soils

Quite a big part of the earth is covered by soil. Soil plays a big part in the well-being of humans, animals, and plants. The term soil refers to a mixture of minerals, gases, organisms, liquids, and organic matter that together support life. Let’s dig in and find out more.

LEARNING OBJECTIVES

By the end of this topic, you are expected to;

• Understand the meaning of soil
• Know the major functions of soil
• Know the characteristics of soil
• Know the types of soil
• Describe soil horizons

Pedosphere is the name given to the body of the soil of the earth. It has four major functions:

• A habitat for organisms
• A means of storage, supply, and purification of water
• A medium of plant growth
• A modifier of the atmosphere of the earth

In turn, all these functions modify the soil.

The pedosphere interfaces with the atmosphere, hydrosphere, and lithosphere. The term pedolith (which is commonly used to refer to the soil) translates to ground stone. Soil is made up of a solid phase of minerals as well as organic matter (soil matrix), and a porous phase responsible for holding gases and water.

Soil is a product of various factors. The influence of relief (slope of terrain, elevation and orientation), climate, the parent materials of the soil and organisms interacting over time. Soil continually undergoes development through numerous physical, biological and chemical processes such as weathering with associated erosion.

Soil science has two main branches of study: pedology and edaphology. Pedology studies the formation, description and classification of soils in their environment. Edaphology is the scientific study of the soil, normally with respect to plant growth.

DESCRIPTION

A typical soil is made up of approximately 50% solids (45% mineral and approximately 5% organic matter), and 50% pores or voids of which approximately half is occupied by gas and the other half by water.

The texture of the soil is determined by the relative proportions of the individual particles of silt, sand and clay that make up the soil. The interaction of individual mineral particles with water, organic matter and gases through both biotic and abiotic processes, causes the particles to stick together (flocculate) to form peds or aggregates. Where these aggregates are identified, soil can be said to be developed. It can, therefore, be described further in terms of reaction, porosity and consistency.

The nutrients can be adsorbed on the surface of clay minerals. Nutrients are either bound in clay minerals (absorbed), or bound in organic compounds as part of dead soil organic matter or of the living organisms.

Soil PH has an effect on plant nutrient availability. Soil pH is defined as the measure of the activity of hydrogen ions in a soil solution.

SOIL FORMATION

Soil formation is also called pedogenesis. It refers to the combined effect of physical, biological, anthropogenic, and chemical processes working on soil parent material. Soil is formed when there is an accumulation of organic matter and colloids are washed downward, leaving behind clay, gypsum, carbonate, humus and iron oxide deposits, producing a layer known as the B horizon. Animal activities and water move these constituents from one level to the other. As a result, a soil profile is formed from the layers. The movement and alteration of materials in the soil lead to the formation of distinctive soil horizons.

FACTORS OF SOIL FORMATION

The process of soil formation is influenced by five classic factors intertwined in a soil’s evolution. These factors are: climate, parent material, topography or relief, time and organisms.

Parent material refers to the mineral material from which soil forms. Typical soil parent mineral materials include calcite, quartz, feldspar and mica.

The principal climatic variable influencing the formation of soil is effective precipitation. Effective precipitation is gotten by, precipitation minus evapotranspiration. Precipitation is defined as the net flux of water from the atmosphere to the earth's surface. Temperature is also a major climatic variable affecting soil formation. Both affect the rate of physical, chemical and biological processes.

Relief or topography is characterized by the elevation, slope, and orientation of the terrain. Topography influences the rate of runoff or precipitation and the rate of erosion of the surface soil profile.

Animals, plants, bacteria, fungi and humans affect the formation of soil. Soil animals mix soils as they form pores and burrows, allowing gases and moisture to move about. This process is called bioturbation. Plant roots penetrate soil horizons in the same way.

Time is a factor in the interactions of all the above.

