Is Reusing Potting Soil A Good Idea?

This article is republished from UK Issue 7 of Garden Culture Magazine. It originally appeared under the title, Reusing Your Potting Soil.

The idea that reusing potting soil is taboo has been in place since the early days of cultivating plants in containers. But so has the direction that one should never reuse the pots themselves. Yet, millions of people recycle pots, so why not get more use from the potting soil too? After all, backyard garden and farm field soil isn’t tossed every time a new crop is planted.

If you are a serious grower, you have easily invested thousands of dollars in potting soils over the years. Even worse, you have probably thrown thousands of dollars of potting soil in the garbage. On its face, throwing soil away after one use doesn’t make much sense. But to a grower-focused on expediency, and not wanting to put determined effort into a garden only to end up with hidden issues, or potential contamination from a previous grow -using new soil is a powerful convenience.

While there are good arguments for using fresh soil, the purpose of this article is to communicate some ideas and methods for reusing potting soil. Growers do it all the time, and with great success.

Look at it this way… Mother Nature doesn’t start over, why should you?

Properly reusing potting soil calls for good physical structure, proper biological diversity, mineral balance through soil testing, and consistent methods. We have been helping people reuse their potting soil for years, and while it is not as simple as removing the roots and replanting, it is well worth the time and money saved. You also get the satisfaction gained from using your resources more sustainably.

Reusing Potting Soil Builds Biological Diversity for Plant RootsThere is a sweet spot in the soil where life thrives. The forest grows trees with no fertilizer because the soil in the forest is naturally balanced, mature, and organized. I’m not suggesting that we expect to grow trees in our gardens, but I am suggesting we consider how to perceive the metrics, and harness the abilities of the life forms that allow this to happen. Much of what we experience as failure in the garden is due to human influence, not bad luck. I would argue that much of the integral part of the growing process, and at the same time get us out-of-the-way. This is the art of making soil. The soil has physical, mineral, biological, and energetic capacities that need balance. Physical balance tends to come with attention to the other three but is generally addressed in potting soils by using amendments like perlite or rice hulls for drainage, and peat moss or coir fiber for water retention. You may need to add these materials back to soil mixes over time due to loss.

The merits of inoculating as much microbial diversity as you can muster accounts for biological balance. In other words, as long as you are introducing proper diversity, microbes self-organize. Seek out a mother of as many natural or farm-based sources as possible for your mix, and deliver them consistently to your soil over time. Like workers on a construction site, they need fresh materials to continue building. The consistency is much more important than the concentration.

Energy defines life. That may sound hokey, but it is 100% true. The more intention we pay towards this reality, the greater the result. The living system’s capacity to produce and thrive off of subtle energies is innate in the life force itself, but can also be encouraged through concepts such as potentization and resonance using techniques like vortexing, paramagnetism, frequency farming, implosion, and others.

We can expand upon an energetic balance in future articles, but mineral balance takes center stage when a grower is considering reusing potting soil. First, I use the term mineral loosely, as a way of capturing all the possible forms of elemental nutrition. Materials such as seawater, clay, rock dust, etc. have value beyond recognition of essential nutrition because they contain broad spectrum minerals, and they are also balanced.

Life can simply receive the elements it needs in order to thrive when all elements are present in the first place, and when they are in balance. Even when elements are not identified as essential for plants to grow, they could be vital for a microbial process, or in order to make the elements required by plants more available.

Why would Mother Nature create an element not needed in the garden?

So, the strength of the system and your success in re-using your potting soil is reliant on the diversity and balance of microbes and minerals. For the sake of agriculture, it is not possible to measure all Earth-bound elements, any more than it is feasible to measure all soil microbes, so soil testing as a lab-based process is typically limited to essential elements.

My company performs custom soil testing for growers, farmers, and landscapers all over the US. We have developed a system of soil testing that not only generates complete raw data for all essential agricultural elements but also provides custom instructions on what materials and products to add in order to account for deficiencies. We’ve done testing on many premium bagged organic potting soils, and most stack up nicely in regards to proper mineral balance. What we also know is that if we try to grow in this soil over and over without using diverse microbes and refortifying mineral balance – things fall apart.

