Understanding Plant Nutrition: Calibrachoa

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Slideshow: Calibrachoa

Calibrachoa are often described as a “high feed” or “high iron” requiring crop. This is not exactly true.

Calibrachoa are an iron-inefficient crop and are prone to iron deficiency because they lack the ability to take up iron from the soil solution if the media pH is too high. Once iron deficiency sets in, calibrachoa will often lose vigor and become susceptible to secondary problems like overwatering or root diseases.

Therefore, to succeed with calibrachoa, you need to monitor media pH regularly and take the proper corrective actions when the media pH gets too high. Here are some pointers for growing calibrachoa.

Points To Consider

- With normal fertilization practices, the acceptable pH range for iron-inefficient crops like calibrachoa is 5.5 to 6.2. Once the media pH increases above 6.2, iron deficiency is likely (Figure 1).
 
- Make sure the iron deficiency symptoms are being caused by high media pH. Root damage caused by overwatering, high media EC, fungus gnats or root pathogens such as Pythium can give foliar symptoms similar in appearance to high media-pH induced iron deficiency because the plant does not have healthy roots to take up nutrients from the growing medium (Figure 2). In the case of root disease problems, careful irrigation and a fungicide drench are required to re-grow a healthy root system.
 
- If you have determined high media pH (greater than 6.2) is the problem, you should try to 1) lower the media pH using ammonium-based fertilizers, 2) acidify the irrigation water to near zero alkalinity and 3) apply supplemental iron drenches. 
 
All three steps can be applied separately, or combined into a single drench. These steps can also result in phytotoxicity, so trial on a small group of plants before applying to the entire crop.
 
1. Using ammonium-based fertilizers to lower media pH. Check with your fertilizer supplier to select a high-ammonium, acid-reaction fertilizer (such as 21-7-7 or 9-45-15). The effect on media-pH can sometimes be slow (more than one or two weeks) especially in cool wet conditions, or with small plants growing in large containers. 
 
Concentration of this corrective fertilizer drench is also important. We suggest 200 to 250 ppm for plugs and liners, and 300 to 400 ppm for finished plants as a one or two-time drench application. 
 
Make sure media-EC is not already high or you can cause salt damage of roots at those high fertilizer rates. Repeated applications of ammonium in cool, dark conditions may also cause toxic levels of ammonium to accumulate in leaf tissue. You may also see lush or softer shoot growth with the high ammonium fertilizer, requiring additional growth regulator applications.
 
2. Acidify the irrigation water to near zero alkalinity. Alkalinity can be thought of as the “lime content” of the irrigation water. The reason you want to remove as much alkalinity as possible from the water is the presence of high concentrations of alkalinity will reduce the effectiveness of an acid fertilizer. The acid residue produced from the fertilizer will neutralize the water alkalinity, rather than react with the media to lower the pH.
 
At a water-pH of 4.5, all the alkalinity is removed from the water, and any additional acid will cause the water pH to decrease rapidly. In general, when there is less than 80 ppm alkalinity in the water, there is no need to remove any additional alkalinity because the concentration is not enough to affect the acidification from the fertilizer. For alkalinity concentrations greater than 80 ppm, consider injecting acid in the irrigation water to bring water-pH down to about 5.0 (giving an alkalinity concentration of about 40 ppm).
 
You can base the amount of acid needed to get to a water-pH of 5.0 on trial and error, or you can calculate the appropriate acid rate for your water source from the North Carolina State University website. For example, 2.8 fluid ounces of 35 percent sulfuric acid will neutralize 100 ppm calcium carbonate of alkalinity in 100 gallons of irrigation water. Ensure you follow safe handling practices when working with acid, and that your injector equipment can handle corrosive chemicals. 
 
3. Apply a supplemental iron drench. Always remember applying supplemental iron by itself is not correcting the underlying problem of high media pH. Rather, it is covering up the problem. Failure to correct the high media pH problem may cause the plants to rapidly deteriorate once the supplemental iron applications are stopped.
There are four types of iron available to growers: iron sulfate (inorganic salt) and three chelates − FeEDTA (13 percent iron), FeDTPA (10-11 percent iron) and FeEDDHA (6 percent iron). The letters are important, because they describe the type of chelate being used. Iron forms decrease in solubility above pH 7 in the order from EDDHA (best) > DTPA > EDTA > sulfate (worst).
 
The amount of supplemental iron to apply depends on the degree of iron deficiency seen in the crop and the form of iron used. In general, FeEDDHA is the most effective iron form for supplemental applications (Figure 3). The amount of FeEDDHA to use may start at 1-2 ppm iron (0.2 to 0.4 ounces FeEDDHA per 100 gallons [1.5 to 3 grams/100 liters]) for the initial symptoms of iron deficiency to 10-25 ppm iron (2 to 5 ounces FeEDDHA per 100 gallons [15-38 grams/100 liters]) for severe iron deficiency. Always try supplemental drenches on a small number of plants first to test for phytotoxicity before applying to the overall crop. 
 
FeDTPA is another less effective iron chelate that’s often used for supplemental drenches. In general, the same amount (ounces/100 gallons) of FeDTPA should be used as with FeEDDHA. If the media pH is greater than 7.0, do not use FeEDTA or iron sulfate for supplemental iron drenches.
 
- Foliar iron sprays (to supply iron), acid drenches (to lower media pH) or drenching with high concentrations of iron sulfate (lower pH and supply iron) are also commonly recommended for correcting iron deficiency. However, after much research, it is our opinion that these methods are not as effective and are more likely to cause phytotoxicity problems than the method outlined above.
 
- Calibrachoa are sensitive to environmentally-induced boron deficiency, especially during propagation, just after transplanting or when the crop is grown under cool, humid and/or low-light conditions. Boron is taken up passively by the plant, so anything that reduces transpiration will reduce boron uptake. However, extreme care should be taken when applying additional boron to the crop. The difference between deficient concentrations, adequate concentrations and toxic concentration in the tissue is relatively small, so care needs to be taken in applying enough boron without over-application.
 
In general, the constant application of boron at 0.2 ppm is adequate, as long as transpiration is not limiting. This is equivalent to the “average” boron concentration applied by a peat-lite formula at 200 ppm nitrogen. If transpiration is limiting, the boron concentration can be increased in the fertilizer solution to 0.5 ppm (similar to summer pansy recommendations). Supplemental drench rates of 2 to 4 ppm boron can also be used if severe boron deficiency is present. 
 
Be careful about applying a boron drench more than two times during the crop because of the high risk of inducing boron toxicity. Foliar sprays of boron are not recommended.

Bill Argo is technical manager of Blackmore Co. You can eMail him at bargo@blackmoreco.com.

Paul Fisher is an associate professor and Extension specialist in the Environmental Horticulture Department at the University of Florida. You can eMail him at pfisher@ufl.edu.

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