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The building block of soil

Soil scientists are putting more focus on the value of organic matter for feeding important microbes and boosting moisture-managing capacity

Without it, soil is just dirt. There’s a new recognition of the importance of soil organic matter, and not only for improving crop yields. It’s also a tool in the effort to mitigate climate change.

Blain Hjertaas, a holistic management farmer and grazier at Redvers, Sask., participates in the Soil Carbon Challenge, an international “competition” that monitors carbon changes on sites where farmers or other land managers are trying to increase soil health and increase soil organic matter (SOM). The Soil Carbon Coalition, which monitors the North American sites at three-, six- and 10-year intervals, reports that Hjertaas increased his SOM by 18.54 tonnes per hectare from 2011 to 2014. That’s equivalent to 22.88 tonnes of carbon dioxide sequestered per hectare each year. Each Canadian’s CO2 footprint is 18.9 tonnes per year.

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“Every hectare I operate more than sequesters one Canadian’s footprint,” says Hjertaas.

Food for microbes

While SOM has always been recognized as important for soil tilth, there’s also a new understanding of the value of the soil microbes which depend on it for food.

“SOM is an organic form of carbon in the soil, which drives the microbial population,” says Diane Knight, a soil science professor at the University of Saskatchewan. “They use carbon for their metabolism. When the micro-organisms die and decompose they slowly release nutrients for the plants to take up. But we want to protect SOM because we don’t want it to be super-available for micro-organisms or else it would degrade right away and there wouldn’t be any SOM left.”

Knight says there’s no “ideal” target level for SOM —it depends on many factors such as climate, soil type and crop rotations.

“There may be some ideal target SOM levels in a particular area, but it’s not across the board and it will vary based on region as well as what farmers are able to grow on their farm.”

In the brown soil zone in the generally hotter, drier region of the Prairies, it’s harder to build — plants don’t grow as large, so there’s less SOM going back into the soil. In the black soil zone with more moisture, there is more available.

Knight says it’s particularly challenging to increase SOM in sandy soils, which are lighter and tend to retain less nutrient and water, whereas clay soils build SOM more readily.

“There’s something magical about clay particles. Clay particles are really small and they can form shells around organic matter, and protect it from being further degraded,” she says.

Input versus output

AAFC research stations across Western Canada have been conducting long-term crop rotation studies for decades, assessing rotational effects on yields, disease, weed competition and other agronomic factors. These studies have not measured carbon until more recently, and so there is no clear understanding of which crop combinations could be most effective at building SOM. It is known that higher-biomass plants such as cereals can help build SOM, but lower-biomass crops such as pulses may not be as effective. However, a combination can help.

Simply put, soil carbon status is determined by the balance between carbon inputs and outputs.

“If you increase the carbon inputs relative to the outputs you should start to gain some soil carbon, or vice versa if you hold your inputs the same but reduce the outputs, the same thing should happen,” says Reynald Lemke, a research scientist at Agriculture & Agri-Food Canada’s Saskatoon Research and Development Centre.

The rotation studies have shown that carbon inputs from a canola crop are generally higher than inputs from wheat.

“Carbon outputs from growing a wheat and canola crop shouldn’t differ greatly because the management of those crops is not so different,” says Lemke. “But if carbon inputs increase by growing canola versus wheat, then there should be a benefit in terms of soil carbon stock, and we have some data from the long-term rotations that is demonstrating this. Although farmers’ decisions to include canola in their crop mix may not necessarily be related to its benefit in terms of soil carbon, it’s a nice bonus.”

Some rotations, such as wheat and lentils, have proven to boost yields and soil carbon in long-term studies at Swift Current. Although the lentil crop has a lower carbon input than the wheat crop, it provides a yield benefit to wheat the following year. That balances the system as the wheat crop grows more biomass and residue to help build more SOM.

Lemke has just introduced soybeans into one of his long-term rotation studies and early indications are that it may have the same synergistic effect.

“Because soybeans fix their own nitrogen, there’s a real benefit in terms of nitrous oxide emissions during the soybean year and probably in the following year because you get some nitrogen benefit,” says Lemke.

“In terms of building soil carbon, it looks to me at this point that the indications are pretty good, but it’s early days and we won’t be able to say that for sure until we have done many more years of studies.”

Extending the season

The decline of SOM started when the first plow hit the Prairies, and was accelerated by decades of summerfallowing. Modern practices have slowed the trend.

“Reduced fallow and reduced tillage was a very good combination in terms of improving soil quality and soil carbon over the past few decades,” says Lemke.

But Knight emphasizes that however farmers choose to continuous crop on their land, they need to make sure they have optimal fertilization. “Farmers want to get the most growth on their land, so that means maximizing root growth as well as the crop residue material that will be returned to the soil,” she says. “That means having a well-managed fertility program, not over-fertilizing but applying what the plants need to support that growth.”

Some farmers such as holistic practitioner Hjertaas are wary of chemical fertilizers.

“Commercial fertilizers are oxidants so they help decompose the organic matter and the bacteria must breed up to get rid of the nitrogen. It’s complex but basically it also depletes the SOM,” he says.

His SOM-building program is designed to produce as much material for as long as possible through the growing season.

“The key to building SOM is photosynthesis — you have to have green plants that are capturing sunlight which then creates sugar which sends roots exudates down and feeds the micro-organisms, and that’s what ultimately creates the organic matter,” says Hjertaas.

“The longer you can do that — the longer you can keep it green — the better it’s going to be. The problem with most cropping systems is that they are only green for about 70 days of the year, whereas perennial forage, if it’s managed well and has a chance to regrow after grazing, can be green for 220 to 250 days of the year.”

Perennials for straight grain

Hjertaas says it can take up to five years to get to the point where SOM increases can be measured but farmers should notice positive changes in their soil within a couple of years, starting with improvements to the water cycle.

“Every gram of carbon holds eight grams of water, so as you get more carbon in the soil, you can have a hard rain shower and you won’t see any water lying around. It’s gone immediately.”

Hjertaas says farmers who work perennial crops such as alfalfa and clovers into their rotations will see rapid SOM improvements. In practice, that can be a challenge, especially for grain farmers who don’t have livestock and grazing land.

But he says there are different ways to include perennials in a grain-only system and more and more farmers are adopting some innovative practices — such as cover crops, winter cereals and intercropping — to try and make sure there are growing roots in the soil from spring thaw to fall freeze-up.

“We just have to change our mindset to look at diversity and think about ‘Once I get this crop harvested in the middle of August, what can I seed into it that will grow for a couple of months and either the frost kills it, or if it was green, the cows can eat it, which would even be better,’” says Hjertaas.

“Soil is life. We depend on soil for our lives, so part of our job as farmers is to be stewards of that and improve it as we use it. We need to farm so that we put carbon back in and become more regenerative in our approach to agriculture.”

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Angela Lovell

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