To Dig or not to Dig

Stable Organic Material in Soil

By: Ann Northrup

Ann Northrup spent her undergraduate years at the University of Michigan, where she earned a Bachelor of Science in microbiology….

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Compost turns yard waste and even kitchen scraps into a valuable soil amendment. Photo: Shutterstock

Compost turns yard waste and even kitchen scraps into a valuable soil amendment. Photo: Shutterstock

Gardening advice often includes the directive to add more organic material to soil. Especially for those who are trying to condition an otherwise hard clay, the optimism gained by fluffing it up with plant-based compost is dashed when the very next year the soil looks like it is right back to it’s old hard, compacted self. What happened to the organic material that was added? The answer is that most of it turned into carbon dioxide upon breaking down, but some of it was converted to stable organic molecules that can’t be seen. Traditional belief is that “stable” organic material in soil is composed of recalcitrant plant-based molecules that are either protected in aggregates or chemically complex and resistant to decomposition. However, evidence is mounting that stable molecules of organic soil material are a product of plant material but are of microbial origin, particularly from fungal metabolism. It takes around 18 months for stable organic carbon molecules of microbial origin to accumulate and improve soil structure by amending pure clay with simple sugars. The take-home message is that amending garden soils with plant compost will improve soil structure with time, however, the remnants of the initial plant residue will disappear. Best not to work the soil too much and disrupt those beneficial fungal communities or destroy the hard-fought, albeit gradual, improvements in the soil structure.

Nature Communications 7, Article number: 13630 (2016)