Accounting for commercial fertilizer is an important part of a nutrient management plan. Fertilizers can be entered and modified from the Fertilizers tab. This tab contains a data table where fertilizer data lives:

SnapPlus includes the following options as "standard" fertilizer sources:
Standard fertilizers cannot be modified once they are entered. If you have a custom blend, you'll have to enter it using the Add custom fertilizer button. Only a name is required to save a custom fertilizer to a nutrient management plan in SnapPlus V3.