Soaking will have ups and downs

Author: Andrew McFadyen | Date: 01 Nov 2016

GRDC northern panellist Andrew McFayden says understanding water quality is important.

The recent heavy rainfall across inland areas of New South Wales has been a mixed blessing for the region, with some low lying areas and valleys being flooded and higher, well drained country receiving a huge boost.

In terms of where to now for those growers, who have experienced flooding, there are a number of considerations for soil nutrients and weeds, coupled with potential opportunities created by good moisture profiles.

Flooding can bring a lot of silt across land, meaning there may be some soil nutrient benefits, but equally there is potential for denitrification.

Soil nitrate can be lost through denitrification, a process that can occur when soils are approaching saturation and become depleted in oxygen. 

In normal, aerated soils, bacteria break down organic matter in the presence of oxygen to produce carbon dioxide (CO2), water and energy. But in very wet or waterlogged soils, oxygen is rapidly depleted, so bacteria use nitrate instead of oxygen for respiration. 

Soil doesn’t have to be under water to have low oxygen availability. This can occur in any soil where internal drainage is restricted; for example, soil with a high clay content that drains slowly, or sandy soil over an impervious layer that impedes drainage. It can be influenced by factors such as the amount of rain, duration of rain, how wet the soil profile was, and capacity for the soil to drain internally.

Given denitrification can be extremely variable, growers need to test wetter areas separately from drier areas. The best way to determine nitrate levels is through soil test. This test should include the subsoil too, as flooding and heavy rains can also cause nitrate to leach down the profile.

The other major issue is what weeds have been brought onto your property with the floodwater? Flooding is a common cause of new weed infestations, through the transport of seeds and vegetative propagules such as stolons, rhizomes and tubers. 

Effective monitoring to identify new weed incursions is needed. Hand removing a few plants every year can help when weed numbers are very low, even on very large properties.

Any weeds are easier to kill when they’re small and require lower chemical rates. Some growers prefer to wait to ensure all weeds have germinated, but if there’s another rain event, those emerged weeds will grow rapidly to a point where they are more difficult to control.

By controlling weeds through summer, growers can help protect their nitrogen levels and not waste stored water.
In the GRDC-funded 2011 and 2012 NSW DPI trials, carried out with the Central West Farming Systems group as part of a ‘Rain and Grain’ project, excellent summer fallow weed control lifted yields by up to 1t/ha for canola and 0.5-1.7t/ha for wheat.

Growers may find more useful information in the following link IWM manual on managing paddocks following flood.

There are also potential opportunities created by flooding in terms of lifting the soil moisture profile.

Dual-purpose crops can often be sown earlier than conventional grain-only crops, increasing the sowing window. Dual-purpose canola can also provide a disease break and additional options for weed control, as does summer cropping.

Before the opportunities can be realised though, growers are strongly urged to address the management issues. With both nutrition and weeds, the key point post flooding is that growers go back to basics in terms of soil testing, managing weeds and checking for resistance.

Andrew McFadyen is a Lake Cargelligo based agronomist with more than 18 years’ agronomy and practical farm management experience. He is an active member of the grains industry with former roles on the Central East Research Advisory Committee, NSW Farmers Coolah branch and planning committees for GRDC Updates.

Contact Details 

For Interviews
Andrew McFayden
0436 191 186

Ellen McNamara, Cox Inall Communications
0429 897 129

Region North