Repairing waterlogged soils

This can result from heavy localised rains, flood waters slowly flowing across the landscape, a rising water table or a combination of all three. The persistence of the waterlogging is influenced by local topography and drainage properties of the soil.

Compacted soils and soils with shallow impermeable layers are particularly prone to waterlogging. Compaction is generally more of an issue in soils with high clay content, especially soils with a high sodium-to- calcium ratio.

Waterlogging may continue even after surface water has dissipated, particularly in clay soils and soils with a shallow impermeable layer below the soil surface. Digging a hole or installing a test well can help monitor the water table and the degree of waterlogging.

Soils should not be driven on or cultivated while the soil is very wet, as that will result in compaction. Only work the soil once the excess water has drained away, but take care as cultivation has the potential to pulverise the soil and create further damage to soil structure.

Waterlogging is known to increase the impact of soil salinity. Salts in the subsoil can move up the profile into surface soils and the root zone area through rising water tables and flooding, becoming highly concentrated and causing damage to roots, especially as the soil dries out. In addition, when a lack of oxygen impairs root function, plants have difficulty keeping sodium and chloride out of the roots, and they end up accumulating more salt than usual.

A gypsum (calcium sulphate) application to the soil can assist with displacing sodium from the soil profile and, as a result, improve the soil’s structural stability.

Waterlogged soil restricts gas diffusion in and out of the soil, leading to the depletion of soil oxygen levels and an accumulation of carbon dioxide. The anaerobic conditions created by waterlogging increases loss of soil nitrate as nitrogen gas which is then lost from the soil. Nitrogen, potassium and boron are also lost from waterlogged soils through leaching in waterlogged soils.

In addition, nutrients such as nitrogen, phosphorus and calcium can have their uptake inhibited, and waterlogging can also cause changes in soil pH, affecting some nutrients’ availability.

In the right conditions, an agricultural lime application may be considered if soil pH needs to be corrected.

Adding gypsum (calcium sulphate) to the soil can assist with improving gas exchange in the soil. The calcium in gypsum will increase the activity of soil microorganisms and earthworms, which aids in soil aeration and macropore formation.

The advantages of gypsum application include increasing the calcium: sodium ratio in soil, especially in dispersive soils (which are structurally unstable in water due to their chemistry). In addition, gypsum assists with greater stability of soil organic matter and more stable soil aggregates, improved water penetration into the soil and more rapid seed emergence.

Talk to your local fertiliser advisor for advice on application rates based on a recent soil lab test result.