Information from original Agriculture Victoria Note Number: AG1442. Published: February 2011.
Flooding of crop areas can occur as a result of rainfall or from flooding from rivers or streams. If soil remains waterlogged (saturated) for extended periods, plant health will be affected, with the potential for yield reduction or total crop loss. Potatoes are one of the most sensitive crops to damage caused by waterlogging.
Waterlogged soils
Water logging may continue in clay soils and duplex soils that have a shallow clay layer below the surface soil. Areas remaining waterlogged for extended periods, need to be identified for remedial action in the future. Such actions may be installing draining lines or subsurface pipe drainage.
If the soil remains waterlogged for an extended period, plant health will suffer. The roots are restricted and may die, crops become stressed and nutrient uptake is reduced. The air between soil particles is displaced by water causing root and eventually plant death due to lack of oxygen. Symptoms of waterlogging stress include wilting and chlorosis of leaves due to lack of nitrogen. In potatoes, waterlogging may also increase the incidence of tuber diseases if occurring for even a short period of time.
Shallow, stagnant water can heat up quickly in hot weather and kill root systems so it is important to remove water quickly.
Water Quality
Avoid irrigating from water sources that drain from flood affected paddocks and avoid reuse of water from flood-affected areas as the water can carry diseases. Dams may become muddy and they can be treated with Gypsum (CaSO4) to clear the water. Salinity may also be a consequence of water logging because salts in the subsoil move into surface soils due to flooding and rising water tables. This can concentrate salts near the surface of the soil, impairing root function.
Soil Management
Most soils, especially those with a high clay content, become compacted and slump after heavy rainfall and flooding. Floods can also deposit a fine clay layer or crust on top of the soil that prevents oxygen and water penetration of the soil.
It is important not to drive on the cropping area while the soil is very wet because compaction of the soil from wheels will result. Cultivation while the soil is too wet should be avoided. If the soil has set hard, once it is dry (or at field capacity), a light cultivation can be used to break up the compaction to allow water and oxygen to penetrate. Care should be taken during cultivation not to pulverise the soil and further damage the soil structure.
It will be important to plant a green manure crop in flood affected paddocks to help improve soil structure and increase the organic matter. Ideally a cereal crop should be grown, such as oats or rye grass, as these will act as disease break.
If slaking or salinity is expected to be an issue, perform a dispersion test.
Diseases
If potato crops are waterlogged for any extended period, they should be ploughed in to reduce the potential for disease development. Waterlogging may increase the incidence of tuber diseases and affect yield. Wet, humid conditions are also likely to increase the incidence of a wide range of diseases and it will be important to increase the frequency of crop monitoring for the presence of diseases.
Good hygiene practices are required after flood events to prevent spread of soil and water born diseases. The likelihood of spread is high, especially to areas down stream of the event. Good examples are the need for specific hygiene measures that would need to be undertaken for bacterial wilt.
Bacterial Wilt of potato can be a problem after a flood occurrence. Floods can spread bacterial wilt through movement of contaminated water, infested soil, plant debris, and rain splash. The bacterium can be spread in contaminated irrigation water. Irrigation dams and channels can be contaminated by run-off from infested paddocks and washdown areas, and infected weed hosts growing in paddocks, along channels and in dams. If dams are downstream of infected paddocks if possible avoid using these to irrigate subsequent crops. In addition, if possible, avoid planting in paddocks downstream from flood affected areas particularly if bacterial wilt has been a problem.
Weeds
Controlling weeds is important as some weeds are hosts for diseases such as Bacterial Wilt. Weed propagules can be dispersed in flood waters. Monitor areas previously flooded for 12 months afterwards to detect new weed infestations. Weed propagules can easily attach to vehicles and equipment in muddy conditions and when being moved between properties to repair flood damage and assist with recovery. They can pose a high risk of weed spread. Ensure that vehicles and equipment of contractors/advisors are clean and free of weeds before entering and leaving your property.
For weed management talk to your local agricultural chemical supplier for advice. Alternatively, look up the Australian Pesticides and Veterinary Medicines Authority website to determine which chemicals to use for weed control.
Nutrition
Heavy rainfall and flooding can cause nutritional problems. Elements such as nitrogen and potassium can be leached from the soil and nutrient levels are likely to be reduced. Fertiliser applications should be adjusted to make up for and shortfalls, however adjust rates with caution as over application will also need to be avoided. Silting may also cause denitrification of the soil by bacteria as a result of less oxygen being present in the soil. This becomes significant when the soil has been waterlogged for 36 hours or more.
Prior to planting the next seasons crop soil analysis should be undertaken to identify any issues and action that needs to be taken to manage the soils prior to cropping. It is important to tailor the fertiliser applications to the needs of the crop and based on the soil test results. The following season crop nutrient health should be monitored using leaf petiole analysis and results interpreted by a consultant to target and adjust fertiliser applications to the crop needs.
Dams
Flooding can lead to a high deposit of silt in dams. Desilting dams should be done by an experienced earth moving contractor. It is important that the carrying capacity (water volume) of licensed dams is not increased. For more information on managing license dams and dams on waterways contact your local water authority.
Southern Rural Water 1300 139 510
Goulburn Murray Water 1300 469 469
Removing Sediment from Farm Dams - General Guidelines for Earthworks
Works are to be conducted in a safe manner using appropriate equipment and experienced plant operators. The contractor must ensure that all works undertaken on the construction site comply with current occupational health and safety legislation. The dam must remain at original capacity. Any work that expands capacity requires a permit.
Batter slopes of three horizontal to one vertical must be maintained below the dam full supply level. Material should be left in a neat stable heap, above the full supply level, out of the waterway and away from the dam excavation and associated spillway. The heap should be left with a domed top and be free draining. Batter slopes on the heap should not exceed two horizontal to one vertical. If suitable, the material can be used to topsoil the dam embankment.
Contact/Services available
Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available. For further information, phone Crop Health Services on (03) 9032 7515 or fax (03) 9032 7604.
AgriBio Specimen Reception
Main Loading Dock
5 Ring Road
La Trobe University
Bundoora VIC 3083
Acknowledgements
The original Agriculture Note was prepared by Agriculture Victoria in May 2011.
Disclaimer
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication
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