Knowing the nitrogen (N) status of paddocks will be critical if grain growers are to fully capitalise on the wet start to autumn across parts of the cropping belt in northern New South Wales and Queensland.
With storm rain and Cyclone Debbie delivering good falls during March 2017, a number of growers will start the winter season with a full profile of soil moisture and a vastly improved crop yield outlook.
However, 2016’s unseasonably wet winter followed by a dry summer is likely to impact on plant available N (nitrate and ammonium) for 2017 winter crops, according to Professor Mike Bell from the University of Queensland’s Alliance for Agriculture and Food Innovation (QAAFI).
Understanding the impact of wet weather on a soil’s fertility status is imperative if growers are to maximise the yield potential of 2017’s crops through the effective planning of a fertiliser strategy, including rate and method of application.
Growers are likely to focus their planning on N given that it is the most likely yield limiting nutrient and N fertiliser represents one of the most significant input costs in farm budgets.
“The key message for growers is `know your nitrogen levels’. Running nutrient budgets to determine how much was removed in 2016 and therefore how much needs to be returned is fine until something unusual happens and last winter was the `something unusual’,” Professor Bell said.
“In these situations, growers will need to reset their nutrient calculators in order to obtain an accurate assessment of plant available nutrient levels.
“The biggest issues with N are likely to be in paddocks that were wet or waterlogged (big N losses) or conversely yielded well above average (high removal rates). Fields where there were failed chickpea crops may also add uncertainty, with the N contribution varying according to how well the crop had grown, and fixed N before being overcome by waterlogging or disease.
“Feedback suggests that there are a number of soil tests coming back with no detectable nitrate, meaning that the reliance on fertiliser N will be higher than usual in 2017. So there may well be growers who would normally expect to have 50/60/70 kg of N in their profile and it simply isn’t there.
“Where chickpea crops failed but vegetative growth had been good, there may be higher-than-normal N reserves, but the lack of summer rain (until March 2017) may mean this N is still in undecomposed residues and not showing up in soil tests. Availability to a winter cereal will depend on temperature and moisture availability from now on, making planning for preseason N applications more challenging.”
The Grains Research and Development Corporation (GRDC) advocates that one of the best ways to assess plant available nitrogen (nitrate and ammonium) levels in the soil profile is to take soil cores and test for nitrogen levels.
Soil sampling to at least 60 cm is the preferred coring depth and this is particularly important for areas that have been fallowed from 2016 as the nitrate can be placed deeper in the profile.
The challenge for growers is that N applied now will not all be available to 2017’s winter crop – distribution through the soil profile is dependent on moisture moving deeper into the soil profile and taking nitrate with it.
If the profile is already full, that movement will be limited and fertiliser N may be marooned in topsoil where roots can’t access it. Whether that unused N is still there for the next crop season will depend on seasonal conditions.
“Put simply, in seasons where residual N is limited like after 2016, the earlier fertiliser N is applied the more chance it has to move down the soil profile to where crop roots can access it,” Professor Bell said.
“Later applications of N are very dependent on seasonal conditions to encourage lots of shallow root activity, or to move that N deeper into the soil profile after a dry period.”
More information on soil testing is available in the GRDC Northern Fact Sheet available from the GRDC website www.grdc.com.au or by following this link, as well as the GRDC crop nutrition extension hub at https://extensionhub.com.au/web/crop-nutrition.
Additionally, the Making Better Fertiliser Decisions for Cropping Systems in Australia (BDFC) website at http://www.bfdc.com.au provides information about soil test critical levels for the four key nutrients – N, phosphorus, potassium, and sulphur.