Plant Cover Crops

Learning about the benefits of planting cover crops.

Lower Inputs

“Where is my nitrogen?”

Austrian Winter Peas add as much as 100 untis of nitrogen to the soil if they hav egood growth in a good stand.

This question keeps coming up on a regular basis in cover crop meetings….”where is my nitrogen?”

There are a number of things we know:

  • Legumes produce N and some of that N is available for the next crop.
  • Most cover crops scavenge N.  Some of that N is available for the next crop.
  • Some of the N is not available for the next crop.

How do you determine how much N is available for the next crop?

According to Managing Cover Crops Profitably: (p. 22)

 How Nitrogen is Released

How much N will soil really acquire from a legume cover crop? Let’s take it from the point of a freshly killed, annual legume, cut down in its prime at mid-bloom.The management and climatic events following the death of that legume will greatly affect the amount and timing of N release from the legume to the soil. Most soil bacteria will feast on and rapidly decompose green manures such as annual legumes, which contain many simple sugars and proteins as energy sources. Soil bacteria love to party and when there is lots to eat, they do something that no party guest you’ve ever invited can do—they reproduce themselves, rapidly and repeatedly,doubling their population in as little as seven days under field conditions (306). Even a relatively inactive soil can come to life quickly with addition of a delectable green manure. The result can be a very rapid and large release of nitrate into the soil within a week of the green manure’s demise. This N release is more rapid when covers are plowed down than when left on the surface. As much as 140 lb. N/A has been measured 7 to 10 days after plowdown of hairy vetch (363).Green manures that are less proteinrich (N-rich) will take longer to release N. Those that are old and fibrous or woody are generally left for hard-working but somewhat sluggish fungi to convert slowly to humus over the years, gradually releasing small amounts of nutrients. Other factors contribute significantly to how quickly a green manure releases its N.Weather has a huge influence.The soil organisms responsible for decomposition work best at warm temperatures and are less energetic during cool spring months. Soil moisture also has a dramatic effect. Research shows that soil microbial activity peaks when 60 percent of the soil pores are filled with water, and declines significantly when moisture levels are higher or lower (244).This 60 percent water-filled pore space roughly corresponds to field capacity, or the amount of water left in the soil when it is allowed to drain for 24 hours after
a good soaking rain. Microbes are sensitive to soil chemistry as well. Most soil bacteria need a pH of between 6 and 8 to perform at peak; fungi (the slow decomposers) are still active at very low pH.Soil microorganisms also need most of the same nutrients that plants require, so low-fertility soils support smaller populations of primary  decomposers, compared with high-fertility soils.Don’t expect N-release rates or fertilizer replacement values for a given cover crop to be identical in fields of different fertility. Many of these environmental factors are out of your direct control in the near term.Management factors such as fertilization, liming and tillage, however, also influence production and availability of legume N.

So while know that cover crop legumes produce nitrogen we also know that the nitrogen will be released at different times and in different amounts according to the circumstances in individual fields and with different management practices.

In 2010 many producers across the Midwest  shared that they could see a “big difference” in their corn yield where they had a legumes as a cover crop.  While we cannot easily (or maybe not totally) quantify the pounds of N available for the next crop we can count on some level of improved performance.  Can we reduce our inputs?  Maybe.  Can we always get higher yields?  No.  Can we anticipate an improved yield?  Probably.  Some folks will tell you “yes” to those questions!  Some others will have a different experience.

So what do you do?  Plan for success, scout your corn, use a pop-up and/or starter with nitrogen…and scout…oh, yeah, I guess I already said that!

Crimson Clover…Great Cover Crop for Corn Producers

After talking to hundreds of row crop producers about cover crops, I was reminded of how much crimson clover offers to the corn farmer.  Not only will crimson clover produce somewhere between 70-140 units of N (with approximately 1/2 of that being available for the next crop…the other percentage goes to building soil organic matter), it also makes for a great companion to other cover crop species.  It mixes well with radishes, annual ryegrass, turnips, oats, winter cereal rye, etc…  The nice thing about crimson clover is that when using it with the other species it pretty much eliminates the need for adding additional nitrogen to those crops.  Most farmers I’ve talked to say that the cost of using crimson clover is less than applying the usable nitrogen equivalent.

Using improved varieties makes a difference too.  Last spring we saw where “Dixie” and some lesser expensive VNS crimson clovers died out over the winter.  New varieties like AU Robin and AU Sunrise looked very strong coming out of the winter with beautiful nitrogen producing stands this spring.

Cover crops May 2010 001
My son Edward in the cover crop plot in mid-May 2010. This is the AU Robin crimson clover plot he's in.

Hairy Vetch- a Top Nitrogen Producer

Hairy Vetch is a terrific nitrogen producing cover crop species.  For Hairy Vetch to produce significant Nitrogen it needs to be established in adequate time to have good growth before going dormant in winter.  Hairy Vetch must also be inoculated to be productive.  One challenge with using Hairy Vetch is that it contains significant amounts of hard seed that will most likely germinate in future years.

Advantages of Hairy Vetch

  • Tolerates lower fertility
  • Can produce significant amounts of N
  • Relatively easy to kill

Disadvantages of Hairy Vetch

  • Hard Seed – can become a weed
  • Needs 5-6 weeks good growth before winter for best results

Crimson Clover; a jewel among the cover crop legumes

bee on crimson clover
Crimson clover is a top nitrogen producer and beautiful as well.

Crimson clover is a real jewel among the cover crop species.  Crimson clover is an excellent producer of nitrogen and there are some newer varieties on the market that are very winterhardy even into Michigan.  Dr. Eileen Kladivko measured how much nitrogen was produced by crimson clover in a test plot in western Indiana.  What she found was quite amazing!  After around 90 days the crimson clover had produced over 140# of nitrogen per acre!  Now, this was a limited test of one plot but the best of the other plots came in at just over 50# N produced.  That is a significant amount of nitrogen.

Crimson clover mixes very well with annual ryegrass (to help the ryegrass be healthier and growing better when there is no manure applied).  It also mixes well with cover crop radishes and or turnips.  Crimson clover is generally considered more shade tolerant so it works well aerial applied into standing corn.

Crimson clover MUST be inoculated for best success.

I have listed some advantages and disadvantages below:

Advantages of Crimson Clover:

  • There are some excellent new varieties on the market that are more winterhardy.
  • There are earlier maturing varieties that mature early enough to make more N in the spring before killing.
  • At around 90 days after wheat it can produce upwards towards 140 units of nitrogen.
  • Earthworms love the environment around the roots of crimson clover…it’s creates “earthworm heaven!”
  • It is easy to kill in the spring.

Disadvantages of Crimson Clover:

  • VNS and MOST older varieties will probably not survive the winter in the Midwest.
  • It has some hard seed so you may see some coming up some months after planting.
  • If it is a very late wet spring there is a slight chance the clover will re-seed itself.

Crimson in ARg
Crimson clover mixed with annual ryegrass is a great cover crop combination.
Crimsom roots
Properly inoculated crimson clover produces large and multiple nodules. Note the root hairs as well.