land surfaces are constantly changing through weathering and erosion. The soils on steep mountain slopes are shallow and often stony and contain many primary minerals.

The role of topography in soil formation is mostly indirect. It influences climate and vegetation. It controls the rate and nature of weathering, soil erosion, surface runoff, drainage, and removal and deposition (redistribution) of the soil parent materials. The most important aspects of topography, as a factor of soil formation, are slope, altitude, aspect and location along the slope.

Soil erosion is rapid on steep slopes and less on gentle slopes. Therefore, on steep slopes, soil profiles are shallow while on gentle slopes they are expected to be deeper. Also leaching is more pronounced in the upper-slope areas leading, among other things, to a well-drained soil.

Altitude affects soil mainly through the action of climate and vegetation. Altitude lowers temperature and increases precipitation. Thus, it leads to zonation of climate, vegetation and soil along hillsides.

In terms of aspect, the side that receive more sunshine and high rainfall tends to have well developed soil than the side which receives low amount of sunshine and rainfall since precipitation and insolation (solar radiation received at the earth’s surface) accelerate plant growth and the weathering process. This influences the soil formed on each slope (leeward slope and windward slope).

On very flat landscapes where there is poor drainage, swampy conditions develop. Soils forming in such areas do not develop to maturity. This is because the rate of leaching is very low. These soils are also devoid of air, leading to poor aeration. This affects microbial populations, such as bacteria, millipedes, nematodes and earthworms, and the influence of these organisms on soil formation. Also there is low chemical disintegration of various materials making up the soil, e.g. humus.

5. Time

Soil formation is a very slow process requiring thousands and even millions of years. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually they may change from one soil type to another.

When soil formation has taken a long and enough time, the soil tends to be more mature and it is usually deep and well developed.

The age of a soil is determined by development and not chronological age. Degree of aging depends on intensity of the other four soil forming factors.

The Importance of Soil

Assess the importance of Soil

Soil is a very important natural body without which life on Earth could probably not exist. Soil is essential for life, in the sense that it provides the medium for plant growth, habitat for many organisms, supports animal life, it is a source of building materials, acts as a filtration system for surface water, carbon store and maintenance of atmospheric gases, among others. Let us take a closer look at each of these:

Medium for plant growth

Almost all plants on Earth grow on soil. Soil is a medium through which water, air and mineral nutrients are made available to plants. It, thus, provides plants with essential minerals and nutrients. It also provides air for gaseous exchange between roots and atmosphere. Better still, it holds water (moisture) and maintains adequate aeration.

Animal life support

Soil supports plants, which are the source of food for animals and humans. Some animals, especially herbivores and omnivores feed directly on plants. Carnivores, in turn, benefit indirectly by feeding on herbivores and omnivores.

Habitat for organisms

Insects and microbes (very tiny single-cell organisms) live in the soil and depend on soil for food and air. The soil is home to a diverse range of organisms such as worms and termites. These organisms are important in the process of soil formation. Soil provides the needed moisture and air for breakdown of organic matter. It also provides a breeding ground for many organisms such as insects to lay and hatch eggs and rodents to give birth to new offspring.

Source of building materials

Think of all the buildings and a diversity of other man-made structures you find on Earth. All of these were and are built by materials obtained from the soil. For example, soil is used in making bricks, cement, tiles and whitewash. All of these materials are used in building houses, bridges and other structures. The iron sheets we use to roof our houses are made from metals extracted from the soil. Also soil is used directly in road construction, just to mention a few examples.

Source of minerals

All known natural minerals are obtained from the soil. The minerals are extracted for commercial purposes. The soil is also used to make fertilizers as it contains mineral nutrients, for example, the minjingu phosphate rock (MPR), mined in Manyara region is used as a phosphatic fertilizer.

Supports agriculture and settlement

Agriculture (crop cultivation and livestock rearing) is carried out on soil. This is because, acting as a medium for plant growth, soil supports the growth of pasture for animals. Likewise, human settlements are established on the soil. Soil influences distribution of settlement for example the areas with fertile soils are densely populated compared to the areas with poor soil.

Provides materials for pottery and ceramics

A special clay soil provides raw materials for pottery (ceramics) and sculptures. Ceramic products are made from clay (or clay mixed with other materials). This helps to generate income to people engaged in pottery.

Filtration system for surface water

After rainfall and snow melt, water flows on the Earth’s surface to water bodies, but much of it soaks and gets infiltrated into the ground. As it continues its way downwards through the many layers in the ground, it is filtered from dust, chemicals and other contaminants. This is why aquifers (underground water) are one of the purest sources of water. Filtered water also provides plants with clean, unpolluted water needed for growth.