Balanced Amendments Make Reusing Potting Soil SuccessfulNo matter who you end up working with for your soil testing, it is essential that you seek out a private lab, or some outlet other than what you find at most State Extension services. Here’s why…

The pioneer of mineral balance and the sweet spot of soil was Dr. William Albrecht. He believed that animals, including humans, provide biochemical photographs of the soils in which their foods are grown. Dr. Albrecht geared his research towards documenting the connection between empty soils and empty people, and he investigated and defined a specific range of positively charged elements, or cations, that soil can hold that has become known as the cation exchange capacity or CEC. Overall, soil is negatively charged, the more complex and biologically active the soil, the greater number of negative binding sites are available to hold positively charged elements. And opposites attract.

The CEC number represents the potential reservoir of the soil to retain cation nutrition for growing plants. A low CEC is the basis for fertilizing and irrigating because if it was high enough, the soil would be able to hold everything that it needs to eat and drink within natural conditions.

Most soil tests we take in residential landscapes will have 1-2% organic matter with a CEC of 5-15. This is a sign of extremely immature soil. Bagged organic potting soil typically shows an organic matter content of 15-20% with a CEC of 15-20. The CEC is lower even in bagged potting soils due to a lack of biological activity and diversity, which you can increase using compost and compost tea, along with humic material, such as worm castings or concentrated humic acids.

The following data comes from Dr. Albrecht’s work, and our observations over the last decade of testing soil and documenting results. This is not a complete list of essential elements, it represents the cations that are held within the soil’s CEC. The information presented here is what we consider as ideal:

  • Cation Exchange Capacity (CEC) = 25-30
  • pH = 6.1 – 6.5
  • Organic Matter = < 4%
  • Calcium (Ca+) = 60-70%
  • Magnesium (Mg+) = 10-20%
  • Potassium (K+) = 2-5%
  • Sodium (Na+) = 0.5-3%
  • Hydrogen (H+) = 10-15%
  • Other Bases = Variable

Reusing Potting Soil Builds Minerals Crops NeedThis data is presented in ranges because it is never a matter of hitting a mark when testing soil. Soil is a dynamic substance that will never be the same. All we can do is use the data while observing local conditions, and the results of plants to establish where within this acceptable range is better. Growing plants can always get better.

The State Extension service is going to look for some of this data, but not all of it. They’re approach is very pH driven, as conventional agriculture is not motivated by mineral balance. They put too much focus on the amount of lime needed to raise the pH on paper, for instance, as opposed to investigating the deficiencies of elements and accounting for them through observing crop growth.

Positively charged hydrogen ions (H+) being present defines the pH of a substance. The reason soil becomes acidic is because it is demineralized and all the other positive elements are no longer present, not because someone poured acid on it. We tend to think of pH in terms of some concrete thing, instead of a metric of the energetic representation of available elements. For example, lime is calcium. Calcium is a cation, so when used in the soil it replaces hydrogen in the CEC, which makes the pH go up. What happens if you have a potassium deficiency?

As Dr. Albrecht identified, “plants are not sensitive to, or limited by, a particular pH value of the soil.” In other words, it is possible to have a perfect pH, and have your minerals entirely out of balance and, therefore, not be addressing your deficiencies.

The pH should really be an afterthought to the soil health conversation, a value that communicates the success of balancing the minerals in your soil, not the other way around. The takeaway is that if you have all of your minerals balanced properly – the pH is always within range.

The name of the game when reusing your potting soil is to trust in the microbes’ ability to construct a dynamic neighborhood for growing plants, but verify that you are bringing the right building materials to the job site through soil testing. Then listen to your plants to get it right.

Images courtesy of Gathering Together FarmSjef Garden Tips, and All the Dirt (respectively).

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Evan is a regenerative agricultural consultant with a background across every facet of the farming and gardening spectrum. He has founded and operated many businesses over the years - including a retail hydroponics store he operated for over 14 years, a wholesale company that formulated beyond organic products and vortex-style compost tea brewers, an organic lawn care company, and a commercial organic wheatgrass growing operation. He now works as a consultant in his new project Be Agriculture where he helps new and seasoned growers take their agronomy to the next level. What we think, we grow!