Bringing the Benefits of Legume Cover Crops to Northern Midwest Climates

Final report for LNC14-364

Project Type: Research and Education
Funds awarded in 2014: $114,497.00
Projected End Date: 01/31/2019
Grant Recipient: University of Minnesota
Region: North Central
State: Minnesota
Project Coordinator:
Dr. Julie Grossman
University of Minnesota
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Project Information

Project Objectives:

Learning outcome 1: Growers will learn if application of nitrogen fixing rhizobia inoculant is necessary with selected cover crop ecotypes. We will characterize nodule occupants among the cover crop varieties in Y1. Mature plants will then be evaluated for nodulation, total biomass N, nitrogen fixed, and rhizobia occupancy.

Learning outcome 2:  Growers will learn basic soil ecological principles to help them manage legume cover crops for optimal function and performance. Blending farmer needs with available data on cover crop biology and soil science, we will develop two hands-on workshops including evidence-based responses to grower knowledge needs. Workshops will be taught as part of grower conferences in the region (i.e. MOSES, Sustainable Farming Association of Minnesota Annual Conference) in the final year of the project.

Action outcome 1:  At least 25 farmers will plan to use a legume cover crop in their farming operation in the three years following project termination. To conduct the formative evaluation, surveys will be provided to workshop attendees and undergraduate course attendees that ask about the degree to which these offerings were useful, met their needs, and what additional questions they still have. Additionally, in the months following the workshops, attendees will again be surveyed and asked to rate their knowledge about utilizing cover crops and the degree to which they are utilizing (farmers) or recommending (those who work with farmers) such practices. Those farmers who identify having most successfully implemented these approaches will be interviewed by phone to understand how and why they were able to engage in these practices. (i.e., what were drivers of their success).

Action outcome 2: At least 60 students will be exposed to examples of legume cover crop use. To meet this action outcome we will develop 4-8 media-rich online case studies showcasing exemplary growers who use cover crops in the North Central SARE region, and highlighting both their successes and challenges in using cover crops. Dr. Grossman will be leading a course in biological principles for use in organic farm management as part of the University of Minnesota’s recently developed Food Systems major, and the development of the case studies dovetails beautifully with this new and innovative program. The case studies will be developed as a course assignment for Dr. Grossman’s class in Y1 and Y2 of the grant, with students conducting all interviews, taking video footage, editing, and developing study questions to be part of the case-study package.


Click linked name(s) to expand
  • Dean Baas, PhD
  • Carmen Fernholz
  • John Mesko
  • Helene Murray, PhD



Year 3 (2016-2017): 

Due to low crop survival in Y1, this study was repeated for a third year (Y3) of data collection at the same two research station locations. This will allow better generalizations about legume performance across multiple site-years.

With regards to cover crop species, nodulation, and nitrogen fixation, we hypothesized: 1) inoculation will increase cover crop nodule number, nodule mass, total shoot biomass, total shoot N, and %Ndfa; 2) the biculture MIX treatment will have the highest biomass, shoot N, and %Ndfa; 3) soil residual N will inhibit nodulation; 4) nodulation will determine N fixation and cover crop biomass; and 5) total shoot N will determine sweet corn yield.

With regards to cover crop production and impact upon soil organic matter, we hypothesized: 1) MIX would have intermediate biomass and biomass N levels, compared to legumes (high N, low biomass) and RYE (low N, high biomass) monocrops and 2) MIX treatment would increase both soil C and N, whereas legumes primarily would increase N and biological C parameters (EXT-N, PMN, MB-C) and rye would influence physical and chemical labile C parameters (POM-C, POX-C).

We strive to understand the relationship between nodulation and nitrogen fixation with the goal of learning best management practices for increasing biomass, N, and N fixation so farmers realize more benefits from their cover crops. Understanding the dynamics between legume cover crop biomass accumulation, soil organic matter transformations, and agronomic performance is a key step in recoupling C and N cycling for sustainable agroecosystems.

