Deep N soil tests are showing low available nitrogen (N) to start 2017 winter crops. Just over half of the tests coming through Nutrient Advantage Lab Services show < 30 kg/ha of plant available nitrogen in the root zone. Three- quarters show < 60 kg/ha of nitrogen.
With less than 30 kg/ha of nitrogen in the soil, crop yields would be low without additions of N fertiliser. If 40 kg/ha N becomes available from in crop mineralisation and no N is added, APW grade wheat would yield no more than 1.6 t/ha.
Southern NSW & Victoria
In parts of NSW and Victoria, there is likely to be enough soil water for low N to limit yield. Some growers have already topdressed N early this season in winter cereals and canola. The risks of haying off or high screenings after topdressing are reduced with low soil N.
Earlier topdressing is likely to favour yield increase while later N application may partition to both yield and protein. However, good results need rain to move the N into the rootzone.There may be few topdressing windows this season. Current Bureau of Meteorology forecasts predict below average rainfall prospects for the rest of July and into September. Topdressing is usually most effective for crops sown within the ideal sowing window. Deep N soil testing is the best way to quantify nitrogen reserves (and can be taken in growing crops) and develop an appropriate fertiliser strategy, although it is getting late in the season for taking deep soil N samples
How late is too late?
Applying fertiliser N after booting in cereals (GS40) or bolting in canola is less likely to increase yield. There may be a boost to grain protein, but the economics need to consider what, if any, protein price premiums are on offer and accepting that yield increases may be small. Topdressing late generally results in lower N-use efficiencies.
If you are unsure of paddock N levels, take deep soil cores to test for nitrate. Crops must be actively growing to take up applied nitrogen. Any plant stresses will reduce fertiliser uptake.
South Australian grain areas have the triple whammy of low soil N, low soil moisture and not great prospects for spring rain. Soil N values recorded this season by APAL across South Australia vary, so paddock soil testing is important to evaluate in season N applications.
Soil N in the northern region is also looking low. Northern growers need to be even more careful about topdressing opportunities to avoid inefficient applications. The active root zone tends to be a bit deeper in the soil because surface soils are often very dry. Opportunities for tactical N applications are restricted, because even more following rain is needed to get N into root zones. As such, Northern region growers might focus on long-term maintenance of soil N reserves.