By Kristin Williams, Agronomy Outreach Professional

We just finished a two-year, multi-farm study on the health of clay soils, funded through a VT Conservation Innovation Grant through the NRCS. Measures of soil health (using Cornell’s soil health test) were not consistent, and we found that comparing practices over time was more informative than comparing field to field. One interesting, and maybe
obvious, lesson was the correlation between soil health practices and crop yields.

So, how do soil health practices influence yield? Research suggests soil health can improve yields. It is important to note our project focused on  demonstration, not replicated research. We compared no-till and conventional/reduced till corn silage on 5 farms with clay fields in our region. A simple t-test revealed no significant difference in yield between no-till (19.1 tons/acre) and conventional (19.2 tons/acre). More importantly, we were able to demonstrate that a farmer can grow no-till without yield losses, and be successful with good management practices. A yield gain might take time as the soil builds up its condition.

We also wondered how cover crop species or mixes might affect corn silage yield. We had an opportunity to use a field where the corn was accidentally killed. We planted 15 different combinations, including 4 single species, 6 two-way mixes, and 5 three-way mixes. This project was a slight anomaly in that the cover crops were planted with a drill in late August, which allowed for a more vigorous production of all cover crops. Radish was a star in the fall, maximizing both phosphorus and nitrogen uptake. We did not measure phosphorus content in the spring, so we do not know how much was retained in the soil. It seems to have allowed
for more available nitrogen in the soil at the time of a pre-sidedress nitrogen test (PSNT), therefore requiring less nitrogen. Surprisingly, legume mix covers had good fall biomass, but that did not translate into more N mineralization.

We applied nitrogen to each plot as per the PSNT recommendation for 20 tons/acre corn silage. At the end of the season, we measured corn silage
yield and compared that to nitrogen applied (see graph). The winter rye plot had a lower corn silage yield and required more nitrogen. Other than the nutrient effect of less uptake and slower decomposition, there may have been a physical barrier created by the standing rye crop, which was particularly vigorous in the spring. However, our three-way mix (winter rye – oats – radish) actually had the highest average corn silage yield, even though it required more N at PSNT time than the pure radish stand.

So, do not go abandoning your winter rye just yet. In fact, we think this three-way mix has promise and we are looking for a mix that gives both fall and spring soil conservation. Radish alone will winter kill, which may be good for mineralization, but not as good for spring soil conservation. Oats also winter kill but provide faster fall soil cover than rye by itself.

When using an over-wintering cover crop, it is clear that timing and success of termination is critical for subsequent crop yields. Nitrogen mineralization may happen later in the season with a plant such as winter rye that has a heavier carbon content. In a no-till system particularly, you may need to adjust your nitrogen rates/timing and put more on upfront. If you are using cover crops, a PSNT seems like a wise investment.

It is also important to remember that soil health is a long game, and it may take time to see the results of your labors with cover crops. We have replicated this project by replanting these cover crops in the fall of 2016, this time planted in September, and will look at this again this coming season.

More info about UVM’s PSNT test can be found at:


Getting More Out of Your Cover Crop

By Kirsten Workman

UVM Ext. Agronomy Outreach Professional

Vermont farmers are on target to plant over 20,000 acres of cover crops this year.  The majority of these acres will be Mixed species cover crops up closeplanted to winter rye, but there is still time (even now) to get a little more from your cover crop.

Legumes are unique because of their ability to fix nitrogen, utilize that nitrogen themselves reducing fertilizer requirements, and contribute it back to the soil for use by the following crop. Agricultural legumes are plants that are in the family Fabaceae.  Most farmers are familiar with the list of legumes that comprise their forage legume species like alfalfa, clover, and trefoil, or those that are grown for grain like soybeans, peas, lentils and even peanuts. And don’t forget the vegetable legume crops like green beans and snap peas.

Legumes also have a much lower carbon to nitrogen ratio (C:N) than cereal grains, so they decompose quicker making that nitrogen more available to the subsequent crop.  If you have ever plowed down (or killed) a nice stand of alfalfa and then planted corn, then you know just how beneficial a legume in your crop rotation can be.  Legumes can provide over 100 pounds of nitrogen credit per acre, which is why they are often called ‘green manures’.

