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How to mix your own potting soil for your container garden

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Container gardening is a very popular topic for Southern California gardeners. Most homes do not have a lot of gardening space, either because of tiny lot size or no lot at all (in the case of condos or apartments). Successful outdoor container gardening depends on using larger pots that are slower to dry out, less likely to tip over, and give the plant’s roots enough room to grow.

Large containers need a lot of potting soil and this can get expensive if you purchase bags of ready-made planting mix. Making your own potting mix can save money as well as allow customization. The internet has a wealth of recipes for potting mixes appropriate for any type of plant. All the recipes call for many of the same ingredients, just mixed in different ratios.

Here is an overview of the most common ingredients and what they do. Once you know this, you can customize your own planting mix.

A good planting mix should be compressible (enough to enable the plant to remain upright), hold water and nutrients, allow for aeration and drainage, and be free of pathogens, weed seeds, and herbicide contaminants. 

A common recommendation is to sterilize ordinary garden soil by baking it at 200 degrees Fahrenheit for 20 minutes. Not only is this inefficient, but it will make your house smell terrible. Don’t do this!

Sand is used in almost all the recipes I’ve found. Although it has no nutritional value, it will improve drainage and add weight to the container (which will prevent it from tipping over). Use coarse builders’ sand instead of finer types of sand.

Bark, particularly finely ground pine bark, holds water and can be used as a substitute for peat moss. It will gradually break down and this process will decrease nitrogen availability. If you use bark, add a nitrogen source such as blood meal.

Sphagnum moss or peat moss will increase acidity. We grow our blueberries in a peat moss-rich mix since they prefer a somewhat acidic soil. It also holds water but is light enough to drain freely.

Coir can be used as a substitute for peat moss and is commonly sold in highly compacted bricks. It is a byproduct of coconut processing and can be more expensive than peat moss. Unlike peat moss, it has a neutral pH. It takes time to rehydrate and should be rinsed several times due to the possibility of salt contamination.

Perlite is a sterile, neutral product obtained from volcanic rock. It improves drainage but holds water and can be used as a substitute for sand. It’s lightweight and looks like pulverized Styrofoam.

Vermiculite is derived from a mineral that has been heated in an industrial process. This heating causes the particles to expand into a fluffy, metallic-looking substance. It aids in drainage, holds water, and can add calcium and magnesium ions to the soil (which plants need).

Los Angeles County

[email protected]; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/

Orange County

[email protected]http://mgorange.ucanr.edu/

Riverside County

[email protected]https://ucanr.edu/sites/RiversideMG/

San Bernardino County

[email protected]; 909-387-2182; http://mgsb.ucanr.edu

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