For most croppers, nitrogen (N) represents the biggest dollar investment. And the greater the investment, the greater the financial risk.
So, what can we do to de-risk the investment in N fertiliser?
The obvious answer is to put it where we need it, and not where we don’t. And of course, we want to maximise the returns from the N we do put on.
The demise of legumes and pastures from our increasingly intensive cropping rotations has increased our reliance on N fertiliser. Two of the last three years have produced our biggest crops on record leaving many paddocks drained of N. And high stubble loads after last year’s bumper crop increases the risk of N tie-up – especially in the absence of significant summer rain.
Crops need a lot of N.
Every tonne of wheat with 10.5% protein contains about 18 kg of N. The amount of N we need to apply will be determined by what the soil can’t supply, and the nitrogen use efficiency (NUE) of what is applied.
Assuming an NUE of 35%, we need to apply about 50 kg N/ha to grow a tonne of wheat, 50 kg N/ha per tonne - over and above what the soil can’t supply for the demand of the crop.
Demand will obviously be dictated by the season, but increasingly we are seeing exceptional yields (and much higher yield potentials) where we have overcome soil constraints – whether it be through liming, deep tillage, soil inversion or correction of nutrient deficiencies.
Overcoming soil constraints has the potential to increase NUE, but CSBP trials indicate that N requirements are often just as high due to the extra yield potential unlocked.
Newer varieties such as Scepter wheat and Planet barley also have higher yield potential than older varieties and therefore higher N requirements, as have modern varieties of canola which I struggle to keep up with!
The NUE of applied N can vary widely. CSBP trials have shown that deficiencies of nutrients like potassium (K) and copper (Cu) can severely limit NUE and therefore returns on investment. Similarly, CSBP trials have shown the big gains from banding N or delaying N applications – depending on the environment and likely losses.
Big stubble loads will likely reduce the effectiveness of surface applied N in 2019, and increase the efficiency gains from banded product.
In a paddock with a high stubble load at Scaddan last year, NUE was twice as efficient banded at seeding compared to streamed onto the crop at Z30 (start of stem elongation).
So, if high stubble loads are being faced this year, and there is a capacity to band N, it makes sense to me to be banding more N this year – especially where there has been significant depletion of soil reserves.
Image: : High cereal stubble loads = increased nitrogen use efficiency (NUE) from banding N.