Pores with water or air
The solid material consists of either mineral particles of different size classes or of organic material. The important pores are filled with either air or water depending on how wet the soil is at a particular time, its structure and soil tillage. In the ideal case, half the pores are filled with water and half with air. However, in soils with an aggregated structure such as clays, the pore volume is somewhat higher (40-60%) than in single-grained soils such as sands (35-45%).
Particle size classes
The texture of the soil refers to the distribution of mineral particles into different size classes. Different countries use different classes, but a common internationally acceptable system classifies the texture into blocks, stones, gravel, sand, silt and clay according to the particle size ranges shown in table "Particle size distribution" below.
Figure shows the difference in size between some of the mineral particles in a soil and the significance of this. Clay and humus particles are the smallest constituents in the soil. Their average diameter is less than 0.0002mm (i.e. 1000 times smaller than a grain of sand) and they are called colloids. The surface of clay particles has a negative electrical charge. This means that positively charged nutrients such as potassium, calcium and magnesium ions can bind to the clay particles. The clay particles therefore comprise nutrient reserves for plants.
1) Fine sand 2) Very fine sand 3) Coarse silt 4) Fine silt 5) Coarse clay
Silt holds water
A cultivated arable soil is often a mixture of different particle size groups. If gravel and sand dominate the soil texture, this gives permeable, dry and relatively infertile soils, while an inclusion of sand in a clay soil makes it warmer. Silty soils are often cold and water-retaining and can easily take up water through capillary rise. The finest mineral particles, clay, have a strong influence on the soil even at concentrations of around only 5%. Clay soils shrink and swell and impart an aggregated stucture to the soil, with cracks and crevices where roots can grown through the soil profile. The typical characteristics of various soils are often a function of the clay content, which strongly influences soil type and tillage.
Organic material positive
The organic material in the soil also has a very clear influence on the character of the soil. It consists to almost 60% of carbon (C) and originates from plant residues that have been decomposed by microorganisms. In this decomposition process (see image below), plant nutrients such as nitrogen (N), phosphorus (P) and sulphur (S) are released. The organic material can have enormous significance for soil properties and its influence is nearly always positive from the farmers perspective. It affects:
- structure and aggregate stability
- water management
- nutrient reserves
- silting and crusting
The decomposition process
- Soil fauna start decomposition of dead organic material, partly through breaking it down into smaller pieces and partly through digging holes in the soil, increasing the supply of oxygen. Earthworms play a specific and very valuable role by breaking down the material and mixing it into the soil.
- Bacteria and fungi continue decomposition in stages. The last phase, the formation of simple end-products that are available to plants (e.g. nitrate, phosphate and sulphate), is called mineralisation.
- Humus formation. The decomposition of various organic compounds proceeds through a series of intermediate products that are of an increasingly simple nature as decomposition advances. These intermediate products react with each other and with the compounds created by soil organisms. This leads to the formation of new chemical compounds that are transformed into high molecular weight, dark-coloured matter known as humic substances. These humic substances have the ability to bind positively charged ions of potassium, calcium and magnesium, for example.
Smallest particles have large specific area
Fine clay (<0.0002mm) and some organic material are colloids and represent the smallest components in the soil. However, they have a large specific area (see Particle size distribution table), i.e. a large surface area in relation to their weight. The specific area increases with decreasing particle size. The surface of clay particles is negatively charged, so nutrients in the soil that are cations bind to the surface, creating a store of nutrients for the plants.
Particle size distribution table
Particle diameter (mm)
A characteristic of all clay minerals is their flattened shape. This, together with their extremely small size, means that clay colloids have a very large surface area in relation to their mass – a high specific area.
For example, one gram of sand has a combined area of around 1.5-2cm2, which is equivalent to a small stamp. However, one gram of clay can have a combined area of several 100m2 – an average-sized house.
Capillary = capillary water is water that can rise upwards in the soil within the fine pores through binding of the water molecules in the pores, adhesion, but also through attraction between water molecules, cohesion. Silty soils have high capillarity and combine a large height of capillary rise with a high rate of capillary rise
Cations = positively charged ions in the soil, e.g. potassium, calcium and magnesium
Colloid = colloids are the finest particles in the soil, with an average diameter of less than 0.0002 mm. The colloids include some organic material and fine clay
Mineral particles = soil mineral particles are the inorganic smallest constituents, which have been formed at the site through weathering of various minerals and rock types or have been transported there, e.g. by glaciers. The properties of the soil are strongly dependent on soil particle size according to table Particle size distribution
Pore = soil pores are the spaces, channels and cracks in the soil, which are filled with either water or air depending on the actual water content of the soil
Soil fauna = earthworms, woodlice, springtails, centipedes, mites and other animals that open the door for bacteria and fungi through dividing and breaking apart plant residues in their mouth, stomach and intestines
Specific area = the combined surface area of the soil particles is expressed as square metres per gram of dry soil and is an important characteristic since it indicates the amount of nutrients the soil can release through weathering and bind to its surfaces
Texture = soil texture refers to the proportions of mineral particles with different average diameter, i.e. the relative proportions of sand, silt and clay in particular according to table Particle size distribution