https://byo.com/mead/item/1908-yeast-nutrients-techniques

Diammonium phosphate, or DAP, (a commonly used plant fertilizer) supplies nitrogen and phosphorus to yeast, but nothing else. Therefore, DAP is useful as a basic yeast nutrient to supply nitrogen and phosphorus, but will not help rectify a deficiency of any other essential mineral or compound. DAP can be added to the wort during the boil, or added to the boil of a yeast starter wort. A half teaspoon of DAP is generally recommended for addition to a 5-gallon (19-L) batch of wort. Barely a pinch is all that should be added to starter wort of a gallon (3.8L) or less in size if it is needed.