Sometimes dirt is too dirty. For starting seeds or growing perennial plants in containers, a soil-free potting mixture can provide better nutrition, support for root development and drainage than soil from your garden. Soilless media also protect new or vulnerable plants from soil-borne diseases and other harmful organisms. Making your own soilless fertilizer mixture lets you provide exactly the nutrition your plants need.
Why Make Soilless Mix
One reason to make your own soilless mix, Cornell University and University of Arizona extensions point out, is that the contents of commercially available seed-starting, potting soil and soilless planting mixtures are not regulated by any state or federal agency. Labels may indicate ingredients in order of amount, but proportions are not required. Ingredients can include many organic and inorganic materials, unspecified amounts of fertilizer or none at all. Mixes for organic gardening should contain endorsement from the Organic Materials Review Institute. The only yardstick for measuring quality is the performance of your seeds or plants. You don't know, and your seeds or plants may not grow well in, what you're buying.
Other Do-It-Yourself Reasons
Making your own mix lets you expand gardening territory beyond the size of your garden beds. Soilless mix is excellent for indoor or greenhouse seed-starting and extends your gardening season as well as its scope. Soilless mix may also be a nutritional or moisture-managing improvement on your garden soil. Containers of soilless mix may allow you to grow plant varieties that do poorly in your soil. Soilless mix also lets you control plant nutrition in ways you cannot in garden beds. Testing and maintaining the pH of your mix guides you to providing the exact nutrients plants need.
Components of Soilless Planting Media
According to Cornell University Extension, peat moss, alone or mixed with shredded pine or fir bark, sand, vermiculite and/or perlite are all you need to make a soilless planting mixture. Peat moss/bark provides a mineral component, while sand enhances drainage capacity and vermiculite or perlite both increase air spaces and retain moisture. Cornell's several recipes begin with a basic mix of peat moss and vermiculite or perlite in a 1:1 ratio. A lighter seed-start mix can be compounded from 1 part peat, 1 part vermiculite and 1 part perlite, with smaller amounts of ground limestone and fertilizer added. Increasing the proportion of peat moss/bark produces a heavier mix for containers and shrubs.
Matching the Mix to Plant Needs
A soil's pH determines nutrition available to plants. While many kinds of plants thrive in neutral planting media with a pH of 5.5 to 6.5, conifers obtain the nutrients they need from acid soil with a pH below 5.5, while lilacs (Syringa spp.) need the nourishment of alkaline soil with a pH above 7.0. Each component of homemade soilless media contributes to the ultimate pH of the mix. Peat moss is acidic, with a pH of 3.5 to 4.5, while vermiculite has a pH of 6.5 to 7.2 and perlite has a pH of 7.0 to 7.5, both considered neutral to alkaline. Before adding ground limestone to increase alkalinity or fertilizer, you need to know the pH of your mixture. Using commercially available test kits of pH probes, or sending your mixture to a lab, lets you adjust your mixture for what you want to grow before planting. Your local county extension can also advise you on mixtures for your plants and local growing conditions.
Once soil testing is done, you are ready to add fertilizer. This step is not always recommended for short-term seed-starting. If you need fertilizer, Cornell recommends a slow-release balanced 5-10-10 fertilizer, with the added caution that your fertilizer should supply a full range of nutrients: nitrogen, phosphorus, potassium, lime, calcium, magnesium, sulfur, iron, manganese and zinc. Additions like ground limestone can be mixed together, then incorporated into the mix.
A Small-Quantity Recipe
Pennsylvania State University Extension suggests combining 2 gallons each of peat moss and vermiculite or perlite in a large container, then moistening the mixture thoroughly. For this quantity of mix, 4 teaspoons of ground limestone will have an impact on pH. Fertilizer amounts can also be measure in teaspoons, according to package labels.
Rub your mixture through 1/2-inch hardware cloth to break up all lumps and make it light. Warm water moistens soil mix more easily than cold. Remoisten stored leftovers before using.
- Pennsylvania State University Department of Horticulture: Homemade Potting Media
- Mohave County Arizona Cooperative Extension: Potting Media for Containers
- Washington State University Clark County Extension: Fun With Containers
- University of Idaho Extension: What Your Substrate Is Trying to Tell You, Part 1
- Cornell University Extension: Container Growing Mediums and Amending Garden Soil
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