by W. Garrett Owen, Josh Henry, and Brian E. Whipker
Fertility management of marigolds
Marigolds are considered to require a low to medium levels of fertility. Most growers supply between 100 to 200 ppm N. The lower end of the N rate is commonly used to help control excessive growth.
Figure 1. Lower leaves of a marigold exhibiting yellow and bronzing leaf coloration due to a low substrate pH. (photo: Brian E. Whipker)
Marigolds should be grown with a pH range of 5.8 to 6.2. This range will help avoid two of the most common nutritional issues with the crop. The most disastrous is a low substrate pH induced iron and manganese toxicity that occurs if the pH drifts lower than 5.5 (Figs. 1 to 3). Lower leaves will develop a bronze speckled appearance. This is due to toxic levels of iron, and sometimes manganese, accumulating in the tissue. Albano et al. (1996) found that lower leaf symptoms developed when leaf tissue iron levels exceed 1000 ppm (Table 1). One should denote that both iron and manganese predominately accumulate in the lower foliage and then progress up the plant. It will take a few weeks for plants to grow out of this condition once the pH is corrected to the 5.8 to 6.2 range.
Figure 2. Low substrate pH results in the lower leaves developing a bronzing coloration due to the accumulation of iron and manganese (toxicity). (photo: Brian E. Whipker)
Substrate pH values above 6.5 can also inhibit iron availability. This is why the optimal substrate pH range is between 5.8 and 6.2 (Fig. 4). The range of 6.2 to 6.4 is the point in which corrective procedures should begin.
Figure 3. Progression of leaf symptoms due to a toxic accumulation of iron and manganese induced by a low substrate pH.
Both nitrogen deficiency (Fig. 5) and phosphorus deficiency can occur with marigolds. Nitrogen deficiency develops as an overall yellowing of the lower foliage or overall plant. This disorder occurs when the fertilization rate is too low. Also note, water stress and tight spacing can also result in lower leaf yellowing. Phosphorus deficiency usually appears as a lower leaf purple coloration. It is most commonly observed when the growing temperatures are cool and low fertilization rates are used.
Figure 4. Reported and optimal pH range for marigolds based on literature indicating upper and lower pH limits inducing deficiencies and toxicities. (photo: W. Garrett Owen)
Summary
Therefore, providing low to moderate levels of fertility of 100 to 200 ppm N and providing a substrate pH range of 5.8 to 6.2 will prevent most nutritional disorders of marigolds.
Management
Monitor marigolds to make sure that the substrate pH is within the recommended range 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.
Figure 5. Low fertilization rates will result in an overall pale yellow coloration due to insufficient nitrogen. (photo: Brian E. Whipker)
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 gallons 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!
- Albano, J.P., W.B. Miller, M.C. Halbrooks. 1996. Iron toxicity stress causes bronze speckle, a specific physiological disorder of marigold (Tagetes erecta L.) J. Amer. Soc. Hort. Sci. 121(3):430-437.
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