Diammonium phosphate
DAP
Inorganic nitrogen for the yeast, the direct fix for a starved ferment.
Typical dose
Staged, commonly up to about 1 g/L in total, split across additions rather than dumped in.
When to add it
In stages, at the start and around a third of the way down, not all at once.
In practice
Pure, cheap, yeast-available nitrogen, and the direct answer to the hydrogen sulphide a nitrogen-starved ferment throws. Added all at once it drives a hot, aggressive ferment and can leave a residue; staged, it carries the yeast through the risky middle third. It supplies nothing but nitrogen and phosphate, which is why complex nutrients exist.
Linked faults are the ones this addition either treats or, used wrongly, causes. The fault page says which, and whether it can be undone.
Others in nutrients and enzymes
Complex yeast nutrient
Nitrogen plus the amino acids, vitamins and sterols the yeast also needs.
Rehydration nutrient
Primes dry yeast with micronutrients before it is pitched.
Thiamine
A micronutrient that helps a ferment finish and lowers SO2 binding.
Yeast hulls
Detoxify a stuck ferment and give the yeast a surface to work from.
Pectic enzyme
Breaks down pectin to clear haze and free juice and colour.
Glucanase
Breaks down Botrytis glucans and speeds lees ageing and filtration.
Beta-glucosidase
Frees the bound aroma compounds locked up in aromatic white grapes.