This question and answer are part of our Ask CSU Extension Anything (About Native Plants) series. We appreciate CSU Extension for answering these questions to expand our community’s knowledge of native plant landscaping. Send us your questions to the Ask CSU email account.
FULL QUESTION:
We heard at the Landscaping with native plants conference that a research experiment is showing that expanded shale is a very good additive for native plants. The downside to it of course is that it has to be mined, which contributes to environmental problems. Do you recommend adding expanded shale when planting native plants?
ANSWER:
A handful of studies have been conducted on the use of expanded shale in “permanent” landscape plantings, and some have demonstrated positive effects, where 3-6mm diameter expanded shale can help native plant performance in the short term (for example, the work presented at the Landscaping with Native Plants conference, but see also Mechleb et al 2014). At least one similar study, though, showed that expanded shale conferred no benefits for plant growth over other amendments (Anderson, C 2018). Even studies showing benefit 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?
Teasing out the subtleties of soil hydraulic conductivity is tricky; certainly the benefits (or non-benefits) of any amendment will depend on the soil texture and soil structure that was there to begin with. In the Sloan (2002) and Mechleb (2014) studies, the soil was described as silty clay and plastic-clay, respectively, which naturally drains quite poorly. In the East Texas study area, natural precipitation kept the soils saturated. Indeed, Mechleb recommends expanded shale as an amendment to improve water-relations in the soil of green infrastructure designed to mitigate rainwater runoff in an area where the native soil is naturally fertile with high water holding capacity. Soil across the Front Range is “dominantly well drained” (yes, really) regardless of soil depth (NRCS 2022) so any claimed benefits of expanded shale may not be based on the same mechanism as in less well drained, wetter climates.
Expanded shale has been shown to be a valuable addition to many “manufactured soil” blends to improve drainage in urban landscapes, particularly for those soils classified as “structural” and for the management of stormwater from impervious surfaces (Sloan et al. 2012).
Should it be used in long-term, single-family lot-scale native plant gardens? Given the environmental costs, monetary cost to the consumer, and inconclusive benefits, (not to mention the labor!), mixing in expanded shale may not be worth your time. Instead, focus on:
- Choosing plants that are suited for your soil conditions without amendment (many natives will be).
- Remembering that a native plant garden is also typically a dry garden (meaning you want to maximize soil water storage, not minimize it by promoting drainage).
- Using mulch (wood chip or gravel), the benefits of which have been conclusively demonstrated.
References
Anderson, C. 2018. Expanded shale as a soil amendment for the rocky mountain region (Order No. 10838377). Dissertations & Theses @ Colorado State University; ProQuest Dissertations & Theses Global. (2116576464).
Mechleb, G. et al. 2014. “Use of Expanded Shale Amendment to Enhance Drainage Properties of Clays,” in Geo-Congress 2014. [Online]. pp. 3444–3454.
NRCS 2022. Land Resource Resource Regions and Major Land Resource Areas of the United States, the Caribbean, and the Pacific Basin. United States Department of Agriculture, Agriculture Handbook 296 p 154
Sloan, JJ et al. 2002. The Suitability of Expanded Shale as an Amendment for Clay Soils. Hort Technology 12:4 646-651
Sloan, JJ et al. 2012. Addressing the need for soil blends and amendments for the highly modified urban landscape. Soil Science Society of America Journal 76:4 1133-1141