SOIL

What is soil?

Soils are the loose mineral or organic materials found on the earth's surface, usually (or averagely) made up of about 25% air, 25% water, 45% mineral and 5% organic matter (humus, tiny living organisms and sometimes plant residue).


It is the stuff that supports rooted plants in a natural environment. There are soils practically on every land that is not covered by water.

There are many types of soils, usually placed in classes (types) based on their color, profile, texture, composition or structure. Each soil type is formed differently and can be found in specific places on the earth’s immediate surface, mid and deep under the surface. Soils on the surface (a few millimeters deep) are usually exposed to direct climatic and environmental factors, and are easily blown away by wind, washed away by water or even broken down by temperature changes, human and animal activity. There are also soils found deep down the earth, often protected from climatic and environmental factors. (see more under soil profile)

Because soils are formed from a variety of ways and in infinite conditions, it is hard to give a number for the types of soils we have on earth. However, they can be grouped using the stuff that they are made of.

For this lesson, we shall look at Sandy, Silty, Clay, Loamy, Peaty and Chalky Soils.

One important subject that many farmers and soils scientists look out for is soil chemistry. This includes soil pH (the acidity of the soil), nutrient level, its organic content and the chemical composition of the minerals found in it. This is partly because different soils are used for different things and it is important to know something about the soils you choose. These are usually determined by the geographic location of the soils, the types of plants growing in them, and even the environmental factors (water and air) that the soil is exposed to.

The study of soils as naturally occurring phenomena is called pedology, and a person who studies soils (soil scientist) is called a pedologist.



Importance (Functions) of soils

Soils are essential for life, in the sense that they provide the medium for plant growth, habitat for many insects and other organisms, act as a filtration system for surface water, carbon store and maintenance of atmospheric gases. Let us take a closer look at each of these:

Medium for plant growth:
Soils support roots and keep them upright for growth.
Soils provide plants with essential minerals and nutrients.
Soils provide air for gaseous exchange between roots and atmosphere.
Soils protect plants from erosion and other destructive physical, biological and chemical activity.
Soils hold water (moisture) and maintain adequate aeration.

Habitat for many insects and other organisms:

Insects and microbes (very tiny single-cell organisms) live in the soils and depend on soils for food and air.
Soils are homes to a diverse range of organisms such as worms and termites. They provide the needed moisture and air for the breakdown of organic matter. (learn more about soil ecosystem)
They provide a home for many organisms such as insects to lay and hatch eggs and rodents to give birth to new offsprings.
A Filtration system for surface water:
After rainfall and snowmelts, 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. During humification (a process where soil organisms form complex and stable organic matter) some organic matter breakdown do not occur completely, especially in soils like peat, owing to its high acid and water content.
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. Nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled in the soil.



                                                          Soil Profile

If one could dig a massive trench (hole), about 50-100ft vertically downwards into the ground, you will notice that you would have cut through various layers of soil types. A look at the layers from a distance gives one a cross-section view of the ground (beneath the surface) and the kind of soils and rocks it is made up of.

This cross-section view is called a Soil Profile. The profile is made up of layers, running parallel to the surface, called Soil Horizons.

Each horizon may be slightly or very different from the other above or below it. Each horizon tells a story about the makeup, age, texture and characteristics of that layer.

Most soils have three major horizons. These are A Horizon, B Horizon and C Horizon. Aside from these three, there are also the O, E and R horizons. How are they different?

The O-Horizon:
The O horizon is very common in many surfaces with lots of vegetative cover. It is the layer made up of organic materials such as dead leaves and surface organisms, twigs and fallen trees. It has about 20% organic matter. It is possible to see various levels of decomposition occurring here (minimal, moderately, highly and completely decomposed organic matter). This horizon is often black or dark brown in color, because of its organic content. It is the layer in which the roots of small grass are found.

The A-Horizon:
The A horizon may be seen in the absence of the O horizon, usually known as the topsoil. It is the top layer soils for many grasslands and agricultural lands. Typically, they are made of sand, silt and clay with high amounts of organic matter. This layer is most vulnerable to wind and water erosion. It is also known as the root zone.

The E-Horizon:
The E horizon is usually lighter in color, often below the O and A horizons. It is often rich in nutrients that are leached from the top A and O horizons. It has a lower clay content and is common in forested lands or areas with high quality O and A horizons.

The B-Horizon:
The B-horizon has some similarities with the E-horizon. This horizon is formed below the O, A and E horizons and may contain high concentrations of silicate clay, iron, aluminum and carbonates. It is also called the illuviation zone because of the accumulation of minerals. It is the layer in which the roots of big trees end.

The C-Horizon:
The C horizon lacks all the properties of the layers above it. It is mainly made up of broken bedrock and no organic material. It has cemented sediment and geologic material. There is little activity here although additions and losses of soluble materials may occur. The C horizon is also known as saprolite.

The R-Horizon:
The R horizon is bedrock, material, compacted and cemented by the weight of the overlying horizons. It is the unweathered parent material. Rock types found here include granite, basalt and limestone.
                                                 Types of soil



Sandy Soils
Sandy soils are free draining, with the largest, but fine and hard particles. It has a gritty feel. It does not bind very well. It is poor in holding water and easily warms up in the spring season. Sandy soils are very low in nutrients, as they are usually washed away. Its degree of aeration depends on the sizes of the particles, which vary a lot in size.
It is usually formed from the weathering or disintegration of bedrock such as shale, limestone, granite and quartz.

