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Sand will add air space in the potting mix, improving drainage in the dirt. Choose builder’s sand, which has a coarse texture. Do not use fine sand or plaster sand, as these are too fine and add a denser texture. Perlite is a good sand substitute. Made from volcanic rock, perlite is pH neutral and can improve how water drains from the soil. It is not as heavy as sand. Peat moss, or sphagnum moss, improves water retention in your potting mix. This can be especially helpful for plants that require more water over a sustained period of time. Peat moss is readily available at garden centers and does not cost much.  Peat moss does have high acidity, however, and may need to be counterbalanced to improve the pH balance in your soil. You can also use ground-up newspaper in place of peat moss, which will also help retain water.  Coir fiber is another option to take the place of peat moss. Coir is the fiber from coconut husks and will improve water retention. It is usually sold at garden centers as a compressed brick that expands when moistened.  Bark can also be used as a partial substitute for peat moss. It creates a lot of air space in the soil, although it does not retain water quite like peat. Some experts suggest avoiding bark, however, because it can immobilize nitrogen, a crucial component of good potting mix. Vermiculite is a volcanic rock material that is silvery gray in color. It is coarse, resembling mini pebbles, and it can improve water retention. Handle vermiculite gently to ensure that it does not lose its air holding capacity.  Choose either a medium grade or coarse grade of vermiculite. Good rich potting mix requires fertilizers and nutrients to give your plants food to help them grow strong, healthy and productive. Some of these materials might include blood meal (for nitrogen), bone meal (for phosphorus), greensand (for potassium), and other minerals. These are all readily available at garden centers.  Limestone is another typical nutritional ingredient. Limestone is used to increase calcium or magnesium levels in the potting mix. Dolomitic limestone is best to get a combination of both calcium and magnesium in your soil. The exception to fertilizing would be if you want to use your potting mix to start seeds. Skip fertilizers for sensitive seedlings.

Summary:
Purchase or acquire sand. Get peat moss. Get vermiculite. Assemble fertilizers and nutrients.