The Basics of Potting Soil

The Basics of Potting Soil

Posted by Megan Nichols on Aug 22nd 2019

When it’s time to repot an indoor plant, often the easiest decision is which new container to pot it in while the most difficult can be which soil to choose. Any basic potting mix will be OK for most plants, but it won’t necessarily be the best.

The Dirt on Potting Soil

There is a lot that goes into a good potting mix, so which one is “good” is dependent on which plant will be sitting in it. Potting mixes are usually soilless, meaning they are made up of a mix of ingredients, none of which are actually soil like you would find outside. The following are common ingredients you might find in different potting mixes.

Peat Moss
Usually the largest quantity component in mixes, peat is a soil amendment used to increase water and nutrient retention and adjust pH for acid-loving plants

Peat Humus
Decomposed organic matter found in peat bogs

Tiny, sharp particles that improve drainage

A form of volcanic glass, heated until it expands, used for aeration in soil

Increases water retention, good at keeping seedlings from drying out when added to a starting mix   

Earthworm Castings
Broken down organic matter (thanks to worm digestion), packed with minerals and nutrients

Alkaline, used to buffer acidity of other growing medium (peat moss, pine bark)

Western Fir Bark
Increases aeration and improves water retention

Decomposed organic matter, rich in nutrients, improves water retention and drainage

Horticultural Charcoal
Made up mostly of carbon, used to improve drainage and acts as a filter to remove impurities that can be harmful to roots.

In general, the higher the percentage of lightweight ingredients the sharper the drainage will be and the better the mix will be for plants that are intolerant of too-wet conditions. Cactus and succulents in their natural environments can handle a heavy watering (drenching rain) but then are immediately relieved by the sharp drainage of the desert sand, so well-drained soil is a must. Orchids are epiphytes and grow on trees, so they like good aeration and a barky mix. Ferns want to remain moist but not soggy, a mix that retains more moisture is better.

There are some slight exceptions, of course. Though aroids (Monstera, Philodendron, Spathiphyllum, and many others) can be propagated in water and can handle periods of intense moisture they are best grown in well-drained soil mix where the moisture levels can be more fully controlled.

Outdoor/Indoor Potting Soil

Tonka Terra
Our very own specialty blend mixed to our specifications. This is the very best there is when it comes to planting outdoor pots. With a mix of organic compost, sphagnum peat moss, perlite, and fine composted bard, Tonka Terra provides the best medium for plants in an outdoor container. This mix is also suitable for many indoor plants, but for those that like more aeration perlite and peat moss are good amendments.  

Espoma Organic Mixes

Orchid Mix
Contains 65-75% Western fir bark, perlite, and horticultural charcoal. Plants with epiphytic growth habits require superior drainage and aeration. Good for Orchids and Bromeliads.

Cactus Mix
Contains 40-50% peat humus, sphagnum peat, sand, perlite, worm castings, and limestone. This mix has sharp drainage, is the correct pH for making semi-desert plants happy, and contains mycorrhizae, a beneficial fungi that creates tiny little root hairs on the plants existing roots and allow for more nutrient uptake. For cactus, palm, and citrus.

African Violet Mix
This mix contains 50-60% peat moss, pet humus, perlite, and limestone. African violets like to remain on the alkaline side, and the limestone in this mix ensures that happens. This mix also improves moisture retention while still providing good drainage. Be sure to water the soil with room temperature water and avoid getting water on the leaves as this can cause unsightly damage. For African violets and other houseplants.

Potting Mix
With 45-55% peat moss, humus, perlite, worm castings, limestone, and alfalfa, kelp, and shrimp meal, this mix is a good all-around, light-weight potting mix for a large variety of indoor plants. The specially formulated mix provides excellent drainage and organic fertilizer and is sure to keep indoor plants happy. General, all-purpose mix is good for any plant.  

Seed Starter Mix
This completely sterile mix is perfect for seed starting as it will not contain any of the pathogens that can be detrimental to tiny seedlings. Made of 75-85% peat moss, perlite, peat humus, worm castings, and limestone. Notice the mix of lightweight material is very high, making this extremely lightweight and just right for tiny plant roots. For seedlings or cuttings and anything that should be kept in a sterile mix until well-established.   

Other Soil Mixes

Bonsai Soil Mix
This sharp draining mix is the perfect combination of ingredients to keep bonsai (which are quite particular and a bit diva-ish) growing well. Haydite (expanded shale), sand pebble, and aged pine bark help with drainage and allow moisture to hang around just long enough, but not too long.

Make Your Own
If you’d like to try your hand at making your own custom potting mix, add to what is already available. We have the raw ingredients, individually in bags.

Happy indoor planting!