PHYSICAL PROPERTIES OF SOIL

The physical properties of soils are texture, bulk density, structure, consistency, porosity, color, resistivity, and temperature.

• Soil texture refers to the proportion of sand, silt, and clay. Other properties that are influenced by soil texture include permeability, porosity, infiltration, water holding capacity, and shrink-swell rate.
• Soil structure refers to the arrangement of soil aggregates. Using the soil structure you can predict some soil properties like organic matter content, texture, biological activity, and human use.
• Consistency refers to the ability of the soil to stick to itself or other objects and its ability to resist rupture and deformation. It is important for predicting cultivation problems and in the engineering of foundations.
• Pore size determines the rate of porosity. Clay soils have smaller pores than sands.
• Soil bulk density is equivalent to the dry mass of the soil divided by the soil's volume. It includes organic materials, air space, and soil volume. Therefore, soil bulk density is always less than soil particle density. This is an indication of good soil compaction. Soil particle density is lower in soils that have a high organic matter content.
• The temperatures of soil depend on the ratio of energy absorbed to that of energy lost. The range of soil temperatures is between -20 and 60 degrees celsius. Soil temperature is important as it regulates the following processes, plant and root growth, breaking of seed dormancy, seed germination, and the availability of nutrients. Heavy mulching (providing soil cover) slows down the warming of the soil during hot seasons.
• Generally, soil color is determined by, drainage conditions, organic matter content, and the degree of oxidation. While soil color is easily discerned, it has little use in the prediction of soil characteristics. It is mainly used in distinguishing different horizons in a soil profile.
• Soil resistivity is the measure of the ability of soil to retard electric current conduction. Higher content of moisture or increased concentration of electrolytes lowers the resistivity and increases conductivity. This increases the rate of corrosion.

TYPES OF SOIL

Soil is classified into six major groups. They are; clay, sandy, silty, loamy, peaty, and chalky.

• Clay soil. Characteristics of this soil include; a lumpy feeling, sticky when wet and hard when dry (cracks), few air spaces, and poor drainage.
• Sandy soil. This type of soil has a gritty feeling, drains easily, is easy to cultivate, and dries out quickly. You can use organic amendments such as adding organic manure to improve the quality of this type of soil.
• Silty soil. This soil feels soapy and soft. It is normally very rich in nutrients and holds moisture.
• Peaty soil. This type of soil is dark and has a damp and spongy feeling. This is because of a high level of peat in the soil.
• Chalky soil. This soil is made up of large grains and has stones in it. This soil overlays limestone bedrock or chalk. It is free draining and alkaline in nature.
• Loamy soil. This soil is slightly damp and fine-textured. It has a great structure, enough drainage, retains moisture, has nutrients, and is easy to cultivate. This soil can be said to be a good mix of sand, silt, and clay.

SOIL HORIZONS

If you dig deep into the soil, you will see it is composed of layers or horizons. When you put these horizons together, they make a soil profile. The main soil horizons are;

• Organic or humus. This layer is made up mainly of organic matter like decomposing leaves. It is thick in some soils but absent in some.
•  Topsoil. This is made up of minerals from the parent material together with organic matter.
•  Eluviated. Made up of silt and sand particles of resistant materials to leaching like quartz.
•  Subsoil. Materials from the A and E horizon that have leached are found here. This layer is very rich in minerals.
•  Parent material. Made up of earth's material deposit from which soil develops hence parent material.
•  Bedrock. A mass of rock like basalt and granite that form parent material in some soils.

SUMMARY

We've learnt that;

• Soil is a mixture of minerals, gases, organisms, liquids, and organic matter that together support life.
• Edaphology is the scientific study of the soil.
• A typical soil is made up of approximately 50% solids and 50% pores.
• The process of soil formation is called pedogenesis.
• The factors that affect soil formation are: climate, parent material, topography or relief, time and organisms.
• Soil is classified into six major groups ; clay, sandy, silty, loamy, peaty, and chalky.