What are Soils?

Soil can be defined as :The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.

Soil really extend a few inches to approximately 6 feet below the Earth’s surface. This thin layer of material, however, supports all plant life on the planet, animals and humans alike

Soils play an important role in the environment

Soils influences the following

• The filtering of animal and human wastes in preventing them from reaching surface and under ground water
• Amount of nutrients in the soils and thus the amount and type of plants grown
• The placement and life span of man made buildings
  

Well since soils influences these things, we need to understand what they are made of.

Soils are made up of many things which are grouped in the following ways:

• Solid materials such as minerals, organic materials
• Liquid elements like water
• Gaseous components like nitrogen and oxygen
  

The most common organic materials making up soils are the remains of dead plants and animals. Other materials comes from the air like dust or rainfall, snow. The gases occurring in the soil are like those in the air (nitrogen and oxygen) which may be produced by microorganisms, bacteria and plants.

Soil Properties

The Properties of Soils are as follows

Soil Texture
Soil texture refers to the size of the grains of sand, silt and clay size particles. Clay particles are the smallest,( less than .002 mm in size).Soil texture can in turn affect other properties such as chemistry,permeability,porosity,

The porosity of the soil refers to the amount of pores, which are the open space between soil particles. Because of this fine textured clay soils hold more water than coarse textured sandy soils.

Soil Chemistry
Plant materials, animal waste, and animals remains adds organic matter called humus to the soil. Humus increases amount of moisture the soil can retain as well as soil chemistry such as pH (acid or alkaline). Soil pH ranges from 3 to 10. Pure water has a pH of 7 which is considered neutral, pH values greater than seven are considered basic or alkaline, below seven acidic. Most good agricultural soils have a pH between 5 and 7. Acidic and alkaline soils causes an agricultural problem due to their lack of nutrients.

Parent Material

Parent material comes from rock that broke down gradually and remained where it was , or material that has been deposited by wind, water, or ice. The properties of the parent material plays an important role in the properties of the soil.

For example soils that have a coarse grain texture most likely came from parent material that is coarse grained. Fine grain soil develops would have come from a parent material which contain minerals that are weathered easily.

Parent material composition has a direct impact on soil chemistry and fertility. Parent materials rich in soluble ions-calcium, magnesium, potassium, and sodium, are easily dissolved in water and available for plants.

Climate

Soils and climate go hand in hand. Why? Well the climate whether rainy,hot and dry or cold affects the physical and chemical reactions on the parent material, causing different degrees and types of weathering. Climate also controls the amount of vegetation which also affects the amount of weathering, erosion and chemical makeup of the soils. Rainfall or precipitation also impacts on the amount of and type of chemicals in the soil, since the water has a dissolving action on the chemicals. Cold tundra regions also inhibits the breakdown of vegetative matter causing it to build up. In the warm and wet tropics, bacterial activity proceeds at a rapid rate, thoroughly decomposing leaf litter. Under the lush tropical forest vegetation, available nutrients are rapidly taken up.

Topography

Topography/ relief of the land, influences the runoff of water, A steeped sloped land in most cases has little surface soil since it is washed away by surface run off.

Effect of soil erosion

First, the slope of the land affects the degree of runoff that is generated when rain falls to the surface. Examine the diagram below showing the relationship between hill slope position, runoff and erosion. Erosion causes stripping of the soil thus preventing parent material to stay in place to develop into a soil.

Soil organic matter consists of a variety of components. Raw plant residues, on the surface, help reduce surface wind speed and water runoff. Removal, incorporation or burning of residues predisposes the soil to serious erosion. Organic matter or Humus is limited by plant and tree type.

The "active" and some of the resistant soil organic components, together with microorganisms (especially fungi) are involved in binding small soil particles into larger aggregates. Aggregation is important for good soil structure, aeration, water infiltration and resistance to erosion and crusting.

Organic matter in soil serves several functions. From a practical agricultural standpoint, it is important for two main reasons. First as a "revolving nutrient bank account"; and second, as an agent to improve soil structure, maintain health, and minimize erosion.

As a revolving nutrient bank account, organic matter serves two main functions:

Since soil organic matter is derived mainly from plant residues, it contains all of the essential plant nutrients. Accumulated organic matter, therefore, is a storehouse of plant nutrients. Upon decomposition, the nutrients are released in a plant-available form.

The stable organic fraction (humus) adsorbs and holds nutrients in a plant available form.
Organic matter does not add any "new' plant nutrients but releases nutrients in a plant available form through the process of decomposition. In order to maintain this nutrient cycling system, the rate of addition from crop residues and manure must equal the rate of decomposition.

