The One Where the Nitrogen is Released
So, you know how legumes fix nitrogen into their tissues, but how does that nitrogen get back into your soil? That takes other friends in the soil known as the microbes...
The Terminator -
When the nitrogen that your legumes fix leaves the soil and enters plant tissues, imagine it saying “I’ll be back.” (Come on, do the voice.) When your cover crop is terminated, whether that be from winterkill, chemical, or tillage, it is then exposed to soil microbes. Those critters go to work on the residue and break it down into inorganic N for the plant. The peak time that this occurs is 5-6 week after termination. This is why it is beneficial to seed legumes the season before a grass crop (hence, the usual corn and bean rotation).
Maximizing Nitrogen -
We know we need to terminate the legume, but when is the best time to do that? Minnesota State studies have found that when the plants are at 50% flowering, the nitrogen content is the highest. Any earlier than that, and we have not reached the peak. Any later than that, and the legume will be more mature and have a negative effect on nitrogen increase because it will use some up to be able to break down in the soil.
How much nitrogen will my legume supply?
This can be estimated by getting a biomass measurement and popping some numbers into a formula, or you can use this Nitrogen Calculator and it’s even easier. When we get our pounds of biomass per acre, we can multiply that times a constant to get an estimate of how much nitrogen those plants created for us per acre. The amount of this that we actually capture in the soil depends if it is incorporated or not. We retain 50% of this fixed nitrogen when the cover crop is incorporated, we hold on to 40% of it when the cover crop remains on the soil surface. This is mainly to volatilization from the surface of the cover crop exposed to the air and sunlight.
Moral of the story is consider using legumes to grow your nitrogen!
Visuals created by Associate Professor Julie Grossman and her team of grad students at the University of Minnesota Department of Horticulture Science.