Why ammonium toxicity can cause chlorosis in pansies

Added on 08 June 2023

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Why ammonium toxicity can cause chlorosis in pansies

Ammonium toxicity in ornamentals occurs when too much ammonium is available in the substrate, and plants take it up excessively. Water-soluble fertilizers may contain more than one form of nitrogen such as nitrate (NO3–), ammonium (NH4+), and urea [CO(NH2)2]. Ammonium and urea (urea converts to ammonium [NH4+]) can result in ammonium toxicity during the winter and early spring when light levels and temperatures are low, and substrates are saturated, oxygen deprived and between 55 to 65°F.

During the early production season, bacteria that usually help convert ammonium to nitrate are not as active in cold substrates, which can lead to ammonium toxicity. While plant cells can store high levels of nitrate nitrogen without any harm, accumulating ammonium can damage cells. Therefore, acidic fertilizers such as 20-10-20 containing a high percentage of ammoniacal nitrogen should be avoided during cold and cloudy weather, especially in northern latitudes. Instead, nitrate-based fertilizers such as 15-0-15 should be used from November to March.

Fertilizers that are high in nitrate nitrogen and low in ammoniacal nitrogen are sometimes known within the industry as dark weather feeds. Additionally, ensure that plants are receiving adequate amounts of potassium, as it helps convert NH₄+ to proteins, which lowers the foliar ammonium levels. Certain crops such as coleus, eggplant, lettuce, pansy, pepper, salvia, tomato, and zinnia are especially sensitive to too much ammonium.