Soil structure is the arrangement of the soil particles into larger aggregates of different sizes and shapes and the pore spaces left between them. The pore spaces allow the root hairs to grow and extract water and oxygen from the soil.
A good soil structure has stable aggregates, a good network of soil pores for good aeration and drainage to allow for easy exchange of air and water by plant roots.
Sandy soil aggregates are not stable due to low organic matter content, low clay content and the inability of the particles to bind. Clay soil aggregates are usually large with insufficient pore space to allow the growth of root hairs.
Factors affecting soil structure
- Organic Matter Content
- Soil Organisms
- Soil Colloids
- Freezing and Thawing
- Water Movement
1. Organic Matter Content
Increasing organic matter
- You can increase the organic matter content of soil by adding manure. Manure can help degraded soil regain its structure as an effective growing medium. It also fertilises the soil providing plants with all the major nutrients like nitrogen.
- Cover cropping is another way of increasing organic matter content in the soil. Cover cropping amongst its numerous benefits enhances the activities of organisms that help to improve the soil. In addition, cover cropping protects the soil surface and when the plants die, they add to the organic matter in the soil.
Decreasing organic matter
- Tillage is one of the ways you can decrease the organic matter in the soil. Tillage is the preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning. The continuous practice of tillage has a long-term implication on the soil, making it ultimately non-productive. This remains one of the major factors affecting soil structure and productivity.
Read more on tillage: What is Soil Tillage, Types and Soil Tillage Effects on Soil and Crops?
- High rates of decomposition also lead to a fast decrease in organic matter. Studies have shown high rates of decomposition of organic matter with increasing temperature and humidity. Therefore areas with high temperatures and humidity experience a high rate of decomposition. This explains why decomposition of organic matter is faster in the tropics than in the temperate areas. In such areas, organic matter is required in high quantities and so there is the need to adopt sustainable agricultural technologies.
2. Soil Organisms
- Bacteria are very important organism contributing to the structure of the soil. Besides helping in decomposition of organic matter, their sticky property help to hold soil particle together. According to James J. Hoorman, Assistant Professor and Extension Educator Agriculture and Natural Resource, many bacteria produce a layer of polysaccharides or glycoproteins that coat the surface of soil particles. These substances play an important role in cementing sand, silt and clay soil particles into stable microaggregates that improve soil structure.
- Further, the majority of fungi decompose the lignin and the hard-to-digest soil organic matter. Fungi also produce thread-like structures called mycelia which bind soil particles and peds making larger particles.
3. Soil Colloids
Factors affecting soil structure also include soil colloids.
- Soil colloids are small particles and the most active portion of the soil. They largely determine the physical and chemical properties of a soil.
- Inorganic colloids (clay minerals, hydrous oxides) usually make up the bulk of soil colloids. The organic colloids include highly decomposed organic matter generally called humus.
- Soil colloids have a very important property to adsorb, hold, and release ions. With this property, water molecules are also adsorbed to soil colloid surfaces.
- Soil colloids are a very important determinant of the physical and chemical properties of the soil.
Tillage is the preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning.
It is done to create air spaces in the soil to promote the growth of organisms which require oxygen. Additionally, there is the perception of allowing for easy manual planting operations and others.
However, tillage has dire consequences on soil structure and productivity. Tillage destroys the soil structure and exposes and kills soil organisms like earthworms. Tillage also reduces the organic matter in the soil.
5. Freezing and Thawing Cycles
Compaction of soils is a big problem in crops production. Studies have shown that this factor can reduce crops yield up to about 50%. Field compaction could be caused by general field operation whether mechanical or physical. Activities like planting, spraying, harvesting, and tillage can cause soil compaction. Farmers adopt several methods to reduce compaction including deep tillage.
A research talks about how moist soils compact easier than dry soils. He advises against entry into fields when the soil is wet. The freezing and thawing processes, prevalent in soils in the temperate regions, are known to reduce soil compaction.
Results of the research show a significant reduction in soil compaction after the freezing and thawing cycle. When the water in the soil freezes, it expands and opens up soil particles creating air spaces. These airspaces remain even after the water thaws allowing more oxygen and soil organism activities.
6. Water Movement
One of the factors affecting soil structure is water movement. Water movement in the soil can influence the soil structure. When water moves vertically (infiltrates) in the soil, the soil particles trap some of the water to the crops to use.
However, the movement of water in soils can also have some negative effects on the soil when excessive.
- It can cause leaching of minerals
- During the movement of water soils that are freshly tilled may collapse and reduce the pore spaces
- In clayey soils, this may create a hardpan
- Horizontal movement of water may cause erosion if not controlled. The fine particles of the soil are easily washed away.