Essay on Soil for School and College Students | Essay | Soil Science

Here is a compilation of essays on “Soil” for class 5, 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on “Soil” especially written for school and college students.

Essay on Soil

Essay Contents:

1. Essay on the Meaning and Importance of Soil
2. Essay on the Classification of Soil
3. Essay on the Physical Properties of Soil
4. Essay on the Layers of Soil
5. Essay on the Formation of Soil

1. Essay on the Meaning and Importance of Soil:

Meaning of Soil:

Soil (sometimes called dirt) is the combination of rock, mineral fragments (pieces), organic matter (dead and living things), water, and air. It is mostly made up of grains of rock weathered by wind, rain, sun, snow, etc., and varying amounts of humus. The type of soil depends on the mix of humus and on the size of the grains of the rock. The grains can be very small and smooth, such as clay, or they can be larger, like grains of sand or even a piece of gravel.

Importance of Soil:

Soils are important to our ecosystem for six main reasons:

1. Soils are a place for plants to grow;

2. Soils control the speed and the purity of water that moves through them;

3. Soils recycle nutrients from dead animals and plants;

4. Soils change the air that surrounds the earth, called the atmosphere;

5. Soils are a place to live for animals, insects and very small living things called microorganisms;

6. Soils are the oldest and the most used building materials.

The climate is very important when soil is made. Soil from different climates can have very different qualities. Soil is a natural body consisting of layers that are primarily composed of minerals which differ from their parent materials in their texture, structure, consistency, color, chemical, biological and other characteristics.

It is the unconsolidated or loose covering of fine rock particles that covers the surface of the earth. Soil is the end product of the influence of the climate (temperature, precipitation), relief (slope), organisms (flora and fauna), parent materials (original minerals), and time. In engineering terms, soil is referred to as regolith, or loose rock material that lies above the ‘solid geology’.

In horticulture, the terms ‘soil’ is defined as the layer that contains organic material that influences and has been influenced by plant roots and may range in depth from centimeters to many meters. Soil is composed of particles of broken rock (parent materials) which have been altered by physical, chemical and biological processes that include weathering (disintegration) with associated erosion (movement).

Soil is altered from its parent material by the interactions between the lithosphere, hydrosphere, atmosphere, and biosphere. It is a mixture of mineral and organic materials in the form of solids, gases and liquids.

Soil is commonly referred to as “earth” or “dirt”; technically, the term “dirt” should be restricted to displaced soil.

Parent Material:

The mineral material from which a soil forms is called parent material. Rock, whether its origin is igneous, sedimentary, or metamorphic, is the source of all soil mineral materials and origin of all plant nutrients with the exceptions of nitrogen, hydrogen and carbon. As the parent material is chemically and physically weathered, transported, deposited and precipitated, it is transformed into a soil.

Typical soil mineral materials are quartz:

SiO₂ Calcite: CaCO₃, Feldspar: KAlSi₃O₈, Mica (biotite): K (Mg, Fe)₃ AlSi₃O₁₀(OH)₂.

Organic Matter of Soil:

The organic soil matter includes all the dead plant material and all creatures, live and dead. Most of the living things in soils are including plants, insects, bacteria and fungi. Soils have varying organic compounds in varying degrees of decomposition. Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water.

Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat (histosols), are infertile. Humus refers to organic matter that has been decomposed by bacteria, fungi, and protozoa to the final point where it is resistant to further breakdown.

Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants and animals, microbes begin to feed on the residues, resulting finally in the formation of humus. Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present.

Humus usually constitutes only five percent of the soil or less by volume, but it is an essential Source of nutrients and adds important textural qualities crucial to soil health and plant growth. Humus also holds bits of un-decomposed organic matter which feed arthropods and worms which further improve the soil.

The degradation of water-soluble constituents contains cellulose, hemicellulose and nutrients such as nitrogen, phosphorus, and sulphur. Humus has a high cation exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.

2. Essay on the Classification of Soil:

Soil is classified into categories in order to understand relationships between different soils and to determine the suitability of a soil for a particular use. One of the first classification systems was developed by the Russian scientist Dokuchaev around 1880. It was modified a number of times by American and European researchers, and developed into the system commonly used until the 1960s.

