Soil in Containers Should Be a Good Mix

I work hard to ensure that the soil in my garden is the best I can give my plants, and they reward me with robust health. Yet that same good soil if transferred to a container would cause the plants in it to languish. That’s because garden soil doesn’t offer enough air, water, or nutrients to a plant growing in a container. Potting soils are specifically formulated to overcome these limitations.

Potting soil needs to drain well but still hold moisture

One of the most important things a potting soil needs to do is provide roots access to air by letting water drain away from them. In the ground, the soil is usually deep enough to let excess water drain beyond root zones. In pots, however, water tends to accumulate at the bottom, despite drainage holes. The smaller the pore spaces of the soil in the pot, the higher that water layer will reach. Larger pores, formed by adding mineral aggregates to potting soils, readily admit water into the soil, then carry it through the medium and out the bottom. Then, all those large, empty spaces can fill with air.

Perlite, vermiculite, calcined clay (kitty litter), and sand are the mineral aggregates most commonly used in potting soils. Perlite and vermiculite are lightweight volcanic rocks naturally filled with air. I prefer perlite over the others because it does not decompose with time nor lose its aerating ability if the potting mix is compressed. Vermiculite is a valuable additive because it prevents some nutrients from leaching away, and it even provides a bit of potassium and magnesium.

A potting mix also must have ingredients that help it retain moisture. This is where organic materials—usually peat moss, sphagnum moss, or coir—come in. They cling to some of the water that the aggregates are helping to drain. Organic materials also hold on to nutrients that might otherwise wash away.

In addition to peat moss, vermiculite, and perlite, commercial mixes often contain sawdust or various grades of shredded bark. Lime may be added to help balance the acidity of the peat moss, and a small dose of fertilizer can often make up for the lack of nutrients.

Adding compost or garden soil can be beneficial

Most gardeners make potting soil by combining perlite or vemiculite with peat or sphagnum moss. Two other organic materials that you could add to your potting mix are leaf mold and compost, which offer a wide spectrum of nutrients.

Adding some garden soil to a homemade potting mix contributes bulk while buffering against pH changes and nutrient deficiencies. The reason that garden soil is rarely added to commercial mixes is because of the difficulty in obtaining a steady supply that is consistent in quality and free of toxins such as herbicide residues.

Soilless potting mixes are relatively free of living organisms, but mixes made with soil or compost are not. Some gardeners talk about “sterilizing” their potting mixes by baking them in the oven to rid the soil of harmful organisms, limiting the hazards of damping-off and other diseases. What I hope they mean is that they “pasteurize” their mixes. Heating homemade potting mixes to sterilizing temperatures wipes out all living things, beneficial and detrimental, leaving a clean slate for possible invasion of pathogens and causing nutritional problems such as ammonia toxicity. Pasteurization, which occurs at lower temperatures, kills only a fraction of the organisms. The best way to pasteurize your soil is to put it in a baking pan with a potato embedded in the soil. Bake it at 350°F for about 45 minutes. When the potato is cooked, the potting mix is ready.

I don’t pasteurize my potting mix. I rely, instead, on healthy container-gardening practices such as timely watering, good air circulation, and adequate light to avoid disease problems. Beneficial microorganisms in compost and garden soil also help fend off pests.