Nitrogen Fertilizer Sources – What’s Best?

Q.  I was browsing a store the other day and stumbled onto a 20# bag of Ammonium Nitrate 21-0-0. I bought it right away as I hadn’t seen any at other stores in my area. Isn’t this the best form of nitrogen for plants?
A.  What you called AMMONIUM NITRATE (34-0-0) is actually AMMONIUM SULFATE (21-0-0).  There is a rather substantial difference between them – even more than the % of nitrogen.  Following are some comparisons between the three most common sources of nitrogen.
AMMONIUM  SULFATE ((NH4)2SO4 – – 21-0-0-24S)
1.  Supplies two vital nutrients for crop growth – nitrogen and sulfur.  This can be good or bad, depending on whether or not you need the extra sulfur.  Sulfur lowers pH, and if you live in a high-rainfall area you do not want that.  HOwever, if you receive less than 20″ of rain per year, the sulphur may help improve your high pH soil conditions.  And the sulfur is in the sulfate (with oxygen) form, which is the only form in which it is available to plants.
2.  Low-cost and readily available most places, which means you can get nitrogen wherever you are.
3.  Only 21% nitrogen, and not immediately available to plants because it must go through a chemical change.  This can be good in some situations and bad in others.  If your plants need an immediate shot of N you may be disappointed – especially in cold weather – because the change happens very slowly in cold weather.  Conversely, in hot weather you may be happy with NH4 because the nitrogen is more stable and does not volitalize so quickly, thus providing nutrition for a longer time.
1.  Combines the stability of the ammonium form with the immediate availability of the nitrate form to provide immediate nitrogen – even in cold weather – with a much higher % of nitrogen than the sulfate form.
2.  Usually costs more than either sulfate or urea (46-0-0), and is not available in many places, because of the potential use in making a powerful explosive.
3.  Only provides one nutrient – nitrogen.  Again, depending on your situation, this can be positive or negative.  In places receiving more than 20″ of annual rainfall more sulfur is often not desirable, and therefore ammonium nitrate is the better solution.
UREA ((NH2)2CO – – 46-0-0)
1.  Not available to plants until several chemical changes take place – therefore the least favorable choice, especially if an immediate “fix” is needed.
2.  Losses from volitalization during the chemical changes make it very important not to apply on the surface of the ground. 
3.  Often the least cost, and with the highest % of nitrogen it appears to be the best value, but it is usually the least desirable of the three nitrogen sources.
Because we feed our plants weekly for several important reasons, the ready availability of the nitrate (NO3) form is our first choice, but any of the three above sources can be used.  Other compounds, such as calcium nitrate and sodium nitrate (mined naturally in Chile), are also good possibilities for using in your Weekly Feed mix, if they are more readily available or less costly.