Carbon store and maintenance of atmospheric gases

Soils help regulate atmospheric carbon dioxide by acting as a carbon store. On a global scale, soils contain about twice as much carbon as the atmosphere and about three times as much as vegetation. This results in the accumulation of organic matter in the soil which is high in carbon content. Also nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled in the soil.

…….

Soil Composition and Properties

Illustrate soil composition and properties

(a) Soil composition

Soil is a complex body composed of five major components namely:

1. mineral matter (inorganic particles) obtained by the disintegration and decomposition of rocks;
2. organic matter, obtained by the decay of plant residues, animal remains and microbial tissues;
3. water (moisture), obtained from the atmosphere and reactions in the soil (chemical, physical and microbial);
4. air or gases, from atmosphere, reactions of roots, microbes and chemicals in the soil; and
5. living organisms, both big (e.g. worms, insects) and small (microbes).

The typical soil consists of approximately 45% mineral, 5% organic matter, 25% water, 25% air, and <0.1% living organisms by volume. These percentages are only generalizations at best. In reality, the soil is very complex and dynamic. The composition of the soil can fluctuate on a daily basis, depending on numerous factors such as water supply, cultivation practices, and/or soil type.

All these five components are completely mixed together forming what we know as soil. Because they are mixed together, chemical and physical reactions normally take place between them. Such reactions create an environment which is suitable for the life of plants as well as other organisms. Each of the five components of the soil is explained in detail below:

Inorganic particles (mineral particles)

Inorganic or mineral particles form about 45% of the total volume of the soil. These inorganic particles are mainly small pieces of rock and different kinds of minerals. The inorganic part of the soil was formed from the parent rock by the action of weathering.

The size of inorganic particles of the soil has a big influence on the properties of the soil. Particles that are small in size have a large surface area on which chemical and physical reactions can take place. For this reason, generally, the smaller the size of the soil particles, the more reactive is the soil.

In addition to this, the size of the individual pore spaces (spaces between soil particles filled with water and air) in any soil depends to some extent on the size of the particles; the smaller the size of the soil particles, the smaller the size of the pore spaces.

Organic matter

The organic matter present in the soil is formed from the remains of plants and animals. It forms about 5% by volume soil and consists of two main components. These are plant and animal remains which have not yet been completely broken down and those which have already been broken down completely.

Organisms and microorganisms present in the soil feed on the remains of plants and animals in the soil. In this way, the soft parts of the remains are broken down. That is they are decomposed. When the rather soft parts of the plant and animal remains are broken down, humus is formed.

Humus consists of very small particles which are usually black or brown in colour. Since these particles are very small in size, they have a very large surface area on which chemical and physical reactions can take place. Humus is actually a complex substance which consists of a mixture of those parts of dead animals and plants which are very difficult to decompose and substances which have been formed through the action of microorganisms.

The quantity of organic matter in the soil is small but, because of the presence of humus, it has got a great influence on the physical and chemical properties of the soil. Humus contains mineral salts. Plants normally get these minerals from the soil. Humus is, therefore, a source of mineral salts which are used by plants.

Humus is also a good binding agent for soil particles. It acts as a glue for soil particles so that they are joined together. When the soil particles are joined to each other, soil structure is formed. Soil structure is very important because it determines the amount of water and air which a soil can hold as well as the movement of water and air in the soil.

Living organisms

The soil contains living organisms of different sizes. While some are large enough to be seen by naked eyes (macroscopic) others are invisible to the naked eyes (microscopic). These organisms give life to the soil. They include members of the animal and plant kingdoms.

In general, organisms make less than 0.1% (<0.1%) of the total volume of the soil. Though they make such a small proportion of the soil, these organisms have a very important influence on the physical and chemical properties of the soil. Without the action of organisms, remains of plants and animals in the soil cannot break down. For example, insects, earthworms, bacteria and fungi feed on the plant and animal remains. When they do this, the remains are broken down and nutrients are released into the soil. This process is called mineralization. The nutrient elements which are released in this way can then be absorbed by plant roots. Humus is also formed by the action of soil microorganisms.

Soil water

Soil water accounts for 25% of the soil volume. Most of the soil water comes from rain. Water dissolves various substances like salts that are derived from animal remains to form soil solution. Plant elements such as calcium, phosphorus, potassium, and nitrogen are absorbed by plants from soil solution. Soil water, therefore, helps plants to absorb mineral nutrients from the soil.

