The properties of soil can be broadly divided into physical and chemical domains. In this article, you will learn about the physical properties of soil. Soil physical properties are those you can see with your eyes and feel with your fingers.

You are going to learn about soil texture, soil structure, soil colour, soil depth, soil temperature and the profile.

There are many properties of soil that are usually considered in the scientific study of soil. You will recollect that we mentioned that those and soil properties are either physical or chemical.

We will now consider the most common soil physical properties in this unit. These will include texture, structure, colour, depth, profile and temperature.

1. Soil Texture

Soil texture refers to the degree of coarseness and fineness of the soil material. In some textbooks and publications, it is also referred to as particle size composition of the soil.

The soil texture is commonly described in terms of percentages of sand, silt and clay grades. Soil texture can be roughly determined by the ‘feel’ of the finger.

It is usually represented as a triangle in which 22 textural grades are identified using combinations of sand, silt and clay grades.

Note that the soil structure is very important because it influences soil properties such as porosity, permeability, structure and consistence.

2. Soil Structure

You can consider soil structure either: as the various arrangements of the primary and secondary soil particles. Or: as the aggregation of the units of the soil mass into various shapes and sizes.

Note that individual aggregates or structures are called peds. When aggregation is absent, as in loose sand, soil is said to be structure less.

Soil is structure is usually described in these three ways

Types of ped,

Size of ped, and

Strength or resistance of ped.

The types of structure are defined by their shape. They are granular, crumbly, platy, columnar, prismatic and blocky.

Soil structures are described as fine, medium or coarse depending on their sizes. Soil structures may also be graded as structureless, weak, moderate or strong, depending on how well formed and how resistant to pressure they are.

You remember it was said that textural classes are quantifiable. In the case of structure

it is not so. Although soil structure is difficult to determine quantitatively, it has significant implication on soil fertility.

A good soil structure is depicted by high water holding and aeration capacities. In essence having good structure will facilitate the activities of micro- organisms and make soils to be loose, friable and easy to cultivate.

3. Soil Colour

Colour is the most conspicuous property of soil that has been used widely to describe and classify it. In identifying soil colour, the terms used are dark, brightandlight.

You need to know that two main substances produce colour in soils. These are the organic matter and mineral matter.

In general, organic matter, especially humus, produces the dark colour while mineral matter produces the light and bright colours depending on their states of weathering.

Soil colour is usually determined in the field by using some kind of colour system. The most widely used colour system is that of Munsell Colour Chart. Soil colour charts are indicated in hue, value and chroma.

The advantage of colour charts is mainly to bring about some objectivity into soil colour determination and enhance comparability.

4. Soil Temperature

Soil temperature is the degree of coldness or hotness of the soil. It is another important property of the soil as it regulates, to a larger extent, the rate and intensity of the biochemical processes of soil formation.

Therefore, when temperature is high biochemical processes will be rapid but when the temperature is low, biochemical processes will be slow.

Similarly, when soil temperature is high micro-organic activities are very active whereas when soil temperature is very low, micro-organic activities are almost non-existent.

The result of the latter is that soil organic matter is hardly broken down to become humus such as in the cold regions of the world.

It is also necessary to know that the nature of the soil surface considerably affects the amount of solar energy absorbed by the soil. Dark soils generally absorbs more solar energy than light-coloured soils, while sols with a smooth surface reflect more radiation than those with a rough surface.

5. Soil Profile

Soil profile is the arrangement of soil in different layers right from the top to the base where the parent rock or parent material is found. Each layer of the soil profile is called horizon.

How do we distinguish one soil horizon from another? It is by noting the characteristics of each horizon. These are indicated through varying texture, structure, colour and constituents.

The ideal soil profile consists of three major horizons viz: A, B, and C. The A horizon is the topmost layer, where leaching or eluviations of substances takes place. This is followed by B horizon which is a one of accumulation or illuviation of substances.

It is, therefore, referred to as a zone of enrichment. The last horizon is the C horizon. It is a zone that is not so much affected by soil forming processes. It is the soil parent material.

It is necessary to indicate that not all soils have well developed horizons such as soils that are developing on young parent materials like alluvial and glacial deposits. Some of them may not have any horizon at all.

6. Soil Depth

Soil depth refers to how deep a soil profile is. It is mainly dependent on the nature of soil parent material, rate of weathering processes and the majority of the soil-forming or pedological processes.

Soil depth may also depend on the nature of topography and slope. For example, soils of flat surfaces especially with easily weatherable materials will be quite deep whereas soils on sleep slopes will be shallow.

In conclusion, this article has looked at some important physical properties of soil namely soil texture, soil structure, soil colour, soil temperature, soil profile and soil

depth.

While soil texture is the degree of coarseness and fineness of the soil material, soil structure refers to the aggregation of the units of the soil mass into various shapes and sizes known as peds.

Soil structure could be granular, crumbly, platy, columnar, prismatic and blocky while soil texture is usually described in terms of sand, silt and clay percentages. The latter are combined to give 12 textural grades of soils in the soil textural triangle.

You have to remember that the most conspicuous property of soil is colour and that the Munsell Colour Chart is mostly used to identify soil colours usually indicated in hue, value and chroma.

Soil temperature largely regulates the rate and the intensity of the bio-chemical processes of soil formation and also controls micro-organic activities in the soils.

Since soil is formed over time, it is observed that different layers are seen as we dig the soil down the surface. This is called soil profile or horizon. You should not forget that there are A, B, and C soil horizons.

Remember also that the A horizon is the elluviation layer while the B horizon is the layer of illuviation. That is, while substances are removed in the A horizon, they are deposited in the B horizon.

The C horizon is the parent materials. Quite related to the soil profile is soil depth. It is defined in terms of how deep or shallow a soil profile is. While soils found on flat surfaces are usually deep, those on sleep slopes are usually shallow.

The Six (6) Physical Properties of Soil