Understanding The Impacts Of Waterlogged Soils

Very few rural areas have escaped this summer’s inundation of arable soils caused by prolonged rainfall, overland flows or rivers bursting their banks.

Soil has become saturated to the point where there is insufficient space in the soil profile for oxygen and just like humans, plants need oxygen for respiration and can only survive for a short period without it.

The impact on the plant depends on the duration and frequency of the waterlogging event; crop tolerance (eg rice is very tolerant, bean crops are more sensitive); crop growth stage (emergence and vegetative stage are likely to suffer the most dramatic yield loss).

Apart from the impact on crop and pasture growth, waterlogging can have a severe impact on soils, including:

  • Loss of top soil
  • Increased compaction
  • Increased soil salinity
  • Dramatic changes to soil biology

Under normal soil conditions, aerobic soil microbes depend on oxygen in the soil. In waterlogged soil these beneficial microbes can’t exist. Anaerobic soil microbes, which can survive without oxygen, become dominant.

Changes to availability of soil nutrients

With a changing population of soil microbes, some soil nutrients will be converted from plant-available into the unavailable form, and even into a harmful toxic gas.
For instance, Sulphur is converted to hydrogen sulphide (H S), giving off the 2 characteristic post-flood smell. Other nutrients will be changed into forms that can be lost from the soil – such as Nitrogen via the denitrification process.

Loss of nutrients and organic matter

Top soil and organic matter washed away in floods means reduced nutrients available for future crops. Mobile nutrients (Nitrogen and Sulphur) and immobile nutrients (Phosphorus, Potassium and most trace elements) can be lost. Soil testing is recommended to assess what soil nutrition remains.
To restore soil health, it is important to get a crop into the soil as soon as possible. An application of animal manure or composts can help to restore depleted organic matter.