What is the best product for planting in containers or do you have a DIY recipe? The products I have tried recently seem to resist water rather than absorb it, even though I start with a damp medium when initially planting.
The basic potting medium ingredients are either soil- or peat- based. What is needed is a growing medium (e.g., soil), something that retains moisture and nutrients (e.g., peat moss) and something to ensure proper drainage (e.g., perlite, vermiculite or sand).
For soil-based media, the basic recipe uses sterilized loam soil, sphagnum peat moss, and coarse sand (or perlite or vermiculite).
Soilless or peat-based media usually consist of peat moss plus vermiculite and/or perlite, along with added fertilizer. These mixtures are good for germinating seeds, and likely would not be appropriate for your plants.
For both types of media, ground limestone and fertilizer (slow-release fertilizers are useful) should be added. Also top-dress the plants from time to time with compost.
Penn State Extension’s Homemade Potting Media sets out the basic “recipe”, which includes the advice to adjust the texture of the mix by feel. eXtension’s Organic potting mix basics provides lots of detail about what should go into a homemade potting mix and why.
If you are concerned about the environmental impact of using peat moss, a relatively new alternative is coir, a coconut shell fibre that holds water and nutrients well and has a pH (acidity/alkalinity) that is suited to all plants. (One environmental concern about coir is that it is shipped a long distance to reach us!)
And if in the end you decide not to make up your own mix, consider speaking with your local garden centre about which commercial potting soils they recommend. Mention your concerns about the potting soils you have tried. They may have suggestions on how to “amend” the potting soil to maximize its effectiveness. Note that we are not permitted to recommend particular products.
Remember also to consider the soil in the context of the types of plants you will grow (e.g., do they require heavy fertilizing to thrive or are there any special nutrient requirements) and the pots you select (e.g., any growing medium you use will dry out faster in (porous) terra cotta pots than in plastic pots.)