What is a good formula for making soilless potting mix?

Years ago Cornell University published a simple and effective recipe for a soilless potting mix that works well for most plants. It is 1 bushel of peat moss, 1 bushel of perlite or vermiculite, 1/2 pound of dolomitic limestone, 1 pound of 5-10-5 fertilizer, and 1-1/2 ounces of 20 percent superphosphate fertilizer. Most commercial mixes now on the market began with this recipe and have since been amended to create a plethora of combinations suited to specific plants. There are special commercial combinations available for seed starting, houseplants, cactus, violets, orchids, and many many more for general container plantings.

You can create your own soilless potting mix using the basic recipe as a start, depending on your particular needs. For instance, using perlite allows for needed air pockets, along with quicker drainage and drying, but probably will require more frequent watering. Using vermiculite, on the other hand, creates air pockets, but holds moisture longer for those plants that do not like to dry so much between waterings.

Another option is to use composted woody fines or ground bark in place of some of the peat moss. The woody fines and bark absorb water more readily than peat moss. Coir (coconut husk) is also an excellent ingredient in place of or in addition to peat moss. Dry peat moss resists wetting and you will find that commercial mixes include a wetting agent to facilitate the wetting of the peat moss. You should always wet peat moss well before use, and if it ever dries completely be sure to water the container several times until you know the water is being absorbed and not just running off the top and down the insides of the container, as dry peat moss will shrink inward and pull away from the edges of the container, allowing too much runoff.

The addition of builder’s sand will create a mix that drains very quickly for cactus or succulents, and also add weight to the container if you have a tall plant that is prone to tipping over in a lightweight mix. In terms of fertilization, if you are less inclined to do it regularly, try substituting a slow-release fertilizer that will last for several months. If you are an erratic waterer, try adding some of the new crystals made of polymers that hold water and release it gradually. A way to help yourself decide what ingredients a particular mix should have might begin with reading the ingredient labels of the commercial products. You will get some good ideas for amendments that suit your special needs.

As you concoct your own version of soilless mix, keep a few things in mind. Your goal will be to have water move through the media quickly, leaving water absorbed by the media and at the same time leaving air pockets in the media to supply oxygen for the roots. The one big “don’t” is don’t use real soil from your yard or garden in the mix. It is heavy, doesn’t drain well in containers, and brings in pathogens and possible chemical residuals. You may also want to compare the cost of your chosen ingredients to the cost of commercial mixes. Making your own may or may not save money.