Final report for GNC23-379
Project Information
High tunnels (HT) are being rapidly adopted by vegetable producers in Minnesota due to resulting increases in vegetable crop production and economic gains. However, in recent years HT sustainability has been called into question, driven by intensive agricultural practices that strain the soil’s productive capacity. Nutrient cycling dynamics have been shown to differ greatly from the open field, due to lack of natural rainfall, temperature differentials, and increased photosynthetic activity. These challenges could offset the economic gains that HT production offers.
The goal of this proposal is to understand the degree to which winter cover crops impact key long-term soil-health factors in HT environments. My objectives are based on the extensive research of the effects of cover crops on soil health and crop productivity, with the aim to quantify and compare soil health metrics between HT soils under long-term cover crop management practices, and those that have never been cover cropped. Specific objectives include 1) Measure and interpret physical, biological and chemical soil health assessment indicators associated with long term winter cover crop production, and 2) Create educational resources and disseminate via in-person events, with an emphasis on emerging farmers. HTs are a particularly appealing approach to enable emerging farmers, such as immigrant and Black, Indigenous, People of Color (BIPOC) farmers, to tap into high-value niche markets, since cost-share payments for HT construction are increased for this population. Implementation of activities targeting emerging farmers will cross-cut all project activities. Outreach activities include 1) a hands-on workshop for emerging farmers to learn about general principles of cover crops, and 2) a presentation of final research results to a farmer audience at a regional conference. Tangible outcomes include practical, research-based information that farmers can use to increase long-term economic gains by making cover crops a part of their HT rotations, and associated educational resources to extend our findings.
This project leverages an existing Minnesota Department of Agriculture (MDA) AGRI-Crop Research Grant aimed at engaging emerging HT farmers in winter cover crop rotations. Under this currently funded project, optimal management of timing of fall planting and spring termination is being investigated to maximize economic and soil health building benefits, such as plant-available mineral N. This proposed SARE project will build off of the information obtained from the MDA AGRI-Crop Research Grant (explained further in the Approach and Methods section).
Objective 1. Measure and interpret physical, biological and chemical soil health assessment indicators associated with long term winter cover crop production. Learning Outcome #1: At least 50 HT Farmers will learn about the impact extended cover cropping has on soil health factors that play a central role in crop productivity. Action outcome #1: At least 20 farmers will plant cover crops in their HT as a way to build and maintain soil health.
Objective 2. Create educational resources and disseminate via hands-on field presentations, farmer conference poster sessions, and University of Minnesota Extension articles. Learning Outcome #2: At least 50 farmers will learn to weigh long term impacts on soil health in their decision to adopt winter cover crop rotations. Action outcomes #2: At least 20 HT farmers will use soil health indicators in their decision to adopt a winter cover crop species that contribute to increases in sustainable practices and decreases in soil degradation.
Cooperators
Research
Materials & Methods
Site descriptions & exterior climate
This study was conducted at two sites over three years from 2021-2024 on certified organic land in Saint Paul, MN (LAT: 44.99028, LONG: -93.17348) and Lamberton, MN (LAT: 44.23836, LONG: -95.3016544) (Fig. 2.1). The Minnesota Agricultural Experimental Station (MAES) in St. Paul, MN has a regional mean annual rainfall of 813 mm. The soil is classified as a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed, superactive, mesic Typic Hapludolls) and is considered prime farmland with 120-170 frost-free days annually (NRCS, 2024). The research site was certified organic in 2017. The Southwest Research and Outreach Center (SWROC) research site in Lamberton, MN was located on a portion of the Elwell Agroecology Farm, characterized by a calcium carbonate fine silt superactive, mesic Typic Hapludoll and is considered prime farmland with 140-180 frost-free days annually (NRCS, 2024). The research site was certified organic in 1993.
