by Josh Henry (graduate student in floriculture) and Brian E. Whipker (Professor of Floriculture at North Carolina State University)
Monitoring substrate pH is an important step in growing a healthy crop. This is especially true for plant species that are sensitive to low pH and develop symptoms in response to suboptimal pH values (typically lower than 5.2). Common symptoms include necrotic spotting, chlorosis, and marginal necrosis of the lower leaves. These symptoms are a result of iron (Fe) and manganese (Mn) toxicity, as these elements become available in excess at low pH (Pilon, 2006).
Figure 1. Symptoms of necrotic spotting and marginal bronzing on the lower leaves could be seen on a number of plants throughout the crop. (Photo: Josh Henry)
A crop of St. John’s Wort (Hypericum perforatum) was recently observed with the typical symptoms associated with low pH (Fig. 1). These symptoms were a black spotting and marginal bronzing of the lower leaves while the upper foliage remained green and healthy (Fig. 2). In more advanced cases, the tips of symptomatic leaves were necrotic (Fig. 3).
Figure 2. These symptoms only affected the oldest leaves. The younger growth appeared vigorous and healthy. (Photo: Josh Henry)
These symptoms indicated that the substrate pH was low, so a PourThru test was done to determine if this was the case. The pH was 6.5 and the electrical conductivity (EC) was 1.24. This pH was actually on the high end, contradicting the initial diagnoses of low substrate pH. Since the PourThru test did not confirm our suspicions, we collected tissue samples from several plants throughout the crop. Symptomatic leaves were collected to determine the foliar nutrient concentrations. The tissue concentrations of Fe and Mn were excessively high at 1090 and 2400 ppm respectively, confirming a toxicity of these two elements (Table 1). Although specific tissue nutrient recommendations are not available for St. John’s Wort, these values are at least 2 – 3x higher than maximum range for most crops.
Figure 3. Tip necrosis could be seen on the oldest, most symptomatic leaves.
This particular grower had been struggling with low pH in several crops, and had applied corrective measures to raise the pH. It was likely that this crop had been suffering from low pH, but a recent lime application had increased the pH to 6.5. The new growth appeared vigorous and healthy, indicating that the crop was growing out of this issue. Tissue analysis confirmed that this problem was a low pH induced Fe and Mn toxicity.
Table 1. St. John’s Wort (Hypericum perforatum) tissue nutrient levels.
Management
Monitor St. John’s Wort to make sure that the substrate pH is within the recommended range for most perennials of 5.8 to 6.2. Limiting or reducing other sources of plant stress will also help to prevent these symptoms.
Corrective procedures
Corrective procedures for low pH are listed below. Switching to a basic fertilizer when the substrate pH is nearing the lower limit will help stabilize the pH. If the pH is below the recommended range, then corrective procedures will need to be implemented. Flowable lime is one option. Typically, a rate of 2 quarts per 100 gallons of water will increase the substrate pH by roughly 0.5 pH units. The flowable lime may be applied using a fertilizer injector. Additional applications can be made if needed.
Potassium bicarbonate can also be applied. The rate of 2 pounds per 100 gallons of water will increase the substrate pH by roughly 0.8 pH units. This treatment will also provide excessive amounts of potassium and cause a spike in the substrate EC. The day following a potassium bicarbonate application, a leaching irrigation with clear water is required to restore the nutrient balance (the ratio of K:Ca:Mg) and lower the EC level. As always, remember to recheck your substrate pH to determine if reapplications are needed.
Flowable lime
- Use 1 to 2 quarts per 100 gallons of water.
- Rinse foliage.
- Avoid damage to your injector by using rates of 2 quarts per 100 gal of water, or less.
- Can split applications.
- Mix 1 pound in 3 to 5 gallons of WARM water. Mix twice. Let settle. Decant liquid and apply through injector at 1:15.
- Caustic (rinse foliage ASAP and avoid skin contact).
- Use 2 pounds per 100 gallons of water.
- Rinse foliage ASAP.
- Provides 933 ppm K
- Leach heavily the following day with a complete fertilizer to reduce EC levels and restore nutrient balance.
- Rates greater than 2 pounds per 100 gallons of water can cause phytotoxicity!
- Epstein, E. and A. Bloom. 2005. Mineral Nutrition of Plants: Principles and Perspectives. 2nd ed. Sinauer Associates, Sunderland, MA. p. 400.
- Pilon, P. 2006. Perennial solutions: a grower's guide to perennial production. Ball Publishing, Batavia, IL. p. 546.