While there are many choices of potting soil for sale, some even OMRI listed organic, I am completely hooked on making my own potting soil on the farm. I believe it is akin to whipping up a batch of minestrone from the garden vs. buying it in a can. It's not really practical, but does save a little money and guarantees that I have enough of a supply for the whole year's potting needs. Plus I know exactly what's in it and can tweak the recipe if needed.
So what is in potting soil? Much like minestrone, the ingredients can vary considerably based on your area, what you have access to, and what you are trying to grow. The basics are as follows:
A nice rich moisture-holding structure-providing backbone- I use coconut coir, but others may use a fine aged compost.
To this I add composted manure with some native soil from the farm. No matter what base you use, native soil should be used as an inoculant to keep it lively!
Perlite or vermiculite are the next ingredients, used to help hold and regulate moisture. Since the first two are finite resources and vermiculite has been know to be rough on young roots, biochar is a fantastic alternative. Biochar can be costly however, unless you make it yourself or have a great source nearby.
Sand is another ingredient that you can use to tweak the structure and moisture retention. If you're trying to grow succulents or desert-dwellers, sand should be used in abundance.
Minerals: colloidal rock phosphate, limestone, oyster shell, wood ash, Azomite, and others can be added in small amounts to ensure micronutrients are accounted for and help with pH. Our soil is slightly acidic here, so I do like to bring up the pH.
I also add a balanced organic fertilizer in small amounts. Some prefer a lot of fertilizer in their potting soil, but I don't like to overdo the food for seedlings. If you're growing hungrier species such as from the Solanaceae family, up the fertilizer!
There are many more additives to make a nice potting soil: shredded bark, spent grains, grass clippings, etc. For each and every one there will be someone speaking for or against it. Some people love to use only topsoil with fertilizer in it or only biochar. There are also a plethora of pre-made substrates for rooting cuttings and starting seeds for indoor grow operations. My advice is to start simple with what you can easily source and do your homework! Also use your senses to determine the quality of your soil- how does it feel, smell, look, hold moisture, etc? Your plants will let you know if they're happy or not, so you can always reassess and alter your ratios.
Mix all your ingredients together. When you think they are mixed, do it again for another 10 minutes or more to ensure a homogeneous end product. Then I rub the mix through a large stainless sifting screen (if you don't have this, make sure to remove any clumps) and away we go...
In the pic I have added two layers of ingredients and have one more to go before the final mixing.