Certain soils in southwestern Oregon pose a challenge for crop production. Known as serpentine soils, they are formed from the mineral serpentinite and contain high levels of magnesium and other metals that can restrict plant growth. Hemp is a key local crop, but these soils make successful cultivation difficult.
The results offered useful insights for other southwest Oregon farmers navigating the complexities of serpentine soils.
Adding calcium in the form of gypsum is commonly recommended to reduce magnesium levels in soils. However, this approach had not been tested on Oregon’s serpentine soils, and research in California has shown inconsistent results.
In 2022, a major hemp producer in Josephine County reached out to Gordon Jones, Oregon State University Extension agricultural faculty and associate professor of practice in the Department of Crop and Soil Science in the College of Agricultural Sciences. The grower had been applying large amounts of gypsum to their mixed serpentine fields over four years based on advice from a fertilizer dealer—but the benefits were unclear.
Jones reviewed the limited research on serpentine-influenced soils and consulted with colleagues Rich Roseburg, former director of the Southern Oregon Research and Extension Center, and Dan Sullivan, Extension nutrient management specialist and professor emeritus. Together, they synthesized existing theory and research, which Jones shared with the grower.
Based on those discussions, the grower agreed to discontinue gypsum applications in parts of their field and participate in a simple, farmer-led on-farm trial. Jones designed a side-by-side comparison of hemp grown with and without gypsum. At harvest, he collected plant tissue samples to evaluate nutrient levels, while the grower and their team assessed flower quality and plant performance.
The crop performed equally well with and without gypsum. Nutrient analyses showed no problems in untreated areas, and visual observations suggested the 10-acre field looked better than the previous year. The grower reported saving $5,000 to $6,000 in material and labor costs without sacrificing yield or quality.
“As you suspected, we saw no reduction in plant health, vigor or yield in areas where we didn't apply gypsum,” the grower said. “Between customer testimonials, test results, and our cultivation team’s observations, we were thrilled with how the crop turned out.”
The results offered useful insights for other southwest Oregon farmers navigating the complexities of serpentine soils — helping reduce unnecessary costs while improving soil management.
This work was supported by the Agriculture and Food Research Initiative, Sustainable Agricultural Systems, Grant No. 13333755/Project Accession No. 1027531 from the USDA National Institute of Food and Agriculture.
