Mixing compost with potting soil allows gardeners to stretch expensive commercial potting soil without sacrificing the health of their plants. Not only does compost provide a boost of nutrients for bedding plants and seedlings, but it also improves the drainage of commercial compost, making it easier for your plants to access the moisture necessary for proper growth. Don't allow compost to account for more than approximately one-third of the volume of your potting soil. According to the University of Minnesota Cooperative Extension, this proportion of compost helps minimize potential aeration problems and keeps your plant soil from becoming overloaded with water.
Measure out the amount of compost soil that you need; for example, if you need 12 cups of potting soil, measure out no more than 4 cups of compost soil. Check the consistency of the compost; it should be about as damp as a wrung-out sponge. Pour the dampened compost into a 1/2- or 1-gallon black plastic garden pot and encase the pot in a clear plastic bag. Seal the plastic bag with a twist-tie.
Place the sealed plastic bag of compost in direct sun to sterilize it using the energy of the sun, as recommended by Judy Pray, author of the book "Garden Wisdom and Know-How." Leave the compost to sit for at least two weeks to allow the sun's heat to destroy weed seeds that could compete with your seedlings for nutrients.
Sprinkle two cups of commercial potting soil in the bottom of a 5-gallon bucket. Measure out 1 cup of your sterilized compost soil and add it to the 5-gallon bucket. Stir the two ingredients together with a trowel; use circular motions and mix the materials until they're thoroughly blended.
Dump the blended potting soil and compost soil into your second 5-gallon bucket. Repeat the blending process with additional potting soil and compost soil until you have a sufficient amount of enriched potting soil for your planting needs.