September 10, 2012

Ammonium Toxicity~~11/1997 AVM

Ammonium Toxicity in African Violets
November/December 1997 African Violet Magazine, pgs 26-27
By Kent and Joyce Stork, Fremont, NE
     For some time we have received questions from individuals who have grown African violets successfully for years who now find that their plants will no longer grow as large as they once did. The plants described exhibited a number of symptoms related to foliage, but those symptoms were not consistent with the usual problems associated with violets. We consulted several sources who suggested the possibility of ammonium toxicity... a condition linked to the use of urea or ammonium based fertilizers.
     At this time, we can find no studies of ammonium toxicity specifically targeting African violets. The condition has been studied in florists’ gloxinias, and we thank Dr. Paul Nelson of North Carolina State University for sharing the results of his studies. He believes that ammonium toxicity does affect African violets. The symptoms he described closely resemble the problems violet growers brought to us.
     In florists' gloxinias, the ammonium toxicity causes lower leaves to curl downward stiffly (not limply). The lower leaves will exhibit irregular and highly unpredictable patterns of chlorosis (lighter green patches). The leaf margins of the most mature leaves will be burned, and as the toxicity becomes more pronounced the leaf burning shows up on younger and younger leaves until it affects the crown itself. The plant may die completely. The root structure will be reduced in size and will have an orange- brown tone that is distinctly different from normal root color.
     Dr. Nelson stated that other plants which have been studied show similar patterns, although some plant types react by leaf curl that goes upward rather than down. The lighter patches of color on the leaves are consistently unpredictable on all plants studied.
     Ammonium toxicity can have a phantom-like quality. The grower may not note any changes in growing methods or conditions when plants develop symptoms. Similarly, symptoms can disappear temporarily before returning. This can lead a grower to believe that a fungus, an unseen insect, or a virus has invaded. It can also cause a grower to believe that something caused an improvement, when in fact it had no effect at all.
     The extent of the reaction can vary in different hybrid strains of one plant type. Since many African violet growers have several hybrids in their collection, it would be
expected that the pattern of symptoms would vary from plant to plant.
     Several growers who attended the 1997 AVSA convention in Florida reported very similar symptoms. They were completely puzzled until they discovered that their standard fertilizer had changed. The company producing it was bought out by a larger corporation who changed the source of the nitrogen to a much cheaper ammonia form. This change was not advertised or promoted but it had serious consequences for growers.
     Ammonium toxicity is generally linked to the use of urea-based or ammonium fertilizers along with the absence or ineffectiveness of soil bacteria. The active soil bacteria can break ammonia down into a usable nitrate form as long as the soil temperature stays above 70 degrees Fahrenheit and as long as the soil pH is 6.0 or above. When the soil temperatures cool to below 70 degrees or the pH drops into a more acid range, the soil bacteria becomes progressively less active and less able to process ammonia. This results in a build-up of ammonia to toxic levels.
     Growers in areas that experience wide seasonal temperature swings might note the problem during the winter season, but not as much in the summer. Farmers in areas with very acid water conditions (often areas that receive lots of rain yearly) report very serious problems with ammonium toxicity in field crops. Violet growers can almost certainly predict similar problems where water supplies are acid. Similarly, if the soil or potting mix is very acid (peat moss can vary in pH in somewhat unpredictable ways), the ammonium toxicity can become a problem even if the water is neutral.
     If you grow violets in an area with well-regulated room temperatures and do not have an acid water supply, you will probably not have to deal with this problem. There is no reason to panic if you see no symptoms.
     If you must deal with seasonally cool temperatures, be aware of how it may affect your violets. Do not use the lowest shelves in your growing area during the cool months since the coolest air is always closest to the floor.
     You may wish to use less fertilizer since the soil bacteria will be less efficient at processing the ammonia. If possible, warm the room so that soil temperature does not sink below 65 degrees Fahrenheit. Avoid using cold water to water plants or refill reservoirs. (Hot water is also not acceptable.. .do not use water warmer or cooler than ten degrees from the air temperature.)
     If you must deal with persistent acid pH in your soil or water, you would be wise to consider using one of the nitrate-based fertilizer products. Read the ingredients on the label. You will need a brand that lists at least one of its sources of nitrogen using the word 'nitrate'. These will not add the excess ammonia to your soil, and are not so dependent upon soil bacteria.
     If you suspect a problem, try leaching the soil. Pour an amount of water at least equal to the size of the pot into the top of the soil and allow it to run through the plant. It is best to do this when the plant is somewhat moist so that all salts will be dissolved and flushable. Do not allow plants to stand in the runaway water. If you note an orange tone to the water that is running out, you may assume that there is some sort of fertilizer build-up in the soil. Leaching should reduce the toxic build-up and may cause symptoms to go away. It is wise to leach until the runoff water is no longer colored.
     Once recognized, ammonium toxicity should be easy to control and avoid. Best of all, the treatment is inexpensive and safe. There's no reason to panic! 

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