Too much water in the soil leads to leaching and hence loss of nutrients. It also causes the soil to be waterlogged. This causes a reduction in the supply of oxygen in the soil, thus causing a problem to soil microorganisms. It should be noted that most of the soil organisms cannot thrive in such anaerobic environments. Such a condition also hampers root respiration and it can lead to dead of plants.

Soil air

The soil also contains air. Soil air is a mixture of gases such as carbon dioxide, oxygen, etc. Generally, soil air contains more carbon dioxide and more water vapour than atmospheric air. Soil air forms about 25% of the soil by volume. Air in the soil occupies the pore spaces (air spaces between soil particles) which are not occupied by water. This means that a soil which contains a lot of water has very little air and one that contains a lot of air has very little water.

(b) Soil properties

Properties of the soil can be categorized as physical, chemical and biological properties. The properties of each soil determine its characteristics. The physical properties of the soil influence plant growth and soil management. The chemical properties determine soil fertility in relation to mineral content of the soil. Soil biological properties are interconnected with other soil physical and chemical properties; e.g. aeration, soil organic matter or pH.

Physical properties of soil

The major physical properties of the soil include texture, structure, colour, porosity, temperature, consistence, density (particle density and bulk density), pore size, soil depth, and permeability. Each of these physical properties is described in detail below:

Soil texture

Soil texture refers to the composition of a particular soil in terms of the size (diameter) of its particles. The particles that make up soil are categorized into three groups by size: sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture. Therefore, soil texture can be defined as the relative proportions of sand, silt, and clay in the soil. These particles are often known as soil separates. The proportions of the separates in classes commonly used in describing soils are given in the textural triangle shown in Figure 3.6

In soil science, soil particles of different diameters are given different names. According to a system used by the International Society of Soil Science, soil particles are classified according to their diameters as shown in Table 3.1.

Soil structure

Soil structure is the arrangement of individual soil particles (soil separates) into small clumps, called peds or aggregates. Soil particles (sand, silt, clay and even organic matter) bind together in a repeating pattern to form peds.

Between the peds are cracks called “pores” through which soil air and water are conducted. Soil structure is most commonly described in terms of the shape of the individual peds that occur within a soil. Depending on the composition and on the conditions in which the aggregates formed, the aggregates may be in the form of plates, blocks, prisms, columns, granules, crumbs, etc, hence the terms platy, blocky, prismatic, columnar, granular and crumby structure, respectively.

The natural processes that aid in forming aggregates are wetting and drying, freezing and thawing, microbial activity that aids in the decay of organic matter, activity of roots and soil animals, and adsorbed cations.

The wetting/drying and freezing/thawing action and root or animal activity push particles back and forth to form aggregates. Decaying plant residues and microbial byproducts coat soil particles and bind particles into aggregates. Adsorbed cations help form aggregates whenever a cation is bonded to two or more particles.

Soil colour

Soil colours range from black to red to white. Sometimes it can even be blue. Soil colour mostly comes from organic matter, manganese and iron. Red soil indicates the presence of oxidized iron. Oxidized iron is also observed on metal objects that have been exposed to the atmosphere. We call it rust. Yellow soils contain hydrated iron. Grey soils indicate chemical reduction of iron and/or manganese due to wetness and lack of oxygen. Dark brown or black colour in soil indicates that the soil has high organic matter content. The dark soil colour from organic matter at the soil surface aids in the absorption of heat from sunlight to warm the soil.

Wet soil will appear darker than dry soil. However, the presence of water also affects soil colour by affecting the oxidation rate. An even, single colour indicates the soil is well drained. In contrast, rusty spots and grey patches indicate poor drainage.

Generally, the factors that influence soil colour include the following:

1. Type of the parent material from which soil has developed.
2. Chemical composition of the soil.
3. The organic matter content of the soil.
4. The drainage of the area where soil is found.

Soil temperature

Soil temperature is simply a measure of the warmth in the soil. Ideal soil temperatures for growing most plants are 18 to 24°C. The temperature of a given soil varies according to soil depth, time of the day and season. It also depends on the amount of solar energy received and the ability of the soil to absorb solar radiation (this depends on soil colour). Nighttime and daytime soil temperatures are both important. Ideally, dark-coloured soils absorb more heat than light-coloured soils. Soils under vegetation cover are generally cooler than those exposed directly to the sun. In this respect, soils can be described as being hot, warm, cool or cold, based on the average soil temperature.

Soil temperature is the factor that drives germination, blooming, composting, and a variety of other processes.

Soil density

Density represents weight (mass) per unit volume of a substance.

Soil density is expressed in two well accepted concepts as bulk density and particle density.

