Replacing Dirt
Stop us if you’ve heard this one before: in order to grow, plants require sunshine, water, and soil. Right? Well, actually, maybe not. It’s a bit more complicated than that. Turns out that soil is a bit of a wild card, and a lot of smart people in the agtech industry right now are challenging this age-old notion.
Note that this article is purely concerned with hydroponic farming, which by definition, is a soilless growing method. What exactly should be used in its absence, is a topic of much debate that will be addressed below. So, fair warning for the easily bored: this post is literally going to be about dirt, and just how interesting it, or the lack thereof, can be.
What Plants Really Want
When we talk about a plant needing soil, what we’re really talking about is a plant needing grow media, which soil has largely taken the conventional role of. By definition, grow media is anything a plant grows through. Plants are picky though, and not just anything will suffice as a suitable media. A Rock, for example, is too dense for a plant to grow in, but, as you’ve surely seen in cool nature pics, a plant can grow through the cracks in a rock. This tells us that grow media must be porous. Like your ex, they need some space to grow, for their roots to spread; but not so much porousness that the plant structure is unsupported. Dirt has classically cornered the market on grow media because it is abundant, and riddled with microscopic space where its tiny particles don’t line up with each other. It’s these porous spaces — or just pores, for those in-the-know — that facilitate the flow of adequate air and water to a plant.
So, it follows that if we can identify an alternative grow media with the appropriate porousness, we can foster healthy plant growth, in an eco-friendly and economically viable way, without the use of soil.
Before we get ahead of ourselves, there are a few questions we must raise when vetting any material for its grow media usefulness. First of all, pore size, as we mentioned. It turns out that there’s no one consistent pore size that’s perfect for a plant. In fact, a healthy, heterogeneous mix, called loam, is generally seen as being the best distribution of sizes in traditional farming (we’ll spare you the inevitable eye-glazing that comes with inspecting the USDA soil texture triangle).
We also want to know how well the material in question holds water, and if it reacts to chemicals in the water. We want to know if it decomposes over time, too. For example: tomatoes, when grown indoors, can take about 11 months to mature — far too long to be grown in organic materials, which will render themselves useless and decompose into mulch, potentially even clogging the irrigation system. Additionally, we must make decisions on a plant-by-plant basis. Microgreens and large lettuce heads need vastly different amounts of support, and have differing ideal oxygen transfer rates that must be factored in.
The most important factor, though, is the cost; not just the financial cost — it’s the 21st century after all — we must also weigh the environmental cost of any input into an agricultural system. Is my grow media reusable? If so, how do we treat it beforehand, and clean it after to be used again? If it isn’t reusable, how can we ethically and responsibly dispose of it?
Now, let’s take our concerns, and apply them to the four most accepted types of alternative grow media, shall we?
Organic Grow Media
Organic matter is so hot right now. Corn husks, coco coir (coconut husk fiber), rice hulls, wood shavings, burlap; most are byproducts of food production and other processes proving to be potentially useful as alternative grow media. It seems like a great way of recycling byproducts into something that’s productive. But the more we tinker with them, the more we find organic material to be a mixed bag for fostering plant growth. All organic matter decomposes, which is great for sustainability. But decomposition, as mentioned earlier, can leach chemicals and debris into the growing system, clogging irrigation, creating nutrient imbalances and affecting pH levels. On this note, when sourcing organic matter, it’s important to know (and sometimes difficult to discern) what chemicals have been applied to it in its past life in case any of them could be toxic to plants.
Oh, and a note about peat moss. While it’s often lumped in as a solid alternative grow media, and its fast decomposition is helpful during the germination process of plant growth, the harvesting of peat moss devastates wetlands, and is thus highly unsustainable. Please, don’t mess with wetlands.
Mineral Grow Media
Minerals are an interesting grow media. Soil, after all, is a mix of organic matter and minerals. Most are chemically inert, or non-reactive, which can be helpful and non-intrusive to a system. Some, like gravel, never decompose and can be cleaned again and again, which is good. Others, like mineral wool, are effective, but can’t be cleaned at all. Sand can be reused but has much smaller pores than desired. Expanded clay is light and inert, but also expensive. In short, a hodgepodge of several different minerals mixed together is likely an effective grow media. But minerals are harder to modify than organic matter, and the truth is that since the dawn of agriculture we have been playing around with minerals looking for the right balance and there’s still plenty of room for experimentation, but perhaps not as much as other media options. Maybe it’s just the right mix of perlite, vermiculite, sand, recycled glass, brick shards, and gravel. We don’t know. Try it out yourself and get back to us.
Polymer Grow Media
Polymers, AKA plastics, have a lot of potential. They’re super long molecules and can be more easily manipulated than minerals, but they have lots and lots of little pores, which makes cleaning impractical, and throwing them out after each use is unsustainable and terrible for the environment. However, as we learn more about how to get a seemingly chemically inert substance like plastic to decompose, that the days of your kooky aunt negging you about plastic sitting in a landfill forever may be over… but for now, she’s mostly right. Another problem is that in their molecular form, polymers, such as reticulated foam, bind to oil, not water. This can be problematic with regards to a system’s water retention needs. There is some hope, however, with biopolymers and recycled plastics such as polyester clothes. Biopolymers flaunt organic and polymer properties, and can be decomposed at the grower’s whim, which allows for nearly indefinite growing and eventual composting. Pretty neat, eh? This can be more commercially viable moving forwards, and frankly has us pretty excited.
No Grow Media At All
Finally, you could theoretically attempt plant growth with no grow media. As long as you had a support structure, such as, say, the mesh of a window screen holding up a plant right at the base of its stem, the roots can branch out and lock into the screen. This minimizes grow media to nearly nothing (therefore, less waste, less cost, less labor required), but creates other problems such as requiring more inventive water distribution methods. If you want to know more about irrigation challenges like that, we’ve got you covered here.
The Dirt on Dirt
So what have we learned about soil? Mainly that we don’t need it. Alternative grow media should be at the forefront of any innovative farmer’s mind. We can and will find a grow media that best balances high plant yield and low cost. Something environmentally sustainable that allows for easy cleaning and reusability. As growers continue to innovate, we may find all sorts of well-balanced alternatives (Gels! Polyethylene! Peanut butter?). But we won’t get there without a little ingenuity and a lot of experimentation.
