How to manage soils after prolonged waterlogging

Friday, 19 April 2024

Our soil expert, Joanna McBurnie, recently answered farmers’ questions about waterlogged soil.

What happens in a waterlogged soil?

When water fills pore spaces, it pushes air out. As soil biology (the engine of the soil) is sensitive to depleted oxygen levels, it may not function as effectively in waterlogged soil.

It may also take a while for biological activity to bounce back. As nitrates will be washed out or released as gas (by anaerobic soil organisms), the soil nitrogen supply will also be reduced.

As water loosens soil aggregates (clumps), the soil may lose structure and slump.

Cultivated layers are usually most vulnerable to slumping. However, sustained waterlogging would likely displace any aggregate, whether in a cultivated layer or not. The degree of slumping also depends on surface cover/residues.

When slumped soil dries, it can lead to a massive hard-set layer. Hard pans could also develop because of accumulations of calcium carbonate, silica or iron, manganese and aluminium compounds that then cause particles to bind together tightly.

What helps soils recover faster?

Following waterlogging, soil recovery is largely dependent on the inherent soil texture. For example, heavy clay soil will be waterlogged for longer periods than medium and light-textured soils.

Good soil management can help soils recover faster and increase their resilience to waterlogging.

The key is to have a soil structure that allows a balance of oxygen and water available to plant roots and soil organisms while allowing excess water to drain.

Will ploughing help soils recover?

No. While ploughing might help dry out the soil, it won’t help it recover.

While worked soil can speed up drying, it has greater pore capacity and holds more rain. Therefore, depending on the amount of rain received post-ploughing and the following seedbed preparation, soil conditions might not be the best for seed growth.

Of course, wet soils are especially prone to compaction. Ahead of any cultivation, always check soil moisture conditions.

Aim to avoid trafficking before soil conditions are ready and reduce cultivations, where possible.

How should slumped soils be managed?

If your soil has slumped, be careful not to over-cultivate it.

Slumping is more common in sandy and silty soils, especially where organic matter is low and in soils with high concentrations of sodium (relative to other cations).

On lighter textured soils:

  • Increase soil organic matter
  • Maintain surface cover (roots will develop structure and promote an active soil ecosystem (soil biology), which will then develop structure further
  • Reduce soil trafficking
  • On soils with high sodium, add lime or gypsum to help improve soil structure

Ensuring good levels of soil organic matter (appropriate to the soil texture) can increase the capacity of the soil to hold water and the ability to drain through the soil profile.

When should subsoiling be considered?

Only consider subsoiling if compaction has been found.

Wet soils are especially vulnerable to compaction. Ahead of cultivation, check for compaction.

You can carry out a simple visual evaluation of soil structure (VESS) on a spade full of soil to see if compaction is present and how deep it is.

If subsoiling is needed, then soil moisture at the operating depth should also be assessed.

Consider using a simple ribbon test to determine the moisture status of your soil and the appropriate mechanical interventions.

Are soils likely to be colder this spring?

Soils that have lain wet for several weeks could be colder than usual, so soil temperatures should be something to bear in mind this spring.

If you are direct drilling or using min-till, be aware that soils may appear dry on the top but not underneath.

To make informed decisions, check soil moisture down to the anticipated drilling depth.

What other things should be considered?

Potential nitrogen deficiency from flood losses could be tested in the soil (N-min).

RB209 has guidance on determining your Soil Nitrogen Supply (SNS) status. It also has guidance on calculating Grass Growth Class (GGC) which describes the ability of a site to respond to nitrogen depending on soil type and rainfall.

If field drainage exists, check outfalls and ditches. Clear them if needed, which is best done in the autumn.

Our field drainage resources have lots of information, including a good section on signs to look for on the surface that might suggest a deeper problem.

Finally, it is important to remember that run-off and soil erosion can occur on all soil types and all slopes (no matter how gentle), especially where there’s little crop cover of the soil.                                         

At present, soil will need particularly careful management. As always, it is difficult to make blanket recommendations.

The best approach will be based on your soil assessments. The road to recovery also requires a healthy amount of patience.

Certainly, any fieldwork undertaken before conditions are right may only make things worse.