Visual assessment of nutrient deficiency symptoms is not good enough for an accurate diagnosis because symptoms can have many causes.
Purpling is one example. Purpling is often considered a tell-tale sign of phosphorus deficiency, but purpling is actually a response to any stress. The plant is not producing chlorophyll in those areas, so the underlying purple pigment comes through.
Proper diagnosis of nutrient deficiency should use most of the following tools:
1. Soil test. Are any obvious shortages evident? Other soil test parameters such as texture, pH and electrical conductivity may also provide clues in the diagnosis.
2. Fertilizer history. Over the past 5 years, what rates and source products have been applied on that field? Include crop yields and consider whether rates have been adequate to match removal.
3. Tissue test. Do not use this alone. A tissue test may show that the plant is deficient in phosphorus or calcium, for example, but heavy rains and saturated soils may have stopped nutrient uptake. The nutrients may be in the soil at adequate rates, but the plant simply can’t access them. So even if the tissue test shows deficiency, a rescue application of these products will not help. Plants may recover on their own when soils dry out again.
4. Herbicide history. Is there any chance of carryover from products applied one or two years ago? In dry conditions or very wet conditions, herbicides can take longer than expected to break down to safe recropping levels for canola.
5. Environment. Cold, wet, hot and dry can all stress canola plants, creating symptoms that may look like nutrient deficiencies. If neighbors have similar symptoms, the cause is probably environmental — frost, excess moisture, etc.
6. History of the land. Recently broken forage land is likely to be depleted in a lot of nutrients. Canola seeded into a field that was in long-term alfalfa, for example, is one case where you may see severe crop stunting and delayed maturity as a result of phosphorus deficiency. Otherwise it is rare to see severe phosphorus deficiency in canola, but it can occur with high yields and tight rotations that will start to mine the soil of its reserves.
7. Look at other fields for similar symptoms. When diagnosing for a specific nutrient, target the crop that tends to be most sensitive to that nutrient. If your farm is depleted of copper for example, this deficiency is likely to show up in wheat before any other crop.
Phosphorus deficiencies look like:
If phosphorus deficiency is diagnosed to be the problem, there is virtually nothing one can do to correct the deficiency in-season. You may try a top dress anyway, because even if nothing good comes of it for this year, any unused phosphate will remain in the soil and be available to future crops.
Nitrogen deficiencies look like:
—Leaves at the bottom of the plant are yellowing prematurely.
—No substantial biomass. Plants are thin and pathetic.
Nitrogen deficiencies can be rescued. Ideally, growers want to apply a nitrogen top up before the 5- to 6-leaf stage, ahead of peak uptake. Dry or liquid is fine, but rain must follow to wash nitrogen into the root zone.
Sulphur deficiencies look like:
—Top leaves are small and narrow, and are often cupped.
—Short pods with little or no seed set.
—Pale yellow leaves
—Prolonged flowering if the crop is having trouble setting seed.
—Patchy look to the field. Sulphur is highly variable across a field, so deficiencies will usually show up in patches.
—Sulphur deficiency is more typical in sandy soil with low organic matter.
If sulphur is short, growers can top up with ammonium sulphate right up to flowering. Rainfall is required to wash the nutrients into the soil. Don’t use elemental sulphur for a rescue treatment either in crop or the following year. It may not be available soon enough. Elemental sulphur is better for long-term maintenance. Start including sulphur in the fertilizer mix for all canola crops on the farm. Sulphur rates of 10 to 25 lb./ac. can provide a very good yield response.