Silty Soils
This kind is finer, smoother in texture and holds water better than sandy soils. It also holds up nutrients and makes it better for crop cultivation. Silty soils are heavier than sandy soils, and almost midway between the properties of sandy and clay soils.
It is formed when fine sediments (dust, organic matter and debris) are carried by water or ice and deposited. When silt is deposited and cemented with time, it forms siltstone. Silt particles are so small and not easily seen by the eyes. It leaves a bit of residue after you touch them.

Clay
The particles that make up clay are the finest and they bind very well. It has very little air spaces. Clay very sticky when wet, and can be molded into any shape and form. When they dry, they are rock hard. Clay soils do not drain very well. Clay is believed to form in places where the rock is in contact with water, air or steam. Example, sediments on sea or lake bottoms may become clay soils with time.

Loamy
This soil is a mixture of sand, clay and silt particles and has the ability to retain water. It is high in calcium, aeration and ideal for most crops and vegetables. It is the soil all farmers dream of, as it is full of nutrients from decomposed organic material. It is soft and easy to cultivate. Peaty
Peaty soils are acidic and as a result, does not support decomposition very well. It is dark in color, rich in organic material, although contains less nutrients than loamy soils. It retains water very well.

Chalky
Chalky soils are alkaline with a pH of about 7.5. It is not acidic and often stony with chalk or limestone bedrock. It is free draining because of its coarse and stony nature. Not the best for crops to grow in as they lack manganese and iron.
 
                 What is soil conservation?

From the pages earlier, we learned about the role of soils and how essential they are for life on earth. Unfortunately, soils are under threat in many ways, from excessive farming practices, use of chemicals during agricultural practices, water, land and air pollution, erosion and so on. These upset the natural function of soils and affect many ecosystems that depend on it for survival.

This is why soil conservation is very important. It is the actions we can take, or things we can apply to our use of soils and lands to ensure their sustained health and quality.

Here are a few:

Planting vegetative cover:



The root systems of vegetative cover hold soils in place and prevent wind and water erosion. It also ensures its supply of organic matter from dead leaves and dropping of animals waste. The vegetative cover also shelters the soils from excessive heat from the sun. It helps to reduce evaporation and retain soil moisture, needed for the breakdown of organic matter.

Careful waste disposal and management:
When we recycle more and compost our food waste, we reduce the amount of contaminants that we introduce to soils. We also give back to the land, rich humus from composts that we do. This is why we need to manage our waste well to ensure that our soils are alive and healthy.

Farming practices:
No-till farming, terrace and contour farming are all great ways to conserve soil quality. Regarding no-tilling, crops are allowed to stay after the harvest season, to shed off naturally, thereby holding the soil together and sheltering the soils from wind and water action. Terrace and contour farms take into account the slope of the land to reduce run-off after the rains. In many places, windbreaks, usually composed of trees or shrubs planted along specific distances in farms are effective ways of controlling wind erosion.



                            Soil Ecosystems

There are some really cool ecosystems that many of us forget to mention when we talk about ecosystems. If you could turn yourself into a small soil burrowing insect, you would be amazed by the life and interactions that go on under the surface of soils. All the members of this wonderful ecosystem are so tiny that it is hard for the eyes to see.

Look at this illustration below. It shows a basic food web in the soils. The arrow shows the direction of energy flow.



In the soils, there are roots of plants. That is a "biotic" factor. The roots depend on nutrients, moisture, air and temperature (abiotic factors) to survive. The roots also depend on fungi and bacteria to protect the roots from harmful bacteria attaching to it. There are also tiny organic matter existing there.

Nutrients and air are provided by tiny animals such as arthropods shredders, (weevils, millipedes, termites and worms) that dig and turn the soils as they feed on fungi and bacteria on dead plant material. The waste (droppings) of these arthropods provide the chemicals needed for the decomposition of other organic material.

Birds and small animals like moles also depend on mites, weevils and insect-eggs for food. Together, they all depend on moisture and air to live in the soils.

It is important that all the members of this ecosystem, together with their abiotic factors are active to ensure the survival of the ecosystem.
 

Factors affecting soil formation Soils form from the interplay of five main factors namely Parent material, Time, Climate, Relief and Organisms. Parent material: This refers to the mineral material or organic material from which the soil is formed. Soils will carry the characteristics of its parent material such as color, texture, structure, mineral composition and so on. For example, if soils are formed from an area with large rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as its parent material. Time: Soils can take many years to form. 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. Climate: This is probably the most important factor that can shape the formation of soils. Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. A warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation and more water erosion. They all help to determine the kind of soils in an area. Relief: This refers to the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopes, and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render it less fertile. Organisms: The source and richness of organic matter are down to the living things (plants and animals) that live on and in the soils. Plants, in particular, provide lots of vegetative residues that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help break down organic matter and aid decomposition. Animal droppings, dead insects and animals result in additional decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions. GO TO LINK:https://www.eschooltoday.com/soils/factors-that-affect-soil-formation.html AND LEARN MORE