Many amateur gardeners know that compost provides a readily available soil amendment, but few are aware that it can be used to create a nutrient-rich potting soil. Whether you're on a tight gardening budget or you simply want to explore your composting options, making your own potting compost provides an efficient, low-cost source of potting soil for bedding plants and potted plants. The University of Minnesota Cooperative Extension cautions that pure compost should make up no more than approximately one-third of the volume of your potting compost to minimize potential problems such as waterlogged soil and lack of adequate root aeration.
Gather equally sized heaps of fresh grass clippings and dead leaves at a well-draining, sunny area of your property. Mix the grass waste and dead leaves together thoroughly with a garden rake, then build the mixed materials into a 3-foot-by-3-foot-by-3-foot heap. Wet the heap down with a gentle spray of water from your garden hose to make the waste about as damp as a wrung-out sponge.
Stir the heap of compost with your garden rake once or twice weekly to aerate the waste and speed up the composting process. Monitor the moisture level each time you mix the waste by squishing a handful of it tightly in your fist; you should be able to wring out no more than two drops of moisture, as recommended by the University of Illinois Cooperative Extenson. (see reference 2, "moisture factor" section) Repeat these maintenance techniques until your compost is finished, or mature, a condition indicated by the presence of fluffy, dark brown, earthy-smelling humus material; generally, this process takes approximately two to six months to complete.
Measure 5 cups of the finished compost into a 5-gallon bucket. Add 5 cups of plain sand--generally available for purchase at your local garden or home improvement center--to the bucket, followed by another addition of 5 cups of plain topsoil. Mix the three materials together thoroughly with a hand trowel; blend the ingredients well by scooping and dumping the waste back into the bucket then moving the trowel in a circular motion through the materials for up to five minutes. Sift the potting soil through your fingers to ensure that the materials are thoroughly mixed together.
Pour the potting compost into bedding plant containers or pots, filling each container about 7/8 full of the potting soil before planting and watering seeds, seedlings or transplanted houseplants. Mix additional potting compost soil up in the 5-gallon bucket using equal parts of the leaf compost, sand and topsoil.