Because of cool temperatures some growers have been fertilizing geranium hangers at very low rate. However with the increase in the angle of the sun and increasing day temperatures in the growing the plants are pushing more growth therefore requiring more nutrients. This has resulted in plants showing nitrogen and magnesium deficiency. Nitrogen is an important component of nucleic acids, amino acids, enzymes and chlorophyll in the plant. Deficiency of nitrogen may cause the plants to appear stunted with light green lower leaves while upper leaves remain green. Prolonged deficiency may cause yellowing (chlorosis) or reddish pigmentation of older leaves depending on cultivars. See photo.
Nitrogen deficiency can be corrected by increasing the fertilizer rate.
Magnesium is also important component of chlorophyll. Magnesium deficiency occurs on the older leaves as greenish yellow chlorosis along the leaf margins and chlorosis between veins. Prolonged deficiency may lead to death of leaf tissue (necrosis) between the veins and leaves tend to curl downwards. Magnesium deficiency can be corrected by using magnesium sulfate (Epsom salts). Mix Epsom salt at the rate of one pound per 100 gallons of water and use as a drench. Growers can also use fertilizers that contain magnesium such 17-5-17 cal-mag, or 15-5-15 cal-mag.
Geoffrey Njue, University of Massachusetts Extension
Growing Geraniums, UMass Extension
Geranium Nutrient Deficiencies: A Visual Primer for Grower Diagnosis and Corrections, OFA January/Feb 2006, number 894