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Can potting soil be reused?

April 6, 2012

By OSU Extension Services

With warm temperatures on the horizon, many gardening enthusiasts are making plans for their spring and summer gardens.

With consumers facing rising costs in just about every area of their lives, some gardeners may wonder if it is OK to reuse potting soil.

Kim Toscano, host of Oklahoma Gardening, said generally there is nothing wrong with reusing potting soil.

“It can be expensive to replace the soil every year. However, there are certain considerations to be made when reusing soil,” Toscano said. “Used soil will require fertilizer applications to replace nutrients that have leached from the soil or have been utilized by plants previously growing in the medium. Incorporating a slow release fertilizer at the proper rate will take care of nutrition needs for several months. If you’re using a slow release fertilizer, liquid fertilizer isn’t necessary.”

Toscano also cautioned about the buildup of salts in the soil. Excessive levels of salts can be detrimental to plant development. Keep in mind that all water sources are not the same and salts may be a problem in some city water systems but not others. The same holds true for well water and some wells may have a higher concentration than others.

“Typically, rainwater, such as water collected in a rain barrel, will have lower levels of salts and is excellent for watering container plants,” she said. “Gardeners may often see salt accumulation (crystal formation) when you examine your soils and the salts also tend to form a white ring on clay containers. These are good indicators that salt accumulation may be problematic if reusing soil.”

Commercial potting soils have agents added to the mix that help the potting soil take up and hold water. These agents break down over time. Likewise, peat and other organic agents mixed into soils for the same purpose of holding water, decompose over time. As such, older soils tend not to hold water as well and often appear “compacted.” This could be addressed by mixing new and used potting soils together, adding organic matter to the potting soil or adding a water-holding agent.

Another option is to add your used potting soil to your compost pile. A good compost pile has mixture of green material, brown material and soil. Finished compost can then be used to fill your gardening containers. The compost will have the water-holding properties for which you are looking. Composting used potting soil will also help with salt accumulation problems.

“If insects, disease or weed seeds are a concern, pasteurize your soil and compost in your oven,” Toscano. “It’s particularly important to pasteurize potting soil that will be used to start seeds. Simply bake moistened soil at 180°F for at least 30 minutes.”

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