Closing the Loop

We are exploring exploring how to turn agricultural, urban, and industrial wastes into useful fertilizers and other amendments for the soil

Closing the Loop

We are exploring exploring how to turn agricultural, urban, and industrial wastes into useful fertilizers and other amendments for the soil

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Closing the Loop

by Laura Crothers last modified May 15, 2018 01:42 PM

"Closing the Loop" in Context

    • Agricultural and municipal waste products can be used by farmers as soil amendments and mulches

    • By recycling nutrients in wastes and using them for agricultural production, farmers seek to “close the loop” and create a more sustainable cycle of food production

    • Farmers and researchers are exploring how to use new wastes—like biochar and biodigestate—at commercial scales, while ensuring there are no unintended consequences of adding waste products to food crops


How we look at closing the loop

Organic wastes such as manure and crop residue can be composted and used as soil amendments.

We are seeking to understand how different waste products impact crop productivity, soil chemistry, and soil biologyKey research questions we are exploring at the Ranch include:

        • Can byproducts of biogas generation (called biodigestate) provide enough fertility to grow commodity crops like tomatoes and corn?

        • What effect does biochar application to the soil have on crop yields and soil fertility?

        • What does applying biochar, biodigestate, and other wastes do to the complex communities of bacteria, archaea, and fungi in the soil?

        • How can we process wastes into forms that are compatible with agricultural management practices?

Featured Research Projects

      • Biochar and crop yields

        Biochar photo by Oregon Department of Forestry
        Biochar is derived from cooking down wastes (like nut shells and wood) at extremely high temperatures in a low- or no-oxygen environment, producing brittle charcoal that can be added to soil.

        Since 2012, Russell Ranch has been studying how adding biochar and compost to tomato and corn fields affects the soil. These experiments investigated (1) how long biochar’s impact on the soil lasts after a one-time application, and (2) how does using biochar as a soil amendment affect crop yield and the nutrients in the soil?

      • Piloting commercial-scale biodigestate fertilizer production

        • Biogas is produced in oxygen-free tanks through the digestive action of billions of microbes. The nutrient-rich sludge that is a byproduct of biofuel production — called biodigestate or “bioslurry” — is a promising fertilizer.

          Biodigestate solids produced during biogas production. Photo by Bob Nichols of USDA.

          Russell Ranch and UC Davis researchers have developed a pilot process to produce biodigestate fertilizers at commercial scales. Our ongoing research is exploring the nutrients found in different solid and liquid biodigestates, and testing the most cost effective ways growers can produce and transport these fertilizers.

          Early research results at Russell Ranch indicate that biodigestate-based fertilizers contain valuable nutrients and microbes not found in many synthetic fertilizers. And our pilot studies with irrigated tomatoes and short-season corn have shown that farmers may be able to use biofertilizer products as the sole source of fertilizer.

Event Details


  • Biochar applications can boost crop yields in the short term, but the effect attenuates quickly

  • Biochar boosts yields and nutrient availability in both conventional and organically fertilized systems

  • Biodigestate fertilizers can be used as the sole source of fertility by farmers without sacrificing yields


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