Materials and methods:

Year 3 (2016-2017):

Cover crop and corn management
The same cover crop treatments as in Y2 were planted at Grand Rapids, Minnesota (GR) on August 25, 2016 and Lamberton, Minnesota (LAMB) on September 2, 2016, to overwinter until spring.  Cover crops were terminated on May 24, 2017 in Grand Rapids and May 26, 2017 in Lamberton to ensure adequate development of sweet corn. In Grand Rapids, cover crops were terminated with a Toro Z Master Z253 riding mower with a custom fit expanded metal cover in 2015-2016 (The Toro Company, Bloomington, MN) and a FM62-H flail mower in 2016-2017 (Woodmaxx Power Equipment Ltd., Akron, NY). Residue was raked evenly over the plots and was allowed to dry for 7 d prior to incorporation with a TG-60-YK tiller (King Kutter Inc., Winfield, AL). Cover crops in Lamberton were terminated with a 90M flail mower (Loftness, Hector, MN) and allowed to dry for 4 d prior to direct incorporation by a JD-235 tandem folding disk (Deere & Co., Moline, IL). Corn was planted by a model 90 single-row seed drill (HEGE Equipment Inc., Colwich, KS) at Grand Rapids and a 4-row precision cone planter with twin finger units (Kincaid, Haven, KS) at Lamberton. Four 7.6-m rows were seeded per plot.

Biomass and soil sampling
Biomass was sampled 0-2 d prior to termination at all site-years. Four 0.25 m2 quadrats were randomly cut to ground level and pooled from each plot. Biomass was separated by cover crop species or weed and dried for 2-7 d at 60°C. Dried plant biomass samples were ground to 2 mm, and then further ball ground for homogenization. Ground plant biomass was then weighed into aluminum tin capsules (EA Consumables, Pennsauken, NJ) at 10 mg ± < 0.040 mg.  Samples were analyzed using an Elementar vario PYRO cube EA-IRMS CNS analyzer (Elementar, Mt. Laurel, NJ) for C and N content. No nodule sampling was done in Y3 of the study as data had been taken in Y1 and Y2. Eight-to-ten 2.5 cm diameter soil cores were taken per plot at the time of biomass sampling at 15 cm depth and aggregated into a bucket. 

Nitrogen fixation quantification
The percent nitrogen derived from the atmosphere (%Ndfa) is determined using three values: the δ15N from the field-grown legume species (δ15Nfix), the δ15N from a non-fixing reference crop accessing the same soil N pool (δ15Nref), and the δ15N from the legume species grown in the absence of substrate nitrogen and therefore entirely dependent on biological nitrogen fixation (B-value) where δ15N is the natural abundance of the 15N isotope in parts per thousand relative to atmospheric N2 (Unkovich et al., 2008).  B-values were determined with native rhizobia inoculants after preliminary analysis suggested that field-grown legumes were nodulated by endemic strains.

Soil carbon cycling properties 
Permanganate oxidizable carbon (POX-C), is a fraction of active C, chemically defined by the quantity of potassium permanganate reduced in reaction with a quantity of soil (Weil et al., 2003). Coarse fraction particulate organic matter (CF POM) of organic matter > 53µM was determined using size fractionation (Wander, 2004). Microbial biomass C (MB-C) was determined by chloroform-fumigation extraction technique (Brookes et al., 1985, Vance, 1987) and total C determined using a Shimadzu TOC-L/TN analyzer (Shimadzu, Kyoto Prefecture, Japan). Potentially mineralizable nitrogen (PMN) was determined using a 7-day anaerobic incubation technique, based on Drinkwater et al. 1996 and extractable N (EXT-N) calculated at the pre-incubation time point.

Research results and discussion:

Legume cover crops can contribute new nitrogen: Percent nitrogen was determined via elemental analysis and extrapolated to biomass production in order to determine the total nitrogen contributions of legume and non-legume cover crops.  Crops in Y3 contributed significantly more plant tissue N as compared to Y1 (where biomass levels were negligible), and was within range of values from Y2 (Table 1).  In Lamberton, both vetch varieties and the V2/rye biculture contributed between 81-85 kg N ha-1.  In Grand Rapids, values were much reduced in Y3 (ranges of only 20 kg N ha-1). Variety V1 was not planted in GR in Y3 due to lack of seed availability.  The effects of cover crop, environment, and the interaction of cover crop and environment on cover crop biomass, weed biomass, cover crop C/N ratio, and total N content of treatments were significant at all environments (P<0.001).  New nitrogen derived through nitrogen fixation ranged 30-54 kg N ha-1 in Lamberton, and was extremely negligible in Grand Rapids, ranging from 1-10 kg N ha-1, across all cover crop treatments (Table 2). 


