Potting soils are generally made out of light materials known as “soilless medias”. Almost every gardener I’ve run into that mixes their own has their own recipe, but here is a basic one to get started with:
- 1-2 parts plant compost, homemade or from a trusted source.
Plant compost can be purchased or made at home in a backyard compost bin. Make sure the compost is mature before use. It should be broken down into unidentifiable bits and have a pleasant “earthy” aroma. Composted animal manure can be substituted for up to half of the total compost.
- 1 part peat, or coco coir.
Since the peat and coir are used interchangeably, a combination of the two may be used. For example, ½ part peat and ½ part coir.
- 1 part perlite, or vermiculite.
The compost and coir hold water well, and adding perlite or vermiculite helps lighten the mix.
It is optional, but helpful to add a healthy helping of earthworm castings.
Another option is to add any quantity of a high-quality commercial potting mix. Several of the beneficial additives in these mixes can be helpful in even small amounts, so including some with your homemade mix can improve the overall quality of the product.
Mix the three (or four) ingredients well, and use as you would normal potting mix.
There are amendments that you may want to consider, which are used in much smaller amounts.
By adding nitrogen, phosphorus, and potassium sources to the starting mix, reliance on early additional fertilization is reduced. As these initial sources are used up, additional nutrients can be top dressed and watered into the mix or replaced by normal plant feedings.
Nitrogen additives such as alfalfa meal, blood meal, cottonseed meal, seabird guano, feather meal, or fish meal can give the plants a starting source of nitrogen.
Phosphorus can be added in forms like rock phosphate (very long term and slow release, so a good choice to add to the starting mix), bone meal, or high P guanos and manures.
Potassium is found in potash and langbeinite.
Silica sand or rock powder can be included to add weight to the mixture and for the slow release of trace minerals over the course of the season.
Kelp adds many helpful micronutrients, beneficial plant hormones, and small amounts of other plant growth aids.
Limes such as shell meal, limestone, or dolomite can be used to raise pH in acidic conditions (such as mixes using peat), and are a source of calcium (and in the case of Dolomite, magnesium).
Adding mycorrhizal fungi powders can introduce a colony in an otherwise sterile mix.
Starting with a good potting mix can make a world of difference in how well a container garden performs.