Crush Nutrient AdditionsWhy add nutrients at crush?
Nutrients are required by yeast to ferment sugar into alcohol. The primary yeast nutrient is ammonia (NH3), added as Diammonium Phosphate (DAP).
There are lots of other amino acids, minerals and other so-called micro-nutrients required by yeast that may or may not be present in the juice. In this addition calculator they are referred to as "Generic", since you're adding a rate of the overall mixture, not a particular component of the mixture.
If yeast don't have enough of a particular nutrient, the fermentation will likely become stuck, so you want to make sure you give the yeast the food they need to complete the fermentation. Adding excess nutrient, however, can leave behind enough nutrients to encourage unwanted microbial growth.
A good rule of thumb is higher nutrient levels for higher brix levels, and also higher nutrient levels for moldy fruit.