This was a clear message delivered by Mario Tenuta, Canada research chair in applied soil ecology and professor at the University of Manitoba, at a recent Manitoba Farm Writers and Broadcasters Association lunch.
“What the heck is soil health?” asked Tenuta. “You hear a lot about it these days, especially this year, the 2015 International Year of Soils. A few years ago everyone was talking about soil quality, but that’s been exchanged for ‘health.’ But these two concepts are different. Soil quality is about the ability to produce food, whereas soil health has a different definition more akin to our own personal health — the continued capacity of soil to function.”
Soil management is an important research priority in Canada, and most farmers have implemented conservation practices such as zero-tillage in an effort to get the most out of their fields.
But Tenuta said we are only scratching the surface when it comes to understanding the role of the soil microbiome in promoting plant health.
What researchers do know is that “soil critters” like bacteria and fungi improve plant health by decomposing organic matter, cycling and redistributing minerals, maintaining reservoirs of nutrients, degrading pollutants, and naturally regulating pest species.
Plants and soil bacteria and fungi have a symbiotic relationship; plants also contribute to soil health. Tenuta said researchers are discovering that the roots of plants leak carbon that is utilized by soil bacteria and fungi. “Plants are actually farming soil organisms for their benefit around the root system,” he said.
It seems plants naturally cultivate beneficial bacteria and fungi near the surface of the root by sending out materials that promote their development. In turn, these micro-organisms have co-evolved to be able to utilize material emitted by plants.
“We’re learning that it’s the bacteria in people that determines our auto-immune responses. I think we’ll learn that it’s the organisms in the soil that determine soil health.”
Respecting soil complexity
Tenuta said increasing soil organic matter does not immediately lead to improved soil health if nutrients are “tied up” in forms that can’t be utilized by the plant.
The “soil food web” comprises many complex, layered relationships between soil organisms, and each layer is essential to ensuring nutrients cycle through the soil.
There are a variety of ways of accessing those nutrients — some organisms can draw nutrients directly from plant roots, while others get nutrients through decomposition and others through feeding on soil organisms. In healthy, biodiverse soils, lower-level trophic organisms are consumed by higher-level trophic organisms. The process gradually “unties” soil nutrients like nitrogen, phosphorus and sulfur so they can be utilized by plants.
“If you’re bad to your soil you truncate a lot of that diversity — the larger trophic organisms are the canary in the coal mine and you get rid of them in unhealthy soils,” said Tenuta.
A worst-case scenario is one in which producers get rid of higher-trophic organisms, which means lower-trophic organisms like bacteria and fungi increase to unhealthy levels. The release of some nutrients is dependent on bacteria and fungi dying. “Massive amounts of tillage will destroy earthworm burrows; pollutants could kill higher trophic organisms,” said Tenuta. “We’re getting better and better with agrochemicals, whereas a lot of them in the past were indiscriminate.”
Microbial nitrogen is released from bacteria and fungi to plants, Tenuta explained. “What we don’t want is for the nitrogen to stay in the organisms. We want this feeding to happen so nitrogen can be released.”
Tenuta also pointed to a particular group of fungi — mycorrhizal fungi — as playing an essential role in soil health. “Without these fungi most plants wouldn’t grow in natural soil because of lack of phosphorus. More than 80 per cent of plants are dependent on mycorrhizal fungi,” he said.
What can producers do to promote soil health? Tenuta says rotation is key, and producers should intentionally increase soil organic matter. But a greater respect for the importance of the soil microbiome is fundamental. “Have a mindset that you’re ‘growing soil,’ not just plants. Make it part of annual farm plans to think about feeding soil.”
Time is another key ingredient in understanding soil health. “Using soil tests, track soil organic matter, inorganic nutrients, yield and protein over many years,” he says. “Observe improvements in structure.”
Soil tests Tenuta noted include the Solvita Soil Health Test, Cornell Soil Health Test, Visual State Assessment Test, and the Soil Food Web Inc. Test.