Fertilizer can contribute to the maintenance of this revolving nutrient bank account by increasing crop yields and consequently the amount of residues returned to the soil.

Soils made of many layers of particles and the structure of the soil changes with depth. The soil closest to the surface contains the most humus (plant and animal materials). The further from the surface (deeper), the less humus and more rock particles there are. If a cut is made through the soil down to the underlying bedrock, it is possible to see layers, or "soil horizons" in the soil. There might be as many as five different soil horizons. The thickness of each horizon changes with different soil types. In some soils, a horizon may be very thin or absent.

"O" horizon

"O" horizon is the top most layer which consists of fresh or decaying organic materials, such as leaves, dead plants and animal remains and animal droppings. The characteristic dark color of the O horizon is the result of humus formation from the decomposition of organic matter. Soils in forested area usually have a thick "O" horizon as these forest areas produce large amounts of organic waste each year from the leaves from the tress above. Deserts regions produce very little organic waste and therefore have very thin, or even no "O" layers.

"A" horizon

"A" horizon is located directly beneath the "O" horizon or the“living layer,” often called “topsoil.” It is dark due to organic matter. This is the coarsest, most fertile layer where most roots grow. It consists of a dark mixture of organic materials and rock particles. Organic materials are broken down slowly by bacteria and other creatures and eaten by decomposers. It is in this layer that most animals live. Plant roots are plentiful in the "A" horizon although many roots grow much deeper into the soil in search of moisture.

"B" horizon

B" horizon, or subsoil is located directly under the "A" horizon often called “subsoil” made of more COMPACTED material.. It contains many of the nutrients which have been washed further down by the rain from the "A" horizon as the rain water seeps downward. Often nutrient rich but low in organic content This layer also contains the remains of the humus.

"C" horizon

"C" horizon is the next layer which has no organic material. This layer determines natural fertility, pH, and soil depth. Partially disintegrated parent material and mineral particles are in this horizon consisting of weathered stone from the parent material. This parent material is the rock from which the minerals in the soil are removed as well as the source of most of the rock particles found in the soil.

"R" horizon

"R" horizon is made up of parent material. This level is the lowest level it may be the bedrock or the parent material may have been deposited from another area by moving ice, by rivers and seas or by the wind. It can be within a few inches of the surface or many feet below the surface.

Zonal Soils is the classification of the different soils types and may occur in different regions of the world. The main ones that we focus on are Chernozems, Latosols, Podsols, Tundra.

Chernozems

Chernozem soils, usually occur under grassland-forest areas, grasses and shrubs, usually develop in cool to cold, subarid to subhumid climates. The cool climate region combined with the grasslands creates a favorable environment for chernozems to develop. They develop highly under dryland grasses and low under wetland grasses, trees and shrubs.

Latosols
Latosols have a characteristic presence of a thin humus layer due to intense bacterial activity. The A-Horizon in these soils often contains much aluminum and Iron oxides which gives it a reddish colour from the concentration of iron aluminum oxides. The leaching effects found in latosols make it deficient in plant nutrients. Latosols are usually deep due to rapid weathering of the underlying parent rock and the C-Horizon . There are also very pervious . In addition to that the horizons in the soil profile are not distinct
Latosol can be found in savannah and equatorial areas which receives relatively heavy rainfall that causes excessive leaching. This causes valuable mineral nutrients to be leached to the lower layers and the soils remain poor. Since latosols are poor soils, they support short grasses and bush. Rearing of animals is a main activity in such areas.

Podsols
The name podsol comes from the derives from the Russian for “ash like”. They can be found in more northern areas, where it is associated with the boreal forests, but can also be seen within the deciduous forest areas. These soils are known to have an ash coloured horizon. Podsols can be found in coniferous forests areas of the northern latitudes.

In a podsol soil the soil constituents are moved downward by percolating rainwater which dissolves and extract further constituents when it passes through the soil. Within the in the podsol profile distinctive horizons can be observed dues to the removal or iron and aluminium to the B horizon leaving behind traces of silica.

Tundra

In tundra soils there are low annual temperatures in the tundra climate and the subsoil is permanently frozen. There is a slow evapouration rate and a very short period for plants to grow. Chemical weathering and biological activity are restricted. The surface layer of the tundra soil is mainly peat. Followed by a horizon of grey mud. Below this area the ground maybe permanently frozen.

Soil erosion is a natural process. Soil erosion can occur as a slow process that continues relatively unnoticed, or it can occur rapidly causing much loss of topsoil.

Causes of Erosion
Wind and water are the main agents of soil erosion. The amount of soil they can carry away is influenced by two related factors:

• Speed - the faster the wind and water moves, the more soil it is able erode;
• Plant cover - plants provides a protective covering to the soil , when the vegetation is removed wind and water can do much more damage.