(i) Bulk density

The oven-dry weight per unit volume of soil is called bulk density. The bulk density of a soil is always smaller than its particle density. The bulk density of sandy soil is about 1.6 g/cm³, whereas that of organic matter is about 0.5 g/cm³.

Bulk density is expressed as a unit of weight per volume, and is commonly measured in units of grams per cubic centimetres (g/cc).

Bulk density is of greater importance than particle density in understanding the physical behaviour of the soil. Generally, soils with low bulk densities have favourable physical conditions.

Factors that influence the bulk density include the following:

1. Organic matter content: Organic matter is light and therefore lowers the weight of the soil. If it present in the soil, it lowers the weight of the soil. The addition of even a small percentage of organic soil material to a mineral soil can affect the bulk density of that soil.
2. Granulation: A soil that is well granulated (one that has formed into grains or particles) has a lower bulk density than one which is not well granulated.
3. Pore space: Since bulk density relates to the combined volume of the solids and pore space, soils with high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space. A compact soil has very little pore space. As a result, it has a high bulk density. Any factor that influences soil pore space will affect bulk density.
4. Texture: Fine textured surface soils such as silt loams, clays and clay loams generally have lower bulk densities than sandy soils.
5. Cultural practices: Continuous cultivation without the addition of organic materials tends to raise the bulk density of the soil while the addition of organic materials lowers the bulk density.

(ii) Particle density

A soil particle has no pore space, and is nothing more than a very small piece of rock. The weight per unit volume of the oven-dry solid portion of soil is called particle density. Usually, particle density is expressed in units of grams per cubic centimetre (g/cm³). An average value for particle density is 2.66 g/cm³. This means that a soil particle that is 1 cubic centimetre in volume weighs 2.66 g. In comparison, water has a density of 1 g/cm³, and organic matter has a density of 0.8 g/cm³.

The particle density is higher if large amount of heavy minerals such as magnetite; limonite and hematite are present in the soil. With increase in organic matter of the soil, the particle density decreases. This means that the amount of organic matter present in the soil determines, to some extent, the particle density of the soil. The higher the organic matter contained in the soil, the lower the particle density of that soil. For this reason, surface soils generally have lower particle density than the subsurface soils.

Porosity

The term porosity refers to the percentage of the soil volume that is occupied by water and air. It is inversely related to bulk density. Since water and air occupy the non solid space of the soil, the arrangement of the solid particles (that is the soil structure) in the soil determines the total pore space to a great extent.

Porosity or % pore space is calculated as a percentage of the soil volume:

% solid space + % pore space = 100

% pore space = 100 – % solid space

Loose, porous soils have lower bulk densities and greater porosities than tightly packed soils. Porosity varies depending on particle size and aggregation. It is greater in clayey and loam soils than in sandy soils. A large number of small particles in a volume of soil produce a large number of soil pores. Fewer large particles can occupy the same volume of soil so there are fewer pores and less porosity.

Compaction decreases porosity as bulk density increases. Aggregation also decreases porosity because more large pores are present as compared to single clay and silt particles that are associated with smaller pores.

Pores of all sizes and shapes combine to make up the total porosity of a soil. Porosity, however, does not tell us anything about the size of pores.

Soil Consistence

Soil consistence refers to the ease with which an individual ped can be crushed by the fingers. Soil consistence, and its description, depends on soil moisture content. Terms commonly used to describe consistence are explained below:

Moist soil:

Loose – noncoherent when dry or moist; does not hold together in a mass.

Friable – when moist, crushed easily under gentle pressure between thumb and forefinger and can be pressed together into a lump.

Firm – when moist, crushed under moderate pressure between thumb and forefinger, but resistance is clearly noticeable.

Wet soil:

Plastic – when wet, readily deformed by moderate pressure but can be pressed into a lump; will form a “wire” when rolled between thumb and forefinger.

Sticky – when wet, adheres to other material and tends to stretch somewhat and pull apart rather than pulling free from other material.

Dry soil:

Soft – when dry, breaks into powder or individual grains under very slight pressure.

Hard – when dry, moderately resistant to pressure; can be broken with difficulty between thumb and forefinger.

Pore size

Pore size is probably one of the most important physical features of a soil. It controls water and air movement and storage. Pores come in all sizes, although clays have predominantly small pores, and sands have large pores. Most soils are a mixture of sand, silt and clay particles, so there is a mixture of different soil pores (Figure 3.3).

An ideal soil condition is one with an equal number of large and small pores. Large pores allow for soil aeration. Aeration is needed for the exchange of oxygen from the atmosphere and carbon dioxide given off by plant roots and microorganisms. About 10 percent of the pores must be large enough for aeration so that root growth is not restricted.