High tunnel description
Preexisting HTs varied in size depending on location. In Saint Paul, there was one tunnel (27.5 x 11 m) and in Lamberton, the experiment was conducted across two tunnels (14.5 x 9 m each). All tunnels were oriented east-west. The HTs were covered with a double layer of 0.1mm polyethylene film. For ventilation, sidewalls were opened or closed automatically based on a preset threshold temperature (18 ℃) via AegisTEC greenhouse controllers (Aegis Technologies, Ashburn, VA). Treatment plot size varied by location due to variations in tunnel size. In Saint Paul, plots were 1.83 x 3.048 m, and in Lamberton 2.13 x 2.13 m.
Experimental design
Two legume cover crop species, Austrian winter pea, (AWP, Pisum sativum) and hairy vetch (HV, Vicia villosa), were evaluated at two planting dates (September and October) and two termination date combinations (April and May), resulting in four treatment timings (SA [September planted, April Terminated], SM [September planted, May terminated], OA [October planted, April terminated] and OM [October planted, May terminated]). Table 2.2 specifies the exact field operation dates. Plots were arranged in a randomized complete block design. Four replications were used, with all combinations of cover crop species, planting date, and termination date represented in a block, for a total of 32 plots (4 controls with only cash crop sequences and 28 plots with summer cash crop and winter cover crop rotations). A control rotation included sweet bell pepper (Capsicum annuum) and no winter cover crop, replaced by spinach (Spinacia oleracea). This rotation was intended to reflect industry standards, however, spinach cultivation failed at both sites in all years, either due to pest pressure or early season bolting and freezing. No plots received any additional inputs (i.e., compost or fertilizer) for the duration of the experiment.
Cover crop planting & termination
Cover crop seed was pre-treated with peat-based Rhizobium inoculant by moistening seeds with a 1:4 sucrose solution, mixing with the recommended rate of inoculant, and allowing seeds to air dry overnight before planting (Exceed Pea and Vetch Inoculant, Visjon Biologics, Henrietta, TX). Before planting, beds were rototilled (25 cm FRT, Troy-Bilt, Valley City, OH) to 15 cm and hand-raking evenly spaced furrows in the soil. Cover crop seed was hand-broadcasted per industry-recommended seeding rates and raked lightly. Overhead irrigation occurred weekly until nighttime temperatures reached below freezing. At both sites, low tunnels were then installed for deep winter insulation and consisted of steel hoops placed about 60 cm above the soil and covered with a white, Agribon+ fabric (Johnny’s Selected Seeds, Winslow, ME).
In early spring, when nighttime temperatures rose above freezing, low tunnels were removed, and weekly overhead irrigation resumed until one week before each respective termination time. Due to small plot sizes, cover crops were terminated by hand-cutting with rice knives to the ground. Each plot was then incorporated by rototilling to 15 cm, covered with plastic mulch, and left to decompose for three weeks before planting the subsequent summer cash crop.
Bell pepper planting & management
After the respective cover crop termination and 3-week rest period, 7-week ‘Sweet Sunrise’ bell pepper plugs were planted and drip irrigation tape was installed. Peppers were planted 0.46 m apart and 0.3 m from all plot barriers with 12 plants per plot. Pepper plants were trellised for support, and blossoms pinched off until plants reached mature size (~1 m tall). In May of 2022 at the St. Paul site, irrigation failure led to drought stress, stunted growth, and, in some cases, seedling death. Unhealthy or dead plants were replaced with similar-sized plugs no more than one week after seedling death to avoid diluting treatment effects.
Biomass sampling
Biomass from cover crops was sampled on termination day at all site years. Two 0.1 m2 quadrats per plot were randomly cut to ground level and pooled. Biomass was separated into live cover crop, dead cover crop, or weed species and dried for 2-7 d at 60°C. Dry weight per hectare was calculated for all treatments. All plant material was ground through a 2 mm sieve and further pulverized with a 2010 Geno/Grinder ball grinder (SPEX SamplePrep, Metuchen, NJ) using a 0.5 cm diameter stainless steel ball bearing (Craig Ball Sales, Seaford, DE) for 2 min at 1500 rpm and stored at room temperature before elemental analysis.
