What's your soil like to begin with?
Aged horse manure is great for corn,
peppers, tomatoes and squash...use less, or none at all, with mint and lavender, they like slightly poor soil, and you'll have enough problems with the mint running wild as it is.
Horse manure won't alter your pH enough to worry about for a while.
I like to add bone meal to boost phosphorous content, tomatoes in particular benefit from it.