Avoid Ammonium Toxicity in Vegetable Crops

Get your Crops off to a Strong Start. Avoid Ammonium Toxicity this Spring.

Different factors, including wet weather and low soil temperatures can impact planting and crop plant establishment. We are seeing cold and saturated soils which could present challenging conditions to get the crops off to a quick, uniform and healthy start.

One common issue we should be aware of under these current growing conditions (cool soils and partially saturated soil moisture conditions) is ammonium toxicity. Ammonium toxicity occurs when fertilizer choices including Urea, UAN 32 or AN 20 are used in these conditions. What occurs is that there is too much ammonium available from the conversion of urea to ammonium in the soil solution for the plant to take up. Also, the time necessary to convert from ammonium to nitrate is soil temperature dependent and can be delayed for some time. The result is an accumulation of ammonium for plant uptake from the soil and plant roots absorb higher ammonium levels than can be metabolized at one time in the leaf. Remember that the preferred form of nitrogen for plant uptake is nitrate instead of ammonium. All forms of nitrogen are available by plant roots for uptake, but all forms are not equal in the plant for rate metabolism. The conversion of urea and ammonium into nitrate nitrogen in the soil is a biological process governed by soil biology, enzymes, moisture, soil pH and ultimately soil temperature. 

hydrolysis and nitrification illustration

Ammonium toxicity affects most plants but the threshold at which symptoms of toxicity show differs among plant species. The most sensitive plants include strawberries, lettuces, brassicas, tomatoes, potatoes and some citrus species. The symptoms of ammonium toxicity include chlorosis of leaves, decreased growth and poor root development. Depending on plant species, the edge of leaves may curl upward or downward. Some plants may senesce, and the surviving plants and their crop marketability is reduced. Also, seed germination and seedling establishment can be inhibited by ammonium toxicity.


Lettuce plant in the middle is wilted in the afternoon from ammonium toxicity-affected roots. Photograph as seen on Ammonium Toxicity on Lettuce blog by Richard Smith & Steve T. Koike, University of California

Ammonium toxicity can be prevented by following appropriate cultural practices. When it comes to choosing your spring nitrogen fertilizer, the use of ammonium containing fertilizers or fertilizers that are converted to ammonium from urea/UAN 32/AN20 sources can have a negative impact on crops due to toxicity from ammonium. Manures or compost can also release ammonium. One strategy is to reduce the amount of nitrogen in the ammonium form when these cool/wet soil conditions exist. A second, begin using a higher nitrate-based nitrogen fertilizer choice such that we can avoid ammonium buildup and thereby possible toxicity. There is clear evidence from many trials that better root growth is obtained when plants receive a higher ratio of nitrate to an ammonium nitrogen source. Observation: a 2/3 nitrate or higher to a 1/3 or lower ammonium form offers the best for rooting and plant growth.tomato - Hanninghof.png

Tomato roots
REF.: Research Centre Hanninghof, Yara - 2004


Improved root growth in citrus trees fertilized with YaraLiva CALCINIT
Quaggio et al., unpublished data

Nitrate has many benefits to a crop. It does not require biological conversion in the soil and is thereby immediately available for plant uptake. Field research confirms that using nitrate-nitrogen forms can significantly increase yields compared to using predominant ammonium-nitrogen forms. Also, nitrate-nitrogen promotes the uptake of critical essential nutrients such as calcium, magnesium, and potassium, increasing crop quality.

YaraLiva® Calcium Nitrate fertilizers provide fast-acting nitrate nitrogen, along with strength-building calcium. In combination, these nutrients fuel prolonged growth and are the preferred choice to prevent the incidence of ammonium toxicity.

Contact your local Yara representative to find a YaraLiva distributor near you.

Eddie Muro
Eddie Muro
Sales Agronomist

Central Coastal California

Patricia Dingus
Patricia Dingus
Regional Sales Manager

Central Coastal California