What is Soilless Potting Mix?

Ignore the name on the bag – potting soil contains no soil.

To the casual observer, soilless potting mix may look like soil. But it’s not – it can’t be called soil because it has no clay or silt. There are several materials it may be made from, but typically, it includes some or all of the following: coco coir (the outer husks of coconuts), peat moss, composted wood chips (usually pine bark), vermiculite, and perlite. Some mixes also include builders sand and a slow-release fertilizer. Soilless potting mix is used for indoor growing, from starting seeds to greenhouse gardening to houseplants to hydroponics.

Why use soilless potting mix?

When growing indoors, controlling pests and diseases is very important. If you were to bring soil from your yard or garden indoors to grow plants, you’d also be transporting any number of weed seeds, insects, molds, fungi, bacteria, and viruses. Once indoors, these pests would multiply quickly with or without pesticides and herbicides to control them. It can turn into a disaster quickly. Soilless potting mix is sterile and avoids these problems.

You also don’t have to guess at the nutrition in a soilless mix. Unless fertilizer has been added (indicated on the bag as NPK ratios), there isn’t any nutrition. This is great for hydroponic growing since that method of indoor growing requires a finely tuned nutrient solution. In organic potting mixes, fertilizer may be listed as cottonseed meal, soybean meal, blood meal, crab meal, fish meal, or animal manures.

Soilless mixes are superior for root growth, too. This lighter-than-garden-soil medium allows germinated plant seeds and seedlings to root faster and develop larger, healthier root systems than if they were growing in a pot with clay soil. It also allows water to drain fast. In a pot with garden soil, water can be slow to drain – this can actually drown roots and cause fungal diseases, among other problems, especially if used indoors.

Much of the porosity is thanks to two ingredients: perlite and vermiculite.

• Perlite is the white stuff in the potting mix. It’s actually superheated and “popped” Obsidian, a type of volcanic rock. It’s very porous and provides tiny holes in the mix through which water, gases and air pass.
• Vermiculite is the gold-brown flecks you see in the mix. It’s superheated mica and has a high water holding capacity – it soaks up water and very slowly releases it back into the mix for the plants to use.

What is the ideal mix of ingredients?

Generally, most container plants will thrive in a mix that contains about 40 percent peat moss, 20 percent pine bark, 20 percent vermiculite, and 20 percent perlite or sand.

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Feel like making your own soilless potting mix?

That’s cool, just mix together the ingredients above into the consistency you need. But please test the pH before using it – some materials can make the mix acidic and you want the pH to be between 6-0 and 7.0 (neutral), to encourage plants to uptake nutrients. The vast majority of plants will do well in this neutral pH range. Learn how to make your own potting mix.