Ok, this may be an odd question, but I know someone out there has the answer to this. I am going to be adding as an additional service to my mulch blowing business, which will be the ability to add any type of granular substance to the mulch while I am blowing it in. The first product I would like to offer is a slow release fertilizer. This is the question I have: I would like to add in a cheap "filler" material (preferably lower density than fertilizer) to the fertilizer in order to make the mulch/fertilizer mixture come out more evenly. I think I can do it without adding the filler, but I don't have much room for error. My first thought would be a granular limestone - I thought someone else would have another suggestion on what to use. This filler material would be mixed with the fertilizer before adding the mix to the mulch. Don't know if this makes a difference - most of the material I use is shredded hardwood based mulch. Any suggestions/opinions would be greatly appreciated.