With five irrigation years in a row and with current adequate nitrogen and other fertilizer nutrients we have grown some big wheat crops with big straw left over. In 2014 some folks had problems with proper canola seed placement through the straw load on the surface. We might even hear the odd whisper about looking for a match.
In the early days of continuous cropping the physical aspects of dealing with the straw of crops like wheat was the issue. The old straw choppers of the day did not do the job. Tillage was excessive in those days but without proper combine choppers even tillage was a challenge after a big crop. Modern combines mostly make “dust” of the straw on a dry day.
The other issue with incorporating large amounts of straw was the “immobilization” of the available nitrogen in the soil. Wheat straw is generally quoted as having 0.5 per cent nitrogen or less. As soil organisms break down the straw they use the soil nitrogen as the source to allow decomposition to proceed. Nothing new here.
Soil test benchmarks for nitrogen were based on field experiments that included straw incorporation. But when big crops came along adjustments were made for the extra straw. The “Rule of Thumb” on Page 39 of Henry’s Handbook concludes that we should add an additional 20 pounds of nitrogen per acre for each additional ton of wheat straw. But we now leave the straw on the surface so immobilization is not the issue it once was.
The current situation
I reviewed data from our irrigation experiments with wheat and large nitrogen rates. In wet years, as grain yield goes up, the straw produced goes up even more. We can easily be dealing with a few tonnes per acre of straw and instead of 0.5 per cent nitrogen it can have as much as one per cent nitrogen, and occasionally even higher.
Deep brown colored wheat straw like the straw shown in the picture can have as much as one per cent nitrogen and a total of 25 to 50 pounds of nitrogen per acre present in that straw.
This straw was in a slightly lodged area — leaning but not down. It was only a small part of the field. The combine was able to get all the heads but left a lot of straw. It was passed through a gyro mower prior to anhydrous ammonia application. Then a regular tine harrow at seven mph on a very hot day and it is ready for canola seed, I hope!
In the early days of soil fertility work in England they had a recipe for procuring a field suitable to compare nitrogen fertilizers: “take two white straw crops and the field is sure to be nitrogen deficient.” White straw means low nitrogen in the straw.
It is an occupational hazard — I still listen to reports that prove nothing because the soil does not need what is being studied.
That straw sitting on the surface with a high nitrogen content means that nitrogen tie up (immobilization) by the straw is a thing of the past. That nitrogen will, over time, become useful for future crops.
So, if in a weak moment we think about using the match we might as well burn a $50 bill for every couple of acres that we burn.
There is a good correlation between wheat grain protein and straw nitrogen content. For CWRS wheat a low protein of 10 per cent likely means a low straw nitrogen of 0.25 per cent — that is, a white straw crop. A straw nitrogen of 0.5 per cent and higher likely means a grain protein of 14 per cent or higher. Many with good black soils and a long history of high nitrogen use could well have high yield and high protein at the same time. If so, the straw nitrogen will be high and the rewards will be reaped in future years.