Cropping in the wet

Bill Manning, North West Local Land Services - Advisory Services (Cropping)

As well as large losses in yield and quality a wet start to summer raises a number of other issues for crop production.

Wet conditions greatly assist in replenishing moisture profiles but floods in themselves may not result in a full profile. Moisture accumulation in vertosols generally proceeds from the topsoil down once cracks have closed up. Rainfall wets from the top of the profile down to a certain level depending on the size of the rain event. When dry conditions follow a rainfall event the topsoil dries out again resulting in a portion of the rainfall event being lost. For this reason, heavier rainfall events are more “efficient” at filling profiles than smaller events. In the summer, rainfall events under 15 mm followed by several weeks of dry weather are unlikely to contribute to soil moisture storage. Frequent rainfall events are also more efficient at refilling profiles as the topsoil does not need to be refilled.

Maize crop

Floods however, can result in surface soil losing its structure and sealing, similar to what occurs in a dam. At this point the infiltration rate of water falls dramatically and surface water may drain away or evaporate before the profile fills. For this reason, it is important to check soil moisture levels before making cropping decisions.

Wet years may provide an opportunity to increase the intensity of cropping through the use of double cropping or shorter fallows, but it is important to check moisture profiles first particularly if flooding has occurred. If additional cropping is to occur it is important to consider the greater demand made on soil nutrition, the weed pressure likely to be faced and the presence of any residual herbicides. In general, wetter conditions should aid the breakdown of herbicides but be sure to check the label first.

The other main impact of flooding is on nitrogen (N) cycles in the soil. Where soil is inundated it becomes depleted of oxygen (O2) and denitrifying bacteria use Nitrate (NO3) as a source of oxygen instead.  In this process the Nitrate is converted to Nitrogen gas in various forms which is lost to the atmosphere. Local Land Services and DPI put together a short factsheet on this earlier in the season. You can the fact sheet here.

Rainfall may also have moved Nitrate down the profile meaning crops will not access it until later growth stages. As with moisture determination soil sampling is useful to gauge Nitrate levels.

Flood water also has the potential to spread soil borne plant disease and pests such as root lesion nematodes, fusarium wilt in mungbean and growers should watch for the emergence of these over the next few years.

If you have any questions about cropping during the wet, contact Bill Manning on  0428 607 731 or william.manning@lls.nsw.gov.au.

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