It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on soil morphology instead of parental materials and soil- forming factors. Since then it has undergone further modifications.

The World Reference Base for Soil Resources (WRB) aims to establish an international reference base for soil classification. Taxonomy is an arrangement in a systematic manner. They are, from most general to specific: order, suborder, great group, subgroup, family and series. The soil properties that can be measured quantitatively are used to classify soils. A partial list is: depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content.

In the United States, soil orders are the top hierarchical level of soil classification in the USDA soil taxonomy. The names of the orders end with the suffix -sol. There are 12 soil orders in Soil Taxonomy.

The criteria for the order divisions include properties that reflect major differences in the genesis of soils:

a. Alfisol:

Soils with aluminium and iron. They have horizons of clay accumulation, and form where there is enough moisture and warmth for at least three months of plant growth. They constitute 10.1% of soils worldwide.

b. Andisols:

Volcanic ash soils. They are young and very fertile. They cover 1% of the world’s ice-free surface.

c. Aridisol:

Dry soils forming under desert conditions which have fewer than 90 consecutive days of moisture during the growing season. They include nearly 12% of soils on Earth. Soil formation is slow, and accumulated organic matter is scarce. They may have subsurface zones of caliche or duripan. Many aridisols have well-developed Bt horizons showing clay movement from past periods of greater moisture.

d. Entisol:

Recently formed soils that lack well-developed horizons. Commonly found on unconsolidated river and beach sediments of sand and clay or volcanic ash, some have an A horizon on top of bedrock. They are 18% of soils worldwide.

e. Gelisols:

Permafrost soils with permafrost within two meters of the surface or gelic materials and permafrost within one meter. They constitute 9.1% of soils worldwide.

f. Histosol:

Organic soils, formerly called bog soils, are 1.2% of soils worldwide.

g. Inceptisol:

Young soils. They have subsurface horizon formation but show little eluviation and illuviation. They constitute 15% of soils worldwide.

h. Mollisols:

Soft, deep, dark fertile soil formed in grasslands and some hardwood forests with very thick A horizons. They are 7% of soils worldwide.

i. Oxisol:

Oxisol are the most weathered, are rich in iron and aluminum oxides (sesquioxides) and kayolin but low in silica. They have only trace nutrients due to heavy tropical rainfall and high temperatures. They are 7.5% of soils worldwide.

j. Spodosol:

Acid soils with organic colloid layer complexed with iron and aluminium leached from a layer above. They are typical soils of coniferous and deciduous forests in cooler climates. They constitute 4% of soils worldwide.

k. Ultisol:

Acid soils in humid climates, tropical to subtropical temperatures, which are heavily, leached of Ca, Mg, and K nutrients. They are not quite Oxisols. They are 8.1% of the soil worldwide.

l. Vertisol:

Inverted soils. They are clay-rich and tend to swell when wet and shrink upon drying, often forming deep cracks into which surface layers can fall. They are difficult to farm and on which to construct roads and buildings due to their high expansion rate. They constitute 2.4% of soils worldwide.

3. Essay on the Physical Properties of Soil:

The physical properties of soils, in order of decreasing importance, are texture, structure, density, porosity, consistency, temperature, colour and resistivity. Most of these determine the aeration of the soil and the ability of water to infiltrate and to be held in the soil.

Soil texture is determined by the relative proportion of the three kinds of soil particles, called soil “separates”: sand, silt, and clay. Larger soil structures called “peds” are created from the separates when iron oxides, carbonates, clay, and silica with the organic constituent humus, coat particles and cause them to adhere into larger, relatively stable secondary structures. Soil density, particularly bulk density, is a measure of soil compaction.

Soil porosity consists of the part of the soil volume occupied by air and water. Soil consistency is the ability of soil to stick together. Soil temperature and colour are self-defining.

The properties may vary through the depth of a soil profile. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5. The effect of pH on a soil is to remove from the soil or to make available certain ions.

a. Soil Texture:

The mineral soil particles differ widely in size. Some are seen with naked eye, while others are small enough to exhibit colloidal properties. The term soil texture is an expression of the size range of the individual particles and it has both qualitative and quantitative connotations.

