Soil is the thin layer of material covering the earth’s surface

Soil is the thin layer of material covering the earth’s surface and is formed from the weathering of rocks. It is made up mainly of mineral particles, organic materials, air, water and living organisms—all of which interact slowly yet constantly. Most plants get their nutrients from the soil and they are the main source of food for humans, animals and birds. Therefore, most living things on land depend on soil for their existence. Soil is a valuable resource that needs to be carefully managed as it is easily damaged, washed or blown away. If we understand soil and manage it properly, we will avoid destroying one of the essential building blocks of our environment and our food security.

soil layers. Soil Formation and Soil Horizons. Soil is a mixture of plant residue and fine mineral particles, which form layers. Vector diagram

The soil profile

As soils develop over time, layers (or horizons) form a soil profile. Most soil profiles cover the earth as 2 main layers—topsoil and subsoil.

Most soils exhibit 3 main horizons:

A horizon—humus-rich topsoil where nutrient, organic matter and biological activity are highest (i.e. most plant roots, earthworms, insects and micro-organisms are active). The A horizon is usually darker than other horizons because of the organic materials.
B horizon—clay-rich subsoil. This horizon is often less fertile than the topsoil but holds more moisture. It generally has a lighter colour and less biological activity than the A horizon. Texture may be heavier than the A horizon too.
C horizon—underlying weathered rock (from which the A and B horizons form).

Some soils also have an O horizon mainly consisting of plant litter which has accumulated on the soil surface.

The properties of horizons are used to distinguish between soils and determine land-use potential.

A soil texture triangle is determined by the proportion of sand, silt and clay. Texture is important because it influences:

the amount of water the soil can hold
the rate of water movement through the soil
how workable and fertile the soil is.

Factors affecting soil formation

Soil forms continuously, but slowly, from the gradual breakdown of rocks through weathering. Weathering can be a physical, chemical or biological process:

physical weathering—breakdown of rocks from the result of a mechanical action. Temperature changes, abrasion (when rocks collide with each other) or frost can all cause rocks to break down.
chemical weathering—breakdown of rocks through a change in their chemical makeup. This can happen when the minerals within rocks react with water, air or other chemicals.
biological weathering—the breakdown of rocks by living things. Burrowing animals help water and air get into rock, and plant roots can grow into cracks in the rock, making it split.

The accumulation of material through the action of water, wind and gravity also contributes to soil formation. These processes can be very slow, taking many tens of thousands of years. Five main interacting factors affect the formation of soil:

parent material—minerals forming the basis of soil
living organisms—influencing soil formation
climate—affecting the rate of weathering and organic decomposition
topography—grade of slope affecting drainage, erosion and deposition
time—influencing soil properties.

Interactions between these factors produce an infinite variety of soils across the earth’s surface.

Parent materials

Soil minerals form the basis of soil. They are produced from rocks (parent material) through the processes of weathering and natural erosion. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material. The types of parent materials and the conditions under which they break down will influence the properties of the soil formed. For example, soils formed from granite are often sandy and infertile whereas basalt under moist conditions breaks down to form fertile, clay soils.

Pie chart showing soil composition — Soil composition Copyright : normaals

Organisms

Soil formation is influenced by organisms (such as plants), micro-organisms (such as bacteria or fungi), burrowing insects, animals and humans. As soil forms, plants begin to grow in it. The plants mature, die and new ones take their place. Their leaves and roots are added to the soil. Animals eat plants and their wastes and eventually their bodies are added to the soil. This begins to change the soil. Bacteria, fungi, worms and other burrowers break down plant litter and animal wastes and remains, to eventually become organic matter. This may take the form of peat, humus or charcoal.

Diagram of nutrients in organic fertilizers Copyright : fancytapis

Erosion

Climate

Temperature affects the rate of weathering and organic decomposition. With a colder and drier climate, these processes can be slow but, with heat and moisture, they are relatively rapid.

Rainfall dissolves some of the soil materials and holds others in suspension. The water carries or leaches these materials down through the soil. Over time this process can change the soil, making it less fertile.

Topography

The shape, length and grade of a slope affects drainage. The aspect of a slope determines the type of vegetation and indicates the amount of rainfall received. These factors change the way soils form.

Soil materials are progressively moved within the natural landscape by the action of water, gravity and wind (for example, heavy rains erode soils from the hills to lower areas, forming deep soils). The soils left on steep hills are usually shallower. Transported soils include:

alluvial (water transported)
colluvial (gravity transported)
aeolian (wind transported) soils.

Soil structure refers to the way soil particles group together to form aggregates (or peds). These aggregates vary in size and shape from small crumbs through to large blocks.

How soil particles may be arranged

Good structure is important, as it allows water to soak into the soil and excess water to drain away. It also allows air movement through the soil. Soil, air and water are vital for healthy plant growth and nutrient supply.

Peds are made up of mineral particles (clay, silt, sand) and organic matter. Peds are held together by the electrical charges on the surfaces of the minerals and organic matter.

Clays and soils with a lot of organic matter are more likely to form strong peds. Sandy soils or soils with little organic matter often have little or no ped development.

Examples of different types of soil structure: a) blocky, b) columnar, c) massive, d) single grain, e) platy.
Peds are described by their shape : a) blocky, b) columnar, c) massive, d) single grain, e) platy.

Activities

How to determine soil texture

Take about 2 tablespoons of soil in one hand and add water, drop by drop, while working the soil until it reaches a sticky consistency.
Squeeze the wetted soil between thumb and forefinger to form a flat ribbon.
Determine the texture based on the length of the ribbon that can be formed without breaking — see following table.

Texture	Length of ribbon (mm)	Soil properties and management implications
Sandy	<15	Little resistance to root growth High infiltration rate Low plant available water
Sandy loam	15-25	Root growth not restricted, but highly susceptible to mechanical compaction May be hard setting Moderate infiltration rate Moderate plant available water
Loam	25	Root growth not restricted Moderately susceptible to mechanical compaction Moderate plant available water Moderate infiltration rate
Silty loam	25	Root growth not restricted Moderately susceptible to mechanical compaction Moderate plant available water Low to moderate infiltration rate
Clay loam	40-50	Root growth not restricted Moderately susceptible to mechanical compaction Moderate to high plant available water
Clay	50-75	Root growth frequently restricted Moderately to highly susceptible to mechanical compaction Some restriction on water movement leading to periodic waterlogging Moderate to high plant available water
Heavy Clay	>75	Root growth moderately to severely restricted High susceptibility to mechanical compaction Water drains very slowly except in self-mulching soils

What is Soil?