you can use egg shells, coffee grinds, corn syrup deluted in warm water, cow shit will work as a top coating and hard wood ash like from your fire place. these are all good things to use that you may either have around your house or that you can usually get for free.
the only thing you need to keep in mind is that the coffee grinds might bring up your ph, the corn syrup might attract ants, and the cow shit has to be rotted not fresh. alittle lime, ant protection and smelly flowers planted near by will solve those minor things.
use them all at once or each seperately it don't matter but just in moderation so you don't mess up your ph.
good luck with your grow!!!!