Potting soil can make or break the success of indoor plants

Proper potting soil makes the difference between a struggling plant and a healthy, thriving plant. The term “potting soil” is a misnomer, however, as the mixtures used for indoor plants really no longer contain any soil. The very finest outdoor garden soil hardens quickly, clogs easily, and lacks oxygen when confined to the confines of a houseplant.

Before today’s soilless potting mixes, houseplant growers would mix their own mixes, typically using one-third each of garden soil, sand, and an organic component such as peat or fertilizer. Since the components usually contained weed seeds and pathogens, there have been many recipes for sterilizing the mixture in the oven or microwave.

After going through the days of soil mixing, I can say that life is better now. Good quality potting soil is readily available in garden centers and is a huge improvement over homemade mixes.

Today’s pot mixes are backed by science. In the 1960s, Cornell University gardeners investigated ways of providing potted plants with a medium that would provide support and nutrition, but also the necessary drainage and ventilation. Their research led to the peat-based potting mix we use today.

What makes good potting soil? Such a mixture will hold just the right amount of moisture and allow the excess to drain away easily. It needs to be porous for root aeration, oxygen exchange and good drainage, but it also needs to hold the right amount of moisture and absorb nutrients. Since the roots of a houseplant are limited to a pot and can’t seek friendlier soil, a houseplant is completely dependent on the mix we offer.

Today’s potting soil is made from ingredients like peat moss, composted bark, coconut, vermiculite, and perlite. Slow release fertilizers are added to many mixes because most of these soilless components provide good ventilation and a root-friendly environment, but they do not have natural nutrition.

All-purpose potting mixes are suitable for almost all indoor plants. However, some plants require a mix that is tailored to their particular needs, such as succulents, cacti, and orchids that can be found at local garden centers.

Garden centers sell special mixtures, for example for orchids. Michael Vosburg / Forum picture editor

Is there a difference in pot mixes? Certainly! Houseplants can grow in the same mix for years, so this is not the right area. The cheapest potting soil often feels heavy in your pocket and lacks the best ingredients for successful indoor plant growth. High quality mixes are light and well ventilated.

Locally owned garden centers usually run successful grower mixes along with national potting mixes. Since most high quality mixtures are very dry in the bag, moisten them the day before use by adding water and stirring to distribute them. The mixture will be soft, easier to use, and easier to rewet than if it was used dry. Pre-wetting is an important step.

When potting a houseplant, be sure to fill the pot with enough mixture, leaving only about half an inch of clearance, which is the depth from the surface of the potting soil to the edge of the pot. When I see ailing houseplants, I have often observed headspace that is too deep, which contributes to moist soil, reducing airflow over the soil surfaces, and providing less soil volume for root growth.

Research has also shown that potted plants have better drainage if the pots don’t have a layer of stone or gravel in the inner bottom, as has been added many times in the past. The pebbles create a layer of change and create a physical property that actually hinders the drainage they are supposed to improve. For the best drainage, fill the pot from top to bottom with good quality potting soil. Coffee filters, diapers, or other screen materials are not needed to cover drain holes when high quality potting soil is used.

Potting soil can easily be infested by fungus gnats, the pesky little black flies that fly around indoor plants. The adult flies lay eggs in the ground, from which larvae hatch, which in turn grow up, creating a never-ending cycle. Mosquito Bits is a product that is labeled to combat mosquito fungus. The granules are sprinkled on the surface of the soil and contain beneficial bacteria that kill fungal mosquito larvae and interrupt the mosquito life cycle.

Mosquito Bits also combats the mosquito's pesky little black flies.  Michael Vosburg / Forum picture editor

Mosquito Bits also combats the mosquito’s pesky little black flies. Michael Vosburg / Forum picture editor

Don Kinzler, a Lifelong Gardener, is the North Dakota State University Extension gardener for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.

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