Table 1. Cover crop biomass and aboveground shoot nitrogen (N).  Fallow plots were kept weed- and crop-free (data not shown).  Letters represent means separation by post-hoc analysis of Tukey’s HSD at α = 0.05 within site-year.  Standard error is represented as ±.






Grand Rapids



Grand Rapids













Mg ha-1

kg ha-1

Mg ha-1

kg ha-1


Mg ha-1

kg ha-1

Mg ha-1

kg ha-1


2.5 ± 0.5 ab

39 ± 8 b

3.1 ± 0.4 ab

79 ± 8 a


1.9 ± 0.3 a

29 ± 5 a

3.7 ± 0.3 a

81 ± 10


1.5 ± 0.1 b

52 ± 6 ab

2.0 ± 0.1 bc

74 ± 5 a




1.8 ± 0.4 b

85 ± 21


1.8 ± 0.2 ab

71 ± 12 a

1.9 ± 0.1 c

73 ± 4 a


0.3 ± 0.1 b

10 ± 6 ab

2.2 ± 0.4 b

82 ± 14


0.9 ± 0.1 c

30 ± 4 b

0.6 ± 0.1 d

23 ± 3 b


0.1 ± 0.0 b

3 ± 1 b

2.0 ± 0.2 b

72 ± 6


3.0 ± 0.4 a

35 ± 5 b

3.7 ± 0.3 a

53 ± 4 a


1.9 ± 0.2 a

29 ± 6 a

3.7 ± 0.4 a

52 ± 5


† Trt, Treatment.

* ND, not determined; V1 was not planted in Grand Rapids in 2016-2017.





Table 2. Field percent nitrogen derived from the atmosphere (%Ndfa) and total fixed nitrogen (N) for legume-containing cover crop treatments.  No differences were found in %Ndfa or total fixed N in Grand Rapids in 2016-2017.  Standard error is represented as ±.







Grand Rapids



Grand Rapids




kg ha-1


 kg ha-1



kg ha-1


kg ha-1


99 ± 5 a

9 ± 1 b

74 ± 3 a

30 ± 2 bc


82 ± 19

1 ± 1

100 ± 2 a

30 ± 5 b


84 ± 14 ab

43 ± 9 a

66 ± 6 ab

51 ± 6 a




65 ± 2 b

54 ± 12 a


38 ± 19 bc

36 ± 17 ab

45 ± 9 b

33 ± 7 ab


103 ± 11

1 ± 0

60 ± 4 b

48 ± 6 ab


36 ± 7 c

11 ± 2 b

60 ± 3 ab

13 ± 1 c


53 ± 6

10 ± 5

53 ± 5 b

37 ± 2 ab


† Trt, Treatment.
* ND, not determined; V1 was not planted in Grand Rapids in 2016-2017.


Do cover crop legumes affect soil organic matter levels? In our study, winter annual legume cover crop treatments did not influence labile SOM levels at both sites, compared to the no-cover crop control, despite significant impacts on extractable and potentially mineralizable N. Differences in labile C were largely due to changes between pre-termination and post-termination, regardless of cover crop presence or absence, let alone treatment (Figure . Time was significant for POX-C, POM-C, POM-N, and MB-C in GR Y1 and LAMB Y1 and Y2 and for POX-C and MB-C in GR Y2. Cover crop was significant for cover crop for POM-C, POM–N and MB-C in LAMB 2015 and only POM-C displayed a significant interaction between termination time and cover crop treatments. The lack of labile C change in general, and especially across different cover crop varieties, indicates that there is likely a biomass threshold that our experiment failed to achieve.  However, higher values of soil extractable N following legumes indicate the importance of legumes as an immediate N source for crop production due to rapid decomposition.


