Assessing soil structure: A step by step guide
Soil structure is a critical component of soil health, directly impacting crop performance, water infiltration, root development, and overall soil productivity.
For both arable and livestock farmers, regularly assessing soil structure is a fundamental practice that helps in maintaining and improving soil conditions for optimal crop and pasture growth. Using results from soil assessments to improve the growing environment can pay dividends in terms of crop yield, the Soil Association has found.
This blog provides a step-by-step guide to assessing soil structure on-farm, and how fertiliser application can support soil structure management.
Why soil structure matters for farmers and growers
The term ‘soil structure’ refers to the arrangement of soil particles (sand, silt, and clay) into aggregates, which are clusters of particles bound together. The way these aggregates are arranged plays a crucial role in determining several key properties of the soil that directly influence its ability to support plant growth. Well-structured soils have good aeration, water-holding capacity, and root penetration, while poor structure can lead to compaction, waterlogging, and reduced yields.
Soil structure also plays a pivotal role in maintaining water quality. A recent report from the Environment Agency found that improving soil structure can reduce surface runoff by as much as 50%, which is crucial for protecting local watercourses from pollutants like sediments, nutrients and pesticides2.
How to assess soil structure
- Choose the right time for assessment
Soil structure assessments should ideally be conducted during the autumn or spring when the soil is moist but not saturated. Avoid periods of extreme wetness or drought, as these conditions can give misleading results. Ensure you select a representative area of the field that reflects the general condition of the soil, avoiding areas that have been recently disturbed or compacted by heavy machinery.
- Take suitable tools
For your soil assessment, you will need to take the following tools to the field:
- Spade or soil auger for digging soil pits
- Knife or trowel to slice through the soil profile
- Bucket for collecting soil samples
- Soil penetrometer/compaction meter
- Pen and paper or mobile phone to record observations
- Dig a soil pit
When you’ve found a suitable soil area, representative of the whole field, dig a pit approximately 30cm deep (or down to the depth of the plough layer) and around 30cm wide with your spade or soil auger. Slice down one side of the pit with the knife or trowel to create a smooth face for examining the soil profile.
- Examine soil layers
Start with examining the top soil, looking for
well-defined aggregates. A well-structured topsoil will have a granular or crumbly texture, whereas poor structure may appear as a solid mass or show signs of crusting. This occurs when the soil surface hardens and forms a crust after drying, which can limit seedling emergence, reduce water infiltration, and increase runoff.
Next, examine the subsoil for signs of compaction or poor structure. A platy structure (where soil forms thin, horizontal layers) often indicates compaction. This can inhibit root penetration and limit vertical water movement, leading to poor drainage and increased susceptibility to waterlogging.
A compacted soil structure is a result of soil particles being compressed and aligned due to mechanical pressure from tillage or heavy machinery. Compacted soils, particularly those with low organic matter content, provide an inhospitable environment for earthworms and other beneficial soil organisms. The absence of earthworms can lead to reduced soil porosity and further exacerbate issues caused by compaction.
- Perform the Visual Evaluation of Soil Structure (VESS) test
The VESS soil test is a widely used method that allows farmers, agronomists, and soil scientists to qualitatively assess soil structure in the field. This test provides valuable insights into the physical condition of the soil, which is crucial for understanding its ability to support plant growth. The VESS test involves examining aggregate size, aggregate strength and soil porosity.
- For optimal soil health, aggregates should typically be 1-10mm in diameter.
- Well-structured soils tend to have aggregates that are rounded rather than angular, and will crumble easily under light pressure.
- Soils rich in soil organic matter (SOM) tend to have more stable aggregates due to the binding action of organic compounds.
- In the VESS test, healthy soils will display visible pores, particularly macropores, which are spaces large enough to be seen without a microscope.
This video from the Farm Advisory Service shows how to conduct a VESS test.
- Assess soil compaction
After completing the VESS test, the next crucial step is to conduct a penetrometer test to quantify soil compaction. This test provides a more precise measurement of the soil’s physical resistance to penetration, which directly impacts root growth, water infiltration, and overall crop performance.
