Understanding waterlogging in soil and crops
Waterlogging happens when the soil becomes too wet and plant roots cannot get enough oxygen. This usually follows long or heavy rainfall. It can lead to:
poor soil structure
loss of nutrients
crop damage or failure.
Waterlogging is not always easy to see. Even if water is not visible on the surface, it can still affect your crops below ground.
Why waterlogging happens
Many soils across NSW are at risk of waterlogging, especially during wet seasons. Soils that are more likely to become waterlogged often have:
low porosity, which limits drainage
high magnesium levels
a low calcium to magnesium ratio, which can lead to compaction.
These conditions reduce how well water drains through the soil. When rain is heavy or prolonged, water builds up quickly. This creates waterlogged conditions.
How crops respond to waterlogging
Crops react differently depending on their growth stage. In early stages, roots need more oxygen. Without it, root tissue starts to break down and plants stop growing. If waterlogging lasts too long, the crop may not recover or produce a usable yield.
Waterlogged soils also reduce access to nutrients. Nitrogen is often leached out or becomes unavailable to plants. This can cause yellow or pale leaves. Other nutrients may also be limited, leading to poor growth and low yields.
Assessing water damage in soybeans
Waterlogged soybeans may not show damage right away. Key factors that affect yield include:
- how long the crop stays waterlogged
- crop growth stage
- solar radiation and temperature
- variety resistance to disease.
How long is too long?
Soybeans can usually cope with waterlogging for 2 days or less. If conditions last longer than 4 days, yield loss can be high, up to 50%, especially during flowering and pod development.
Plants die more quickly when:
- the soil was already saturated before flooding
- the weather is warm and sunny
- drainage is slow.
Cool, cloudy conditions help plants survive longer, but soybeans may still not fully recover.
What to check after waterlogging
Look for:
- wilted or dead plants
- new growth from the main growing point
- root nodules that are healthy and pink inside (cut open to check).
Brown or mushy nodules mean the plant is not fixing nitrogen. In that case, applying nitrogen as a side dressing or foliar spray can help support growth.
Check the plant population and overall health to decide whether to keep the crop. Speak with your local agronomist to get support and make an informed decision.
Assessing water damage in maize
Waterlogging affects maize differently from soybeans. Use the same assessment process, but note these key differences:
- Maize is tolerant to waterlogging from growth stages V6 to V10.
- Maize is more vulnerable before V6 and after V10, during reproductive stages.
- Maize does not fix nitrogen, so it relies on available soil nutrients.
Waterlogging before V5 can seriously harm maize crops. It affects root growth, reduces nutrient uptake (e.g. nitrogen and phosphorus), and increases the risk of root diseases like pythium crown rot. More than 4 to 5 days of excess water at this stage can reduce yields.
From V6 to V10, plants are more resilient and can often tolerate waterlogged conditions for up to 10 days.
From V10 to V16, maize takes up most of its nutrients. Waterlogging during this period can affect yield. Consider applying foliar fertiliser or side dressing if safe to do so.
If the crop looks too damaged to recover, ask your agronomist if it’s worth continuing through to harvest.
Assessing water damage in forage sorghum
Forage sorghum is more tolerant of waterlogging than other crops such as maize or soybeans. While waterlogging can reduce growth and tillering, the crop often recovers well with the right support. Applying nitrogen and sulphur as a top-dress fertiliser can help promote regrowth and tillering.
Risk of prussic acid poisoning
When forage sorghum is stressed, it can produce hydrocyanic acid. This can lead to prussic acid poisoning in livestock, especially cattle. The risk increases:
during rapid growth
after waterlogging
when nitrogen-based fertilisers are applied too soon after stress.
Always test forage before grazing. For more information, see:
What to check after waterlogging
It is often easier to assess water damage in forage sorghum than in grain crops. Keep the crop if:
plant populations are strong
most plants are healthy
there is minimal plant death.
Look for signs of nutrient stress and apply fertiliser if needed.
Before grazing:
check that plants are palatable
avoid feeding if flood residues are still present
wait for light rainfall to wash off sediment if the crop was submerged.
What to do with damaged crops
If a crop won’t meet its intended purpose, consider other options:
- Cut it for hay or silage.
- Use it as a cover crop and mulch it later in the season.
Cover crops can protect soil, reduce erosion and add organic matter. For more advice, read our guide on composting flood spoilage.
Strategies to reduce waterlogging damage
If your paddocks have a history of waterlogging, planning ahead can help reduce the risk. These strategies can help reduce waterlogging.
Use raised beds
Raised beds help remove water from the root zone. They improve drainage and provide better growing conditions. You can use a bed former can be to shape raised beds and furrows.
Change sowing timing and rates
Planting earlier can reduce the risk of seedling damage. You can also increase sowing rates to help offset potential losses.
Control weeds
Weeds compete with crops for nitrogen. Use selective herbicides before rain events to manage weeds effectively.
Apply foliar nutrition
After waterlogging, apply foliar fertilisers that include key nutrients like nitrogen and potassium. These help crops recover from stress.
Improve soil drainage
After soil testing, you can apply gypsum to hard, compacted or clay soils. If calcium levels are too low compared to magnesium (less than 2:1), the soil may have poor structure and need amendment.
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