When to Use Nitrogen – Which Source of Nitrogen is Best

Q.  I want to apply nitrogen (8 oz/30′ row) after transplanting seedlings as directed.  My problem is that the recommended 34-0-0 ammonium nitrate is difficult to find.  This leaves 21-0-0 or 46-0-0.  Since the object from what I understand is to jumpstart the growth, I would think 46-0-0 would be better, with more than double the nitrogen of 21-0-0.  My question is what rate should I apply it since I don’t want to burn the new plants?. 
A.  Nitrogen fertilizer is available by itself in three commonly available compounds.  They are ammonium nitrate (34-0-0) of Oklahoma City infamy, which is the best source of nitrogen, ammonium sulfate (21-0-0), which is usually the second choice, and urea (46-0-0) which is usually the last choice.
Do NOT use nitrogen by itself, except for two times, 1) immediately after TRANSPLANTING young plants into the garden, at which time it is good to give your plants a boost to help overcome transplant shock and give them a quick start.  The amount to use is 1/4 ounce per running foot of soil-bed, or 8 ounces per 30′ bed.  And 2) if you see a nitrogen deficiency in your growing plants.  Consult the Garden Doctor books for deficiency corrective treatments.
Which compound is best to use?  If the weather is cold when you transplant, the 21-0-0 and 46-0-0 will not perform very well.  However, we do not recommend you increase the dosage, but we do encourage you cover urea with some soil.
Here is a brief recap of the reasons why 34-0-0 is the best and 46-0-0 is the worst nitrogen source.  34-0-0 is NH4NO3, including an ammonium portion and a nitrate portion.  The nitrate portion, NO3, is immediately available to plants, and that’s why this source is the best, especially in cold weather.  The NH4 portion must go through a chemical change to become available, and that doesn’t happen much in cold weather. 
21-0-0 is NH4(SO4)2, and thus has only the ammonium portion.  But it does have sulfur, which is an essential nutrient, and will tend to lower the pH, so if you are in a low rainfall area and the weather is warm, 21-0-0 can be good.  Remember, if you are in a high-rainfall area, that 21-0-0 can exacerbate your low pH acidity problems.
Urea – while it shows 46-0-0 as a percentage of nitrogen, has a chemical formula given two different ways – as CH4N2O, or CO(NH2)2 – both of which mean that the nitrogen is not in a form that plants can use it.  Instead the urea must undergo multiple chemical changes before it becomes available to your plants.  Meanwhile, because nitrogen in all it’s compounds is volatile and returns to the air, urea nitrogen is much more susceptible to loss in this way, and farmers who use it are counseled to apply it only beneath the soil surface, to minimize losses during the chemical changes.
Keep agitating your suppliers, and even your lawmakers, to make 34-0-0 available, and in the meantime use the others the best you can.
Two other alternatives you might consider, if they can be found, are calcium nitrate, which can also help with your calcium needs, and potassium nitrate – both of which can provide nitrogen in the NO3 form.  However, they are usually  more expensive than the other alternatives, and are primarily used in the greenhouse/hydroponic industry.
No matter which nitrogen source you use, remember to apply it, as well as all other fertilizers about 4″ from the plant stems and water them in thoroughly.