Soil structure describes the physical configuration of the soil. The sand particles in a sand soil are held together weakly and do not form aggregates, while the clay particles in a clay soil readily form aggregates. These aggregates make clay soil easy to till and improve the transport of air and water.
Soil structures are defined as the 3-D arrangement of primary particles in the soil, i.e. how the primary particles are situated and connected to each other. Determining factors for soil structure include its texture and content of organic material. A distinction is made between soils with a single-grained structure and soils with an aggregated structure.
Sand soils are not held together
A sand soil such as this (see image above) is an example of a single-grained structure. The grains of sand are relatively large and often only held together weakly. Even sand soils with a higher content of colloids fall apart easily when exposed to pressure in the soil. A sand soil with a low clay content often requires deeper cultivation in order to create a good arable soil and growing site. The low clay content gives the sand soil a low internal structure forming capacity.
Clay soils become aggregated
A clay soil such as this (see image above) is held together and often has an aggregated structure. Even at a content of 5%, the clay has a very strong impact on the soil and dominates its properties.
The aggregated structure is the result of a range of processes in the soil that in combination form aggregates. These structure forming processes affect the soil structure in a dynamic interplay with soil tillage.
Aggregates improve the soil
When clay particles are bound together in aggregates, the properties of the soil are almost always improved. An important effect is that soil tillage becomes easier. The movement of air is also better, improving the transport of oxygen to and carbon dioxide from the root system. The aggregate structure also improves soil permeability to water and its water-holding capacity. Good soil structure makes an arable clay soil a more productive growing site, since all essential functions for the plant and its root system are provided. This gives faster growth and higher yield.
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" in chapter The building blocks of soil.
Single-grained structure = in a soil with single-grained structure, the primary particles are held together very weakly or not at all in some form of aggregate.
Aggregated structure = when the primary particles come from the clay particle size group, they are bound to each other and form aggregates that can then be stabilised by organic material, lime and various chemical precipitates.
Colloid = colloids are the finest particles in the soil, with an average diameter of less than 0.0002mm. The colloids include some organic material and fine clay.
Oxygen = element that occurs in air as oxygen gas O2 at a concentration of 21 % – vital for cell respiration in plants and their roots.
Carbon dioxide = gaseous waste product (CO2) of cell respiration in the roots that is also the building brick together with water for sugars created by the plant through photosynthesis.