Soil sampling
Ten 2.5 cm diameter soil cores were taken per plot two weeks after cover crop termination at 15 cm depth and aggregated into a bucket. Cores were homogenized by hand mixing in the bucket. The mixture was divided into two sub-samples. Half were stored in air-tight Ziploc® bags to retain field moisture and kept at 4°C until analysis for ≤7 days. The second subset was stored in paper bags and air dried at 35°C for 5-7 days or until dry to the touch. Both subsets were passed through a 2 mm sieve before analysis.
Microbial biomass
Microbial biomass carbon (MBC) and nitrogen (MBN) analysis used the direct chloroform extraction method (Setia et al., 2012; Soil Sampling and Methods of Analysis, 2010). Two 10 g fresh soil subsamples were extracted with 0.5 M K2SO4 after one subsample received 0.5 mL chloroform. All samples were shaken for 4 hours at 150 rpm and filtered through Whatman No 1 filter paper. Microbial biomass extracts were analyzed for total organic carbon and total nitrogen on a TOC-CPH analyzer with a TNM-1 module and ASI-V autosampler (Shimadzu Scientific Instruments, Columbia, MD), resulting in a measurement of total inorganic N and C in the sample, including possible trace amounts of dissolved organic C and N.
Potentially mineralizable nitrogen
Potentially mineralizable nitrogen (PMN) was determined using a 7-day anaerobic incubation technique where 10 g fresh soil was held at field capacity moisture in the dark at 37 °C (Drinkwater et al., 1998). The samples were then extracted by shaking in 30 mL 1.33 M KCl for one hour at 240 rpm, filtering with Whatman No 1 filter paper, and extracts frozen until analysis. PMN was analyzed by loading 100 μL of each sample extract into a 96-well plate containing a set of NH4-N standards and reacting with 40 μL each of ammonium salicylate (C7H9NO3) and ammonium cyanurate (C3N4O3). Absorbance was measured at 630 nm with a SpectraMax M5 (Molecular Devices, Sunnyvale, CA). The PMN is reported as the net NH4-N after anaerobic incubation.
Permanganate oxidizable carbon
Permanganate oxidizable C (PoxC) analyses were based on the laboratory procedures and calculations of (Weil et al., 2003; Woodings & Margenot, 2023)). Briefly, 2.5 g of soil was mixed with 18 mL of deionized water and 2 mL of 0.2 M KMnO4 solution. The mixtures were shaken for 2 minutes at 120 oscillations per minute. Samples were allowed to settle for 10 minutes, after which 0.5 mL of the supernatant was mixed with 49.5 mL of deionized water. Then, 200 mL of each sample was loaded into a 96-well plate containing a set of KMnO4 standards. Absorbance was measured at 550 nm with a SpectraMax M5 (Molecular Devices, Sunnyvale, CA).
Pepper harvest
Pepper harvest began once fruit met any of the harvest criteria and continued weekly until the respective cover crop planting date each fall. In 2022 and 2024, all plants were harvested except for the 4 corner plants to avoid edge effects, equaling 8 data plants per plot. However, in 2023, pest damage resulted in an unequal distribution of plants per plot. To prevent bias, yield is reported per plant rather than plot totals.
Statistical Analysis
Data were analyzed using R version 4.2.2 (R Core Team, 2022). Data summaries were conducted with tidyverse (Wickham, 2023). Linear mixed effect models of year, site, species, fall planting, and spring termination on each response variable (cover crop biomass production, biomass-N, PMN, PoxC, MBC, MBN, and cash crop yield) were constructed using the lme4 package with block as a random effect (Bates et al., 2023). Analysis of variance (ANOVA) was conducted on each model, followed by post-hoc means comparisons using the multcomp package with Sidak p-value corrections for multiple comparisons (Hothorn et al., 2023; Wright, 1992). Data was separated via site year because of treatment failure in Lamberton in Y2. ANOVA’s assumptions of normality and homogeneity of variance of residuals were checked using Shapiro-Wilk and Levene’s tests. Significant differences were considered at 𝛼=0.05. Figures (except Figure 2.1) were created using the ggplot2 package (Wickham et al., 2023).
