To make your on fertilizer you need 5 decomposible items. These can be obtianed by digging like "You dig and find a carrot!".
Once these 5 items are obtained you can put them in your fertilizer box and shut it, the decomposing takes 12 hours and the end product is 25 fertilizer which can be built on top of the regular 100 just like voting.
We could probably have something like a little map of the garden and you get a certain amount of turns and you can use fertilizer to dig in the ground to find items.