Question
Thankyou for your reply. In the nitrogen cycle when a plant takes up ammonium nitrate it goes from ammonium to nitrite to nitrate, the hydrogens that have been lost then have an acidifying effect. Which on a pH8 will improve the soil in the shallow rooting zone. How does urea behave in the soil to get to plant available form, will it to help the soil? I think it takes longer to convert but am not sure if it or AN has negative effects on the plant/soil relationship.

Both AN and urea will be applied in granular form with suphur combined in the same granule. e.g. AN as 27N+30S03 OR 27N+12SO3 or Urea+sulphur as 40N+14S03 OR 38N+18SO3 Costs are similar, would like to use ammonium sulphate but just to expensive. I can't decide what product to choose, any ideas? I am looking to apply suphur for the crop to because as the atmosphere is being cleaned up plants are showing a requirement for it.

Any help would be appreciated.
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Followup To
Question -
On a soil with pH 8 which is best form of nitrogen in order of preference? (This would be for a crop of wheat).

Urea
Urea and sulphur
Ammonium Nitrate
Ammonium sulphate
Answer -
There are advantages and disadvantages to using the different products:

1) Ammonium nitrate (34-0-0)
contains nitrogen in both the ammonium and nitrate form,
less subject to volatilization losses than urea when surface applied without incorporation, more subject to leaching losses on sandy soils.

Ammonium sulphate (19-3-0-22S; 20-0-0-24S granular products)
22-24% sulphate-sulphur which is the plant available form,
used for direct application as both a nitrogen and sulphur source. They are more acidifying than other nitrogen fertilizers which would help lower your pH. However, they are subject to volatilization losses when not incorporated into the soil on high pH soils (pH 7.5 or higher).

Urea (46-0-0)
highest analysis dry nitrogen fertilizer. This fertilizer is more subject to volatilization losses than ammonium nitrate 34-0-0 when not incorporated into the soil. Significant losses can occur when urea is applied under warm dry conditions to sandy soils, and on alkaline soils (pH 7.5 or higher). This would be a concern on a soil with a pH of 8.0 unless incorporated and watered in immediately.

Ammonium nitrate may be your best choice unless you have very sandy soils. But expensive.

Ammonium sulfate will help lowering pH most efficiently and can thus also be a good choice, but must be incorporated based on your pH.

Urea is typically the cheapest option, but risk of volatilization is high.

If applied properly, all of the above N fertilizers will have an equal effect on wheat yields.

Regarding the volatilization issue:
There is a potential for N loss if urea (46-0-0) or urea-ammonium nitrate (28-0-0) is broadcast on the soil surface without incorporation when soil pH is higher than 7.3, or air temperatures are high. Shallow incorporation of urea or fertilizers containing urea within 48 hours of application is encouraged when these N sources are used for wheat production.

I hope this helped you a bit.

Answer
Urea is an organic compound which is converted by mirobes in the soil to ammonium. Then it follows the nitrogen cycle. The conversion is usually pretty fast especially at temperatures above 60 deg F. A few days difference at most.

The problem using urea on alkaline soil is that it becomes volatile as the microbes convert the nitrogen into ammonium (ammonia due to the high pH) which then escape to the atmosphere as ammonia gases. This can be eliviated by incorporate the urea into the soil after applying and watering in immediately.

Your choice of nitrogen carrier would depend on your ability to incorporate the material into the ground and soil type. If you have rather sandy soil bases I would chance it with urea. If you are sowing on loam or loamy clay then I would go with ammonium nitrate. Adding nitrate to sandy loam or loamy sand is bad news.

Before applying, have a soil test done to determine existing fertility.