July 4, 2023 By Bruce Barker, P.Ag CanadianAgronomist.ca
Soil health has been defined as “the capacity of soil to function as a vital living system, within the ecosystem and land-use boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality and promote plant and animal health” (Doran and Zeiss, 2000). While some countries have developed soil health tests to monitor changes in soil over time, in Western Canada, there was no standardized, Prairie-based system for assessing soil health.
To address this gap, research was conducted by Kate Congreves, associate professor in the department of plant sciences at the University of Saskatchewan, and research technician Athena Wu, with the objective of developing a soil health scoring framework for Saskatchewan soils that integrates biological, physical and chemical indicators. This framework uses these soil attribute values to develop meaningful scores and a weighting system to calculate the overall soil health score.
Soil samples were taken from 55 fields at 26 sites at Agri-Arm locations, producer fields and Agriculture and Agri-Food Canada (AAFC) long-term sites in the fall of 2018. These sites included four in the Gray soil zone, 13 in the Black zone, 21 in the Dark Brown zone and 17 from the Brown zone. Composite soil samples were collected from the zero to six, six to 12 and 12 to 24 inch depths (zero-15, 15-30 and 30-60 cm).
Forty-four per cent of fields were cropped to canola, 29 per cent to wheat, 15 per cent to pulse crops, with three sites in green manure and one site each in flax, barley and potato. One native prairie grassland and one woodland site were also included.
The samples were analyzed for a range of soil chemical, physical and biological attributes. Chemical attributes included pH, EC and total soil concentrations of phosphate, potassium, sodium, magnesium, calcium, manganese, iron, copper, zinc, boron and sulfur. Soil nitrate (NO3-), ammonium (NH4+), soil organic carbon (SOC), total carbon (TC), total N (TN) and potentially mineralized N were also calculated.
The soil’s physical attributes were determined for soil texture, field capacity (FC) and wet aggregate stability. Soil biological attributes measured were soil protein, active carbon, soil respiration (CO2) and nitrous oxide (N2O) production.
Data analysis determined the weighting factors in order to integrate each soil attribute into a soil health scorecard called the Saskatchewan Assessment of Soil Health (SASH).
The most important factors in determining soil health from soil tests were soil carbon and nitrogen factors of soil organic C, active C, total N and soil protein, along with total phosphate.
Carbon and N are important parts of soil organic matter, which is important for nutrient supply and cycling, water supply and cycling, climate regulation and supporting plant growth.
The SASH framework offers a way to measure soil health changes faster than looking at soil organic matter content, which changes slowly over time. This is because the SASH framework measures both labile (active carbon and soil protein) and stable forms (total carbon, total N and soil organic carbon) of soil organic matter.
The overall SASH score in the zero to 24 inch depth ranged from 41.24 per cent to 77.05 per cent. The highest score was on native prairie. This overall SASH score did not differ between soil zones, with a median ranging from 60.17 per cent to 65.68 per cent.
The researchers also compared soil health scores to crop productivity by looking at cereal crop yields in the rural municipalities where the samples were collected over the previous 10 years. During dry years, a positive relationship was observed between higher soil health and higher yields.
A case study was conducted on three sites. The native grassland site had a SASH score of 76 per cent. Farm 1 in the Black soil zone had a history of no-till cereal and oilseed production and a crop rotation that included a cover crop mixture that was periodically grazed by livestock. This farm had a SASH score of 70 per cent, which was close to that of native grassland. Farm 2 had a SASH score of 48 per cent and had a history of intensive potato production with frequent tillage.
Overall, the research found that soil C- and N-indices and total P are the main drivers of soil health differences. Management decisions that support biogeochemical cycling, C and N sequestration and P retention can also improve soil health scores. The SASH framework provides a foundation to help convert soil test data into a soil health scorecard. An online SASH tool is under development and will help inform on-farm management decisions to improve soil health.
Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.