Rhizobium nodulation seen on pea cover crop roots
Rhizobia nodulation seen on pea cover crop roots

The legumes themselves are not responsible for nitrogen fixation, however.  This happens as a result of a symbiotic relationship between the nitrogen-fixing bacteria that invade the plant root and store nitrogen in root nodules.  The plant provides the bacteria with nutrients and energy, and the bacteria provide the plant with a usable form of nitrogen.  These bacteria, called Rhizobia, are able to take nitrogen gas from the atmosphere (N2) and convert it to ammonia (NH3), which is then converted to ammonium (NH4+) and nitrate (NO3) which are the forms of nitrogen usable by plants.  In order for good root nodulation and maximum nitrogen production, it is important to inoculate your legumes with the appropriate species of Rhizobia bacteria at planting.  Some seed is available pre-inoculated, but many times you will need to apply the inoculant yourself.  Whoever you get your seed from should have inoculant available as well.  Beware, however, inoculants have a short shelf-life and are also species specific.  Using clover inoculant on peas or vetch will not be successful.

Plant Available Nitrogen (PAN)

From D. Sullivan, Oregon State Univ. See Reference below.
Graph 1. PAN from cover crop related to date of cover crop termination. Originally from D. Sullivan, Oregon State Univ. See reference below.

The ability of your legume cover crop to supply nitrogen to your subsequent crop depends on how much biomass and when you terminate the cover crop.  This plant available nitrogen (PAN) becomes available roughly 4-6 weeks after cover crop termination.  Oftentimes, a cereal grain terminated at or beyond the boot stage can actually immobilize nitrogen and create a PAN deficit, making it necessary to increase fertilizer/manure nitrogen applications.  This is because microbes are tying up nitrogen temporarily as they break down the carbon rich material. Conversely, a cover crop terminated too early will provide only minimal PAN.  Below is a simple explanation of the differences between cereal grain and legume cover crops and the implications of when you terminate them.

Table 1 from information in the publication by D. Sullivan referenced below.
Table 1. From information in the publication by D. Sullivan referenced below.

Considerations When Planting Legume Cover Crops

Hairy vetch in bloom

Legume cover crops will need to be planted earlier than cereal grains to survive winter and maximize N production.  For clovers, you’ll want them established by August 15th in Vermont, so this limits them to being interseeded or planted after a cereal grain harvest.  The winter annual legumes can be planted as late as September 1st through 15th, which means you can still plant them after a timely corn harvest.

If you are planting legume cover crops only to replace nitrogen, the economics may or may not pencil out.  Usually in organic systems, this is a preferred practice.  However, when commercial nitrogen fertilizer is $45 for 100 pounds of nitrogen and a legume cover crop could cost you $70 per acre for that same 100 pounds the nitrogen benefit may not be financially rewarding.  Certified organic fertilizer, however, could run you $150 per acre, making the cover crop a wise investment.  However, a legume cover crop is more than just nitrogen, and these additional benefits are harder to quantify.  According to USDA this includes “yield improvements beyond those attributable to nitrogen alone.  These may be due to mulching effects, soil structure improvements leading to better moisture retention and crop root development, soil biological activity and/or enhanced insect populations below and just above the soil surface.” (Clark, SARE).  They are also great soil conditioners, and can provide early weed suppression.

There are many legume cover crops, but the table below gives a list of the most common ones planted in the northeast.

Table 2: Information in this table from Managing Cover Crops Profitably, A. Clark (SARE)
Table 2: Information in this table from Managing Cover Crops Profitably,  A. Clark (SARE)


Sullivan, D. and N. Andrews. 2012. Estimating Plant-Available Nitrogen Release from Cover Crops. Oregon State University Extension Service.

Clark, A. 2007. Managing Cover Crops Profitably. College Park, MD: SARE.

Flynn, R. and J. Idowu. 2015. Nitrogen Fixation by Legumes. New Mexico State Unviersity Extension Guide A-129. 

Carbon to Nitrogen Ratios in Cropping Systems. 2011. USDA Natural Resources Conservation Service.

 Other Resources:

Do you have questions about cover crops? Would you like to conduct a trial on your farm? Contact Kirsten [802-388-4969 ext. 347,]