Answer: You can mix your own potting soil, and you'll find it is much less expensive than the packaged kinds. The simplest recipe calls for mixing 1 part sphagnum peat moss with 1 part vermiculite. This is excellent for seed starting. Once seedlings develop their first true set of leaves, it's time to pot them and give them a different growing medium, one that contains some nutrients. For this mix, combine 1 part sphagnum peat moss with 1 part vermiculite, and add 2 parts sifted compost. The compost provides texture and nutrients, which is why fertilizers aren't needed. Instead, feed young transplants each time you water with a weak solution of liquid seaweed.
For container-grown ornamental and vegetable plants, the mix should include 1 part sphagnum peat moss, 1 part perlite and 3 parts sifted compost. This mix works especially well for those who garden on balconies or rooftops because it's relatively light. Perlite can be a bit costly, though, so if weight isn't a consideration, substitute coarse sand.
To 1 gallon of mix, add 2 tablespoons each of blood meal, bone meal and gypsum and 6 tablespoons of greensand. This combination of goodies provides all the nutrients plants need to grow and will last for months. The diet can also be supplemented with compost tea of liquid seaweed at each watering.
Vary the ingredients in this mix to address the specific needs of plants. For instance, for a plant that requires extra-rich soil, double the amount of compost. If the plant requires acidic soil, double the peat moss. For cacti or succulents that prefer drier conditions, skip the perlite altogether and double the amount of sand.
I mix everything in a wheelbarrow, measuring by volume: one 3-gallon bucket of peatmoss, one bucket of sand or vermiculite, two buckets of sifted compost or aged-manure. Mix these all together and then add bloodmeal, bonemeal, etc. (I use a 1-pound coffee can to measure these.) If you don't have a wheelbarrow, use a tarp or sheet of plastic laid flat on a driveway or lawn. Dump everything in the middle of the tarp and stir it together with your hands. Once the potting soil is well mixed, store it in bags in the garage or shed.
Best wishes with your garden!
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