Erosion by Water

• Rainfall Intensity and Runoff
  
  In intense rainfall the impact of raindrops on the soil surface can break down soil particles scattering them. Light particles such as very fine silt, sand, clay and organic matter are easily removed by the raindrop splash and runoff water.
  
  Soil movement by rainfall (raindrop splash) is usually in instances where there is an intense weather system such as a Hurricane .During and after a hurricane the amount of soil eroded is noticeable causing the collapse of roads and landslides. Runoff occurs whenever there is excess water on a slope that cannot be absorbed into the soil or trapped on the surface. The amount of runoff is increased when infiltration is reduced due to soil compaction, or freezing.
  
  
• Soil Erodibility
  
  Soil erodibility is the ability of soils to resist erosion, based on the physical properties of each soil. Generally, soils with faster infiltration rates, higher levels of organic matter and improved soil structure have a greater resistance to erosion. Sand, sandy loam and loam textured soils tend to be less erodible than silt, very fine sand, and certain clay textured soils.
  
  Tillage and cropping practices which lower soil organic matter levels, causes poor soil structure, and result of compacted contribute to increases in soil erodibility. Decreased infiltration and increased runoff can be a result of compacted subsurface soil layers. A decrease in infiltration can also be caused by a formation of a soil crust, which tends to "seal" the surface. On some sites, a soil crust might decrease the amount of soil loss from sheet or rain splash erosion, however, a corresponding increase in the amount of runoff water can contribute to greater rill erosion problems.
  
  
• Slope Gradient and Length
  
  It is logical that the steeper the slope or gradient of the land ,the greater the amount of soil loss from erosion by water. Soil erosion by water also increases as the slope length increases due to the greater accumulation of runoff.
  
  
• Vegetation
  
  Little vegetative cover of plants can increase the effect of soil erosion Plant protects the soil from raindrop impact and slows down the movement and amount of surface runoff and allowing excess surface water to infiltrate. Trees and roots tends to provide breakages on the surface causing water to slow down or divert different directions on a slope. Roots tend to provide crevices and filtration points in the soil allowing water to penetrate into loser layers of the soil thus increasing the permeability of the soil

Erosion by Wind

• Erodibility of Soil
  
  Small soil particles can be carried by the wind great distances. Fine and medium size particles can be lifted and deposited, while coarse particles can be blown along the surface the abrasion that results from a particles blowing along the surface may reduce the soil particle size and further increase the soil erodibility.
  
  
• Climate
  
  The speed and duration of the wind have direct relationship to the extent of soil erosion. Soil moisture levels can be very low at the surface of excessively drained soils or during periods of drought, thus releasing the particles for transport by wind.
  
  
• Vegetative Cover
  
  The lack of permanent vegetation cover in certain locations has mat result in extensive erosion by wind. Loose, dry, bare soil is the most susceptible. Vegetation also serves as a windbreaks whether they be shrubs or trees, preventing the wind from removing soil particles to other location

Windbreaks

Windbreaks are effective when a wall of trees and other plants blocks the wind they also limit violent motions of the wind to those areas closest to the windbreak. Windbreaks can protect areas up to ten times the height of the tallest trees in the windbreak.

There should be at least two lines in each windbreak. One line should be large trees. The second line, right next to it, can be shorter trees and other plants with leaves. Locally grown trees and plants are best for windbreaks.

Steep slope protection

One of the most effective methods of preventing topsoil erosion on steep slopes is to plant vegetation which can provide a natural and attractive barrier against erosion.

In many cases however, soil is washed or blown away before the vegetation has an opportunity to establish a strong root system which is essential in order to provide the new slope with the protection needed.

Ways of Reducing Soil Erosion

One way of reducing soil erosion is to retain or add woody vegetation (shrubs and trees) to shoreline such as beaches,ponds. The tree roots forms an underground network which helps to keep the soil in places and from eroding away easily.

Minimize paved areas that increase surface runoff. The more paved surfaces we have like roads,driveways the more surface run off is increased. These surfaces are often impermeable to the water and usually channel the water in a specific direction which increases the strength and amount of surface runoff on any given surface.

Avoid adding additional weight such as cars, buildings or storage sheds on the hillside slopes. These structures often add weight to a soil and when the soil becomes saturated during an intense downpour it may slip and slide very easily

Gabions is a physical barrier created by man to prevent. Inhibit soil erosion. Gabion baskets are rectangular wire baskets that are filled with stones 4-8 inches in diameter. Similarly retaining walls, seawalls, bulkheads or breakwalls are rigid walls that are placed in such a way to form a barrier between the shore and the water.