by E. Bader – January 2024
Soil testing at the farm reveals the rich soil life and organic matter below the surface and why annual soil tests should focus on more than just the basic nutrients to optimize soil health.
In September, the team at Domaine de Graux was busy digging holes across the farm, not to plant seeds or transplants, but to test the soil. Soil testing is a key management tool for any farm, conventional or agroecological. However, the role of soil testing and the analysis differs vastly between these two types of farming.
History and Types of Soil Testing
Soil testing first became common in the 1940s, driven by the new focus of soil health in the aftermath of the Dust Bowl, mechanization, and the adoption of synthetic nitrogen fertilizers post-World War II. These historic soil tests, and the tests for many decades to follow, focused on a handful of measurements, or “NPK” the basic nutrients of nitrogen, phosphorous, and potassium, and pH. These measures guide the applications of lime to manage acidity, and of fertilizer, the key input for industrial ag production.
Today, common soil tests still focus on the NPK and pH, key minerals, and physical properties such as soil type, texture, structure, and moisture content. However, soil testing has evolved along with our growing understanding of soil health and the diversity of life below our feet. There is a range of soil testing metrics from the minimal “NPK” up to Trace Genomic’s diagnostics that sequence the genome of your soil’s microbiome and identify levels of 250 different pathogens in addition to the NPK and basic soil structure.
Methods for sampling are evolving with technology as well. Testing can still be done with manual tools and processes or can now be technology-powered services that combine robust sampling strategies and precision equipment with testing, like Agricarbon’s solution.
Agroecological Practices Require Measuring Soil Life
Agriculture is just beginning to apply the concepts of soil biology, not just chemistry. Soil is far more than minerals and substrate, it is a rich ecosystem that interacts with crops and climate conditions. Tapping into the benefits of that ecosystem means adopting practices that work with soil biology, not against it. With this new complexity, farmers may wonder “which soil tests are critical?”
The Soil Health Institute (SHI) conducted a 3-year study of over 30 soil health indicators at 124 long-term agricultural research sites across North America, comparing soils from conventional systems to that of regenerative systems. SHI identified three metrics that, at minimum, farmers should consider in addition to the “NPK.” These metrics are:
- Soil organic carbon concentration, an indicator of nutrient cycling and retention, soil structure, and water holding capacity;
- carbon mineralization potential, which identifies carbon and nutrient cycling capacity and is related to microbial biomass and activity; and,
- aggregate stability, a measure of resistance to wind and water erosion and water infiltration.
These three values are strongly tied to the practices in use on the land, including avoiding tillage, integration of livestock grazing, use of cover crops, reducing inputs including fertilizers and pesticides, diverse crop rotations, and historical management in addition to soil type and climate. The land at Domaine de Graux has been under agroecological management since 2011, using organic and regenerative practices.
“Regenerative” can mean different practices depending on the type and location of a farm operation. To simplify, there are four keys to building soil health, according to the USDA Natural Resources Conservation Service:
- Maximize Presence of Living Roots
- Minimize Disturbance
- Maximize Soil Cover
- Maximize Biodiversity
Soil Testing at Domaine de Graux
The team at Domaine de Graux opted for soil testing that would measure soil carbon and measures of biological activity. Marie Delvaulx, General Manager of Domaine de Graux, explains that soil health practices, including regular testing, is part of the mission at the farm.
“We want to prove that regenerative ag principles create better and more resilient soils,” she explains. “This process begins with quantifying the current soil health metrics and continuing to measure over time.”
In all, eighteen samples were taken on Domaine de Graux, across the different areas of the farm from forest to crop fields, buffer strips, and perennial pasture. The soil sampling also included six samples of the same type of land use with conventional practices from the region near Domaine de Graux for comparison.
The soil tests included measuring soil organic matter (OM) and microbial biomass, a measure of the biological activity in the soil. Microbial biomass includes beneficial microorganisms and species of fungi. This measure is related to the increase in soil carbon, and it is greatly increased through agroecological soil management practices such as no tillage, crop rotations and biodiversity, cover crop use, and minimizing inputs such as pesticides. Pesticide use, including herbicides and insecticides, is associated with a decline in soil microbial biomass. Abiotic influences on microbial biomass include soil pH, mineral content, and soil texture.
Soil Test Results
The average soil organic matter on Domaine de Graux is 4.3%. The average organic matter for samples taken near Domaine de Graux, for the same crop, is 3.38%. On average, the results indicated 1.3 times more organic matter on Domaine de Graux than the comparison.
For context, a “good” result for soil organic matter should be between 3% and 6%. Other comparisons between conventionally and regeneratively managed farms also show significantly more soil organic matter with regenerative management. In one study that also controlled for location, soil type, and crop, the range for soil organic matter on conventional farms was 2% to 5% and averaged five times less soil organic matter than the comparison regenerative fields.
The forest and alfalfa field samples had the highest measures of soil organic matter, at 6.91% and 6.06%, respectively.
The microbial biomass measures were the biggest difference between soils sampled on Domaine de Graux and those outside of the farm. On average, soil samples from Domaine de Graux measured 15.62 µg NDA/g soil compared to 1.85 µg NDA/g soil on conventional fields. The results showed a difference of nearly 8.5 times the soil biological activity at Domaine de Graux.
July 2023 soil test results for Domaine de Graux.
Healthy Microbiome, Healthy Soil, Healthy Humans
A rich soil microbiome provides many benefits for crops. First, these beneficial microorganisms help plants access the available nitrogen and phosphorus in the soil. The microbiome also increases plants’ resilience to stressors like drought and salinity, provides biocontrol for pathogens, and improves soil structure, water holding capacity, and carbon sequestration. Additionally, this rich microbiome benefits human health by increasing the mineral content such as iron and zinc and phytonutrients in the foods we harvest.
Despite the positive results for the soils at Domaine de Graux, Marie Delvaulx explains that soil testing at the farm will continue and expand. The team at Domaine de Graux is committed to agroecological practices, including no tillage, use of cover crops, biodiversity and crop rotations, and organic inputs. Ongoing testing will measure the effects of these practices on soil health over time.
“We would like to set goals to make sure we improve the quality of our soils and adapt our way of working to improve the soil quality. We will measure and test every year. And probably partner with experts to help us on this journey,” she says.
Some of those soil health partners have four legs. The farm just welcomed 70 sheep to graze the cover crops in the fields, adding carbon, in the form of manure, to the soil as they go. Long-term, integrated livestock grazing builds both soil organic carbon and microbial biomass.
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