A couple of months ago it was so dry we wondered if we could grow a crop this year. Now we have had reasonable planting rain and everybody is madly sowing. The soil moisture from the March rainfall has stimulated mineralisation of N in the soil and it is still probably going on. Soil samples taken a couple of weeks ago may underestimate the amount of N available to your crop, if ongoing mineralisation has not been factored into your N recommendation.
So why is there this late spike in mineralization? The soil microorganisms that work on converting organic matter to the mineral form of N work best in warm conditions with adequate moisture. In non drought situations this leads to a release of N over the entire summer. When there are dry conditions over summer, mineralisation can come as a pulse after rain falls if temperatures are high enough, like the graph shows for this year.
In some soil samples we are seeing ammonium-N levels higher than nitrate-N levels, indicating that mineralisation was still going on at the time of sampling, as nitrogen contained in stubble and organic matter breaks down into ammonium before converting into nitrate.
Where ammonium-N is elevated it should be considered as part of a seasonal N budget.
In 2011 and 2012, we lost N at the end of the fallow around Australia Day due to the flood conditions. This year we haven’t lost it.
In the northern grain belt, this issue only applies north of about Dubbo. South of there things are about normal for this sowing.
Some of us have only had time to sow so now we want to know when we can apply N and how much do we need. Given that only 20-30 % of the total crop nitrogen requirement is taken up by Z31, early in crop application of N can be considered where logistically practical with rates based on soil samples taken early in-crop or plant tissue analysis at the end of tillering.
For more specific answers for your situation you should talk to a reputable agronomist, or you could look at the following publications.
Prepared by Luke Beange with technical input from Chris Dowling, Back Paddock Company, Queensland.
Photo courtesy John Alexander, Figure provided by Back Paddock Company.