Within an aggregate, the pores are small. Between aggregates, pores are large. Small pores are usually called micropores, and large pores are called macropores. As organic matter is added, the number of macropores increases. This increase results from the increase in aggregation, decay of root channels and creation of earthworm channels. Macropores are crushed when a soil is compacted. Tillage tends to increase macropores in the short-term, but reduces the number of macropores in the long-term because of the loss of aggregation.

Soil permeability

Soil permeability is the ability of the soil to transmit water and air. As the soil layers or horizons vary in their characteristics, the permeability also differs from one layer to another. Pore size, texture, structure and the presence of impervious layers such as clay pan determines the permeability of a soil. Clayey soils with platy structures have very low permeability.

Organic matter, especially crop residue and decaying roots, promotes aggregation so that larger soil pores develop, allowing water to infiltrate more readily. Permeability also varies with soil texture and structure.

Permeability rate or coefficient of permeability is determined in the field by digging a hole of approximately 30 cm diameter, smearing the sides of the hole with heavy wet clay or lining with plastic sheet and measuring the rate of infiltration of water by filling the hole repeatedly with water and noting the time it takes for the water level to go down by a specific depth.

Water held in a soil is described by the term water content. Saturation is the soil water content when all pores are filled with water. Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for about 24 to 48 hours. Free drainage occurs because of the force of gravity pulling on the water. When water stops draining, we know that the remaining water is held in the soil with a force greater than that of gravity. Permanent wilting point is the soil water content when plants have extracted all the water they can. At the permanent wilting point, a plant will wilt and not recover. Unavailable water is the soil water content that is strongly attached to soil particles and aggregates, and cannot be extracted by plants. This water is held as films coating soil particles. These terms illustrate soil from its wettest condition to its driest condition.

Several terms are used to describe the water held between these different water contents. Gravitational water refers to the amount of water held by the soil between saturation and field capacity. Water holding capacity refers to the amount of water held between field capacity and wilting point. Plant available water is that portion of the water holding capacity that can be absorbed by a plant. As a general rule, plant available water is considered to be 50 percent of the water holding capacity.

The volumetric water content is the total amount of water held in a given soil volume at a given time. It includes all water that may be present including gravitational, available and unavailable water.

The relationship between these different physical states of water in soil can be easily illustrated using a sponge. A sponge is just like the soil because it has solid and pore space. Obtain a sponge about 6 x 3 x 0.5 inch in size. Place it under water in a bucket, and allow it to soak up as much water as possible. At this point, the sponge is at saturation. Now, carefully support the sponge with both hands and lift it out of the water. When the sponge stops draining, it is at field capacity, and the water that has freely drained out is gravitational water. Now, squeeze the sponge until no more water comes out. The sponge is now at permanent wilting point, and the water that was squeezed out of the sponge is the water holding capacity. About half of this water can be considered as plant available water. You may notice that you can still feel water in the sponge. This is the unavailable water.

Soil depth

Soil depth shows how thick the soil cover is. The exact soil depth is difficult to assess and is very variable. Soil depth is estimated by means of topography, bedrock outcrops, and observations made when digging pits for soil classification and soil sampling.

Depth of soil profile differs significantly for different soil types. It is one of basic criterions used in soil classification. Soils can be very shallow (less than 25 cm), shallow (25-50 cm), moderately deep (50-90 cm), deep (90-150 cm) and very deep (more than 150 cm).

Soil depth has a great influence on soil fertility and productivity. Deep soils are normally fertile and they hold more water than shallow soils. Deep soils support a wide variety of plants including the deep-rooted and shallow-rooted plants while shallow soils can only support shallow-rooted plants and they are poorly drained.

Chemical properties of soil

The chemical properties of soil include such properties as Cation Exchange Capacity (CEC), soil reaction (pH), salinity, C:N ratio (Carbon-to-Nitrogen ratio), and soil fertility.

Cation Exchange Capacity

A cation is a positively charged ion. Most plant nutrients exist as positively charged ions, or “cations”, in the soil environment. Among the more common cations found in soils include hydrogen (H⁺), aluminium (Al³⁺), calcium (Ca²⁺), magnesium (Mg²⁺), zinc (Zn²⁺), copper (Cu²⁺), manganese (Mn²⁺), and potassium (K⁺). Most heavy metals also exist as cations in the soil environment. These cations are in the soil solution and are in dynamic equilibrium with the cations adsorbed on the surface of clay and organic matter. CEC is a measure of the quantity of cations that can be adsorbed and held by a soil.

Clay and organic matter particles are predominantly negatively charged (anions), and have the ability to hold cations from being “leached” or washed away. The adsorbed cations are subject to replacement by other cations in a rapid, reversible process called cation exchange.

