Ammonium toxicity is a problem that can be avoided with common sense and attention to detail. The causes of ammonium toxicity can be grouped into two general areas: level of ammoniacal-nitrogen fertilization and suppression of nitrifying bacteria.
Avoid the excessive application of nitrogen fertilizers by following the recommendation for the crop that you are growing. Also, avoid using fertilizers with an excess portion of ammoniacal nitrogen. Consider geographic location and season when selecting a fertilizer. A fertilizer containing 40 percent ammoniacal nitrogen, such as 20-10-20, can be used year-round in warm climates but not during cold periods in the northern United States and Canada.
Bacteria that convert ammonium to nitrate function best under certain environmental conditions. Although the optimum pH for nitrifying bacteria is 7.8-8.0, most crops grow best at pH 5.5-6.5. Follow the recommendation for the crop that you are growing; however, if ammonium toxicity has been a problem in the past, stay on the high end of the recommended range.
Low soil temperature will also suppress bacteria growth. If you are growing a crop at a lower temperature, reduce the proportion of nitrogen in ammoniacal form, stay on the low end of the recommendation for total nitrogen and keep the substrate pH high.
Oxygen is necessary for nitrification, so don’t overwater. If you are growing outside where rain can be a problem, use a coarse substrate.
We grew seven bedding plant species (vinca, verbena, snapdragon, salvia, petunia, pansy and dianthus) in substrates amended with the standard lime rate (high pH) or one-third the standard lime rate (low pH). Treatments of 20-10-20 ratio fertilizers containing five different ammonium levels (0, 25, 50, 75 and 100 percent of total nitrogen) were applied at 100-ppm nitrogen. The low substrate pH levels ranged from 4.0 to 5.1.
General symptoms. Generally speaking, high ammonium-treated plants were more compact (shorter and less leaf area). Younger leaves are affected on young plants (bedding plants and plug seedlings). Symptoms include curling of young leaf margins, interveinal chlorosis of young leaves followed by necrosis in the chlorotic areas, less root growth, and the death of root tips. In contrast, mid and older leaves are affected in finish crops, such as pot mums and gloxinia. Finish crop symptoms include curling of leaf margins, various forms of chlorosis (such as whole leaf, large patches, interveinal), less root growth, and burning of leaves and root tips.
Symptoms by crop. Interveinal chlorosis and upward curling of margins appeared on young leaves of vinca. With time, chlorosis spread from the interveinal areas to the entire leaf and faded to a lemon-green color. The tips of these leaves were slightly deeper green.
Young leaves of verbena developed interveinal chlorosis and margins curled upward.
Snapdragon leaves developed chlorotic blotches roughly 1/16-inch wide by 1/8-inch long in interveinal regions toward the terminal end of young leaves. As these blotches increased in number and size and spread toward the base of leaves, necrotic spots developed a short distance from the tip of these leaves. The same syndrome later occurred on young leaves of lateral shoots.
Salvia plants developed chlorosis along the margins of young leaves that spread inward to form an interveinal pattern. Moderate distortion occurred on these leaves. Pin point necrotic spots appeared in the chlorotic areas. Young leaves of petunia became chlorotic between veins.
Young leaves of pansy developed chlorosis along the margins that progressed inward into an interveinal pattern. Margins of these leaves rolled upward, sometimes before chlorosis and other times during or after. Dianthus developed interveinal chlorosis on young leaves. Ends of these leaves curled downward in excess of 360 degrees, giving a “pig-tail” appearance. Young leaves on lateral shoots became generally chlorotic.
Curing the problem. It is best to avoid the problem; however, if ammonium toxicity develops, begin by leaching the ammonium from the substrate. Stop applying ammonium and use an all-nitrate fertilizer, such as 15-0-15. Also, be sure to apply adequate potassium, which helps convert ammonium to proteins in plants, which lowers the foliar ammonium levels.