Qualitatively, it refers to the ‘feel’ of the soil material, whether coarse and gritty or smooth. Quantitatively, soil texture refers to relative proportion of various sizes of particles in a given soil. Most natural field soils are composed of mineral particles, including coarse fragments, gravel, sands of varying sizes, silt and clay. Soil texture is not readily subjected to change.

Many particle size classifications exist, each, of which having different class limits for each size fraction. Classification of International Society of Soil Science (ISSS) renamed as International Union of Soil Science (IUSS) and the United States Department of Agriculture (USDA) are widely followed (Table 4.1).

Physical Nature of Soil Separates:

Coarse fragments that range from 2 to 75 mm dia are termed gravel or pebbles, those ranging from 75 to 250 mm are called cobbles (if round) or flags (if flat) and those more than 250 mm across are called stones or boulders. Sand and gravel may be round or irregular. They are not sticky when wet. These are not plastic.

Water holding capacity of sand is low because of large pores between the particles. Soils dominated by sand are well drained. The specific surface area (total surface area of the particles per unit mass or unit volume of dry soil) may be about 0.1 m² g^-1 for fine sand. Silt particles are intermediary in size and properties between sand and clay. Soil particles posses some plasticity, cohesion and adsorptive capacity, but much less than the clay separates. Silt may cause soil surface compact and crusty.

Clay particles vary in shape from plate like to round. When clay is wet, tends to be sticky and plastic or easily molded. Water and air movement is restricted. Water holding capacity is high. It becomes hard and cloddy when dry. The specific surface area ranges from 10 to 1000 m² g^-1 compared to 1.0 and 0.1 m² g^-1 for silt and fine sand, respectively.

Sand and loamy sand are the two recognised specific textural classes. Silt group includes soils with at least 80 per cent silt and 12 per cent or less clay. The properties of this group are determined by those of silt. A soil must contain at least 35 per cent clay and in most cases, not less than 40 per cent to be designated as clay.

In such soils, the characteristics of clay are dominant. The loam group is complicated in textural class. It is a mixture of sand, silt and clay and the properties are in equal proportion. Accurate and commonly used method for determining the soil textural class is laboratory method based on mechanical analysis. The USDA has developed triangular diagram for determining soil textural classes.

Importance of Soil Texture:

Coarse textured or sandy soils are loose, low water retentive, well drained, well aerated, easily cultivable and are called light soils. On the other hand, fine textured or clay soils tend to absorb and retain much more water as they have large surface area per unit volume. They become plastic and sticky when wet, hard and cohesive when dry, difficult to cultivate and are called heavy soils.

In general, sandy soils have low water and nutrient retentive capacity, low organic matter content, little or no swelling and shrinkage, high leaching of nutrients and pollutants. Fine sands are easily blown by wind. Silty soils have medium to high water and nutrient retentive capacity, moderate aeration, slow to medium drainage, medium to high organic matter content, usually, good supply of nutrients and moderate leaching of nutrients.

These soils are easily blown by wind and susceptible to water erosion, easily compacted, have little swelling and shrinkage and are relatively difficult to work at high moisture content. A loam soil contains a balanced mix of coarse and fine particles with properties intermediate among those of sand, silt and clay.

A loam soil is considered to be an ideal soil for crop growth. Its capacity to retain water and nutrients is superior to that of sand, while its drainage, aeration and tillage properties are often favourable than those of clay. The clayey soils have high water and nutrient retentive capacity, poor aeration, poor drainage, high to medium organic matter, medium to good supply of nutrients and high swelling and shrinkage. These soils resist wind erosion because of excellent sealing properties. They are easily compacted and retard leaching of nutrients and pollutants.

b. Soil Structure:

Soil structure is defined as the natural arrangement, orientation and organisation of particles in soil. It describes the overall arrangement or combination of primary soil separates into secondary groupings called aggregates or peds. Soil conditions and characteristics such as water movement, aeration, heat transfer and porosity are influenced by structure.

Types of Soil Structure:

Based on the arrangement of peds or aggregates, soil structure is classified into four principal types:

Platy:

The aggregates are arranged in relatively thin horizontal plates. It is often formed from parent material and can also result due to compaction by heavy farm machinery.

