Nutrient deficiencies in canola are rarely an issue for canola growers, but mobility and environmental issues can inhibit uptake. Having a good plan, though, goes a long way. Warren Ward, agronomy specialist, Canola Council of Canada, explains.
When they do surface, nutrient deficiencies don’t show up in all corners of the field, but in irregular patches throughout fields. From a distance, deficiencies can look like disease, but closer up differences appear. Diagnosing deficiencies cannot be done from the side of the road, but must be done in the field. In a lot of cases, they can be tricky to determine, said Ward, suggesting that growers combine soil and tissue samples for a better assessment.
The best way to avoid a nutrient deficiency is to have a good plan and to follow best practices. If you do that, said Ward, the chances of having a nutrient deficiency are really quite low.
A good plan will include a regular soil-testing program, so that you know what’s in your soil and what you need to add in order to hit your target yields. From there, it’s just a matter of choosing the fertilizer that’s going to work with your equipment. “It’s not rocket science,” said Ward. “But it does take a little bit of planning. For the most part it’s something most people should be doing.”
Of the major macronutrients, nitrogen, sulphur, phosphorus and potassium, the latter is the least problematic. Deficiencies can occur, but are less likely since canola is a really good scavenger of potassium in the soil. That being said, Ward suggests that those who have poorer soil to begin with may be more at risk for potassium deficiencies than others.
When assessing nutrient deficiency it’s important to know if the nutrient is mobile in the soil or mobile in the plant. Nitrogen, for instance, is mobile in both the soil and the plant. Deficiencies show up when fields are underfertilized and under environmental conditions that don’t allow the nitrogen to move properly through the soil into the plant.
Nitrogen, said Ward, usually isn’t a problem, since growers apply it in higher amounts than other nutrients. Deficiencies will show up in the lower leaves because the plant is scavenging nitrogen to supply the growing point. Nitrogen deficiencies cause yellowing of the lower leaves. Upper leaves, however, will still look fairly healthy.
Timing is really critical with nitrogen because peak uptake starts around the six-leaf stage before the crop bolts. “You want to make sure you have an adequate amount of nitrogen in the soil for that plant uptake prior to that period,” said Ward.
If deficiency shows up at flowering, the window to address the issue narrows. In extreme cases, a late application could get some recovery, but growers need to consider timing, environmental conditions and yield potential.
Sulphur and phosphorus
Nutrients that are not mobile in plants include sulphur and phosphorus. Sulphur is mobile in the soil, but not in the plant, so symptoms will appear in newer growth first. “Often you’ll see cupping of the leaves, maybe paler leaves as well,” he said.
Sometimes purpling will appear in the plant, but Ward says this is a response to stress, not necessarily nutrient deficiency.
Above-ground symptoms are not the only way to tell if your crop is suffering from a nutrient deficiency; below-ground symptoms will appear as well. Root health issues could limit nutrient uptake as well.
When assessing roots, Ward suggests taking plants from good and bad areas. Where compaction issues occur, root systems will struggle to find nutrients, spreading outwards rather than down. “If your nutrients are lower than that compaction layer, your plant isn’t going to be able to access them under those conditions,” he said.
Residual herbicide carry-over can also impact root growth.
Probably the trickiest macronutrient deficiency to detect is phosphorus. “We call it ‘hidden hunger’ because the plant might be deficient or even marginally deficient, but it doesn’t appear that anything is wrong with it,” said Ward.
Phosphorus is not mobile in the soil, but it is mobile in the plant. Because it is not mobile in the soil, phosphorus fertilizer placement is important. It is more important in areas where soil tests have determined low levels of phosphorus.
Phosphorus deficiencies result in smaller, spindly plants that look unhealthy.
To diagnose the problem, Ward suggests assessing soil and tissue samples from good and bad areas in the field.
“Unfortunately with phosphorus because it’s not mobile in the soil, there’s not a lot of rescue treatment for it,” he added. “So that’s one that we really need to focus on planning beforehand to minimize the chances of seeing deficiency.”
The most talked-about micronutrient with regards to canola is boron. Generally speaking, western Canadian soils are pretty good at supplying the required micronutrients. But at certain times of the year, nutrient requirements will increase. “That’s when you’d most likely expect to see a deficiency whether it’s a macro- or a micronutrient,” said Ward.
Oftentimes, micronutrient levels are depleted only in small windows during the growing season. Wait a few days before taking action, as the problem can right itself without intervention.
Finally, Ward advises growers to use the same lab when sending in soil and tissue samples. “Each lab can give slightly different results, and if you do want to keep track of nutrient levels in the field over time, it is useful to stick with the same lab,” he said.