The structure of a clay soil is the result of a number of different processes, which together give the soil profile its character. Soil tillage also affects soil structure through disturbing it to some depth and mixing in harvest residues.
Structure forming processes. Illustration: Graphics from Sigvard Andersson & Paul Wiklert, Swedish University of Agricultural Sciences, 1972
- Chemical processes
- Manure and lime
- Soil profile. The sum of all these processes is a soil profile that changes continually due to the processes above and to tillage. All the processes have an effect on the topsoil, while their effect at depth varies. The result is often a soil profile that has finer aggregates at the surface and coarser aggregates at depth.
- Effects of tillage. Soil tillage has additional effects on structure formation and the placement of plant residues. The tillage layer often has a looser structure, while a denser layer is formed at tillage depth. This applies irrespective of the technique used, but the depth varies.
1. Frost or heat
Frost and cold in the winter and heat in the summer have the same effect on the soil – removal of water. Both processes dry out the soil and when water is removed the clay particles are pressed closer to each other. The result is mechanically created aggregates.
2. Chemical processes
Organic material, iron and aluminium oxides and carbonates stabilise the aggregates through acting as binding agents. In soils that are not tilled, this process plays a major role. However, repeated soil tillage decreases the stability of the aggregates and in an arable soil the structure is therefore more dependent on soil clay content and biological activity.
Earthworms eat plant remains and mix them into the soil as they munch their way through the soil profile. Their casts act as adhesive between the soil particles and increase aggregate stability. Earthworms also stimulate the soil microorganisms, which increase soil stability through producing mucus and other binding compounds.
Plants dry out the soil through their water uptake in the same way as frost and heat. When the soil dries out, the clay particles are forced closer to each other, creating and strengthening aggregates. Plant roots also increase the amount of organic material in the soil and leave root channels behind. Overall, this means that a plant cover on soil builds up the soil structure, while a bare soil without growing plants breaks it down.
Drainage transports away excess water in the soil profile and thus dries out the soil. In this way, drainage helps to improve the soil structure. Very few soils are naturally self-draining and therefore drainage is essential for uniform drying of the soil, decreased soil compaction and early establishment of crops in autumn and spring.
6. Organic material and lime
Animal manure, intercrops, harvest residues etc. supply organic material to the soil. In the short term, this increases biological activity and causes a bloom of soil microorganisms. In the long term, it increases the content of organic material, which improves aggregate stability. Regular addition of lime also improves aggregate formation.
7. Heavy machinery
On arable soils, passes with heavy machinery often cause soil compaction. Soil compaction results in the large pores in the soil being compressed and eliminated. This means that rapid transport of water and air to the roots is impeded. Drainage is also restricted and it becomes more difficult for the roots to force their way down through the soil.
8. The outcome is the soil profile
Together, all the structure forming processes described above result in a soil profile that often has finer aggregates at the surface and coarser aggregates further down. The processes are dynamic and therefore the structure varies over time. For farmers with their sights set on the future and sustainable production, it is important to understand the significance of factors controlling the structuring process. Through diagnosis, it is rather easy to get a picture of soil structure.
9. Tillage affects structure
Cropping system affects the soil structure and the placement of plant residues determines whether to employ conventional or minimal tillage or even direct drilling. The tilled layer often has a looser structure, while a denser layer is created at tillage depth. This denser layer develops regardless of tillage technique used, but the depth at which it arises varies. To avoid creating dense layers in the soil, it is important to vary tillage depth.
Frost = means that the soil water freezes to ice when the temperature sinks – positive from a soil structure perspective since the volume of water increases when ice is formed and this process loosens the soil.
Iron and aluminium oxides = chemical compounds between iron (Fe) and aluminium (Al) on the one hand and oxygen (O) on the other, e.g. common rust is an iron oxide.
Carbonates = carbonate (CO32–) is a salt of carbonic acid (H2CO3) – often found in the soil as calcium carbonate (CaCO3) and also forms the basis of liming compounds, which are usually crushed or milled calcium carbonate.
Intercrops = crops such as grass, mustard, clover etc., which grow in the period between main crops, e.g. between winter wheat and spring barley. Used for various purposes – decreasing nitrogen leaching from the soil, increasing the supply of organic material, attracting wild birds and animals, etc.
Pores = 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.