Ammonium nitrate: a dual nitrogen fertiliser
Ammonium nitrate or ammonitrate, obtained chemically by the reaction of a mixture of ammonia gas and nitric acid, is a fertiliser used extensively to improve agricultural production. Its manufacture, which requires heat, consumes a significant amount of energy and yields a mixture of two ions in equal proportions: ammonium ion (NH4+) which can attach to the clay-humus complex of the soil and nitrate ion (NO3-) which remains soluble in the soil solution.
These two forms of nitrogen combined in a single product have effects not only on plant growth, but also on the soil and the environment.
Nitrate and ammonium: multiple effects
The nitrogen in the ammonium compound must be converted into nitric nitrogen due to action of soil bacteria, when the soil temperature is suitable. This biological reaction creates protons, thus making the soil more acidic locally.
Nitrate ions, being highly mobile and soluble in soil water, can be assimilated directly by the root system of plants as long as they are in the rhizosphere zone.
Nitric nitrogen or nitrates are water-soluble and cannot be retained by the fixing power of the soil due to their negative ionic charge. Without root assimilation, in a soil with low biological activity and prone to erosion and runoff, the nitrates will be carried deep down into the soil. Their concentration in water is the leading cause for the pollution of groundwater, and in some cases, even its eutrophication.
Solutions to counter the carry-over effects of ammonium nitrate
There are several solutions to combat the carry-over effects of the use of ammonium nitrate and to optimise its efficacy, as required by the Nitrates Directive, updated according to the geographical area.
It has been noted that sulphur fertilisers provide good plant nutrition. In fact, sulphur and nitrogen penetrate into plants synergistically and an input of sulphur optimises the nitric nitrogen nutrition of plants.
Liming, even on the surface, will be an effective solution to combat acidification due to the conversion of ammonium ions into nitrates. Liming will therefore have a positive effect on biological activity.
Finally, all the practices aimed at increasing the biological activity of the soil (conservation of residues, reduced tillage, eco-fertilisation, storage of organic matter, etc.) will help to combat erosion and will enhance the structural quality of the soil. These two conditions are essential to ensure the efficacious use of ammonium nitrate in an eco-friendly fertilisation path.