Although not a common occurrence, sulfur deficiencies can appear, especially in poinsettias. Most of the problems are associated with the use of Cal-Mag fertilizer formulas, which are not a significant source of sulfur (S), and therefore supplemental S needs to be provided. In diagnosing a S deficiency, one must differentiate among the other mimics such as low nitrogen and iron deficiency.
For areas with low concentrations of alkalinity, Cal-Mag fertilizer blends are popular. Cal-Mag formulas contain a higher percentage of nitrate-nitrogen and tend to be basic reacting fertilizers, which helps avoid substrate pH drop. In our research studies at North Carolina State University, we favor 13-2-13 Cal-Mag (low phosphorus blend) and 17-5-17 Cal-Mag (neutral pH reacting).
Cal-Mag formulas are a blend of fertilizer salts that rely upon magnesium-nitrate as the magnesium source instead of magnesium-sulfate. This is done to avoid precipitation between calcium (cation) and sulfur (sulfate anion), which will result in a visible solid forming at the bottom of the mixing tank. This results in only trace amounts of S being available if any sulfate form of micronutrients were used in the fertilizer formulation. These trace amounts are insufficient to support the plant's needs. The problem is compounded if your water supply also lacks any significant concentration of S.
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