Commercial potting mixes are formulated to meet the widest possible range of houseplant-growing requirements. It is handy and efficient, but if you are growing specialty plants, you may want to make your own indoor plant soil. You will be able to change the ingredients to increase or decrease moisture retentiveness, manipulate pH and adjust specific nutrients for individual plants. Additionally, you can create a custom germinating mix for seeds, transplants and cuttings.
Prepare Your Soil
Obtain sterilized loam soil at garden centers and nurseries. Look for bagged product, often called garden soil or topsoil.
Sterilize the soil if you are using material acquired from your backyard. According to Barb Fick, home horticulturist with the Oregon State University Extension Service, soil can be sterilized in your home oven. Preheat your oven to 250 degrees Fahrenheit.
Spread moist soil on a heatproof pan and cover it with foil. Test the soil’s temperature with a food thermometer. It must reach and maintain a temperature of 180 degrees Fahrenheit for 30 minutes. This process will kill weed seeds and soil viruses.
Plan for Water and Aeration
Use peat moss to aerate the soil and improve its water-holding capacity. Moisten peat moss thoroughly before mixing with other ingredients.
Improve drainage with coarse builder’s sand. Sand is inexpensive but heavy, and does not improve water retention. Do not use sand with clay soils.
Add perlite for lightweight aeration and drainage. Perlite does not hold water and tends to float to the surface of containers. Soak perlite before using to cut down on dust.
Consider horticultural grade vermiculite for aeration, drainage and water retention. Vermiculite can compact over time, reducing its effectiveness.
Make Your Mixes
Mix equal parts soil, peat moss and sand, perlite or vermiculite.
Adjust the texture of your mix. If medium feels sticky, add small amounts of sand and peat moss. If the mixture feels too gritty, increase the amount of peat moss.
Make a soilless seed starting mix by mixing equal parts peat moss with perlite or vermiculite. This lightweight mix allows seeds to germinate and develop roots quickly.