Magnesium is a common nutrient deficiency that can occur later in the growing season if supplemental magnesium is not supplied.
Deficiencies can occur as a result of many factors. The propensity for Mg deficiency problems varies by geographical area. In much of North America, adequate magnesium (Mg) concentrations of 25-50 ppm Mg are in the groundwater. However, not all locations have Mg concentrations adequate to sustain plant growth and must be supplied another way. Plants can be provided with Mg through a variety of methods, including groundwater, dolomitic limestone included in substrates, and through fertilizers.
In areas lacking magnesium (Mg) in the water supply, dolomitic limestone can provide adequate Mg concentration through most of the season. When relying on dolomitic limestone to provide adequate Mg fertility in warm springs, the extra irrigations can result in leaching the available Mg and resulting in deficiencies. This has been observed to be the case when growers utilize a fertilizer such as 20-10-20 (N-P2O5-K2O), a fertilizer that does not supply calcium or magnesium, and also rely on the Mg from the dolomitic limestone during spring bedding plant production.
The top nutritional problem to avoid with geraniums is pH drop. This results in lower leaf bronzing and necrosis. One of the management strategies is to provide extra lime to keep the substrate pH >6.0. With loading up with all of this lime, it results in excessive calcium (Ca) being available to the plant. The cations potassium (K), calcium (Ca), and magnesium (Mg) must be kept in a 4:2:1 ratio to retain balanced uptake. If one of those elements is provided in excess, it can result in limiting (antagonizing) uptake of another one. Usually Mg is the one that loses.
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