Emma Erler, host of Homegrown NH and lead horticulturist with Kirkwood Gardens, says the best potting mix for potted plants will be lightweight, fluffy and dry, and contain peat, coir, bark, perlite, or vermiculite.
There might be confusion between garden soils or potting mix. And if you’re setting up container gardens, you want to choose soilless mixes usually called potting mix.
Garden soils are typically intended for use in the ground or in raised beds, and they contain minerals and organic matter. They more closely resemble the actual soil that's in the ground. Garden soils can also contain weed seeds, insects and diseases if they haven’t been pasteurized.
Erler says garden soil is not a good choice for containers because the soil can become quickly compacted and waterlogged, which reduces air space around the roots. This can lead to poor or stunted growth.
Garden soil also tends to be very heavy, so if you have to move your containers around, it can become problematic.
Look for a potting mix with materials that are made specifically for growing potted plants. They tend to be lightweight, retain moisture well and provide plenty of air space around plant roots. Air space is actually one of the most critical aspects of potting mix. If the roots don’t have enough air, a plant usually doesn’t survive.
Knowing what is in the potting mix is key to determining whether it will be a good match for the plants you are trying to grow.
Compost is occasionally included in potting mix for added nutrients. It can reduce air space in the soil and should be used sparingly for potted plants. Compost should make up no more than 1/3 of a potting mix.
Bark that has been ground and partially composted is often incorporated into less expensive potting mixes in place of peat. Bark provides good aeration but dries out more quickly than peat, requiring more frequent watering.
Coir is a fibrous material from coconut husks that is sometimes used in place of peat. It is similar to peat in that it retains water without becoming soggy.
Vermiculite is the product of heating mica chips. It is a gray, spongy material that increases water retention in mixes. It also holds on to nutrients and thus keeps fertilizer available for the plant roots for a longer period of time.
Perlite is a white volcanic rock that is reminiscent of Styrofoam. It is light weight and porous and is used to improve the drainage and aeration of potting mix.
Sand is another common component of potting mixes. It can improve drainage and is often added in large quantities to mixes intended for cacti and succulents.
Fertilizer is sometimes added to potting mixes, usually in a slow-release form that breaks down gradually over time when it comes in contact with water. Eventually this initial source of nutrients will be exhausted, and potted plants will require additional fertilizer.
Moisture-retaining treatments come with some potting mixes and are meant to reduce how often you need to water. These “hydrogels” or “water storing crystals” are polymers that have the ability to absorb large amounts of moisture and slowly release it as the soil dries.
General potting mixes will work fine for most annuals and vegetables grown in containers, but they may hold too much moisture for orchids, succulents or cacti.
These plants demand specific conditions around their roots, typically a drier or more aerated soil. Look for a mix that's labeled for orchids or for cacti and succulents.
When in doubt, choose a peat-based general-purpose mix, or make your own potting mix by combining the ingredients above. You can find potting mix recipes online.
The bottom line is to select the highest quality potting mix that you can afford.
See you in the garden.
If you have a gardening question for us, email us or send a voice memo to HomegrownNH@ NHPR.org.
Homegrown New Hampshire is a collaboration between Squam Lakes Natural Science Center and NHPR.