Cations leaving the exchange sites enter the soil solution, where they can be taken up by plants, react with other soil constituents, or be carried away with drainage water.

CEC is dependent upon the amount of organic matter and clay in soils and on the types of clay. The greater the clay and organic matter content, the greater the CEC should be, although different types of clay minerals and organic matter can vary in CEC.

Cation exchange is an important mechanism in soils for retaining and supplying plant nutrients, and for adsorbing contaminants. It plays an important role in wastewater treatment in soils. Sandy soils with a low CEC are generally unsuitable for septic systems since they have little adsorptive ability.

Soil reaction (pH)

By definition, soil pH or soil reaction is a measure of the hydrogen ion (H⁺) concentration in a soil solution. It is an indication of the acidity or alkalinity of a soil.

The pH scale ranges from 0 to 14, with values below 7.0 described as acidic, and values above 7.0 described as alkaline. A pH value of 7 is considered neutral, where H⁺ and OH^– are equal, both at a concentration of 10^-7 moles/litre.

The most important effect of pH in the soil is on ion solubility, which in turn affects microbial and plant growth. A pH range of 6.0 to 6.8 is ideal for most crops because at this pH there is optimum solubility of the most important plant nutrients (Figure 3.4). Some microelements (e.g., iron) and most heavy metals are more soluble at lower pH. This makes pH management important in controlling movement of heavy metals (and potential groundwater contamination) in soil.

In acid soils, H⁺ and Al³⁺ are the dominant exchangeable cations. The latter is soluble under acid conditions, and its reactivity with water (hydrolysis) produces hydrogen ions. Calcium and magnesium are basic cations; as their amounts increase, the relative amount of acidic cations will decrease.

Factors that affect soil pH include parent material, vegetation, and climate. Some rocks and sediments produce soils that are more acidic than others. For example, quartz-rich sandstone is acidic while limestone is alkaline. Some types of vegetation, particularly conifers, produce organic acids, which can contribute to lower soil pH values. In humid areas, soils tend to become more acidic over time because rainfall washes away basic cations and replaces them with hydrogen ions. Addition of certain fertilizers to soil can also produce hydrogen ions. Liming the soil adds calcium, which replaces exchangeable and solution H⁺ and raises soil pH.

Lime requirement, or the amount of liming material needed to raise the soil pH to a certain level, increases with CEC. To decrease the soil pH, sulphur can be added, which produces sulphuric acid.

Soil pH affects plant growth in the following ways:

It affects availability of plant nutrients (in general, optimal pH is between 5.5 and 7.5).

Low pH soils (<6.0) results in an increase in aluminium ions (Al3+) in the soil. Aluminium is toxic to plants.

It affects availability of toxic metals (in general, more available in acidic soils).

It affects the activity of soil microorganisms, thus affecting nutrient cycling and disease risk.

The availability of plant nutrients at different pH levels is as indicated in the following figure.

Salinity

Soil salinity is the salt content in the soil. The process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salination can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean. It can also come about through artificial processes such as irrigation.

Saline soils are high in pH and exchangeable Na⁺. pH is generally higher than 8.5 and exchangeable sodium occupies more than 15% of CEC. The high Na content may affect the growth of phytoplankton, zooplankton and fish. These soils generally occur in arid and semi arid regions. They are reclaimed by treating the soil with gypsum (CaSO₄) or sulphur.

Salinity occurs in the soil as a result of various processes. Described below are the major causes of soil salinity:

Natural occurrence

Salts are a natural component in soils and water. The ions responsible for salination are Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cl⁻. As the Na⁺ ions predominate, soils can become sodic. Sodic soils present particular challenges because they tend to have very poor structure which limits or prevents water infiltration and drainage.

Over long periods of time, as soil minerals weather and release salts, these salts are flushed or leached out of the soil by drainage water in areas with sufficient precipitation. In dry regions with poor precipitation, salts may accumulate, leading to naturally saline soils.

Dryland salinity

Salinity in dry lands can occur when the water table is between two and three metres from the surface of the soil. The salts from the groundwater are raised by capillary action to the surface of the soil. This occurs when groundwater is saline (which is true in many areas), and is favoured by land use practices, allowing more rainwater to enter the aquifer than it could accommodate. For example, the clearing of trees for agriculture is a major reason for dry land salinity in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops.

Salinity due to irrigation

Rain or irrigation, in the absence of leaching, can bring salts to the surface by capillary action. Salinity from irrigation can occur over time since almost all water (even natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and eventually begin to accumulate. Since soil salinity makes it more difficult for plants to absorb water from the soil, these salts must be leached out of the plant root zone by applying additional water. Salination from irrigation water is also greatly increased by poor drainage and use of saline water for irrigating agricultural crops.

