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Soil amendments: Biochar and biodigestate

by Zixin Lu last modified May 03, 2017 03:30 PM
Can we take garbage and other wastes and turn it into fuel for food?

Researchers at Russell Ranch are looking into ways to take agricultural, urban and industrial wastes and turn them into useful fertilizers and other amendments for the soil. 

Using waste products as soil amendments can benefit agriculture in more ways than one: it can add more carbon to soil (and keep it out of the atmosphere), can help growers control soil pH and nutrient content, and can potentially help growers save money.

More broadly, integrating waste products back into food production will help society "close the loop" and turn the food system into a more sustainable cycle of food production.

Russell Ranch is studying how two burgeoning practices — the use of biochar and biodigestate as soil amendments — can be used to turn waste into fuel for food. 


Biochar: A promising source of carbon

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Biochar photo by Oregon Department of Forestry

In the Amazon Basin, a rainforest region characterized by infertile soils, areas of soil have been discovered to be almost black in color and rich in nutrients. The soil’s color is derived from its high organic matter content, believed to originate from historical charcoal applications (“biochar”) added to the soil some 2,500 years ago, either intentionally or as a waste product from cooking.

In the past decade, there has been a growing interest in whether the fertility of these “amazon dark earth soils” can be replicated in modern farming practices.

Biochar can be derived from burning nearly any organic material, including many wastes produced in California agriculture and other industries, like nut shells and wood. These wastes are cooked down at extremely high temperatures in a low- or no-oxygen environment, producing brittle charcoal that can be added to soil.

Benefits of Biochar

Can boost water and nutrient retention in soil 

Can Iincrease carbon storage in the soil

Increases soil pH and nutrient availability in acidic soils

Challenges of Biochar

Soil texture, mineralogy, and organic matter impact biochar's effects

Some types of biochar can inhibit microbial activity in the soil

Added cost to farmer

While there is a lot of excitement surrounding biochar as a cheap and effective way to add carbon and other nutrients to soil, the evidence supporting its widespread use in agriculture is limited. Researchers at Russell Ranch have been testing the effects of biochar on California's fertile soil, asking questions like: how long of an effect does biochar have on the soil? How does biochar impact crop yield and soil nutrient composition? What does applying biochar do to the complex communities of bacteria and fungus in the soil? 

You can explore some of our biochar research results in the interactive below.

Biodigestate: Using biofuel waste as a "biofertilizer" 

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

Through the digestive help of billions of microbes housed in oxygen-free tanks, people can take food waste and create biogas, a clean renewable energy. What’s left behind in the tank is a nutrient-rich sludge — called biodigestate or “bioslurry” — that is a promising fertilizer.

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 sustainable fertilizers.

While our work is ongoing, early results are promising. We’ve found 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.

But biodigestate-based fertilizers are not without their challenges. Manure-based liquid forms of the fertilizer have the potential to clog drip irrigation systems; ongoing research is exploring how to address this problem to expand biofertilizers’ use beyond solid pellets. And it is still unknown how much nitrogen in these fertilizers is being taken up by crops.

Benefits of Biodigestate

Sustainable source of fertility for farms

Can be applied directly to soil as a fertilizer

Can be concentrated and filtered for use in drip irrigation

Challenges of Biodigestate

Unprocessed form has limited shelf life

Composition varies depending on food waste source


Our biochar research collaboration includes Russell Ranch director and UC Davis Professor Kate Scow, Scow Lab graduate student Deirdre Griffin, Professor Sanjai Parikh and Parikh lab postdoctoral researcher Fungai Mukome. Our biofertilizer research collaboration includes Zhang Lab graduate students Tyler Barzee and Hossein Edalati, Scow Lab postdoctoral researcher Daoyuan Wang, and Russell Ranch manager Israel Herrera. Collaborating institutions include CleanWorldCalifornia Bioenergy, New Hope Dairy (Galt, CA), Fiscalini Dairy (Modesto, CA), and Sacramento Municipal Utility District.

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