Tree Bark, Rice Hulls & More—Exploring the Future of Soilless Growing Media

Soilless growing mixes may be made with more regionally-sourced ingredients, new processing techniques over next few years.

by Susan Brackney
PHOTO: Spencer Wing/Pixabay

Plenty of gardeners are rethinking the carbon footprint of all of their gardening practices—including the kinds of soilless mixes they use. Indeed, from a product’s ingredients and processing to its cumulative shipping distance, not all soilless mixes are created equal. In particular, many growers are doing their best to reduce or eliminate their reliance on peat.

“This desire to transition away from peat is happening so rapidly that the research and development into alternatives hasn’t had the full opportunity to catch up with that demand,” says Dr. Brian E. Jackson. A professor at North Carolina State University’s Department of Horticultural Science, Jackson develops both traditional and new growing media.

In part, researchers and manufacturers first must identify high-performing growing media ingredients available on a large scale. Then they must also determine the stability of certain ingredients over time and see how growing plants respond.

Despite the current industry lag, Jackson notes, “Solutions are on the way.”

Read more: Say goodbye to peat moss—why and how to use alternatives.

Regional & Short-Term Ingredients

Researchers are making headway with some regionally available materials. “There materials that are in the South or the Northeast that can be used to formulate mixes,” Jackson says. “In the Northeast, you may have some seafood compost or you may have more use of rice hulls down in the Delta of Mississippi.”

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States such as Louisiana also have bagasse—a waste product from the sugar cane industry—which shows potential. “That material is marvelous, but it breaks down really quickly,” Jackson says. “So, don’t formulate a growing media with that and expect to grow plants that have birthdays in containers.

“Think more about [using it for] short-term crops.”

The Big Three

At the national level, Jackson suggests coconut, bark and wood fibers will top soilless growing media ingredients, by volume.

“I think coconut will remain one of the main constituents in peat-free materials,” he says. “And pine or other bark—either composted or processed in different ways—will have an ever-growing role in peat-free mixes.”

Researchers are also investigating using wood fibers or waste wood from forest byproducts. “Wood is extremely malleable,” Jackson says. “It can be engineered and modified … into so many different sizes of particles.”

He continues, “If you want to process wood chips into a granular material similar to perlite, for example, for great aeration, you can. Or there’s different ways that we now know how to manufacture wood fiber to create a really wooly, true fiber that holds a lot of water. There’s a blank canvas of what wood can offer, and that’s something that’s kind of new for us.”

Read more: You need plenty of air and water for healthy soil.

DIY Mixes

Wish you had access to a green (or greener) soilless growing mix right now? “Look locally,” Jackson suggests. “There could easily be local producers of soil products at individuals’ local garden centers that do indeed have reliable, peat-free mixes.”

And, although we may lack Jackson’s scientific instruments and heavy equipment, we can still run our own soilless growing media experiments. “If it’s for fun or on a small, trial basis, you can try to formulate your own mix,” he says.

Start by looking carefully at the traditional mix you typically use. “One of the things that we do is we’ll reverse engineer it,” Jackson says. “We’ll take that product and run it through a sieve. We’ll fraction out all the different particle sizes and look at those different fractions. We can determine pretty accurately how much perlite was in it, how much bark was in it, how much peat, or other things.”

He continues, “Then, if you have a sawdust pile or bark or other things, you can take that material and screen some into a wheelbarrow or whatever else and kind of mimic those same particle sizes.”

You’ll also want to try to mimic the traditional mix’s ratio of different particle sizes. “Maybe you have 30 percent big, coarse particles and 50 percent fine [particles,]” Jackson suggests. “You can just screen things of different sizes and reconstruct something that’s similar to what you’re used to using.”

Testing, Testing …

Ideally, the soilless growing mix you make should possess the same—or similar—air-porosity and water-retention properties as the traditional mix. “An easy way to [test] that is to get your [traditional] product that you’ve always used and put it into a gallon pot,” Jackson advises. “Then, irrigate it two or three times and let it drain. Pick the pot up and get an idea of how heavy it is.”

Follow those same steps with your DIY mix. “Compare it to the traditional potting mix,” he continues. “If one is much heavier than the other, then you already know you’ve created something that’s not similar.”

Jackson adds, “If the one that you’ve created is half the weight, go back and add more of those fine particles to it that we [previously] sieved out. Maybe increase that by double, mix it again, wet it, and see if the weight then is similar to what you’re used to.”

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