Farmers Test Whether Clay in Compost can Help Mitigate Climate Change

November 1, 2022
A touch of clay may make the compost at Midnight’s Farm even better. Photo by Faith Van De Putte.

On a 100-acre farm in Washington, farmers David Bill and Faith Van De Putte are curing a new compost mix that could provide important clues to fighting climate change.

“We’re adding a bit of clay to see if that increases nutrient density and carbon retention,” explains Bill from Midnight’s Farm on Lopez Island, where he and his wife, Van De Putte, produce compost, raise cattle and pigs, and grow a market garden. The team recently received a $25,000 grant from Western SARE to test whether the minerals in clay could improve compost quality.

Healthy soil is alive with billions of organisms that, when fed and cared for properly, can nourish plants, detoxify pollutants, hold water, and cycle and store carbon from the atmosphere. Compost is rich in organic matter that feeds the bacteria, fungi, insects, worms, and other organisms in healthy soil.

Much research has been conducted on how organic matter like plant roots and compost promotes soil health and carbon sequestration. With climate change intensifying, scientists are urgently exploring ways to augment carbon storage in soil. One mechanism involves boosting mineral-associated organic matter, the largest and most persistent pool of carbon in soil.

“That’s where this experiment comes in,” Bill says. “In general, clay soils hold more carbon than sandy soils. We’ll measure the clay-added compost for mineral concentrations, carbon content, particle size distribution and cation exchange capacity, which influences soil’s ability to supply important nutrients to plants.”

Bill and Van De Putte will also grow lettuce seedlings in a potting mix enriched with clay compost and examine, in triple-replicated trials, whether the new recipe has any impact on lettuce nutrition and yield.

Midnight’s Farm produces about 1,000 yards of compost and 2,000 yards of wood chips each year from yard, wood and agricultural waste collected onsite and from people throughout the community. Theirs is the first compost facility in San Juan County approved by the Washington State Department of Ecology for its high standards and quality product.

“A lot of the debris we collect used to get burned, and you’d see smoke over a good part of the island,” Bill says. “It’s rewarding to, instead, transform it into something that’s such an essential part of soil fertility.”

Bill and Van De Putte were inspired by emerging research on mineral-associated organic matter and wondered whether they could make their compost even better. They will test two types of clay: store-bought montmorillonite clay and clay harvested from an onsite irrigation pond. Both clays will make up about 5 percent of the composted ingredients.

“If we find any benefit from compost with added clay, we also intend to develop a mechanism to disperse clay into compost feedstocks at a larger scale,” Bill says. “We will share our findings widely, and hopefully bring new understanding of carbon stability to the age-old practice of making and using compost. In compost as in life, it’s all about trying to make things better.”

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Topics: Composting
Related Locations: Washington, West