The consequences of salinity include the following:

Detrimental effects on plant growth and yield.

Damage to infrastructure (roads, bricks, corrosion of pipes and cables).

Reduction of water quality for users due to sedimentation problems.

Salinity is an important land degradation problem. Soil salinity can be reduced by leaching soluble salts out of soil with excess irrigation water.

Soil fertility

Soil fertility is the ability of the soil to supply nutrients required by plants in adequate quantities and correct proportions. Plants require at least 16 elements to complete their life cycle. They are: C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Cu, Zn, Mo, B, and Cl. Some of the lower plants in addition to the above elements require Co, V, and Si. Among these C, H, O, N, P, K, Ca, Mg and S are required in large quantities and therefore called macronutrients and the rest called micronutrients. The elements C, H, O are obtained mainly from air and water and the rest from the soil.

In a fertile soil, production is high at the start but diminish rapidly later due to exhaustion of the soil reserve of nutrients. In order to maintain high production, fertilizers need to be applied frequently to the soil.

The amounts and kinds of fertilizers that need to be applied to the soil depend on the natural fertility of the soil.

Soil organic matter

Soil organic matter is the organic matter component of soil, consisting of plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by soil organisms. Organic matter exerts numerous positive effects on soil physical, chemical, and biological properties:

Physical – stabilizes soil structure, improves water holding characteristics, and lowers bulk density; dark colour may alter thermal properties.

Chemical – higher CEC, acts as a pH buffer, ties up metals, and interacts with xenobiotics (foreign substances).

Biological – supplies energy and body-building constituents for soil organisms, increases microbial populations and their activities, source and sink for nutrients, ecosystem flexibility, affects soil enzymes.

Nutrients in the soil organic matter are released through microbial transformations to become available to plants. Release is highest under warm, moist conditions and slowest in cool dry climates. Microorganisms are the driving force for nutrient release to plants.

Carbon-to-Nitrogen ( C:N) ratio

The term carbon-nitrogen ratio or C:N ratio refers to the ratio of the weight of organic carbon to the weight of nitrogen which are present in the soil or in an organic substance. Organic substances which contain a lot of protein (and therefore nitrogen) are said to have a narrow C:N ratio. Those which have low amount of nitrogen are said to have a wide C:N ratio.

Plant tissues with a narrow C:N ratio decompose more rapidly than those with a wide C:N ratio. In other words, plant tissues which are rich in nitrogen e.g. legume plants decompose more rapidly than those with a low nitrogen content. Legume crops have the ability to raise the nitrogen content of the soil through the action of nitrogen-fixing bacteria living in their root nodules.

Biological properties of soil

A large number of organisms live in the soil. Soils harbour bacteria, actinomycetes, fungi, algae, protozoa, nematodes, worms, insects and rodents. These organisms perform a variety of functions for their growth and reproduction. For these functions of soil organisms, soils behave like a living entity. Soil components photosynthesize, respire, and reproduce. In addition, they produce organic matter, consume organic matter, and decompose them. Some of them burrow in the soil, make spaces for their accommodation and movement, and mix surface and subsoil materials together. Soil becomes a dynamic body for the activity of soil organisms.

The changes that are caused by soil organisms have their impact on soil fertility and productivity. Although soil biota, which includes living roots and soil organisms, occupies a very small fraction of the total soil volume (< 0.5%), it has tremendous influences on soil properties and soil processes. 60-80% of the total soil metabolism is due to the microflora (microscopic plants; bacteria are often considered to be microflora). However, soil organisms are usually the most active in the surface soil zone of 0–15 cm, because this zone has accumulation of organic residues and available nutrients. They work best when there is good aeration, a neutral soil reaction, soil moisture content at about half of the water holding capacity, and temperature between 25°C and 38°C.

Soil depth, organic matter and nutrients, microclimate, and physical and chemical soil environment influence the population and function of soil biota.

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The concept of Map and its importance to social economic activities

Map reading is the process of identifying features on a map by using symbols and signs or names. This technical work requires certain skills that any map reader must possess.

Map interpretation refers to interpretation of the symbols and signs used on map into ordinary language by indicating the features they represent and draw logical conclusions from the information as represented by the symbols.

Importance of map reading

Map reading is very important to social and economic activities. Maps are drawn for different purposes and once drawn they can serve as databases from which various information can be obtained and used for a myriad of social and economic benefits.

