Is “potting soil” the same as the soil in my yard? Are both equally okay to use indoors?

Simply, no.

The soil in your backyard is most likely a dynamic mixture of sand, silt and clay – depending on where you live.

Most potting soils sold at garden stores aren’t ‘soils’ at all, but a mix of ingredients. Together, these ingredients can smooth the road to good plant growth. (Topsoil sold at garden centers, however, is more like the soil in your back yard, and the following comments do not pertain to it.)

A poor quality retail growing medium next to a standard product - fb
Tomato growth in a hobby product from the U.K. compared to the market leading brand.

Potting soils are the better choice for indoor plants and starting seedlings for transplant. A good potting soil allows good, constant plant growth.

Good potting soils have an open structure, allowing air to the roots, but at the same time capturing water, essential for plant growth. They need added plant foods, and need to have a suitable pH, the balance between acid and alkaline conditions.

Backyard soils don’t have the right structure or nutrients for indoor plants – and you have the added risk of bringing in undesired weed seeds, diseases, and pests.

coir blocks and reconstituted coir - fb
Compressed coir bricks (for export), and coir reconstituted at a growing media producer.

Many potting soils contain peat as a major component. Peat is the layered accumulation of partially degraded organic material over hundreds of years. Although peat covers about 20% of the surface of the globe, in some countries, peatland habitats have become rare. The harvest of peat may also release additional carbon, possibly contributing to climate change.

For that reason, buying a potting soil that contains less peat and more coir may be a more environmentally-friendly choice. Formerly regarded as a waste product, coir (pronounced “koi-er”) is the outer husk of coconuts. Sri Lanka, India, Mexico, and Costa Rica have made this a valued export to European and North American markets. Once the husks are washed to remove natural salts, coir is dried and compressed. In the country of import, it can be wetted and it then swells to make a good potting soil base. However, coir is about double the price of peat, and its import via ships and containers also contributes to carbon dioxide in the atmosphere.

Coir wets up and retains water well – even better than peat for some uses, especially for seeds and cuttings from plants. In Europe it’s becoming the potting soil of choice for the rapidly-expanding strawberry market.

Bark and wood fiber are other options in potting mixes. They are lightweight and easy to transport. But their age may affect their usefulness. Bark needs to be aged, and potting soils with wood fibers need to be used within months. During their time on the store shelf, bagged potting soils with a high proportion of wood fiber may lock up nitrogen in the wood. That means that once you open the bag, the nitrogen is not available to your plants, starving them of this vital nutrient.

Other composted materials, when not aged or properly matured, can seriously harm plant growth. The same qualities that make them good as weed-killing mulches make them toxic for young plants!

For seeds and young plants, it is always worth buying a good potting soil. In both the USA and Europe, research with both peat and other materials has led to high quality formulations of potting soils for the hobby gardener. In the USA, the Mulch and Soil Council, a manufacturer’s association with independent testing at North Carolina State University, now sets standards for potting soils. Hobby gardeners should choose certified products.

Answered by Bill Carlile, Bord na Mona (translated as “Peat Board”), Ireland.

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