Research conclusions:


Year 3 (2016-2017):


Our study results demonstrate that winter annual legume cover crops influenced labile N parameters. Hairy vetch-rye mixes produced large amounts of biomass in comparison to legume monocultures while maintaining C/N ratios significantly lower than rye monocultures, which can serve to promote active decomposition and release of N to the soil. However, cover crop biomass production and species differences had little effect on labile C parameters. Overwhelmingly, differences in soil parameters were due to changes from pre- to post-termination rather than cover crop treatments, suggestiing tillage impacts. Site history also likely drove C and N dynamics, especially in GR in which the oxidation of labile C from perennial groundcover conversion prior to annual agriculture experimentation likely overshadowed cover crop treatment effects. Yet in LAMB Y1, legumes increased POM-C levels prior to cover crop termination, indicating that N-rich root biomass may be an overlooked input, even with relatively low amounts of aboveground biomass inputs.

Cover crops had a clearer influence on soil N, EXT-N and PMN increases in cover crop treatments, with significantly greater increases in VET and MIX treatments in EXT-N in LAMB Y1 and Y2. Reduced yield in rye treatments indicate potential issues with crop residue and N immobilization (Teasdale et al., 2008), yet low stand count and yield across treatments indicate a broader need to focus on germination rate optimization and seedling diseases, corn earworm control, and N management in integrated cover crop-sweet corn systems.  

Due to the bio-physical limitations in the upper Midwest of 1) short timeframes for cover crop growth during the spring, 2) lack of adequate time for decomposition, and 3) difficulty to achieve corn maturity in short seasons following cover cropping, it is imperative that agronomic science and future research focus on these challenges. Integration of plastic mulch to speed corn development (Kwabiah, 2004), management of low C/N ratio cover crop mixes to produce sufficient biomass, augmentation of SOM, synchrony with crop N demand (Murrell et al., 2017), reduction of tillage for minimal soil disturbance and effective termination (Ginakes, 2017, Lowry and Brainard, 2016, Williams et al., 2017), and breeding for legume winter hardiness (Silva and Delate, 2017) all demand additional experimentation. Poor establishment of fall-planted vetch and uneven maturation when grown in intercropping setting could also be improved through experimentation with frost seeded or early-spring planted vetch following early-spring termination of fall-planted rye or winter-killed oat cover crops. This may facilitate temporal intercropping and the growth of legumes in the absence of competition. Lastly, relatively high contribution of biomass N due to weed populations demand serious consideration of their utility in diversified cropping systems.

Our results suggest that winter annual legume cover crops integrated into the harsh climates of the upper Midwest are able to survive cold winters, accumulate considerable biomass N, and provision significant soil N after termination. While the limited cover crop biomass accumulated in winter annual cover crops in corn rotations in the upper Midwest may play a key role in N provisioning and a minor role in SOM stabilization, it is likely that it must be supplementary to establishment of perennial rotations for long-term sustainability. Further research is needed to combine evaluations of winter annual legume cover crops with longer-term cropping rotation trials to understand how these cover crops may specifically impact labile SOM and N cycling in agricultural soils, especially over longer time scales.


Here we also include further details about Project Impacts resulting from the workshops and trainings we offered as part of this project, in addition to the quantitative reporting format under 'Learning Outcomes'. The number presented under Learning Outcomes (210) is the total number of attendees of our workshops. We made follow-up phone-calls to a random subset of these participants one year following the workshop dates, resulting in more detailed information about changes in knowledge, attitude, skills and/or awareness following our workshops. The results of these phone-calls are here:

  • All 12 farmers reported increasing their knowledge of basic soil ecological principles to help manage legume cover crops as a result of the workshop
  • 8/12 farmers reported increasing their use of legume cover crops as a result of project educational efforts
  • 5/12 farmers reported altering the way in which they terminate legume cover crops, to increase N provision
  • 2/12 report using nitrogen calculations to estimate the amount of N provided by legume cover crops
  • 7/12 report positive experiences with what they have tried; one reported a negative experience as cover crops were too 'wieldy' in their growth

Additional relevant comments from farmers included:
RE N contribution calculations: would like to do this with high school students, but she is horrible at math, so she doesn't
RE termination: bought a flame weeder, trying it out
RE tillage- before workshop, had a standard tilled bed, but now uses a raised, no-till bed
RE inocculation: didn't before, now inocculates
RE termination: Has switched to a no-till system; Presentation was particularly interesting and informative; Also using legumes in pathways

When asked if there were practices they wanted to implement, but couldn't, reasons why included limited land (1) and time and equipment constraints (4)

While the 12 random participants that we called was only a subset of our 210 workshop participants, extrapolation demonstrates that workshop results were likely impacting farmer practices in using legume cover crops to promote soil N cycling and health, our overall SARE project goal. 


