At the past group farmer meetings, we pushed the use of deep soil nitrogen testing by 2-6 weeks before sowing to determine the soil nitrogen at 0-30cm and 30-60cm in the 0-60cm soil profile in a paddock zone.
Deep nitrogen above 80kg of nitrogen per hectare is good. What was yours?
Another way of determining soil nitrogen is from soil nitrogen mineralisation using the formula Nov-March rain x 0.25, plus 17.5 x topsoil organic carbon percentage (that we usually put down as 1.2 OC%.)
I calculated from November fallow for a few sites, and assumed 10mm from this front and another 25mm to the end April.
The calculated kgN/ha was 46kgN/ha. Starter fertiliser mainly used as MAP will add a small amount of nitrogen.
Wheat and barley have low biomass up to late tillering.
The nitrogen needed is low given the soil nitrogen.
If tillering is good, topdressing can be left to the first node stage.
If tillering is low or the crop does not have a good green colour, you should measure the tiller number.
Cereal crops with less than 450-500 tillers/m² should be topdressed at the first node.
Canola is different. It needs increasing biomass up to flowering as more biomass means more yield.
It is usual to apply the first topdressing at the 3-5 leaf stage or pre-sowing.
The optimum flowering date is July 26.
It is common to apply the second topdressing at eight leaves and early elongation.
A reminder that wheat and barley need 50kgN/tonne and canola 80kgN/tonne.
Winter and spring soil mineralisation assists if there is rain or irrigation.
Most of the nitrogen comes from the soil mineralisation.
For spring irrigated crops or dryland crops with good rainfall springs the rule of thumb that soil nitrogen mineralisation is 110kgN/ha (240kgurea/ha).
By the time you apply the first canola topdressing, hopefully the current high urea price will have dropped.
Tiller counts in wheat and barley would greatly assist urea rate recommendations.
• John Lacy is an independent agriculture consultant based in Finley. His columns are based on available data and farmer feedback gathered through his local discussion group events and networks.
