Summer 2024

Container Soil Recipes for Small Spaces Gardening

by David George

All plants require four essential elements: sun, water, air, and fertile soil with space to grow. Container plants are no exception. Because they are not grown in native soil though, container plants have special soil needs. Garden centers sell excellent potting soil mixtures, but they are often expensive or lack certain nutrients. Were you aware that you can create your own container soil mix?

A woman builds her own potting soil. Courtesy Shutterstock.

Special attention must be given to ensuring your soil mix is not too heavy with water-retaining clay. Clay quickly compacts and becomes too soggy, which limits air in the soil and suffocates the roots. Container plants also consume nitrogen more quickly than in-ground plantings, so container soil should be amended with extra nitrogen more often.

I quizzed two Contra Costa Master Gardeners about their favorite container soil recipes that avoid these issues and lead to healthy growth: Terry Lippert, an experienced UC Master Gardener and container-planting enthusiast, and UC Master Gardener, Deborah Christman.

Terry: “My container potting mix has three ingredients: (1) a good compost that contains chicken manure; (2) perlite (vermiculite is an acceptable substitute, or use a mixture of perlite and vermiculite); and (3) coconut coir that has been soaked in warm water. Mix equal volumes of the three ingredients. Recently, I've started adding some topsoil to add more mineral content to native soils. To avoid making the finished potting mixture (with native topsoil) too heavy, add about 10-15% more of the volume of the three ingredients you've already mixed.”

Mixing in white perlite aids water retention. Courtesy Shutterstock.

Chicken manure is a concentrated source of animal-based nitrogen. Compost, in general, adds nitrogen and other vital ingredients for healthy plants and is primarily composed of organic decomposing matter that feeds essential microorganisms in the soil. Perlite and vermiculite are naturally occurring minerals that absorb excess water like a sponge, and then release it as the soil dries out. They moderate container soil moisture levels, so plant roots have more reliable access to water. Perlite absorbs less moisture but aerates soil better than vermiculite. Coconut coir is a fibrous and moisture-absorbing product that comes from discarded husks of coconuts. It can absorb and then release into the soil up to ten times its volume in water. Like perlite and vermiculite, it stabilizes soil moisture to promote a consistent moisture source for roots.

Deborah: “My (container soil) formula is basically: 1/6 (coconut) coir, 1/6 Perlite, and 1/3 commercial potting soil, and 1/3 mix of organic compost with a starter additive or cottonseed meal (for a nitrogen boost). Sometimes I add liquid kelp. I top it all with rice straw mulch. I add extra compost as the season progresses, lifting the straw to “feed” the soil. If I’m growing tomatoes, I add one antacid and 2 aspirin. (For blueberry plants, I add whatever acidifying product or acidic compost I might have. Then I chant or pray over all the containers in the light of a full moon.”

I don’t think, Deborah, that the last part is an official position of the research-based UC Master Gardeners program, but it is fun to dance under a full moon! Seriously, antacid tablets are packed with calcium carbonate, which studies have shown helps tomatoes resist blossom end-rot disease. And in plants, just like in people, salicylic acid from aspirin helps plants cope with stress and disease. The positive effect of liquid or granular kelp on plants is due to high levels of cytokinin, a natural growth hormone that promotes cell division, root and shoot growth, improved photosynthesis, and overall plant health.

Retail container soil professionals recommend a soil mix that contains beneficial mycorrhizal fungi and diversified nutrients such as crab meal, fish bone meal, earthworm castings, humic acid, and more. Mycorrhizal fungi and these other ingredients promote new root development.

A good discussion about container soil recipes by Santa Clara County Master Gardeners can be found at: https://mgsantaclara.ucanr.edu/garden-help/container-gardening/#soil_for_container.

For basic container soil information, check out this UCANR blog: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=29331

Greatly amended planting soil in your pots will increase your chances for great veggies and flowers. Try these recipes and make your own great potting mix!

View All Stories | Return to Spotlight Stories