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Soil health a moving target

There’s no one-size-fits-all measure of soil health, David Lobb says

... the health of one soil can be very different from the health of another and both are healthy.” – David Lobb, University of Manitoba.

Saying a soil is ‘healthy’ isn’t something simple like running through a checklist.

David Lobb, a soil scientist at the University of Manitoba says it’s a moving target that takes many variables into account.

There are hundreds of different soils across the province, thousands across the country and the development of each one moves toward a balance with its natural conditions, its local hydrology and its regional climate, he told the Manitoba Agronomist Conference earlier this winter.

“Consequently the health of one soil can be very different from the health of another and both are healthy,” he said. “I always encourage students and farmers to consider crop yield. It integrates a variety of environmental conditions in the site, it’s probably the best biological indicator of the health of the soil.”

Most western Canadian farms were set up on native grassland and these soils are rich, fertile and ideal for cropping. In a natural system, a grassland soil evolves under a cover of varied perennial plants and it’s fairly stable, evolving to that balance of natural conditions.

Farming changes that. Crop species are annuals and they aren’t native to the Canadian Prairies. Because they’re annuals they’re very productive but they thrive on disturbed soil. When plowed, tilled and harrowed the topsoil becomes susceptible to erosion and an eroded soil isn’t a healthy soil.

A healthy soil does a number of different things besides growing plants. It filters and stores water. It regulates the local hydrology by slowing the movement of water and helps to reduce peak stream flows and flooding. It stores a tremendous amount of carbon and is extremely important to the carbon cycle and climate change.

“Soil stores more carbon than all the carbon that’s stored in the vegetation and more than is found in the atmosphere and in the surface of the ocean,” he said. “How we manage the carbon of soil definitely affects our climate.”

Carbon is also the vital part of the “fuel” that drives life on earth. Soil with a good supply of carbon is home to a wide variety of different microbes, fungi, plants and animals that form a vibrant and complex ecosystem. It’s the foundation of nutrient cycling and has a huge impact on how other ecosystems work. This includes agro-ecosystems as well.

“I would argue that a healthy soil produces an abundant stable, nutritious and profitable food supply,” he said. “When we’re talking about soil health we’re really interested in agricultural soil productivity and crop profitability.”

Lobb has been looking at how Manitoba’s soils have eroded over the last few decades and there’s both good news and bad news. The good news is that soil erosion has lessened since the 1970s.

“The reduction in annual soil erosion rates has been greatest in the Prairie provinces,” he said. “It’s largely in response to the adoption of conservation tillage or zero till and the reduction in the use of summerfallow.”

Overall, the land is healthier than it was 50 years ago. The bad news is that it could, and should, be better. Conservation tillage doesn’t actually stop erosion and loss of soil organic carbon. It simply reduces it. Tillage still causes erosion and should be the focus of future conservation efforts. Conservation tillage must focus on the amount of soil movement during tillage as well as the amount of crop residue left on the soil surface.

“Many crops such as soybeans and potatoes deplete the soil of organic carbon even under conservation tillage,” he said. “It may be very difficult to achieve a net gain in organic carbon within conventional crop rotations.”

Lobb suggests an integrated approach to manage all forms of soil erosion. We have to look at wind and water erosion but we have to look at the combination of wind, water and tillage. Any assessment of soil erosion has to consider the annual rates as well as the cumulative total loss of soil. We need to know the historical impact of soil erosion to target effective management practices to sustain or enhance soil as well as crop productivity and profitability. We also have to build back soil carbon.

“Including cover crops in the rotation may be an effective means of building the soil organic carbon and importing organic carbon to these areas may be necessary,” he said. “That may be a necessary alternative to growing organic carbon on the areas if they’re not successful in producing biomass.

“If I had to assign a grade I would give Canada and Manitoba a C+,” he concluded. “Although considerable improvement has been made to slow the degradation of soil health, insufficient creativity innovation and effort has been made to build soil health, to sustain productivity and profitability for future generations.”

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