Flooding of crop areas can occur as a result of rainfall or from flooding from rivers or streams. If soils remains waterlogged (saturated) for extended periods, plant health will be affected in a number of ways.

What should I do?

The best strategy is to drain or pump water from the crop area as soon as possible. Where flooding is landlocked, a sump can be dug to assist pumping. Over time the water will be removed by transpiration by plants and soil evaporation largely assisted by wind movement.

Waterlogged soils

The full impact of inundation or waterlogging will not be apparent for some time after the event so ongoing monitoring is important to mange crop health. Water logging may continue in clay soils and duplex soils that have a shallow clay layer below the surface soil. Installing a test well or digging a hole can help monitor the water table depth and hence the degree of waterlogging. Areas remaining waterlogged for several weeks, 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 extended periods, plant health will suffer. 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 the following season the potential for growth can be limited by the flood waters leaching nutrients.

Shallow, stagnant water can heat up quickly in hot weather and kill root systems so it is important to remove water quickly.

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.

Water Quality

Avoid irrigating from water sources that drain from flood affected paddocks. Dams may become muddy and they can be treated with Gypsum (CaSO4) to clear the water. Avoid reuse of water from flood-affected areas as water can carry diseases. Turbidity of water in holding dams has poor light penetration (lack of sunlight) which can lead to bacterial cists and virus survival. Water Quality issues to test for are increased Salinity and nutrient loads, as well as boron toxicity in bore water supplies. Salinity may 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.

Diseases

Many diseases are more active in wet, humid conditions. Where water is ponded or soils are waterlogged, humidity will be higher, adding to disease pressure. This should be factored in when considering disease control programs. A monitoring program should be based around identifying areas infected by disease followed by a qualified and quantitative assessment of chemical control measures. Specific information on chemicals, formulation, registration, application rates and withholding periods should be provided by authorised staff at chemical outlets.

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.

Weeds

Controlling weeds is important as some weeds are hosts for pests and diseases. 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 you local agricultural chemical supplier for advice. Alternatively, look up the Australian Pesticides and Veterinary Medicines Authority website to determine which chemicals to use to control weeds.

For grass infestations that harbour insects, use sprays that don’t affect broad leaf plants. Spot spray broad leaf weeds to avoid impacting crops. (Note: With Holding Periods are important)

Nutrition

Heavy rainfall and flooding can cause nutritional problems. Elements such as nitrogen, and potassium can be leached from the soils and 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. To tailor fertiliser applications, soil tests and leaf analysis should be undertaken and results interpreted by a consultant. 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.

Soil Management

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.

Depressed growth of plants under saline conditions is mainly caused by the salt raising the osmotic potential of the soil solution i.e. the salt is making it harder for the plant root to take up water. The chloride component present in saline soils can be leached out of the root zone of the plant by irrigation and rainfall. However, the sodium component can be left behind firmly attached to clay particles. Affected (sodic) soils have poor infiltration and drainage resulting in waterlogging, increased run-off, poor water storage and surface crusting. Sodicity restricts plant growth by limiting the movement of oxygen, water and nutrients to the plant roots. Treat the area by applying gypsum (CaSO4.2H2O) at a rate determined by a soil test.

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 Southern Rural Water on 1300 139 510

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 3 horizontal to 1 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 2 horizontal to 1 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

This Agriculture Note was prepared by Agriculture Victoria in February 2011.

For more information about Agriculture Victoria, visit the website at agriculture.vic.gov.au or call the Customer Service Centre on 136 186

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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