Does Expanded Shale Fix Clay Soil?

Heavy, sticky clay soil is a problem for gardeners who will try anything to make the soil easier to dig. One proposed option is expanded shale.

Does it work? Should you use it?

What is Expanded Shale?

Shale is a type of rock. Expanded shale is shale that has been heated to very high temperatures, which causes gases inside the rock to expand it. This is very similar to popping popcorn – except you can’t eat the shale.

Expanded shale is lighter and contains more air pockets, which can absorb large quantities of water, compared to shale rock. It reminds me a lot of perlite or vermiculite, both of which start as natural ore and are heated to expand them. Expanded shale is more stable than either perlite or vermiculite.

Adding expanded shale to any soil will make it lighter in weight and more porous. It helps the soil absorb more water. It is also added to cement to lighten building materials.

It is claimed that this product will improve clay soil, and that is the main function I want to investigate in this post.

What is Haydite?

Haydite is a specific brand of expanded shale that has become a “genericized trademark” in certain regions and industries, much like “Kleenex” is used for tissues.

The material was patented by Stephen J. Hayde in the early 1900s based on his process of heat-expanding shale to create lightweight concrete.

The words haydite and expanded shale are now used interchangeably by many people.

Does Expanded Shale Improve Clay Soil?

A number of Extension offices claim that adding expanded shale to clay will improve the soil, but most of these are located in or near Texas. This does not seem to be a nationwide practice.

Much of the promotion has been done by Dr. Steve George of Texas A&M Cooperative Extension. He even has an old YouTube video promoting the material, which mentions a scientific field study, but gives no reference to the study. I tried to track it down, but couldn’t find it. He has not provided it by email either.

There are some studies that show expanded shale does make concrete lighter and produces a stable product for the construction industry, but there is very little work done to show it is a good amendment for gardens.

A study entitled “The Suitability of Expanded Shale as an Amendment for Clay Soils” compared small [1 to 3 mm (0.039 to 0.118 inches)] and large [3 to 6 mm (0.118 to 0.354 inches)] diameter expanded shales, quartz sand, sphagnum peatmoss, and cottonseed hulls, in Austin silty clay soil. None of the amendments affected the growth of panies. They also tested Scaevola aemula, which is very sensitive to clay soil, and found that large diameter expanded shale improved growth. Expanded shale did not decrease the bulk density of the soil, but peatmoss and cottonseed hulls did.

In another report, the same principal researcher commented, “Research with expanded shale demonstrated some benefits for improving root growth and flowering, but the results were inconsistent across growing seasons and perhaps inconsequential given the cost of the product. Conversely, expanded shale and slate are primary ingredients in specialized soil blends where there is a need for lightweight or highly porous media such as green roofs”.

A thesis project that looked at various mixtures of expanded shale and compost found no improvement in plant growth for heuchera or penstemon plants in the Colorado area.

The Ask CSU Extension Anything (About Native Plants) concluded, “Even studies showing benefits note inconsistent results between years and plant types (Sloan et al. 2002). Given the expense of the product plus the environmental costs, is it really worth it?”

Is Expanded Shale a Natural Product?

Some articles on the product and its manufacturers claim that it is a natural product. It’s not. It is a man-made product. That does not mean there is anything wrong with the product, but it does take a lot of energy to produce, and claims that it is natural are just wrong.

Other Uses for Expanded Shale

The product is also used in media for plants that like high drainage, such as succulents and bonsai.

The Bottom Line

Expanded shale will loosen clay soil and make it more porous. Unlike compost, it is long-lasting but does not add nutrients.

It is probably a product that could be useful when combined with compost to loosen clay soil. However, the high cost and limited benefits for plant growth do not justify its use.