Expanded shale, not gypsum, has water retention capabilities in soil.
Instructions on this 40-pound bag of expanded shale include blending in a 3-inch layer of compost along with the shale to best amend clay soils.
Expanded shale mixed in soil aids in retention, drainage

February 26, 2009

by Sara and Alton Meyer
Victoria County Master Gardeners

edited by Charla Borchers Leon
Victoria County Master Gardener
Use shale, potting soil and mulch in containers for proper drainage.
With temperatures in the 70s in recent days, some gardeners may be thinking about redesigning flower beds and landscaping. However, with our area of the Coastal Bend hit by drought for more than a year, gardeners must take into account another factor when creating new landscapes - water availability.

Researchers at Texas A&M University and at the Dallas Arboretum use a newly promoted product, expanded shale, which helps with this aspect of soil preparation.

According to Jim L. Turner, director of horticulture research, Dallas Arboretum, "More plant deaths in Texas are due to overwatering and poor drainage." Expanded shale can help with several gardening concerns - clay soil, water conservation, and proper drainage.


Blue Shale, located in the Texas Midway Shale formation, is found 10-15 feet underground in a pattern from Corsicana to Texarkana south to Laredo. Jack Sinclair of TXI Industries says, shale is mined and ground to - to 1-inch size particles and then kiln fired for 40 minutes at a temperature of 2,000 degrees C. Due to the high heat, the 60 to 70 percent silica content changes in chemical make-up causing the material to expand. As the material cools, cavities are left after the gases escape, leaving a porous lightweight chunk capable of absorbing water, which is slowly released later as needed. Basically stone, expanded shale does not decompose like organic matter. Since expanded shale does not decompose, its use is a long-term investment when compared to compost.


For many years, gardeners used gypsum as an amendment for clay soil. Gypsum is also a natural product, but used in powder form when amending soils. Gypsum does not have any of the water retention capability that expanded shale does. Gypsum may promote drainage and aeration of soil, but must be re-applied each year. Gypsum is calcium sulfate, a water soluble form of calcium, said Dr. Gene Taylor, Texas AgriLife Turfgrass extension specialist. He also added that most of the soils in Texas are high in calcium and adding more gypsum "won't improve the soil structure."


Dr. Steve George, Texas AgriLife Extension Service horticulturist in Dallas, recommends the use of expanded shale particularly for heavy, sticky clay soils. After a two-year research study and six years of field trials, he said that expanded shale will "open up and aerate clay soils faster than any other product tested."

Several trials are under way in the city of Duncanville and in the city of Carrollton using expanded shale in each city's perennial flower beds. Trials include working with black gumbo only, lava sand with gumbo, compost with blackland soil and compost and expanded shale in existing soil. He also indicates that the lightweight material might create a permanent physical change in the blackland soil for at least "10 years as a conservative estimate." Expanded shale not only increases soil porosity, but also reduces compaction, insulates from temperature extremes, and absorbs "38 percent of its weight in water to release later during drought periods," according to the results of these trials.


For new flower beds in clay soil, lay down a 3 inch layer of the expanded shale with a 3 inch layer of compost, then till in 6-8 inches deep. Expanded shale must be incorporated into the soil, not added on top of the soil surface. Crown beds to further improve drainage. After planting plants, cover with a 3 inch layer of mulch. If soil is sandy or loam, expanded shale is not a necessary addition to improve drainage although its water holding capacity may be the bonus.

George explained that in preparing beds in this manner, commercial fertilizer is not necessary and compost won't need to be added again as "the mulch will break down and work its way into the soil." A year after preparing the bed, bring the mulch layer up to 3 inches. In subsequent years, the mulch breaks down faster. In the spring and in the fall mulch will need to be added to raise the mulch level back up to 3 inches. We also found out that the Dallas Arboretum personnel use expanded shale in all of their rose gardens to help with drainage and water conservation.

It works great in containers, too. For container gardening, apply a layer of expanded shale on the bottom of the container and then mix 50-50 with potting soil for the balance of the container. In large containers the use of expanded shale reduces the need for watering and the weight of the planted container.


The gardener can find small 40 pound bags of expanded shale in Victoria at A to Z Nursery, Earthworks, Four Seasons Garden Center and Renken's Nursery and in Cuero at Ful-o-Pep Garden Center and Texas Homestead Nursery. If the garden project is larger, expanded shale is available in bulk quantities from the supplier which is more economical for the homeowner. The initial investment is more expensive than other amendments, but with the long lifespan of ten years, the cost of this amendment is less.

Using expanded shale as a water wise soil amendment, Neil Sperry, Texas gardener, horticulturist, and publisher of Neil Sperry's Gardens magazine, states: "I have been a big advocate of expanded shale for many years and can't imagine working my soil without it."

Promotes drainage and aeration

Enhances plant performance

Reduces soil maintenance

Promotes healthier root system

Absorbs and releases water slowly


Newly created flower beds

Window boxes

Raised beds

Large containers or pots

Parking lot islands

Landscaping in park areas

Roof gardens

The Gardeners' Dirt is written by members of the Victoria County Master Gardener Association, an educational outreach of Texas AgriLife Extension - Victoria County. Mail your questions in care of the Advocate, P.O. Box 1518, Victoria, TX 77901; or, or comment on this column at