Ammonium nitrate [photo] 

Ammonium Nitrate

Stomach ulcers, urease and urea

By Dr. Koos Bornman, Venessa Moodley, Kobus van Zyl, Thinus Louw and David Fouché


It has become common practice to use large quantities of pure urea as a pre-plant fertilizer especially when planting maize. This practice is well established in the Free State and North West production areas and is also finding its way into Mpumalanga.

Recently, comprehensive soil analysis surveys by Omnia have shown that this practice poses a significant threat regarding soil condition and fertility. In a previous article the threat of ammonium toxicity as well as the disturbing negative trends in soil analyses, especially subsoil analyses were discussed. In this article the risk of slow urea conversion in sub-soils, and the resultant need for ammonium nitrate supplementation will be addressed.

What does urea and stomach ulcers have in common?

Most gastric ulcers are caused not by stress or spicy food as was once believed, but by a bacterial infec-tion of the stomach, caused by Helicobacter pylori.

One of the hardest organs for a microbe to colonise is the stomach. Besides the thick layer of mucus and peristaltic washing, the stomach is filled with acid (average pH ~2). One way in which H. pylori survives in the stomach is by producing the enzyme urease, which converts urea into ammonium and carbon dioxide; in this way, the bacterium surrounds itself with a layer of ammonium and ammonia, which neutralises the acid in its immediate vicinity.

Likewise urea requires the enzyme urease to convert it to ammonium in soil to become available as nitrogen source to the plant.

Soil surveys

As mentioned, the analyses trends from 5,200 sub-soil samples since 2002 in the areas of Bothaville, Viljoenskroon and Bultfontein were discussed in a previous article. Having noted the significant deterioration of subsoils due to acidification, it was decided to launch a study to investigate the efficiency of conversion of urea to ammonium under the typical subsoil conditions in these areas.  

The study, as mentioned, was initiated by OmniBio™ and a group of senior agronomists within Omnia Fertilizer. Some 220 samples were carefully taken (July 2012) in the above-mentioned areas of the typical sandy soils (effective cation exchange capacity (ECEC) of between 2 to 8) at a depth of approximately 300 to 400 mm and sent for comprehensive soil analyses including physical and chemical parameters, as well as certain soil biological parameters.  

The results of this initial survey gave some disturbing confirmation of the above-mentioned suspicion and also additional information as discussed below.

The primary objective 

The primary objective of the study was to determine and evaluate the presence of urease within the above-mentioned study areas. Urease is an enzyme or protein that is inherently present in organic matter.  

According to literature, there is also a strong relation between urease activity and temperature. The impact of temperature was therefore specifically evaluated in sandy soils known to be low in urease activity. It is becoming common practice for producers to pre-plant urea at a depth of 200 to 300 mm (often in dry soil) fairly close to planting and there is concern regarding the efficiency of such practices in cold, sandy soils with very low or rather no organic matter presence at depth.  

The degree of urease activity in these soils was scientifically evaluated in Omnia’s OmniBio™ laboratories and it was re-established that soil temperature had a significant effect on urease activity, especially if soil temperatures drop to below 20 degrees Celsius. At these temperatures urease acitivity is less than half that of the optimum.  

As mentioned, urease activities are already low in sandy soils at a depth of 30 cm (10 to 15 µg urea per gram soil for a period of 2 hours). In this study a highly significant depression of urease activity was measured in the subsoil samples to levels as low as 3 µg urea/g/2h if temperatures dropped to below 15 degrees Celcius.  

What is of additional concern is that 80% of all the samples had a pH of less than 5.2 (KCl), confirming the general survey mentioned. The lowest pH measured was 3.5 in KCl in the North West and Free State area. More specifically, 40% of the samples had a pH of less than 4.5. Extractable acidity is also high in many cases with acidities as high as 53% saturation reached.  

Interestingly, urease activity could be predicted by a statistical model using soil parameters to the extent of more than 60% variation was explained.  

The parameters that contributed most to explaining variance of urease activity besides temperature (53% overall) were: estimated or effective cation exchange capacity (ECEC), the presence of the cations Ca, Mg and K as well as bulk density or clay percentage. Other interesting parameters also contributed to explaining variance (such as estimates of anaerobic (waterlogged) conditions) in urease activity. Throughout pH and extractable acidity do not seem to play a major role in urease activity as pH was already critically low (urease had optimal activity at a water pH of 7.4).

Valuable benchmark levels have been recorded and models developed from the initial study and soils at risk can be defined making use of a conventional soil analysis and an OmniBio™ urease determination.

What this study has shown thus far is that the use of pure urea without ammonium or nitrate in sandy subsoils is inefficient. The conversion rates of typical pre-planted urea rates could take as long as two months to convert to ammonium, let alone nitrate (refer to the article addressing nitrification rates and ammonium toxicity). The expected conversion rates (to ammonium) as such are also too low to sustain the peak requirement of nitrogen of high yielding maize in the early season (up to 8 kg Nitrogen requirement per ha per day). When urease inhibitors are used, the situation will worsen, adding another 10 days.

The conclusion is thus that pure urea (without ammonium nitrate) is a high risk nitrogen source to use for pre-plant purposes in sandy soils and that other sources rich in nitrate and secondary elements such as calcium, magnesium and sulphur, typically offered by Omnia Fertilizer, should be considered. It would seem from several research sources that nitrogen products for grains should contain approximately 25% nitrate to reduce the risk of inefficiency and optimise nutrition. Contact your Omnia agronomist for a more detailed analysis of your soils and prevent those stomach ulcers due to concern regarding the use.of the wrong nitrogen source under less than ideal conditions.

Verwysings / References
- Bornman, J.J. 2013. Ammonium toxicity: A silent killer. SA Grain, Landbouweekblad, Farmer’s Weekly - to be published.
- Kissel, D.E. 1989. Management of urea fertilizers. North Central Regional Extension Publication. Kansas State University.
- Yamaoka, Yoshio. 2008. Helicobacter pylori: Molecular Genetics and Cellular Biology. Caister Academic Pr. ISBN 1-904455-31-X.

This article was published in the Nutriology® Newsletter, Summer 2013