Is construction-grade expanded clay safe for growing plants?

Q:

Can I use construction-grade expanded clay in place of brands like Hydroton when cultivating plants?

A:

In the pursuit of cultivating plants hydroponically, growers are constantly in search of ways and or methods to keep their cost of production low in order to maximize profits.

There are certainly substitutions, alternative materials, and other components that may be used in place of brands marketed towards growers in order to keep production costs low. I would consider the following before deciding on your source material for expanded clay pebbles as a hydroponic grow medium.

Hydroton and other brands of expanded clay manufactured for use in hydroponics are thought to be of food-grade, lacking other foreign chemical contaminants and materials that may be found in construction materials.

Anecdotal reports that can be found on various grow forums indicate construction-grade materials possessing a higher alkalinity. This would require further buffering and less predictability when soaking the medium and throughout your specific grow cycles. This alkalinity could be a result of exposure to limestone, concrete/cement, construction sand, mortar, and other materials.

Quite often, the construction-grade clay is manufactured or co-packed in diverse manufacturing settings that are catering to other material manufacturers. You can be confident that when buying brands like Hydroton, the clay is manufactured in a dedicated environment free of contaminants.

Beyond raising your pH, these contaminants may act to harm or entirely kill your crop at the drop of a hat with little time to remedy the situation. When buying in retail settings, the grower can also be confident of storage conditions of the clay and whether or not it has been exposed to other harmful chemicals. I would never buy a bag of expanded clay that was sitting next to a dusty bag of mortar mix.

Another thing to consider would be the cation exchange capacity (CEC) of your source of clay medium. Clay manufactured for hydroculture is made to specific parameters suitable for growing plants, with a mostly inert quality. Construction-grade clay may possibly be fired at lower temperatures, resulting in a higher, lingering CEC.

This attribute, if not kept in check, could very well lead to phytotoxicity. As mentioned in the article, the clay is pretty well inert, but even with Hydroton, one still needs to flush accumulated salts from the medium once and a while. I fear this issue could become compounded with higher-CEC clay.

Standards in particle size are also important. Construction-grade clay may be either too small or too large. If too small, the macro pore space may be too tight and thus result in compacted medium with less aeration.

If the pebbles are too large, which is common with construction-grade material, pore space may be too open and would require more diligent control of your irrigation flood cycles (if using ebb and flow).

From what I can see on some manufacturer’s websites, construction-grade clay may also be of a higher thermal conductivity than that of hydroculture clay. What this means is that, should you experience fluctuations in reservoir temperature, whether you are using ebb and flo, DWC, or other hydroponic culture, the clay would then act to hold and store whatever water temperatures it is exposed to.

If you are gone for a couple days and your HVAC system suddenly fails, you may experience a surge in high water temperatures. A higher thermal conductivity in your medium would ensure the roots may be more prone to things like pythium, botrytis, gnats, and other pesky problems.

With all this being said, I do believe in trial and error and further experimentation. I encourage you to trial both options. At the very least, you may benefit from monitoring a smaller identical set-up with construction-grade expanded clay. If you are able to mitigate the potential issues discussed above, then the lowered cost of production would only serve to make you a smart and savvy grower.

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Written by Zach Zeifman
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Zachary Zeifman discovered his love for gardening while working for Homegrown Hydroponics/Dutch Nutrient Formula. Zach now owns and operates Soulgarden Farm, where he grows sustainable hydroponic and traditional soil crops. During the winter, Zach helps homeowners design and build hydroponic gardens to grow food year-round.

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