Someone mentioned urea in a previous post, but I couldn't find it to reply. Thought I'd share this regarding nitrogen and urea - from The Alcohol Textbook:
"Ammonium ion, supplied as ammonia or the sulphate or the phosphate salt, is a preferred nitrogen source for yeast cultivation in laboratory studies. When urea is utilized as the nitrogen source, biotin or other growth factors may be required. Urea, which is not a normal constituent of mashes, is easily broken down into two molecules of ammonium ion and one molecule of carbon dioxide. Many countries have now banned the use of urea as a yeast food ingredient for potable alcohol manufacturing because it leads to the production of small amounts of urethane (ethyl carbamate). This compound is a suspected carcinogen in foods (Ingledew et al., 1987)."
Note: Miracle Grow contains urea.
"The nitrogen content of a yeast cell varies between 3-9% (w/w) and one can thus make a rough calculation as to how much suitable nitrogen nutrient would be needed in a fermentation."
The example in the book gives a ratio of 1.7 grams of DAP per liter of mash to achieve 360 mg/L of usable nitrogen. You can adjust this according to the nitrogen levels of your mash constituents. Wheat for example has 64 mg/L of usable nitrogen, whereas sugar has zero. This is the formula I have used and seems to work fine for my wheat mashes.
I have followed a recommendation that I read somewhere (can't remember where) to break the DAP usage into three separate additions: at the start of fermentation, at one third sugar depletion, and at two thirds sugar depletion. I usually have to add just a little more in the last 24 hours to counter sulfur production. Still trying to find the magic number to avoid it altogether.
DAP is alkaline and will raise the ph of the mash, so I add it before making ph adjustments with potassium carbonate.
Lastly, I have also read that DAP is not recommended in the yeast hydration medium because it is detrimental to the yeast during hydration. I use Go-Ferm for hydration and though I can't quantify its effectiveness, I figure it can't hurt and I have had consistently successful fermentations using it.