Soil’s processes need to be understood to build organic matter

Reading Time: 4 minutes

To understand the paradigm shift in the production of soil organic matter you’re going to have to dig a little deeper.

“Soil organic matter (SOM) is intimately linked to soil health, to ecosystem health, to planetary health and it has influences on the physical, chemical and biological properties of the soil,” Sarah Hargreaves, Ecological Farmers Association of Ontario farmer-led research director, at the organization’s annual conference in December.

Why it matters: More is being uncovered relating to how cycles and microbes affect soil organic matter.

Within the old paradigm SOM has been understood as humification, which was an artifact of how the soil was handled and measured in the lab, which drastically changed the complexity and compounds of soil.

Imagine using Drano to get hair out of the drain and you study that clump as “hair” without acknowledging how much the toxic alkaline material had changed the complexity, compounds and structure of the hair.

The new paradigm incorporates the cycles, connections and relationships within an ecosystem and doesn’t just look at one asset saying that’s what the formation of the ecosystem depends on. That concept is a passion of Hargreaves.

She channelled that passion into the University of Toronto’s ecology program and was captivated by the intricate relationship of bio-geochemical cycles, which led her to the sub-Arctic near Churchill as field assistant where she ultimately did her master’s degree.

“It was here in the Arctic where it clicked for me that soil was the hub of the cycling that I love so much,” she said. “The recycling of nutrients on a global scale was happening in the soil but it was affecting the world.”

When she and her husband moved from the Arctic to Iowa, Hargreaves swapped her sub-Arctic ecologist hat for an agro-ecology one and studied plant-microbe interactions in perennial and annual cropping systems for her dissertation. The natural connections, relationships and cycles that existed in the Arctic were not reflected in the
Midwest soils; instead, she discovered a system that was “leaky.”

“Agriculture in many parts of the world is broken, it’s leaky,” she said. “There’s even leaking on a community level…they were de-incorporating. Towns didn’t want to be towns, the younger generation were fleeing the state.”

Reconnecting cycles

How can we reconnect the cycles and the relationships in an agro-ecosystem?, Hargreaves wondered. Shortly thereafter she and her family moved to a farm in Ontario and began raising food in a way that cycles and relationships remained intact, connected and held sacred.

Hargreaves, husband Drake Larsen and their daughter rotationally graze their small flock of sheep, a small herd of cattle and raise artisanal pasture-raised chickens and ducks and have had pasture-raised pigs in the past on their 50-acre farm, Three Ridges Ecological Farm.

“We use landscape design to guide the movement of animals on the land and the planting of perennials,” said Hargreaves.

The long-term vision for the farm is one that integrates the storage and capturing of water with the storage and building of soil.

“All of this soil organic matter is central to all aspects of ecosystem connection,” she said. “It signals when your ecosystem is healthy and when your ecosystem is sick.”

The new SOM paradigm embraces that connection by acknowledging the impact of diversity within the ecosystem, photosynthesis, plant residues and root exudates, microbial processing and microbial necromass – all play a part collectively and as individual components.

The importance of microbial necromass (dead microbes) and microbial-derived carbon has really changed the paradigm, said Hargreaves.

Research conducted by Dr. Cynthia Kallenbach, of McGill University, explored if microbes alone could make SOM. She took sterile clay with zero organic matter and added some simple sugars and microbes. After 15 months there was two per cent organic matter present.

The study provided clear evidence that dead microbes, or necromass, was stable and able to remain and accumulate in the soil as organic matter.

“Two per cent organic matter in 15 months under ideal conditions – that’s pretty remarkable,” said Hargreaves. “Here the microbes are teaching us their role is as important, if not more important, in death as it is in life.”

Diversity is an overarching theme for the new SOM paradigm whether it’s in crop rotation, integration of livestock, rotational grazing or the introduction of perennials with deep, large roots taping into the water and providing year-round food and habitat for microbes.

The creation and maintenance of SOM matters because it increases water retention, infiltration and increased storage, a chemical benefit as a buffer for nutrients and a source for more SOM due to the biology it attracts.

The basic tenants of SOM are simple — keep the soil covered at all time either through plants or mulch; maximize diversity; minimize soil disturbance; keep live roots in the ground; use organic inputs and it sinks and stores

The more plant diversity on top of the ground the more of a diverse microbe community you’ll be supporting within the soil, she said.

Adding organic matter

“Organic inputs feed the soil, balance out the microbes and it helps maintain a diverse microbial community,” Hargreaves said.

In order to facilitate that on your farm there are a number of options; conservation tillage, crop rotations, cover crops, composting and compost utilization, livestock integration, adaptive multi-paddock grazing, water capture and the four “Rs” of fertilizer use.

Hargreaves said as farmers it’s generally difficult to hit all these targets but incorporating a few of them into the process will help build SOM and stability.

New research into SOM stabilization within the new paradigm will look at microbial necromass recycling as an important precursor for soil carbon stabilization, said Hargreaves.

As we know, decomposition also leads to carbon in the atmosphere, she said. “It’s a balance between how much are we wanting the microbes to die, decompose and release nutrients because that makes SOM but . . . we don’t want them to be processing more than they’re dying because that will lead to a net carbon in the atmosphere potentially.”

Hargreaves hopes ongoing and future research will be practical enough for farmers to use in plotting cropping and management systems to maximize efficient carbon use.

“This is the place where research is happening,” she said.