Prismatic:

Two types of this structure, columnar and prismatic, are vertically oriented aggregates, occurring commonly in surface horizons of semiarid and arid regions. The prisms having rounded tops called columnar structure mostly occur in subsoils of salt affected/sodic soils. Prismatic structures have the tops of prisms angular and are relatively flat horizontally.

Blocky:

All the three dimensions of the peds are more or less equal. They are cube­ like with flat or rounded faces. When the faces and edges are mainly round, they are called sub angular blocky. They are usually confined to subsoil.

Spheroidal:

Rounded aggregates are placed in this category. Relatively nonporous aggregates are called granules and the pattern granular. When the granules are especially porous, the term crumb is used.

Structural Management of Soils:

When the clay is deflocculated, as under the influence of exchangeable sodium, the soil aggregates, generally, collapse. Aggregates are also vulnerable to the effect of water, swelling and shrinkage, beating action of raindrop and scoring action of runoff. Cultivation when the soil is too wet or dry, excessive tillage and soil compaction also cause breakdown of soil aggregates. Close growing perennial plants with extensive root system such as grasses promote soil aggregation.

Unlike soil texture and specific soil surface, which are more or less constant for a given soil, structure is highly dynamic and may change gradually from time to time in response to changes in natural conditions, biological activity and soil management practices. Soil structure can be of decisive importance in determining soil productivity, since it greatly affects the water, air and heat regimes in the field.

Soil structure also influences the soil mechanical properties, which may in turn affect seed germination, seedling establishment and root growth. Soil structure can affect the performance of agricultural operations such as tillage, irrigation and drainage.

4. Essay on the Layers of Soil:

Soil is a thin layer of material on the Earth’s surface in which plants have their roots. It is made up of many things, such as weathered rock and decayed plant and animal matter. Soil is formed over a long period of time. Soil Formation takes place when many things interact, such as air, water, plant life, animal life, rocks, and chemicals.

The soil profile is one of the most important concepts in soil science. It is a key to understanding the processes that have taken in soil development and is the means of determining the types of soil that occur and is the basis for their classification. The soil profile is defined as a vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock.

The soil profile can be as little as 10 cm thick in immature soils and as deep as several meters in tropical areas where the climate is conducive to rapid alteration of the underlying rock to form soil. In temperate areas, the soil/profile is often around a meter deep and in arid areas somewhat shallower than this.

All soil profiles are composed of a number of distinctive layers, termed horizons, interpretation of which is the key to understanding how the soil has formed. Most soils will have three or more horizons. Soils that have not been cultivated will normally have L, F and H layers at the surface.

These layers largely represent different degrees of decomposition of organic matter, the L layer representing the litter layer formed of recognizable plant and soil animal remains, the F layer below, the fermentation layer, usually consisting of a mixture of organic matter in different stages of decomposition, and the H layer, the humose layer, consisting largely of humified material with little or no plant structure visible. Below these, and in cultivated soils occupying the surface layer, is the A horizon composed of a more or less intimate mixture of mineral and organic matter.

(i) Organic Matter (O):

Litter layer of plant residues in relatively un-decomposed form. O horizons may be divided into O₁ and O₂ categories, whereby O₁ horizons contain decomposed matter whose origin can be spotted on sight (for instance, fragments of rotting leaves), and O₂ horizons containing only well-decomposed organic matter, the origin of which is not readily visible.

(ii) Horizon (P):

These horizons are also heavily organic, but are distinct from O horizons in that they form under water logged conditions. The “P” designation comes from their common name, peats. They may be divided into P₁ and P₂ in the same way as O Horizons. This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.

(iii) Surface Soil (A):

Layer of mineral soil with most organic matter accumulation and soil life. This layer is the top 15cm of the soil profile and has the highest percentage of organic matter. The layer was likely formed from decomposing plant and mineral. A” Horizons may be darker in color than deeper layers and contain more organic material, or they may be lighter but contain less clay or sesquioxides.

The A is a surface horizon, and as such is also known as the zone in which most biological activity occurs. Soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are concentrated here, often in close association with plant roots.

A-horizons may also be the result of a combination of soil bio-turbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A- horizon is regarded as a “bio mantle”. This layer eluviates (is depleted of) iron, clay, aluminum, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored “E” subsurface soil horizon is apparent at the base of the “A” horizon.

