Defining Soil Health in 2022

Friday, September 23, 2022

What's the right recipe farmers need to create optimal soil?

by Kristen Lutz

Soil health is a complex subject muddied down by finding the right mix of elements that contribute to optimal conditions.

Whether it's soil structure, organic matter, or good microbial content, what exactly contributes to happy, healthy soil? Has the definition and goal changed in the last few years?

The answer is often dependent on what the producer is looking to get out of their land.

Erosion damage

Wind and water erosion is damaging to the land. The removal of valuable topsoil through erosion can affect crop emergence, growth and overall yield. Adjusting soil management practices can reduce the stripping of topsoil, especially in hilly landscapes.

Ben Ellert, a research scientist in biogeochemistry with Agriculture and Agri-Food Canada at Lethbridge, Alta., states the importance of reduced tillage. "Soil is less prone to wind and water erosion when we use no-till practices."

As most producers are fully aware, with reduced tillage, organic residues are often left on the surface. Creating a buffer of straw and organic matter protects the soil from wind and water erosion. In conventional tillage, the organic residue would be mixed into the soil, leaving the surface bare and at risk to effects from erosion.

David Lobb, a professor in the department of soil science at the University of Manitoba, stresses the dangers of tillage erosion. "Tillage erosion is more degrading than wind and water erosion combined," he says. "There are two different versions of no-till which are commonly referred to in Canadian agriculture."

Lobb explains that no-till is a system that creates very little soil disturbance. It normally uses equipment such as double disc openers and trash whippers to prepare the soil and place the seed. While zero-till creates more soil disturbance due to its use of shoes or sweeps to place the seed, these can move a fair bit of soil and crop residue.

Hilly land can bring special challenges. Lobb explains that with the speed of equipment, the movement of soil is highly variable and more soil would be moved downslope over time. "The big problem with tillage erosion, if they are high disturbance and high speed, is they will cause as much erosion as a mouldboard plow," he explains. "They move less soil but move it farther and with much greater variation between upslope and downslope."

Over time, topsoil will collect at the bottom of the hill, leaving the hilltop nearly bare. The exposed soil will be prone to effects from wind and water erosion. Not to mention little to no topsoil is not the ideal growing environment.

To promote plant growth on hilltops, Lobb recommends producers "pull this topsoil, and put it back on the eroded hilltops and then implement practices to keep it there as long as possible."

"Both no-till and zero-till, if they are low disturbance and low speed, will leave crop residue on the surface and reduce wind and water erosion as well as minimize tillage erosion and promote and sustain plant growth and yield," says Lobb.

Organic matter

Although soil organic matter and no-till practices are closely linked, it's important to note that increased organic matter is not always directly related to reduced tillage. Much has been written about the contribution of no-till to increased organic matter in Prairie soils, but other contributing factors tend to be overlooked.

"Other contributors include changing plant genetics, crop variety, changing fertilizers and finetuning fertilizer recommendations so we're growing bigger crops and our carbon inputs are larger," says Ellert.

He also mentions that the near elimination of summer fallow has drastically increased carbon inputs. "Instead of producing one crop every two years, which is what we were doing with summer fallow, we're producing one crop every year and using the soil moisture resources more effectively."

A higher presence of crop residue or organic matter on the surface can often benefit the soil below. "When we talk about soil health, we consider its physical, chemical and biological dimensions. And (soil organic matter) often improves all three," says Ellert.

When soil aggregates are left intact from reduced tilling practices "we have better infiltration of moisture and gaseous exchange which improves physical properties.

"The chemical aspects of soil health include nutrient exchange between the soil and plants, which is critical for nutrient retention and plant growth. Then there's the biological dimension to support life, including plants and animals and microorganisms.

"They're critical for maintaining soil fertility and protecting the environment."

Bobbi Helgason, a soil microbiologist in the College of Agriculture and Bioresources at the University of Saskatchewan, agrees.

"Increased organic matter is a good indicator that you're improving soil health. It plays a key role in water retention; it acts like a sponge, but it also indirectly helps with regulating water-holding capacity by improving soil structure and increasing porosity," says Helgason. "Importantly, it creates a better ability for the soil to regulate nutrient cycling. For example, it increases cation exchange capacity and aggregation."

In many cases, we associate the health of soils with the yield it produces. However, non-provisioning aspects of soil such as buffering hydrologic fluxes and accepting and holding water are often what contribute to highly productive fields.

Organic residue on the surface of the soil is a balance between what organic matter is coming in and organic matter being lost. In healthy soil, organic matter is lost at a consistent rate as microbial decomposers chew through the residue.

To increase volume and quality of organic matter inputs Helgason says, "it is a really important connection to make that if you have more plant growth, you'll have more incoming plant matter which stands to increase organic matter and have positive feed-back on next year's growth and on and on. Having higher quality organic matter can be built by having a diverse crop rotation and balanced fertility.

Because you have a balance between carbon, nitrogen, phosphorus and all the other nutrients that plants need to grow but microbes need them too."

Although organic matter primarily originates from crop residues, producers can also increase organic matter through added organic elements such as manure and compost.

Leaving organic matter on the surface of the soil can have benefits, including moisture-holding capabilities, while providing a nutrient-rich environment for soil microorganisms.

Microorganisms matter

The types and abundance of microbial populations within the soil are directly related to management practices. A large portion of the organisms within the soil are decomposers, meaning they heavily rely on organic matter as a food source.

Supporting the microbial population in the soil often contributes to improved soil structure and sustained plant health. New roots will grow along paths created in the soil from previous roots or earth burrowing insects.

These channels can also allow roots to access water and nutrients and facilitate gas exchange, all of which are critical for growth. With reduced tillage practices, these channels remain intact.

"By tilling in organic matter, you're actually better feeding the microbes, but they have a boom-and-bust behaviour. By tilling, the food becomes abundant, and the population will grow really quickly, but then they chew their way through it and the population crashes. With no-till you have a greater microbial abundance at a more consistent level all the time," says Helgason.

This is also directly linked to carbon dioxide (CO2) emissions from the soil.

"In no-till, you're going to have slower emissions of CO2 than if you were to till in the organic matter. Breaking it up faster results in a quicker release of CO2 because the microbes are chewing it up and producing CO2, but it will be a short burst rather than a slow burn in no-till. Generally, we say that you're going to have more CO2 loss in a conventional system," says Lobb.

To support microbial diversity, Helgason recommends diversifying crop rotation. "Microbes are like us; they're healthier on a balanced diet. If you're feeding them different kinds of plant residues and giving them balanced nutrition, it can support diversity."

"Some of the microbes favour plant growth, like mycorrhizal fungi associated with the roots, while others may be pathogenic. In pulse crops, we have nitrogen-fixing bacteria within root nodules, where the plant provides sugars in exchange for nitrogen fixed by the bacteria," says Ellert.

"Producers should continue to be mindful that soil is alive and we need to feed soil biota a balanced diet and minimize disturbance through tillage, which to microbes is an extreme disturbance event," concludes Helgason.

"Soil health is not a well-defined term, and the definition might vary quite a bit," says Ellert.

"What we're expecting from our ag landscapes is to produce harvestable plant commodities. But we must also consider the non-provisioning services the land offers us, and too often we take these non-provisioning aspects for granted." BF