Don’t Throw Out Your Used Potting Soil

Take a cue from Mother Nature and add fertility back into spent potting soil instead of throwing it out.

by Evan Folds
PHOTO: iStock/Thinkstock

Even for casual growers, buying potting soil for the container garden every season can be expensive. I know container growers who reuse their potting soil year after year, some carrying it from house to house in tarps when moving like agricultural hobos. But good soil is like gold and not to be taken for granted. It can be refined and balanced with good intention, and with proper care can get better with time.

Putting Life Back Into Soil

At the end of the growing season many growers consider their soil used up and throw it away. But I’d contend that there’s never a point where soil is spent, where it has no value or needs to be discarded. Soil doesn’t work like that.
Soil can always be regenerated. The key to upcycling potting soil is to leverage the ability of microbes to digest organic matter into humus and to account for deficiencies and ensure complete fertility.

Humus is perfect plant food. There’s a reason that the forest needs no fertilizer yet is capable of growing enormous trees, like redwoods and sequoias. Humus doesn’t just happen—it’s created through the digestion of countless microbes, like bacteria, fungi and nematodes, which work together in the soil ecosystem and are collectively called the “soil food web.”
Due to property development and artificial agriculture, the average landscape or chemically treated field is devoid of a proper diversity of soil microbes. You can add them back by sourcing various compost tea recipes found for sale, by using worm castings or even by cultivating them yourself from natural sources. The greater the diversity, the stronger the system.

Soil Microbes To The Rescue

Soil microbes not only digest organic matter into fertilizer, they work toward a sweet spot in the soil mineral balance. This optimum soil ratio was discovered by soil pioneer Dr. William Albrecht, whose research was geared towards investigating and defining a specific range of positively charged elements, or cations, that can be held by the soil: the cation exchange capacity, or CEC.

Physically speaking, soil is comprised of sand, clay and organic matter. Sand is neutrally charged and inert, but clay and organic matter have a net negative charge, making them capable of holding the positively charged elements retained in the CEC—as they say, opposites attract. Organic matter is also capable of holding negative anions.

Think of the CEC in the soil as the size of your gas tank. The higher your CEC and the more efficient the microbes in the soil are in cycling nutrients, the greater ability there is for soil to hold onto water and the elements required for growing plants—i.e., the less you have to irrigate and fertilize.

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It must be done through soil testing, but the epitome of upcycling soil is using soil testing to balance the essential elements within the CEC. With soil testing, it isn’t enough just to balance the pH. As Dr. Albrecht identified, “Plants are not sensitive to, or limited by, a particular pH value of the soil.”

Keeping Your Potting Soil Balanced

What Dr. Albrecht documented was that if you have the correct mineral balance in the CEC, then the pH of the soil is always in proper range. It is the afterthought that tells us the elements found in the soil are balanced, but does not drive any action. Take for instance the use of lime to raise the pH in soil. Lime is primarily calcium, but if that is all you add, what happens if you have a potassium deficiency?

There are good resources and books on the market about soil testing if you want to go that route, but you can also keep it simple and use balanced fertilizers to give your potting soil a boost. Make sure they are organic—microbes do not like cheap artificial fertilizers, plus you will have to flush the salts out over time. And consider a calcium supplement to your primary fertilizer unless you test the soil.

You will have to add volume back to your potting soil over time. Materials like perlite, sand or rice hulls can be used for drainage, and peat moss or coir fiber can be used as filler. You can also use fertility products like bat guano, kelp meal, fish meal, rock dusts, humates and alfalfa meal to value add the soil. Or you can just buy new bags of potting soil and mix it all together.

However you do it, have fun with it and encourage natural biological processes. All growing is trial and error. Upcycling your potting soil is not a science, more of an art. The process may feel complicated, but think about it like composting your soil perpetually over time. Mother Nature doesn’t have to start over, and we don’t either!

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