Here are some things to keep in mind when looking at potential nutrient deficiencies. Generally if you contact someone for help or look up the symptoms these are things you are going to need to know.
Look for patterns in the field that might be related to management operations or soil conditions. Nutrient deficiencies are rarely uniform over the field they will usually show up first in distinct areas of a field. For example some hybrids might have a striped pattern on the leaves which might be mistaken for Mg deficiency. However, if every leaf on the plant and every plant in the field show this pattern, it is probably not a nutrient deficiency, but rather a hybrid characteristic.
Knowing what the deficiency is, is only half of the problem, the other part is knowing why you have the problem. For example, a potassium deficiency can be due to low soil K, but is also a common deficiency caused by compaction.
If the deficiency is due to low soil K, adding more K should solve the problem, but if the problem is due to compaction adding more soil K may not help. An example here might be a pattern that seems to follow tire tracks in the wetter areas of a field would be a strong indication of a compaction problem.
Look for the position of the symptoms on the plant. Some nutrients are mobile in the plant and thus symptoms appear first on older leaves and move up the plant as the deficiency gets worse. Nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) are in this group.
Other nutrients are immobile in the plant and symptoms will show up on the new growth when the plant becomes deficient. Two examples where this is very useful for making a correct diagnosis are Mg and Zn; and N and sulfur (S).
Both Mg and Zn show up as a stripping on the leaves but magnesium deficiency will show up on leaves on the bottom of the plant, but zinc will show up on leaves on the top of the plant. Also, in this case, the Mg stripping is usually very uniform and crisp, whereas the Zn stripes are often rough, starting as short strips that get longer and tend to run together as the deficiency gets worse.
Nitrogen and S both show up initially as pale green or yellowing on the leaves. The main distinction is that N deficiency will show up on the lower leaves and S will be at the top of the plant.
As the deficiency gets worse, N deficiency will become an inverted yellow “V” starting at the tip of the leaf and going back the middle of the leaf. As S deficiency worsens, the top of the plant will turn yellow and there may be some stripping on the top leaves as the deficiency progresses.
Potassium deficiency is the other fairly common deficiency. Because K is mobile in the plant the symptoms will be on the bottom leaves of the plant and will show up as yellowing on the edge of the leaf.
Plant analysis is an excellent tool for diagnosing nutrient deficiencies. When using plant analysis to diagnose visible nutrient deficiency problems, the best approach is to take comparative samples. With this method two samples are taken for analysis.
One sample is taken from the problem area and the second sample is taken from a nearby area that is as near to identical to the problem area as possible but is showing normal growth. This method can be used at any stage of growth and it eliminates the confounding effects of most of the cultural and environmental factors.
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When using this method the two samples must be taken at the same time and the same plant part must be sampled. Usually the newest fully developed leaves or stems are the best plant part to sample for this method. If the problem is nutritional, comparison of the nutrient levels in the two samples usually gives a direct indication of the problem.
Taking soil samples to go with these two plant samples can further help identify the problem. If the plant samples show a nutrient deficiency in the problem area and the soil test shows the same thing, it is clear that the soil is deficient and adding that nutrient should help. However, if the plant samples show a nutrient deficiency in the problem area but the soil samples do not, this indicates that something else is limiting the ability of the plant to get the available nutrient and thus adding more of that nutrient, without addressing the underlying problem will not help.
When using plant analysis it is critical that sampling instructions from the lab are followed exactly. Nutrient levels in plants can vary a lot with stage of growth and plant part sampled and results from incorrect samples can be very misleading.