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Step-by-Step Guide to Soil Structure Assessment

27 May 2024

This article is produced as a part of the FAS Crops & Soils Bulletin. Subscribe now to receive the full report in your inbox monthly.

Recent wet weather conditions could have resulted in damaged soil structure, particularly if the fields bore machinery or livestock during these periods. Research from SRUC highlights the detrimental impact of compaction on agricultural output; the yield of first-cut grass silage was found to be up to 37.7% lower due to tractor compaction and 19.0% lower because of cattle trampling when compared to an uncompacted control area. It is advisable to explore whether poor soil structure is impacting yields on your farm before implementing any corrective measures, as unnecessary use of remediation machinery could potentially create new problems where none previously existed. 

Assessing Soil Structure

  1. When: After the soil begins to dry and before the soil becomes too dry.  If looking at a recently sown crop, wait until well established or after harvest. 

  2. Where: select three or more sites per field that are representative of the field or investigate an area when you suspect there may be a problem. 

  3. At each site, use a spade to dig down to a depth of 30-40cm and remove a block of soil for analysis. Leave a clean view of the soil's profile by only cutting three sides of the pit with the spade. 

  4. Analysing Soil Structure: Refer to the scoring system developed by SRUC: Visual Evaluation of Soil Structure (VESS).  Examine the removed soil block closely. Score the soil structure on a scale from 1 to 5, with 1 indicating excellent structure and 5 suggesting significant compaction that requires intervention. 

    What does good soil structure look like? Porous blocks with rounded edges that can easily break apart when moist, with vertical cracks that encourage root growth downwards. This type of soil promotes effective use of soil nitrogen, and enhances drainage and mineral uptake through the roots.

    What does bad soil structure look like? Large, unbreakable clumps, discoloured patches with perhaps orange mottles or grey colour, and roots concentrated in large pores. A sulphuric smell can indicate severe structural issues.

  5. If there is compaction, locate the limiting layer: Measure from the surface to pinpoint the depth of the compacted layer (a distinct horizontal layer with tight soil from other soil depths). This helps in choosing the appropriate remediation technique. 

Click to Enlarge

Image: Visual Evaluation of Soil Structure – a method to score structure quality of soil

Plan Remediation:  Important: Only VESS scores 4 and 5 require remedial action. Implement remedial actions when the soil is neither too wet nor too dry to avoid further damage. Optimal times for these activities are typically in the autumn when conditions are favourable and field demand is lower. For mild compaction, allow natural processes to help recover the soil structure; consider resting the field for longer or deferring use until conditions improve.

Remedial Techniques

Shallow Compaction (up to 10cm): Use tools like a sward slitter or aerator to alleviate compaction without disturbing much of the soil structure.

Deeper Compaction (beyond 10cm): Employ a sward lifter or, in severe cases, consider ploughing and reseeding the area to completely refresh the soil profile.

Remember, early detection and proper management of soil structure are key to preventing yield losses. It is essential to assess the soil structure before undertaking remedial measures to ensure that interventions are both necessary and appropriately timed to avoid exacerbating existing conditions. Start digging holes to help ensure the health of your soil and the sustainability of your farm's output.

Poppy Frater, SAC Consulting

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