Boosting nitrogen in organic systems

A sweet and red clover cover crop mix. Photo: File/Laura Rance

By John Greig
OrganicBiz staff 

Agriculture and Agri-Food Canada researchers have validated what organic farmers have known for years — that cover crops reduce nitrogen loss and fix nitrogen for use in subsequent crops. However, they also have found that in order to support a corn-soybean-wheat rotation, more cover crops are needed.

Xueming Yang, a researcher with Agriculture and Agri-Food Canada’s Harrow Research Station says that a three-year study shows that “winter hardy legumes provides significant amounts of nitrogen to organic corn.”

Yang used multiple types of cover crops during the trials including crimson clover, red clover, hairy vetch and sesbania. Organic farmers, like many conventional farmers, plant a corn- soybean-wheat (followed by red clover) rotation.

He told the Guelph Organic Conference that they got excellent crop establishment on all of the cover crops when they used a mouldboard plow after wheat. In the first year of the trial, they disced the ground after wheat and that resulted in poorer stands.

They found that the cover crop will scavenge more nitrogen if the cover crop is left growing until the next spring. This means more nitrogen is available for future crops like corn, less environmental impact and increased soil health and organic matter.

Yields of corn in the trials continued to increase year over year, hitting 200 bushels in some plots.

Wheat, however, posed growing challenges. Wheat yield has declined during the trial due to a lack of soil fertility.

“This is a problem in this crop rotation,” says Yang.

Yang and his team are turning their attention to filling that nutrient gap for wheat, including looking at how to intercrop a cover crop with corn.

Organic farmer Dean Martin also has the same challenge with getting his winter wheat to yield well two seasons after the clover put more nitrogen into the ground and much of it was used by the subsequent corn crop.

He generally gets about 50 bushels per acre of wheat. “It’s so far from the nitrogen cycle, you get lower wheat yield.”

Martin shared the stage with Yang at the organic conference to talk about nitrogen management on his organic farm.

Martin and his family farm about 800 acres near Harrow, as well as some on Pelee Island. They have been certified organic since 2003.

He says Yang’s work is valuable as there has been so little replicated trial research completed on organic farm practices.

He also said that the Harrow Research Station has “beautiful, beautiful ground.”

Martin, on the other hand, has a wide variety of soils to work with from sandy soil to untiled Brookston clay loam.

He has become an expert in managing different types of clover as a green manure for his rotation, but mostly uses double-cut red clover. It grows better, but single-cut red clover will provide more nitrogen. He finds most of the double-cut red clover he buys also contains some single-cut red clover, and he’s OK with that mix.

He is, however, concerned about the quality and predictability of his clover supply. There’s very little registered organic clover seed available and the common seed can come from many sources, usually Oklahoma and Manitoba, but Martin has also seen seed from Chile show up.

“You never know what you are really getting. Is there one seed in there you don’t want?”

As a result, Martin has started harvesting his own clover seed.

Unlike Yang’s trial, where he seeded clover and other cover crops like hairy vetch after tillage after wheat, Martin usually frost seeds his clover into wheat in the spring. He uses a spinner spreader on the receiver hitch of his pickup or an ATV.

Some years he might rotary hoe or harrow the crop in order to get some shallow incorporation of the clover seed.

He finds the clover grows well, sometimes so well that he has to move his wheat harvest up a few days or a week in order to get the crop off before the green of the clover gets too thick.

Unlike conventional farmers who usually terminate a cover crop with a spray in the fall or the early spring, organic farmers try to leave as much of the crop growing as long as it can, as Yang’s research shows that crops that are still growing in the spring will continue to fix nitrogen.

Martin usually does not mouldboard plow, but instead uses various other tillage techniques, especially a Salford 5100 high-speed disk, an aggressive primary tillage disk. He says they can use it even on the Brookston Clay in the spring. He says he can get onto the land with clover earlier as the growing plants suck up more deep moisture.

They use secondary tillage to completely cut off the clover, with their favourite being a C-tine cultivator with seven to nine-inch sweeps and a packer behind it. Martin finds it handles the residue well. They go over a field three times with that cultivator before corn planting.

Martin says he’s happy with his clover management system, although he is now considering using some hairy vetch in his system, with the success Yang has had with it as a cover crop. Yang’s research showed good growth and nitrogen fixation with clover and vetch, but the sesbania, a fast-growing, nitrogen fixing plant in the pea family, has not stood up to cooler Ontario falls.