Ammonium Toxicity

Ammonium toxicity is most commonly seen on young lettuce plants that are between thinning and rosette stages. Aboveground symptoms of ammonium toxicity are similar to those caused by root injury: dull, dark green or gray-green leaves, temporary leaf wilting, desiccation, and yellow V-shaped sections. As the symptoms progress, leaf margins may turn brown, and the plant may become severely stunted or die.

Typically, small root hairs and lateral roots are brown and dead, and a longitudinal cut down the taproot shows a reddish-brown discoloration in the center of the root. The root core may be hollow in advanced cases. The internal symptoms may be confused with those caused by Verticillium or Fusarium. The exterior tissues of the larger roots may crack and have a corky appearance, which may be confused with bacterial corky root disease.

Conditions that favor the occurrence of ammonium toxicity include the use of ammonium containing nitrogen fertilizers (ammonium nitrate, ammoniated phosphates) or fertilizers that are converted to ammonium (urea). Manures can also release ammonium. This condition may occur if ammonium-containing fertilizers are applied at excessive rates or if placement is too close to the root. The condition is favored by cool soils because the conversion of ammonium to nitrate is slow. The condition can also occur in warmer times of the years in heavy soils that are waterlogged or become sealed at the surface.

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.