Stop and smell the dirt

University of Alberta researchers develop a soil heatlh test by evaluating the composition of the soil

Most farmers and gardeners recognize healthy soil.

It has a certain look and smell and likely feels softer than poorly conditioned soil.

That sense of quality is mostly based on experience and scientists have struggled to measure the traits of healthy soil — until now.

Researchers at the University of Alberta have developed a test to measure soil health by assessing the aggregates within a soil sample.

Aggregates are groups of soil particles that bind together. Spaces within and between aggregates provide pores and pathways for air and water to move.

“Aggregation is very important because better aggregated soils are better able to hold water available for the plant, and to infiltrate water, to manage (wet) conditions,” said Guillermo Hernandez, a University of Alberta assistant professor in agricultural, life and environmental sciences and one of the developers of the new test.

It seems strange to test and quantify something that is sensory, but farmers and agronomists like metrics.

“There’s a big movement switching toward this idea of having soil health scores,” said Marla Riekman, land management specialist with Manitoba Agriculture.

Producers are already using soil health tests, such as the Solvita test, which measures the amount of carbon dioxide that a soil sample releases.

More carbon dioxide equals more microbial activity in the soil.

Cornell University has a comprehensive method to generate a soil health score, which looks at things like soil tilth, soil drainage, the number of beneficial organisms and aggregate stability.

The University of Alberta test assesses the structure of the soil by evaluating soil aggregation.

“How the soil is put together. How the particles in the soil are adding to one another and building the structure of what we call soil,” Hernandez said.

The U of A scientists used 3D laser scanning to get a detailed look at soil aggregates, ranging in size from .25 to 16 centimetres in diameter.

In a news release, the University of Alberta said the results of the tests correlate nicely with known markers of soil health.

“Recent advancements in laser technology have now enabled accurate, rapid measurement of such fractal aggregation, which was previously unfeasible,” the U of A said. “Validation results showed close agreement with soil carbon and water retention, collectively leading to robust metrics of land sustainability.”

Hernandez said larger aggregates are often “fluffier” than smaller aggregates within soil, likely because the bigger aggregates have more pores for holding water and air.

So with larger aggregates there is more space for water and nutrients to move and additional room for roots to grow.

“That’s what we want to see in soils,” Hernandez said. “The ability of the soil to hold water and air, which are (critical) for plant growth…. People have been able to gain this insight that soil health is what powers the ability of the cropping system, to deliver productivity in the long term.”

However, the scoring system for the U of A test, which doesn’t have a name, is counter-intuitive.

When a sample is tested it re-ceives a score between 0.9 and 1.0. The closer to 0.9, the better the soil quality.

“It (the soil) becomes softer … the number becomes lower,” Hernandez said.

The U of A researchers used the test on a variety of soil types and different cropping systems and reached several conclusions:

  • Native forest and native grassland had the best quality soil.
  • Perennial crops and diverse plants enhanced soil quality.
  • Fallow and annual cropping decreased soil quality.
  • A few years of controlled traffic farming seems to increase soil quality.

Having a test to measure soil health could become essential because consumers and corporations want to know how farmers are managing the land, Hernandez said.

“In order to have access to market, nowadays, and to be able to have social licence, the agricultural sector needs to show those (measures) of sustainability,” he said.

“This would be one way to (quantify) the soil management.”

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