How to assess soil health for better productivity

When should I assess my soil?

Soil condition is the most important factor in timing. It should be moist, friable and warm, thus providing the best environment for biological indicators such as earthworms. Typically, this is in autumn before soils get cold or in spring as soils start to warm again. Having a growing or recently senesced crop present will help to assess rooting. Avoid dry, frozen or fully-saturated soils, as this can lead to poor or false diagnosis and worm activity will be low.

Where to look first for healthy soil?

You can take a local benchmark of how a healthy soil should look and smell from undisturbed areas at the field edge. Field assessment should start on well-performing areas away from headlands and heavily trafficked areas, then move to areas of concern where crops may be performing poorly to give a contrast. Drones or simpler tools such as Google Earth can help identify crop differences from above.

Dig enough holes to give a good representative sample of the whole field (typically 4-5), particularly on variable soil types.

How deep should I dig into the soil?

For a visual evaluation of soil structure (VESS) to assess structural issues and earthworm abundance, holes about a spade’s width and depth will suffice (roughly 20x20x20 cm). Detailed guidance in the form of a do-it-yourself VESS chart can be found here.

Earthworms are a useful indicator of soil health to inform management actions. Five pits should be dug in a ‘W’ pattern across a field, contents hand-sorted, and results recorded. Note the total number of worms and record the presence of surface, topsoil and/or deep burrowing worms. Also check for deep-burrower earthworm signs including surface middens and large, pen-width vertical burrows in and around each soil pit. Guidance is available in Rothamsted Research’s free #30minworms survey booklet and online earthworm identification quiz.

What about the subsoil?

ADAS’s Yield Enhancement Network (YEN) considers that an 11 t/ha wheat crop requires about 330 mm of water between April and July when average rainfall is only 200 mm. Most soils hold between 140-200 mm per 1 m of depth, so good crops need to be well-rooted down to at least 1m to access adequate moisture, even in an average year.

Dig a 1-1.4 m pit in March/April to assess subsoil condition in good and bad areas to inform subsoil management. Look for vertical cracking, deep worm holes and rooting depth.

A good wheat crop should have some roots to 1 m by this time and many roots by early June. The pit should be wide enough to allow comfortable assessment of the soil profile.

A similar visual examination of subsoil structure can be made using the SUBVESS technique, along similar lines to the one for the topsoil.

Should I record the results from my soil findings?

Recording location and assessment data, and taking photos, are essential for improving soil health, allowing future assessments to be planned and progress related to changes in management practices tracked over time.

There are several services and ‘big data’ initiatives that can help with this. Agronomy groups and consultancy firms such as Hutchinsons and SAC Consulting in Scotland offer comprehensive soil health assessments for farmers.

What are key indicators of soil health problems?

6 common signs of poor soil health are outlined below:

1. Poor or stunted plant growth and restricted roots can indicate problems worth investigating. Yield maps or aerial imagery – taken when very dry or very wet – can be useful to identify poorly performing areas.
2. Surface waterlogging and deep ruts confer compaction.
3. Water being held up (given away by a wet sheen) or poor rooting are signs porosity is suboptimal.
4. Cracks in the soil should be predominantly vertical – horizontal cracks indicate a breakdown of structure.
5. A stale smell, blue or grey layers or slow residue breakdown can all indicate anaerobic conditions that need improving.
6. Drains not running after heavy rainfall, soils remaining saturated for long periods after rain and lines of crop canopy differences in line with drainage systems indicate soil water movement issues.

What can I do to fix/prevent structural problems within the soil?

Below, we’ve listed 7 tips for ensuring a healthy soil structure:

1. Maintain drainage systems, including field drains, ditches and outfalls, and use mole drainage if appropriate.
2. Use roots to improve and maintain good structure, which can include cover crops or stimulating volunteer growth post-harvest.  Avoid fallows where possible.
3. Plan to improve organic matter to build structural resilience.
4. Use appropriate mechanical interventions at the right depth to accelerate recovery.
5. Ensure crops are well nourished and healthy – strong, deep rooted plants will provide pore space at depth for the following crop and start a ‘virtuous circle’.
6. Prevent problems occurring by minimising machinery traffic, axle loads and especially tyre/ground pressures, avoiding working wet soils and excessive cultivations that burn-off organic matter and reduce a soil’s resilience.
7. Subsoil improvement is a long-term project, with improved drainage and organic matter key ingredients to success.

Contributors

Thanks to the following experts who helped compile this article:

•          Professor Roger Sylvester-Bradley, ADAS Crop Physiologist
•          Professor Bryan Griffiths, SRUC Soil Scientist
•          Philip Wright, Wright Resolutions independent soils and mechanisation expert
•          Dr Jackie Stroud, Rothamsted Research Earthworm Expert
•          Simon Cowell, Essex grower and Soil Farmer of the Year 2018

This article is taken from Bayer’s Crop Focus magazine - Spring 2019. If you’d like to subscribe to the magazine click here.