Nutritional Factsheet Marigolds
Fertility management of marigolds
Marigolds are considered to require 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.
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 (Fe) and manganese (Mn) 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 Fe, and sometimes Mn, accumulating in the tissue. Albano et al. (1996) found that lower leaf symptoms developed when leaf tissue Fe levels exceed 1000 ppm. One should denote that both Fe and Mn 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. Substrate pH values above 6.5 can also inhibit Fe availability. This is why the optimal substrate pH range is between 5.8 and 6.2. The range of 6.2 to 6.4 is the point in which corrective procedures should begin.
Both nitrogen (N) deficiency and phosphorus (P) 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. Note that 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.
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