Answer:
There are many different things you can add.
Explanation:
I have heard bat guano is really good, others are:
Manure. ...
Garden Compost. ...
Seaweed. ...
Leaf Mould. ...
Mushroom Compost. ...
Blood, Fish and Bone. ...
Bark. ...
Grit or Sand.
But I'm sure that there is many others.