Participation Summary

Educational & Outreach Activities

2 Journal articles
6 Webinars / talks / presentations
4 Workshop field days

Participation Summary:

150 Farmers
125 Ag professionals participated
Education/outreach description:

Outreach and workshops:

Organic Management Systems working group on agroecosystem diversification, Nov 4-6, 2018, Baltimore, MD.
Working group with agricultural professionals to discuss diversification of organic systems, including legume integration.

Sustainable Farming Association Annual Conference 2017; St. Joseph, MN
Workshop taught: Soil Health for Small Farms (40+ participants)
Instructed participants in the basic principles of soil health and application for small farms and gardens, including cover cropping, presenting results from this study.

SWROC Organic Field Day 2017; Lamberton, MN
Workshop taught: Winter annual cover cropping research plots in the Upper Midwest (50+ participants). Instructed community members in thesis research context and design; fielded questions on soil health and cover crop impacts on soil N and C from this project.

NCROC Visitors Day 2017; Grand Rapids, MN
Workshop taught: Winter annual cover cropping research plots in the Upper Midwest (100+ participants) Instructed community members in thesis research context and design; fielded questions on soil health and cover crop impacts on soil N and C from this project.

Wauters, V and Petran, A. 2d workshop organized by Grossman lab graduate students in Red Lake Nation Community Center, as part of the Red Lake Food Summit, Sept 13-14, 2017. All participants were members of indigenous tribes in the USA; in Minnesota, there were representatives from Red Lake and White Earth, as well as from tribes in Wisconsin, and New York. (20 participants). Funded by a SARE PDP, and presented research results from this R&E study.

Presentations and Outreach

Grossman J Designing multifunctional agricultural systems in the Upper Midwest via legume diversification. Soil Science Society of America Annual Meetings, San Diego, CA, Jan 6-9, 2019.

Grossman J and Wauters V.  USDA Sustainable Agriculture Research and Education Executive Council learned about Grossman Lab research funded by their programs during a field presentation on campus in St. Paul, MN, July 18, 2017.

Liebman, A., S. Perrone, T. Sooksa-Nguan, J. Grossman. Linked Crop Production and Soil Organic Matter Impacts of Winter Annual Legumes in Upper Midwest Organic Agroecosystems, Agronomy Society of America Annual Meeting, Soil Health for Agroecosystems session, October 23-25, 2017, Tampa, FL.

Grossman, J., Liebman, A., Perrone, S., Sooksa-nguan, J. Nitrogen dynamics of hairy vetch (Vicia villosa) and red clover (Trifolium pretense L.) legume cover crops in organically-managed agroecosystems in the northern United States. Latin American Scientific Society of Agroecology, (SOCLA), Brasilia, Brazil. Sept 10-15, 2017.

Perrone, S. Grossman, J., Liebman, A., Pfeiffer, A., Sooksa-nguan, T.  Nitrogen contributions from winter annual cover crops in the upper Midwest. Southern Sustainable Agriculture Working Group (SSAWG) Conference, Jan 25-282017.

Journal Articles submitted (in review)

Liebman, A., Perrone, S., Grossman, J., Wells, M.S., Sooksa-nguan, T., Jordan, N. 2019. Effect of legume incorporation on soil organic matter transformations in Upper Midwest row-cropping agroecosystems. Submitted.

Perrone, S., Grossman, J., Liebman, A., Sooksa-nguan, T., Gutknecht, J. 2019. Biological nitrogen fixation of winter annual legume cover crops in Upper Midwest horticultural cropping systems. Submitted.


Learning Outcomes

210 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas taught:
  • basic soil ecological principles to help manage legume cover crops
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.