Reversing organic matter loss using cover crops

In a Canadian province where the loss of soil organic-matter is a long-standing problem, cover crops (and other cultivation strategies designed to improve soil structure) are playing an increasingly important role in cropland sustainability. A variety of factors, however, have thus far prevented such soil management strategies from being widely employed by farmers. Still, experts say improvements in technology and a growing understanding of cover crop management are bringing ever-more opportunities for soil improvement.

Organic matter levels low in southwest

Christine Brown, field crops sustainability specialist with the Ontario Ministry of Agriculture, Food, and Rural Affairs, says the current Ontario average for soil organic matter in arable land is between 4.0 and 4.2%. However, the province’s three most southwestern counties of Essex, Kent, and Lambton – largely grain producing areas – show consistently lower levels.

“Organic matter in Essex, Kent, and Lambton was about 4.25% in 2002. In 2016 it averaged 3.2%,” she says. “It’s likely the largest drop in Ontario […] Some counties have actually gone up though.”

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So, what explains this organic matter degradation? Ms Brown says research, conducted in part by SGS Agri-Food Laboratories, a Canadian agricultural-science research firm, indicates that Ontario’s most common field crop rotation – the corn, soybeans, and wheat regimes so prevalent in Essex, Kent, and Lambton – are insufficient to maintain organic matter levels without the addition of cover crops, organic amendments, or greater crop diversity. This is particularly problematic given that Southwestern Ontario is relatively devoid of livestock – and its associated manure and forages. “Cover crops add organic matter. Organic fertilisers add organic matter. It’s kind of a one-plus-one-equals-three thing. They have a synergy when used together,” she says.

RTK proven useful According to Mark Richards, a grain, sugar beet, and tomato farmer from the province’s southwest, tools such as GPS and Real-Time Kinetic (RTK) technology have proven to be useful modern tools. Richards and his family keeps much of their 2000 acres covered with green material 365 days a year. In the spring they use a 12-row strip-till machine augmented with a customized variable-rate fertilizer wagon to prepare rows for planting the next crop. A combination of site specific soil mapping, as well as regular satellite and drone crop imaging, helps ensure their adjust-on-the-go variable rate equipment is as precise as possible. Text continues underneath image Mark Richards is a grain, sugar beet, and tomato farmer from Ontario’s southwest. – Photo: Mark Richards “We run everything on GPS and auto-steer except the sprayer. We use RTK there […] it’s reducing labour with tech know-how,” says Richards. “One of the biggest reasons we are using cover crops is to decrease [herbicide] resistant weed pressures. They seem to make the herbicide more effective on what’s left as well.” While soil organic matter levels across their fields have not jumped, he says, there also hasn’t been a decline despite harvesting higher yields.

Cover crops offset compaction

“50 years of intensive crop production in Ontario has dramatically reduced our soil organic matter, while we dramatically increased the size of our equipment. It’s made compaction a huge issue”, says Peter Johnson, agronomist with Real Agriculture, a Canadian farm-news company.

Mr Johnsons says cover crops are a key factor in offsetting organic matter loss and its associated increase in compaction risk – something which perpetuates that loss. As a general rule, he says preventing soil compaction requires about 1% organic matter for every 5% of clay.

But even farmers who do take steps to achieve this ratio might not be doing as well as they think.

“Go into the fencerow (or pasture) and take a soil sample, then one from the field you’ve been working for 25 years,” he says. “It gives a pretty good comparison. I rotate, I always use clover in wheat, or seed oats if there isn’t a good stand, and the pasture is at 9% while my field is at 4%.”

Barriers to cover crop adoption

According to Ms Brown, one of the most significant barriers to adoption is the perceived lack of control managing the impact cover crops have on overall crop success.

Questions about the length of time cover crops should be left in the field, what the best burn-down method is, how it will impact pest pressures the following year, and how to handle it in the spring if it overwinters – particularly if there’s a wet planting season – can all prevent growers getting interested in pursuing such management systems.

On top of that, Johnson says grain producers need to be ready for some potentially significant yield losses should they chose to plant into a living, green cover crop in the spring – even if all other factors go well. Indeed, his own work has shown consistently high losses (20 bushels per acre / 1,25 ton per hectare) in corn grown after cover-crops. “There are not too many farmers that can handle a 20 bushel per acre yield loss,” he says.

Making an intensive cover crop system work For Blake Vince, a grain farmer from Kent county who has been using multi-specie cover crop mixtures in a no-till system since 2011, weighing the cost of planting green by yield alone doesn’t provide the grower with a full picture. Indeed, he says one of the benefits his 18-species cover crop system – one designed to mimic the diversity of natural ecosystems – is the additional biological controls it inherently creates. Vince plants corn and soybeans into living greenery each spring using a standard seed drill modified to cut through the cover crop. “We are really starting to see the system come to life in terms of the resilience of our crops in subsequent crop years,” says Vince. “We’ve seen a proliferation of beneficial insects. We have slugs like everyone else, but most times it’s not an issue because of all those beneficial insects. There’s also been an increase in rare or endangered species like the bobolink and meadowlark.” Text continues underneath image Blake Vince standing in his 18 species cover crop. – Photo: Mel Luymes Vince also says the decomposition rate in his fields has skyrocketed. There’s also been noticeable improvements in water and nutrient uptake and retention, carbon sequestering, and significantly more consistent crop growth. He has, however, experienced setbacks. Wet springs and dry summers in the last couple years meant his corn yields were certainly less-than-ideal. In the end, though, Vince says he did harvest a decent crop, and attributes that to the general resilience of his fields.

Opportunities to blend systems

For both Mr Johnson and Ms Brown, the most substantial opportunity exists in combining the current, more commonplace production systems with more intensive cover-cropping. “I think there’s potential opportunities with incorporating strip-till into these systems. GPS and site-specific tech is making it easier [to incorporate cover crops] as well,” says Ms Brown. “I think more people are willing to try cover cropping with strip till than with no-till systems.”

Starting slow

Farmer Blake Vince encourages farmers interested in delving into cover crops to start small. He suggests converting a small amount of wheat acreage and modifying pre-existing equipment rather than buying new.

“As farmers we are visual learners. So much of the detail of soil health requires our other senses and a microscope”, he says. “The only thing we have to invest in is a change of attitude.”

Mr Johnson expresses a similar sentiment, and encourages those interested to first plant test strips in the middle of a field. Doing so can provide a good comparison with their current growing system, and allow farmers to both learn from and weather potential failures before expanding.