(iv) Sub-Soil (B):

This layer accumulates iron, clay, aluminum and organic compounds, a process referred to as illuviation. A horizon, the B horizon may be divided into B₁, B₂, and B₃ types under the Australian system. B₁ is a transitional horizon of the opposite nature to an A3 – dominated by the properties of the B horizons below it, but containing some A-horizon characteristics.

The second layer or B1 horizon is similar to the A horizon and is found from 15-30cm. B₂ horizons have a concentration of clay, minerals, or organics and feature the strongest pedological development within the profile. It is sandy, with a slight increase in clay composition throughout its thickness of 30-60cm. B₃ horizons are transitional between the overlying B layers and the material beneath it.

The B₃horizon is found from 60cm and beyond whether C or D horizon. The A₃, B₁, and B₃ horizons are not tightly defined, and their use is generally at the discretion of the individual worker. Plant roots penetrate through this layer, but it has very little humus. It is usually brownish or red because of the clay and iron oxides washed down from A horizon.

(v) Parent Rock (C):

Layer of large unbroken rocks. This layer may accumulate the more soluble compounds. The C Horizon may contain lumps or more likely large shelves of unweathered rock, rather than being made up solely of small fragments as in the solum. “Ghost” rock structure may be present within these horizons. The C horizon also contains parent material. It forms the framework of the soil. The A and B layers are formed by this layer. The C horizon forms as bed rock weathers and rock breaks up into particles.

(vi) Bedrock (R):

R horizons denote the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer.

R horizons denote the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer.

(vii) Limnic (L):

Horizons or layers indicate mineral or organic material that has been deposited in water by precipitation or through the actions of aquatic organisms. Included are copro-genous earth (sedimentary peat), diatomaceous earth, and marl; and is usually found as a remnant of past bodies of standing water.

Sand and silt are the products of physical and chemical weathering; clay, on the other hand, is a product of chemical weathering but often forms as a secondary mineral precipitated from dissolved minerals. It is the specific surface area of soil particles and the unbalanced ionic charges within them that determine their role in the cation exchange capacity of soil, and hence its fertility.

Sand is least active, followed by silt; clay is the most active. Sand’s greatest benefit to soil is that it resists compaction and increases porosity. Silt is mineralogical like sand but with its higher specific surface area it is more chemically active than sand. But it is the clay content; with its very high specific surface area and generally large number of negative charges that gives a soil its high retention capacity for water and nutrients.

Clay soils also resist wind and water erosion better than silty and sandy soils, as the particles are bonded to each other. Calcium. Magnesium, Sulfur, Potassium; depending upon soil composition.

Nitrogen; usually little, unless nitrate fertilizer was applied recently.

Phosphorus; very little as its forms in soil are of low solubility.

Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from 2.0 to 0.05 mm (0.079 to 0.0020 in) in diameter. Silt ranges in size from 0.05 to 0.002 mm (0.002 to 0.00008 in). Clay cannot be resolved by optical microscopes as its particles are 0.002 mm (7.9 x 10-5 in) or less in diameter. In medium-textured soils, clay is often washed downward through the soil profile and accumulates in the subsoil.

Soil components larger than 2.0 mm (0.079 in) are classed as rock and gravel and are removed before determining the percentages of the remaining components and the texture class of the soil, but are included in the name. For example, a sandy loam soil with 20% gravel would be called gravelly sandy loam.

When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil.

A soil is called organic if:

Mineral fraction is 0% clay and organic matter is 20% or more.

Mineral fraction is 0% to 50% clay and organic matter is between 20% and 30%.

Mineral fraction is 50% or more clay and organic matter 30% or more.

5. Essay on the Formation of Soil:

Soil is the result of evolution from more ancient geological materials. Soil formation, or Pedogenesis, is the combined effect of physical, chemical, biological and anthropogenic processes on soil parent material. Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum.

These constituents are moved (translocated) from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons. Soil formation proceeds are influenced by at least five classic factors. They are parent material, climate, topography (relief), organisms, and time.

The Weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants become established very quickly on basaltic lava, even though there is very little organic material. The developing plant roots are associated with mycorrhizal fungi.

Soil Microbes:

Soil is the most abundant ecosystem on Earth, but the vast majority of organisms in soil are microbes, a great many of which have not been described. There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely.

