We fill all of our raised beds with a slight variation of the “Mel’s Mix” (1/3 Vermiculite, 1/3 Peat Moss, 1/3 Compost) found in Square Foot Gardening (Here’s a video showing how we mix it together easily!). The most important ingredient of the soil mix is compost: decayed organic matter that’s nature’s free source of food for plants. As living things die and break down, the nutrients within them are returned to the soil as compost.
We realized soon after starting that we were going to need A LOT more compost in order to expand our garden. We built our first compost bins by zip-tying shipping pallets together. Instead of making our own compost, we saved some time and filled them with compost from a local municipal compost facility. We now make all of our own compost by combining as many sources of organic material as we can. Here are some examples of the types of free “ingredients” we use: used coffee grounds picked up from Starbucks/7-11, wood chips from the local municipal wood chip pile, leaves collected in the fall, plants I pull up or chop down from the garden that aren’t diseased, grass clippings, and kitchen scraps.