Lower than expected grain protein levels could be a symptom of declining soil organic matter (SOM) levels. Since higher protein wheat can fetch a premium price, it’s worth understanding how SOM fits into in-season nitrogen (N) management. Nitrogen supply is a critical driver of grain protein content.
Higher yield and lower protein
In wheat, N is used to grow the plant and then develop grain protein. During grainfill, N moves from the leaves and stem into the grain.
As yields increase, the N supply is distributed among a greater number of grains and/or is spread out within larger grains. This lowers the protein concentration in the grain. It’s an issue in seasons that favour high yields, with good rainfall and a cool finish. A hot, dry finish, on the other hand, causes smaller grains and lower yields. The same amount of N in fewer, smaller grains leads to higher protein.
Why else might you get lower protein?
Low protein suggests there was not enough N supply during the growing season and grainfill. This can happen because of:
- not enough N overall
- inaccessible N due to dry or otherwise hostile soils
- low supply of another nutrient (like phosphorus) inhibiting N uptake
- inefficient N transport within the plant due to:
- low supply of other nutrients such as potassium, sulphur and zinc
- variety – Australian National Variety Trials suggest some higher yielding cereal varieties produce lower protein grain.
If protein levels are persistently lower than expected, N supply is probably not well managed.
Organic matter and in-season N
SOM breakdown is an important source of N during the growing season. As microbes decompose SOM, N gets mineralized into plant available N. When SOM stocks are low, crops miss out on a potential source of in-season N. Grain protein and/or yield potential drops if low SOM and organic N supply are not compensated by applications of more fertiliser N. Relying solely on applied N is risky. A good buffer of N from soil OM can help N become available to plants when they need it.
Managing organic matter for protein
Continuous grains cropping runs down SOM. In the Midlands area of WA, 87 % of soil samples taken in the last four years have a soil organic carbon level below 1 %. 30% are less than 0.5 %. At this level there will be minimal mineralization regardless of the season.
Pasture and legume rotations can add to SOM and increase the amount of N mineralized from SOM. With fewer legumes in crop rotations, there are fewer opportunities to increase or even maintain SOM. Not using legumes in crop rotations to supplement SOM and N stocks could be having a long-term effect on grain protein levels.