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South West UK | I totally agree; the rate at which Urea is converted into nitrate depends upon the state of the soil and the soil life. If it is cold, frozen or water logged it will be sat there as Urea for a long time- certainly measured in days or weeks in some conditions.
I consider Urea agronomically superior purely due to the fact it is not as instantaneous as using Ammonium nitrate, much less ammonia itself.
If urea is relying upon soil life to convert it, then it is surely proving safe for the organisms involved and can't reach levels that are detrimental to the organisms involved. We know that ammonia is poisonous to a lot of organisms if they are exposed to a high enough concentration (humans etc are no different).
In the UK it is not uncommon for people to be putting urea on land in February, certainly one of my dairy farmers was doing it just over a week ago. He is relying on the urea sitting there waiting for the soil to warm, at which point it will be begin to transform and just by happy coincidence it will be the exact same time the grass begins to grow, too. Compare this to AN and the change is far more rapid. Plants seem to respond to AN very rapidly, urea seems to be less instantaneous.
Downside is urea in very high temperatures can be volatile- and you risk losing a lot of your expensive nitrogen to the atmosphere so we don't use it at all in the hotter months when soils are very dry.
Most of our fertiliser is applied as granules or prills, some people do use liquid nirate based solutions though where they have a sufficient acreage to warrant it.
Edited by ollie1983 2/19/2015 20:13
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