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Using cover crops to manufacture nitrogen

Demonstrations look at different ways to use over crops.


March 5, 2008
By Gordon Leathers

Topics

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Red clover, one of two seedling forages grown to demonstrate nitrogen fixing potential in southern Manitoba. Photos Courtesy Of John Hollinger, MAFRI.

John Heard and Mitchell Timmerman with Manitoba Agriculture, Food and Rural Initiatives (MAFRI) conducted a field demonstration in 2007 designed to examine what legumes can do and how well they will work in southern Manitoba. The idea was to investigate how legumes can fit into different rotational practices, such as relay cropping and green manuring. In one of the demonstrations, they seeded red clover into an established fall rye. “This is a practice that’s been done extensively in other jurisdictions so why not here in Manitoba?” Timmerman asks. “There have been a few attempts here but they’ve met with mixed results at best.”

Once the fall rye is harvested, the legume is left for the rest of the season. It makes use of the remaining heat units to fix nitrogen and to produce some biomass that is worked down in the late fall. The ploughed down legume crop then provides a source of organic nitrogen that will be released during the next growing year. “We want to see those things go down in the fall,” Timmerman says. “The soil bugs have to do their work and we have to give them time to do that,” he adds.

Growing two crops together takes some experimentation. The time of planting is critical. “I seeded the clover in early May but I should have seeded in early April or late March,” Heard explains. “If you give the winter wheat too much of a head start, the clover doesn’t get a chance to establish,” he says.

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Legume crops can deliver over 100lb/ac of N
In the second part of the demonstration, Heard decided to show how other legumes can work as nitrogen fixers for other crops. In this demonstration, the legumes were grown as a normal green manure crop, seeded alone without another crop. Heard tried a variety of legumes including red clover, berseem clover, Indian Head lentils and fababeans. The levels of residual soil nitrogen were estimated based on the amount of dry matter produced. Estimates are necessary since a standard soil test does not provide an accurate reading of the amount of nitrogen because the biomass must break down before it shows up as soil nitrogen.

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Berseem clover. Nitrogen yield came to about 50 pounds per acre, which was similar to the red clover.

To help develop estimates, the researchers looked at an important concept that has come out of some work done at the University of Manitoba, the Fertilizer Replacement Value (FRV) of different legumes. This is determined by growing cereals (oats in this case) on land that produced legumes the previous year. Differences in yield with different rates of fertilizer are compared and the FRV is calculated as the amount of nitrogen produced per thousand pounds of dry matter. The FRV can then be used to estimate the amount of soil nitrogen on the basis of dry matter produced.

“For every thousand pounds of dry matter there are 30 pounds of nitrogen being produced and half of that is available for next year’s crop,” Heard says. “Half of that is your nitrogen credit.”

Different legumes yielded different amounts of nitrogen with fababeans coming in with about 75 pounds of nitrogen available to the next year’s crop. Indian Head lentils also recorded 75 pounds, as did chickling vetch. The big winner was peas at 110 pounds.

The seedling forage legumes did not produce as well in this demonstration, sweet clover came in around 55 pounds and alfalfa trailed at 25 pounds. Berseem and single cut red clover were about the same at around 50 pounds, double cut red clover a little better.-end-n

More information may be found at: www.umanitoba.ca/outreach/naturalagriculture/articles/
frv.html