1. Geological maps provide information about the type and distribution of rocks in an area. This knowledge is of great help to builders who can use it to find out where to obtain certain rocks for construction. This knowledge can also be used by mineral prospectors to locate possible areas where they can obtain mineral. The information on soil types can be used by civil engineers to establish the stability of the on which to build roads and other structures.
2. Relief maps provide information to many people in many ways. For example, civil and architectural engineers need to know relief of an area so that they can plan in advance how to overcome relief barriers in their construction plans. It also important to large scale farmers as they need relief to plan for the extent of farms and also to determine the possibility of mechanization (use various machines in agricultural production).
3. Drainage maps are useful to civil engineers as they can use them to get prior knowledge on how to construct bridges, roads, railways and other infrastructures. It is also useful to agriculture as such maps indicate possible sources of water for irrigation of crops, watering livestock and for other general farm uses.Weather and climate maps are useful especially to people interested in agriculture. They provide information on the kind of crops to be grown and the type of livestock to keep in a certain area.
4. Vegetation maps show the distribution and type of vegetation in a region. This gives a clue on the kind of social and economic activities that can be carried out in an area.
5. Soil maps are very useful to agricultural officers as they can use the information about soil types to advise farmers on the type of fertilizers to use, soil requirements and proper soil management practices.
6. Maps provide information on the relationship between phenomena or events. For example, maps showing the location of volcanoes also indicate the connection between volcanoes and earthquakes. Earthquakes are very common in areas with numerous volcanoes. This will give people crucial information on the possibility of occurring vulcanicity so that they can avoid setting settlements on such hazardous areas.
7. Maps provide background information as compared to present work. For example, maps showing distribution of forests in the past may be compared to the present maps to draw conclusion on the extent of vegetation change through deforestation, afforestation or reforestation.
8. Maps provide valuable information for statistical analysis. Therefore, they are very useful to researchers and any field of study.

Essentials of a Map

Identify essentials of a map

Essentials of a map are the necessary prerequisites that a map should have. All maps in general require the following qualities or essentials:

Title – The most basic component of a map is its title. The title should refer to everything the map covers. It could be a basic name of a country, such as “Tanzania,” or it could be more extensive, such as “Water Tables in the Western Saharan Desert.” The title should clearly state what the cartographer‘s intentions and goals are; it should be specific, and it should not include irrelevant information.

Scale – shows the relationship between map distance and ground distance. For example, the scale 1:100000, indicates that one centimetre on the map represents 100,000 cm (1 km) on the ground, there are three types of scale;

1. Large scale; this is a kind of scale that take place on small area. The size of the features represented is large in size. It can take part on the area like a farm, village, ward etc. The denominator of these scale are also small in figure, e.g. 1:5,000, 1: 10,000.
2. Medium scale; this is a kind of scale which take part on medium size of area. Size of features being represented is also medium in size. It can take part on areas like District, Region or municipals. Their denominator being like 1:20,000 or 1:25,000.
3. Small scale; this is the type of scale that used to represent large areas. The size of the features being represented is small and features can be many as possible. The denominator of the scale is large like 1:50,000 or 1:125,000. types of scales, can be represented differently using the three different ways;
4. Statement scale – this is a map scale stated in words or it is a verbal scale. The words ―one centimetre to one kilometre‖ is an example of a statement scale.
5. Representative fraction – this is a means of expressing the relative size of a map or drawing by a fraction or ratio e.g. 1/100 or 1:100. This means that one unit on the map represents one hundred units on the ground
6. Linear scale/graph scale is a line showing the distance on the map that represents a given distance on the ground. A linear scale is divided into two parts (Figure 5.1):
7. Primary section – it is placed on the left-hand side of the linear scale.
8. Secondary section – this is placed on the right-hand side of the linear scale.

Key – every map must have a key. The key is a vital tool in understanding and interpreting the map. The key should explain every feature or symbol contained on the map. It should reveal what every marking means and sometimes provide additional information. For example, a city may be represented by a large black dot of a certain size on a map, and the key may explain that this represents a city with a population greater than one million people

Margin/boundary/frame – it is essential that all maps be enclosed in a frame for neatness

North direction/compass orientation – this is the direction towards the North in those maps drawn to grid system. All maps must have a compass orientation. Because the primary purpose of a map is to provide and insight into directions, a map has to be able to show which way is which on a compass. Most maps have “North” at the top and “South” at the bottom, but all maps should have an official representation of the compass orientation

Date – to give context to a map, the date of publication should be present. As maps are continually updated with additional information and improved accuracy, it is important to know the time when your map was published. For example, viewing a map of Tanzania published in 1980 might still be useful but will not be as accurate as one published in 2014