Ammonium nitrate [photo] 

Ammonium Nitrate

Ammonium toxicity: The silent killer

By Dr. Koos Bornman (General Manager: Strategic Agricultural Services)

It has become common practice to use large quantities of reduced nitrogen (urea, ammonia and ammonium), especially urea, as a pre-plant fertilizer especially when producing maize. This practice is well established in the Free State and North Western areas and is also finding its way into Mpumalanga. These products pose the risk of build-up of the nitrogen cation, ammonium, which is highly toxic to plants.

This article wishes to convey the risk of ammonium toxicity, explain its effects and suggest how it can be prevented or alleviated.


When large amounts of urea, for instance, are pre-planted at a depth of 200 mm to 300 mm in a typical sandy soil of the North Western or Free State areas, it is usually in a fairly cold, acid environment, low in organic matter. During the early season after the first rains, such soil often becomes water logged. These conditions are conductive to the accumulation of ammonium as the process of nitrification is inhibited.

Ammonium toxicity

Ammonium toxicity has already been described by Charles Darwin in 1882. The plant cannot store ammonium as it does nitrates. Free ammonium in the plant cell is toxic and needs to be bound in amino acid form by combining with sugars as soon as possible. It already has a toxic effect on grain crops at levels as low as 150 mg/kg in the soil, if it is the only nitrogen source. This level is already reached when 80 kg nitrogen is banded in one meter rows in a sandy soil. It causes severe plant chlorosis (often similar to a magnesium deficiency symptom, i.e. yellow striped leaves) with reduced photosynthesis, stunted plants with a low shoot to root ratio and inhibition of especially root growth (see photos). Ammonium toxicity can have a severe impact on seedling growth in particular, but also on mature plants and can lead to between 15% and 60% yield loss. Ammonium toxicity is so aggressive that it can even be used as a herbicide.

Some physiological characteristics related to ammonium toxicity are the suppression of cation uptake (calcium, magnesium and potassium) and enhancement of anion uptake (especially phosphate and chloride). Other indicators are a reduction of intra cellular pH and a drastic decline in sugars and starches in the cell.

Studies on maize in South Africa by the Agricultural Research Council has indeed proven such toxicity showing severe yield depression in maize at high nitrogen application as only urea.

Nitrification and acidification

The oxidation of ammonia is dependent on quite a few factors. Nitrification is dependent on the presence of nitrifying bacteria which are sensitive to low temperatures, pH and basic cations such as calcium. In fact, it is documented that nitrification comes to a complete halt at pH levels measured in water of 5.5. Also, nitrification is severely inhibited by the lack of oxygen and ironically high levels of ammonium. In sandy, water logged subsoils with low pH and temperature, typically associated with the soils of the Free State and North West, 50% nitrification of ammonium may take as long as 50 days. Nitrite toxicity is also often associated with delayed nitrification.

It is well known that, if a reduced nitrogen source is not efficiently intercepted by the root system, it will convert to nitric acid, causing severe soil acidification. This would then also be expected if high concentrations of urea and ammonium are used, especially when pre-planted.

Subsoil analysis trends

An analysis of the data from 5,200 subsoil samples from the Omnia soil analysis database, since 2002 in the areas of Bothaville, Viljoenskroon and Hoopstad, has confirmed the above theory and revealed the following disturbing facts:

A drop in the median pH (KCl) of more than 0,4 units to a critical level of 4.7 (40% of all the samples had a pH of less than 4.5 with the lowest measured being 3.5). A significant increase of extractable soil acidity from zero to more than 10% for more than one in five of the samples in 2011 was measured (the highest being 53%).

What is also of note is the lack of presence of cations in the subsoil. Half the samples had calcium, magnesium and potassium levels of less than 330, 60 and 70 mg per kg respectively.

The remedy

The question may well be asked: what can be done to remedy the situation of the subsoil ammonium toxicity threat, especially as reduced nitrogen sources will remain popular due to its high concentration and availability?

The first suggestion is to get cations into the acidified subsoil zone by means of fertilizers containing high cation concentration and by using combinations of lime and gypsum. It has been proven that higher cation presence (especially calcium) even under low pH conditions, will enhance nitrification significantly.

The second suggestion is to simply add a fraction of nitrate to the reduced fertilizer source. Literature states that nitrogen uptake may be enhanced by 75% and growth rates of crops may be enhanced by up to 40% and even higher, using combinations of reduced nitrogen and nitrate. The ratios between ammonium and nitrate are specific to the crop produced, but in general for grains a ratio of 80% reduced nitrogen and 20% nitrate nitrogen is near the optimum.

Some physiological explanations of this enhance-ment of reduced nitrogen efficiency with nitrates is the stimulation or optimization of biochemical processes, stimulation of the synthesis of the growth hormone cytokinin, physiological alkalization of the rhizosphere (hydroxyl swop from the root for negatively charged nitrate), and preventing carbon drain from the root.

Speak to your Omnia agronomist to get the right tailor made product to manage the risk of ammonium toxicity in your crop, especially for pre-planting and with plant.

Photo 1-3: Ammonium toxicity in barley and maize. Photos from articles by Britto and Kronzucker, 2002 and from Adriaanse, 2012.


  • Adriaanse, F.G., 2012. Efficacy and application of nitrogen sources. Proceedings of the Technical Symposium of the Fertilizer Society of South Africa, Pretoria, 22 August 2012. Pages 4 to 19.
  • Britto, D.T. & Kronzucker, H.J. 2002. NH4 toxicity in higher plants: a critical review. J. Plant Physiol. 159. Pages 567-584.

This article was published in the NutriologyAmmonium toxicity: The silent killer Newsletter, Summer 2013