Soil 3 – Soil Composition

The ideal composition of soil, 25% Air, 25 % H2O, 45% Mineral Matter 5% Organic Matter.

Particles The texture of a soil depends on the

relative mixture of sand, silt and clay particles.

The most common method of classifying soils is based on the percentage clay in the oil.

E.g. Soils that contain 0 – 5 % clay are known as sandy soils.

The particles in the soil are classed on their size.

Anything over 2mm in diameter is referred to as gravel, pebbles or stones.

Particles from 2 mm to 0.5 mm are called sand particles.

From 0.5 mm to 0.002 mm are called silt particles.

Any particle under 0.002 mm is referred to as clay.

Sand and Silt are similar in composition and are formed by physical breakdown of rocks.

Clay particles are formed by both physical and chemical breakdown of rocks.

As mentioned before soils are classified by the amount of clay in the soil. • 0 – 5 % Clay Sandy Soil • 5 – 10 % Clay Sandy Loam • 10 – 20 % Loam • 20 – 30 % Clay Loam • 30 – 40 % Clay Soil • 40 % Up Heavy Clay Soil

A more common and accurate way at looking at the type of soil is by using a soil triangle.

Mineral matter is composed of inert solids, gravel, coarse and fine sand, and silt and clay particles. These different names come about because of the size of each is different.

Soil Texture

Gravel > 2mm

Coarse Sand ~ 2mm 

Fine Sand < 0.2mm

Silt < 0.02mm Clay <0.002mm

The % of each particle in a soil can be estimated using sieves.

 1. A known weight of a dry soil particle is

placed in the top sieve. 2. Shake vigorously3.  Weigh the clay fraction4.  Result:

Clay X 100 = % ClayTotal 1

The clay particle is the most important soil particle as it is the only particle, which is charged.

It holds a negative charge and is called an anion.

It is able to attract positively charged ions towards it (cations)

When lime is spread on land it replenishes the Ca2+ (calcium) in the soil and flocculation occurs.

This is the gathering together of clay particles forming aggregates

It is responsible for giving soil its structure.

A well-flocculated soil will have a desirable crumb structure.

It will be friable.

An undesirable structure is referred to as being plastic or blocky.

Lime promotes flocculation. It is recommended that it be spread on

average every 5 years. Flocculation in soil promotes aeration.

When a soil sample is rubbed between you fingers

it breaks up into crumbs.

This is the ability of clay articles to swap positively

charged ions (cations) from one to another.

Sandy Soils Have large air holes. Free Draining soils Is easy to work with (light) Dries out quickly Minerals are easily leached. Poor soil with little or no nutrients. Is a warm soil.

Clay Soils Holds water easily This protects from leaching of minerals. Is naturally fertile soil. Very poor drainage, which can lead to

water logging Is a cold soil.

Loam Soils Intermediate characteristics of both clay

and sandy soils. More advantages and fewer disadvantages

than sandy or clay soils. A good mixture is 40 % Sand, 40 % Silt

and 20 % clay. While the nature of soil depends on the

particle composition, the amount of humus in the soil is also a major factor.