Insert the penetrometer’s probe into the soil and apply steady pressure. The reading on the device indicates the resistance the soil exerts against the probe. Measure soil resistance at various depths, typically at 10cm intervals down to a depth of 60cm or more, depending on the crop’s rooting depth. This helps to identify compaction layers that may be present at different soil depths.
How to interpret penetrometer readings
- Low resistance (0-200kPa or 0-30PSI) indicates minimal soil compaction and well-structured soil
- Moderate resistance (200-300kPa or 30-45PSI) indicates moderate compaction; root growth may be restricted, particularly during dry conditions when water infiltration is critical
- High resistance (>300kPa or >45PSI): indicates severe soil compaction; root growth will be significantly restricted, leading to stunted plants, reduced nutrient and water uptake
- Evaluate root growth
Roots are fundamental to plant health and productivity, acting as the plant’s anchor and primary means of absorbing water and nutrients from the soil. The evaluation of root growth during a soil structure assessment provides valuable insights into the overall health and functionality of the soil.
What does crop root structure indicate?
Check how deep roots penetrate the soil. In well-structured soils, roots will grow deep and spread out evenly, because the soil offers less mechanical resistance. This allows roots to access deeper soil layers where water and nutrients may be stored. For example, during periods of drought, deep root systems are crucial as they can tap into moisture reserves that are not available to shallow roots. Examine the lateral spread of roots too. In well-structured soils, the horizontal distribution of roots allows the plant to explore a larger soil volume for water and nutrients, increasing the efficiency of resource uptake.
What does crop root colour indicate?
Also look at root colour – healthy roots should be white, firm, and unblemished. Such roots indicate that the soil structure is favourable, allowing for good aeration, water infiltration, and nutrient availability, supporting vigorous plant growth.
Get more advice on promoting crop growth with nutrient management
Roots that are brown, deformed, or stunted points to poor soil structure. For example, compaction can create a dense, impenetrable layer that restricts root growth. In compacted soils, roots may become deformed or ‘flattened’ as they struggle to penetrate hard layers. Additionally, poor aeration in compacted or waterlogged soils can lead to oxygen deficiency around the roots, causing them to turn brown and die off in severe cases.
- Record your soil test findings
Make sure to record your soil structure observations, including aggregate size, porosity, compaction, and root health. It’s a good idea to also use photos to document the soil profile and any problem areas, and contact your agricultural adviser if necessary for further technical recommendations on improving your soil structure. Finally, it is recommended to assign a score to your soil structure using the VESS method, which ranges from 1 (very good) to 5 (very poor).
- Take action based on your soil structure assessment
If your assessment indicates a poor soil structure, consider remedial practices such as:
- Subsoiling to mechanically disrupt compacted layers, creating channels for air and water movement.
- Reducing tillage to preserve soil structure and prevent the breakdown of aggregates, maintaining porosity.
- Cover cropping which, through the action of plant rooting and heightened organic matter, biologically loosens compacted soils and enhances aeration by improving aggregate stability and creating biopores.
Ongoing assessment of your soil structure is crucial to track the impact of these changes.
Find out more about the SFI payments on offer for creating and actioning a soil management plan
The role of fertilisers in soil structure management
Fertilisers, while essential for crop nutrition, also play a role in maintaining and improving soil structure. However, their application must be carefully managed to avoid negative impacts on soil health and the environment.
- Balanced fertilisation
A balanced fertilisation regime helps promote healthy plant growth, which contributes to soil organic matter , improving structure over time. For example, a well-timed application of phosphorus supports root development, which helps maintain soil structure by stabilising aggregates.
- Use of organic amendments
Incorporating organic amendments, such as compost or manure, can significantly enhance soil structure. These materials increase microbial activity which is vital for the formation of stable soil aggregates.
- Timing of application
To avoid soil acidification, which negatively affects soil structure, growers should split nitrogen applications across the growing season to match crop uptake, reducing the risk of leaching and soil structure degradation.
Read our guide to nutrient management for autumn-sown crops
References
- Soil Association (2015) – Runaway maize
- Environment Agency (2023) – Summary of the state of the environment: soil