Capturing soil nitrogen
I WAS contemplating the summer season many of us have had and how much sub soil moisture there may be in our future winter crop blocks.
Those who were under the big rain events in late February and early March would be reasonably happy with their profiles of moisture.
Congratulations on getting underneath these big storms, and given your soil structure and stubble levels were appropriate at the time, sub soil moisture levels would be very handy now.
What do any of us think about our nitrogen levels?
It is the major nutrient which all plants require to survive and of course, as farmers and agros, we do not just need the wheat or barley crops to survive, we want them to yield as much grain as possible.
It is then another step up to achieve a higher protein level in the grain to give better returns to your bank account.
So it is a fairly important decision is the nitrogen one.
However it is not just about rate is it?
Apply 120kg of nitrogen (260kg urea) per hectare with about 50kg per hectare of nitrate nitrogen already in the root growing zone of the soil profile and you could produce just over four tonne per hectare of wheat at 12 per cent protein.
Sounds simple an equation and it is nearly the end of April, so in the next couple of months, surely we will get some rain to give us an opportunity of planting of a cereal crop.
Have you ever given much thought as to how the nitrogen spreads through the soil from where you place it and just as importantly, how long does it take?
The yield equation in the above nitrogen budget itself, is what we have been assuming for many many years.
It is quite correct, however assumes your nitrogen efficiency is running at the 50 per cent level and that is probably low enough to horrify many of you.
So facing up right now to consider a nitrogen application for this winter crop is not so simple and could be somewhat audacious to assume you will get anywhere near 50 per cent efficacy and it could be as low as 20 per cent usefulness to your wheat or barley crop.
First let us assume unfortunately due to lack of the summer rain, you have a dryish topsoil down to 25-30cm and have recently applied nitrogen beneath the soil surface, then you are going to need some serious rain events to spread and mobilise downwards, the nitrogen into a future developing root zone.
So I believe a 25mm rainfall event is not going to cut it, even in very light soil types.
You are going to need moisture and warm soil to start and speed the conversion from urea to a plant available form of nitrogen.
The warm soil is not a problem, however the lack of rain is the issue.
In a typical soil type with around 25 to 50 cation exchange capacity, which describes much of our soil of our winter crop region, I would suggest you are going to need 75mm of steady penetrating rainfall to facilitate conversion and most importantly, for the nitrogen to be moved beneficially deeper into the soil profile.
This wetting front can be more than half again in front of your nitrogen passage.
In other words the wetting front could be at 30cm and the nitrogen may be only half way or less even.
These rough figures could be very dependent on your soil type and I suspect that the heavier the soil, then slower the nitrogen fronts descent.
Let us consider that you got under all that rain of 125-200mm in February and March this year and you did not have your nitrogen applied.
That would have been a fair call not to apply extra N before then, as it was fairly dry and our summer crop profitable economics was fairly sad.
So you have a good profile of moisture now and intend or have applied post rain nitrogen to these future winter crop blocks.
So we need another good rain event and none of us would knock that back.
It is a real conundrum when to apply nitrogen to your cereal grain paddocks and I will not state it is an easy decision time.
Various parts of Queensland received a big rainfall in this mid-February to late April period.
We need to apply nitrogen for our winter crops before these huge downpours occur, to get a reliable and more efficient N response from our wheat and barley crops.
WINTER CROPS: It is important to take advantage of rainfall events to get nitrogen levels right for planting winter cereal crops.