Potting soil is different than soil-less potting mix which is used to germinate seeds. It took me quite a number of years to figure that out. But once I did, I started blending my own.
The best homemade potting soil mix has three basic ingredients:
1) a growing medium
2) something to help retain moisture and nutrients
3) something to promote drainage.
- A growing medium such as garden soil from a home and garden center, which has been pre-sterilized to remove weeds or disease
- Spaghnum peat moss for moisture retention. It is harvested from bogs that have been drained, so the moss has dried and turned a light brown color; you may need to lightly moisten before mixing the potting soil.
- Perlite, vermiculite, or sand for drainage. Perlite is made by heating bits of a glasslike mineral until they expand into puffy, lightweight particles. It holds no water, aside from the little that clings to the surface of each particle.
Mix those three ingredients in equal proportions, adding more of any ingredient until you have a loose but clump-able mix. Keep this mix in a covered storage container.
There's a second way to make homemade potting soil that involves fewer ingredients, and is preferred by organic gardeners. It is compost-based potting soil and it is made by simply mixing equal parts sterilized garden soil and compost (pre-packaged or homemade) and then add sand or pebbles to promote drainage.
Remember, while homemade potting soil is a great growing medium, your plants won't thrive unless you regularly add fertilizer to the the potting soil. You can do this in a number of ways. You can amend your homemade potting soil mixture with limestone before using it. You can also top-dress plants occasionally with any number of types of compost, such as recycled mushroom compost which is what I use. You can also rely on a fertilizer that offers slow-release nutrients in order to help your plants retain their growing vigor.
Martha Stewart's Potting Soil Recipe:
When mixing, start with a clean container, such as a galvanized washtub, large enough to hold all of the ingredients. Use a scoop or small pot to measure ingredients. It is very important that you wear a surgical mask over your mouth and nose while you're working, so you won't inhale dust particles, and that you lightly spray the mixture with water. (Don't overspray, or you'll end up with mud.)
Mix the ingredients together thoroughly and store them in covered, galvanized containers or plastic trash cans. Don't make too much soil mix at once -- over time, the organic content can spoil.
- 3 parts peat (screened). Peat is a natural organic material that helps soil retain air and moisture by increasing pore space. Aged, pulverized bark can be substituted.
- 2 parts perlite (horticultural grade). Made from volcanic material, perlite is a light aggregate that provides aeration, increases drainage, and absorbs little water.
- 2 parts topsoil (screened and sterilized). A complex mixture of minerals, organic materials, microorganisms, and pores filled with air and water. You can purchase sterilized topsoil, or sterilize your own from screened soil from your garden. To sterilize soil, place it in a shallow pan on your outdoor grill, until temperature of soil reaches 160 degrees F to 180 degrees (use an old meat thermometer).
- 1 part vermiculite. Vermiculite is derived from mica; it provides aeration and, unlike perlite, absorbs water.
- 1 part No. 3 coarse sand (not beach or play sand, which is too fine). Helps with drainage and aeration of soil; adds weight to keep pots stable.
- 1/4 part charcoal (horticultural grade). Keeps soil slightly alkaline and absorbs any dissolved salts in the soil; helps provide aeration and drainage.
- 1 cup bonemeal. Adds nutrients and phosphorus, necessary for good root and flower formation.
- 10 tablespoons Dolomitic lime. Helps balance pH of mix.
- 14-14-14 Osmocote (follow package instructions to determine amount). A time-released fertilizer; adds essential nutrients for good plant growth.
Welcome (almost) Spring!