All good potting media should meet the needs of plant roots for air, water, nutrients, and support.
Some common ingredients allowed in organic potting media include:
Soil. For many years, the trend in conventional growing has been toward soilless media. A major reason for this is concern about soil-borne plant diseases and the excessive density of mixes where soil is a dominant ingredient. However, soil is still used in some organic blends. Clean commercial topsoil is an acceptable natural ingredient, but you have to be certain that it has not been treated with prohibited ingredients to kill microbes and weed seeds.
If you are using soil from a farm or garden, use only the best. Consider solarizing, steam pasteurization, or oven heating if the soil has any history of soil-borne diseases.
Sand. Coarse sand—called builder’s or river sand— is best. It adds air spaces to the potting mix. Avoid plaster sand and other fine sands. They tend to settle into the spaces between the other ingredients and make a dense mix. Clean, washed sand has a near-neutral pH and little if any food value for plants. Sand is much heavier than any other ingredient used in potting mixes. The added weight is good for tall, top-heavy plants that might blow or tip over. Sand is the least expensive and most readily available larger-particle material.
Organic Compost. Compost is perhaps the most common potting-mix ingredient among organic producers. Compost holds water well and provides nutrients. The quality of compost depends in part on how it is made, but especially on what it is made from. Composting is not difficult, but it does require some experience and a variety of clean, organically acceptable components. Compost is rarely used alone as a potting medium, because most composts are too porous and the soluble salt levels are often high.
Coir. Coir is a by-product of the coconut fiber industry. Most coir comes from India, and Sri Lanka and is typically shipped in compressed bricks, which expand when wetted.
Newspaper. Ground-up newspapers can be used as a substitute for peat or coir. Newsprint should not be more than 25% by volume of the mix. Avoid the inclusion of glossy paper or paper with colored inks.
Perlite. Perlite is a volcanic rock that is heated and expanded to become a lightweightwhite material. It is sterile and pH-neutral. When added to a soil mix, perlite can increase air space and improve water drainage. It is a hard material that does not break apart easily. Perlite pieces create tiny air tunnels that allow water and air to flow freely to the roots. Perlite will hold from three to four times its weight in water, yet will not become soggy. It is much lighter than—and can be used instead of—sand.
Vermiculite. Vermiculite is a micaceous mineral that is expanded in a furnace, forming a lightweight aggregate. Handled gently, vermiculite provides plenty of air space in a mix. Handled roughly, vermiculite compacts and loses its ability to hold air. Vermiculite holds water and fertilizer in the potting mix. It also contains calcium and magnesium and has a near-neutral pH.
Sawdust. The quality of sawdust used as media depends on the wood. Cedar, walnut, and redwood sawdusts can be toxic to plants. Oak, hickory, and maple are reputed to tie up soil nitrogen more readily than sawdust from evergreens. Sawdust from treated or painted lumber is not allowed in organic production.
Bonemeal. Bone meal is a mixture of crushed and coarsely ground bones that is used as an organic fertilizer for plants and formerly in animal feed. As a slow-release fertilizer, bone meal is primarily used as a source of phosphorus. If you’re a vegetarian or vegan, then this is an ingredient you would probably want to avoid.
Some potting media recipes
Working from tried-and-true recipes is a good idea, especially when starting out, but experimentation is the only sure way of knowing which blend or blends will work best for a particular vegetable in the conditions you are able to provide it.
- 6 parts compost
- 3 parts soil
- 1-2 parts sand
- 1-2 parts aged manure
- 1 part coir (pre-wet ) or newspaper
Classic soil-based mix
- 1/3 mature compost, screened
- 1/3 garden topsoil
- 1/3 sharp sand or perlite
General vegetable mix
- 2 parts vermiculite
- 3 parts coir (or ground-up newspaper)
- 2 parts perlite
- 2 parts cow manure
- 3 parts topsoil
- ½ part bonemeal