Ammonium toxicity damages plant roots and water-conducting (xylem) tissues. As the xylem collapses and roots are damaged, water uptake is restricted and wilting and stunting occur. Some plants may die and the marketability of surviving plants is reduced. In addition, free ammonium can reduce seed germination.
Ammonium-nitrate based fertilizer does not cause ammonium toxicity in grains, as is a strong possibility with urea.
Photo courtesy of Plant Physiology and Agrobiology Research Group
Pea plants (a) and spinach (b) grown with solutions containing ammonium or nitrate. While pea plants tolerated ammonium nutrition, spinach plants grown with ammonium halted growth forming buds (c)