As we start to move into fall and winter flower crop production cycles, it’s a good time to go back through some basics about nutrient deficiencies.
No matter where you are in a cropping cycle, nutrition problems can be tricky to figure out. The good thing is they can be differentiated from disease or pest issues based on a few key observations:
- If the damage is uniform and crop wide, it’s most likely a nutritional issue
- If the damage is localized or more random, it’s most likely a disease or pest issue
Key indicators of specific nutrient deficiencies are easy to spot with regular monitoring, and should be knowledge that just needs a refresher every now and then. Key indicators are listed here, along with a handy infographic (Figure 1).
Nutrient Mobility: When deficient, mobile nutrients will move to the new growth, but immobile nutrients will be trapped in the old growth. A quick scan of where the damage is occurring can help to narrow the potential culprits.
Stunted growth: If plants are looking shorter than normal crop wide this can be a sign of nitrogen (N) or phosphorus (P) deficiency, especially if the older growth also looks yellowed. Individual or clustered groups of stunted plants are more likely to be caused by diseases. Look for small root systems and/or wilted shoots can be a sign of black root rot (Thielaviopsis) or Pythium.
Interveinal chlorosis: Dark green leaf veins with light green to yellowed leaf tissue? Sounds like a deficiency in magnesium (Mg), copper (Cu), iron (Fe) or manganese (Mn). Magnesium deficiency should show up in the bottom leaves first since it’s a mobile nutrient. Copper, iron and manganese are immobile, so expect to see the chlorosis in the newest leaves initially. Of course, as the deficiency progresses, all leaves can show symptoms.
Leaf margins: If a border around the outer edge of a plant’s leaf looks yellow, purple, red or brown it might be a potassium or calcium deficiency. This can also be a sign of pesticide or fungicide phytotoxicity, so review your spray records carefully.
Remember, many deficiencies are not as simple as they first appear. Nutrient interactions can be positive (synergistic) or negative (antagonistic). Synergistic interactions between nutrients can help one or both to have greater uptake by the plant. Most deficiencies occur because of an antagonistic interaction between nutrients, where they compete with each other for uptake (Figure 2).
The negative interaction can occur in one of two ways. In the first, an excess of one nutrient can block others from uptake sites on the plant root, making it deficient because it cannot get to an uptake site. In the second, the excess nutrient is present in high enough amounts that it changes the pH of the growing media, making the other nutrient unavailable to the plant.
In many cases, one nutrient in excess can knock many others out of balance. For example, an excess of Potassium (K) can cause (Figure 3):
- Increased availability of Iron (Fe) and Manganese (Mn)
- Decreased availability of Nitrogen (N), Phosphorus (P), Magnesium (Mg), Calcium (Ca), and Boron (B)
I’ve got just one more important point for this post. Knowledge of your growing media’s pH is essential to keep nutrients available to the plant. If you stray too far from the target range for your crop and media, nutrients will get converted to forms that are not available to the plant, turning your fertilizer solution into very expensive waste! A pH between 5.5 and 6.0 can also help to keep diseases like black root rot (Thielaviopsis) and Pythium at bay.
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