Ammonia: the exception!
As with all rules there are exceptions and no more so than when it come to the environment. Whilst not included in any of the previous groups Ammonia, an inorganic compound that is the product of biological processes, can be extremely significant in modern agriculture. A strongly basic cation Ammonia can occupy a significant proportion of the cation exchange. furthermore Ammonia can have a significant influence on soil pH. Whilst ammonia occurs naturally in the environment, it’s use in Agriculture often originates from a manufactured process.
As a component in Nitrogen nutrition Ammonia can be utilised directly by plants, but at elevated levels causes toxicities. More-often than not Ammonia is utilised by the microbial community who convert it to Nitrate (NO3-). However this process whilst converting the Nitrogen into a more accessible form also results in the production of Hydrogen+ ions which are strongly acidic and result in a rapid lowering of soil pH.
Whilst being natural and beneficial at low concentrations Ammonia is best described as a supplementary nutrient that should never be relied upon as a significant source for the Nitrogen nutrition of plants.
However it is the product of a highly motivated fertiliser industry and whilst there is plenty on the dangers of Ammonia itself there is little on publicly accessible forums at least to warn about the environmental and management consequences of using ammonia as a fertiliser. However here are the few I have found on local and global risks and management issues associated with Ammonia fertiliser use:
Friend and foe: Nitrogen pollution threats environmental and human health threats
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011