Essential Characteristics of Soils

Scientists can learn a lot about a soil’s composition and origin by examining various features of the soil. Colour, texture, aggregation, porosity, ion content, and pH are all important soil characteristics.

1. Colour

Soils come in a wide range of colors – shades of brown, red, orange, yellow, gray, and even blue or green. Color alone does not affect soil, but it is often a reliable indicator of other soil properties. 

In the surface soil horizons, a dark color usually indicates the presence of organic matter. Soils with significant organic material content appear dark brown or black. The most common soil hues are in the red-to-yellow range, getting their color from iron oxide minerals coating soil particles.

Red ion oxides dominate highly weathered soils. Soils frequently saturated by water appear gray, blue, or green because the minerals that give them red and yellow colors have been leached away.

Read Also: Processes of Soil Formation

2. Soil Texture

A soil’s texture depends on the content of the three main mineral components of the soil: sand, silt, and clay. The texture is the relative percentage of each particle size in soil. Texture differences can affect many other physical and chemical properties and are therefore important in measures such as soil productivity. 

Soils with predominantly large particles tend to drain quickly and have lower fertility. Very fine–textured soils may be poorly drained, tend to become waterlogged, and are therefore not well-suited for agriculture. 

Soils with a medium texture and a relatively even proportion of all particle sizes are the most versatile. A combination of 10 to 20 percent clay, along with sand and silt in roughly equal amounts, and a good quantity of organic materials, is considered an ideal mixture for productive soil.

3. Aggregation

Individual soil particles tend to be bound together into larger units referred to as aggregates or soil peds. Aggregation occurs as a result of complex chemical forces acting on small soil components or when organisms and organic matter in soil act as glue-binding particles together.

Soil aggregates form soil structure, defined by the shape, size, and strength of the aggregates. There are three main soil shapes: platelike, in which the aggregates are flat and mostly horizontal; prismlike, meaning greater in vertical than in horizontal dimension; and blocklike, roughly equal in horizontal and vertical dimensions and either angular or rounded. 

Soil peds range in size from very fine-less than 1mm to very coarse-greater than 10 mm. The measure of strength or grade refers to the stability of the structural unit and is ranked as weak, moderate, or strong. Very young or sandy soils may have no discernible structure.

4. Ion Content

Soils also have key chemical characteristics. The surfaces of certain soil particles, particularly the clays, hold groupings of atoms known as ions. These ions carry a negative charge. Like magnets, these negative ions (called anions) attract positive ions (called cations). 

Cations, including those from calcium, magnesium, and potassium, then become attached to the soil particles, in a process known as cation exchange. 

The chemical reactions in cation exchange make it possible for calcium and the other elements to be changed into water-soluble forms that plants can use for food. Therefore, a soil’s cation exchange capacity is an important measure of its fertility.

Read Also: The Origin and Development of Soil (Soil Genesis)

5. pH

Another important chemical measure is soil pH, which refers to the soil’s acidity or alkalinity. This property hinges on the concentration of hydrogen ions in the solution. A greater concentration of hydrogen results in a lower pH, meaning greater acidity. 

Scientists consider pure water, with a pH of 7, neutral. The pH of the soil will often determine whether certain plants can be grown successfully. Blueberry plants, for example, require acidic soils with a pH of roughly 4 to 4.5. Alfalfa and many types of grass, on the other hand, require a neutral or slightly alkaline soil.

In summary, several factors of soil formation act either singly or in combinations to convert geological parent material to the soil through mainly physical and chemical weathering. Soils with different characteristics result from the different combinations of factors and processes.