One estimate put the number at over a million species per gram of soil, although a later study suggests a maximum of just over 50,000 species per gram of soil. The total number of organisms and species can vary widely according to soil type, location, and depth.

Plants, animals, fungi, bacteria and humans affect soil formation. Animals, soil meso-fauna and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about. In the same way, plant roots open channels in soils. Plants with deep taproots can penetrate many meters through the different soil layers to bring up nutrients from deeper in the profile.

Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding organic matter. Micro-organisms, including fungi and bacteria, affect chemical exchanges between roots and soil and act as a reserve of nutrients. Humans impact soil formation by removing vegetation cover with erosion as the result.

Their tillage also mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers. Micro-organisms are able to metabolize the organic matter and release ammonium in a process called mineralization. Others take free ammonium and oxidize it to nitrate.

Particular bacteria are capable of metabolizing N₂ into the form of nitrate in a process called nitrogen fixation. Both ammonium and nitrate can be lost from the soil by incorporation into the microbes’ living cells, where it is temporarily immobilized or sequestered. Nitrate may also be lost from the soil when bacteria metabolize it to the gases N₂ and N₂O. In that gaseous form, nitrogen escapes to the atmosphere in a process called denitrification.

Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that results in surface runoff. Plants shade soils, keeping them cooler and slowing evaporation of soil moisture, or conversely, by way of transpiration, plants can cause soils to lose moisture.

Plants can form new chemicals that can break down minerals and improve soil structure. The type and amount of vegetation depends on climate, topography, soil characteristics, and biological factors. Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location.

Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.

Soil Water:

Water is the chemical substance with chemical formula H₂O. One molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom. One molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom. Water is a tasteless, odorless liquid at ambient temperature and pressure, and appears colorless in small quantities, although it has its own intrinsic very light blue hue.

Ice also appears colorless, and water vapor is essentially invisible as a gas. Water is a tasteless, odorless liquid at ambient temperature and pressure, and appears colorless in small quantities, although it has its own intrinsic very light blue hue. Ice also appears colorless, and water vapor is essentially invisible as a gas.

Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential for plants. It constitutes 85%-95% of the plant’s protoplasm. It is essential for photosynthesis. It is the solvent in which nutrients are carried to, into and throughout the plant.

It provides the turgidity by which the plant keeps itself in proper position. In addition, water alters the soil profile by dissolving and re-depositing minerals, often at lower levels, and possibly leaving the soil sterile in the case of extreme rainfall and drainage. In a loam soil, solids constitute half the volume, air one-quarter of the volume, and water one-quarter of the volume, of which only half will be available to most plants.

The amount of water remaining in a soil drained to field capacity and the amount that is available are functions of the soil type. Sandy soil will retain very little water, while clay will hold the maximum amount.

The time required to drain a field from flooded condition for a clay loam that begins at 43% water by weight to a field capacity of 21.5% is six days, whereas a sandy loam that is flooded to its maximum of 22% water will take two days to reach field capacity of 11.3% water. The available water for the clay loam might be 11.3% whereas for the sandy loam it might be only 7.9% by weight.

Soil Atmosphere:

The atmosphere of soil is radically different from the atmosphere above. The consumption of oxygen, by microbes and plant roots and their release of carbon dioxide, decrease oxygen and increase carbon dioxide concentration. Atmospheric CO₂ concentration is 0.03%, but in the soil pore space it may range from 10 to 100 times that level.

At extreme levels CO₂ is toxic. In addition, the soil voids are saturated with water vapour. Adequate porosity is necessary not just to allow the penetration of water but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower.

Oxygen diffuses in and is consumed and excess levels of carbon dioxide, diffuse out with other gases as well as water. Soil texture and structure strongly affect soil porosity and gas diffusion. Platy and compacted soils impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce nitrate to the gases N₂, N₂O, and NO, which are then lost to the atmosphere.

Aerated soil is also a net sink of methane CH₄ but a net producer of greenhouse gases when soils are depleted of oxygen and subject to elevated temperatures.

Influences on Soil Formation:

Soil formation would begin with the plants, which are supported by the porous rock as it is filled with nutrient-bearing water that carries dissolved minerals from the rocks and guano. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. That assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time.