Results
Cover crop biomass & N accumulation
Year, cover crop species, fall planting date, spring termination date, and interactions between year and cover crop species all affected biomass production. In the longest cover crop production period (SM), biomass ranged from 312.1-5918.8 kg ha-1, while the shortest cover crop production period (OA) ranged from 24.3-4962.5 kg ha-1 across site years and species. Y1 produced the most biomass for both species and sites (p=7.7 e-12). Over-winter pest pressure via herbivory resulted in total cover crop failure in Lamberton in Y2, thus the experiment was replanted the following year. St. Paul, though impacted by pest pressure (as seen in significant decreases in production between Y1 and Y2), still accumulated measurable cover crop biomass. There were two treatment differences in the fall planting x spring termination factor analysis. In Y1 the shortest growth period (OA) accounted for the lowest biomass accumulation for HV (p=0.0004). For AWP in St. Paul Y2, there was a 176% difference between the longest (SM) and shortest (OA) cover crop growth periods, where SM accumulated 1928 kg ha-1 more biomass (p=0.01).
Regarding fall planting time, there were only two occurrences of treatment differences: AWP in St. Paul Y2 (p=0.0019) and HV in Lamberton Y3 (p=0.025). In both cases, September-planted cover crops accumulated 1452.1 kg ha-1 and 1526.3 kg ha-1 more biomass than October-planted cover crops. There was only one occurrence of treatment differences regarding spring termination time, i.e., HV in St. Paul Y1 (p=0.0006), where there was a 165% increase between April and May-terminated HV.
Weed biomass was also measured. More than half of the production period combinations showed higher weed pressure when cover crop termination was delayed until May, but fall planting time did not affect weed biomass production. Regardless, AWP and HV accumulated more cover crop biomass than weed biomass (p=5.058 e-16) across all site years and production periods.
Cover crop biomass-N was impacted by year and fall planting time with interactions between site and species and year and species. Similar to biomass production, cover crops accumulated more N (kg ha -1) at both sites in Y1 compared to Y2 or Y3, respectively (p=1.397 e-12. Austrian winter pea performed similarly across site years except St. Paul Y2, where the longest production period (SM) accumulated the most biomass-N (p=0.01135). Hairy vetch only had one occurrence of treatment differences in St. Paul Y1, where all treatments performed similarly except for the shortest production period (OA), which accumulated the least biomass-N (p=0.0005).
Soil Health Indicators
Four soil analyses were used to quantify soil health impacts of varying winter cover crop production periods, including potentially mineralizable nitrogen (PMN), permanganate oxidizable carbon (PoxC), and microbial biomass carbon and nitrogen (MBC and MBN, respectively). All analyses were conducted on soils at one sampling time point, approximately 3 weeks after the respective cover crop termination (either April or May, depending on the treatment), to capture the decomposition point at which we expected measured soil variables to be impacted. Potentially Mineralizable Nitrogen was affected by site, year, fall planting time, and spring termination time. Interactions between year and fall planting time and species and spring termination time also affected PMN. While there were generally no treatment differences, PMN in St. Paul increased in Y2 relative to Y1 (p=2.2 e-16), with AWP in the SM treatment increasing by 5 times and 4 times for HV (Figure 2.2, panel A). The time at which cover crops were planted in the fall had less of an effect on PMN, with most treatments showing no differences between planting earlier and planting later (Figure 2.3, panel A). When comparing the impact of spring termination time, April-terminated treatments had higher PMN than May-terminated treatments (Figure 2.3, panel B), which was especially prevalent in HV plots. Permanganate oxidizable carbon was affected by site, year, fall planting time, and spring termination time. Where treatment differences occurred, PoxC was greatest in September-planted, April-terminated (SA) treatments (Figure 2.2). While MBC was affected by site and year, MBN was only affected by year, where Y1 in St. Paul accumulated more MBN across treatments than in Y2 (p=6.894e-6). There were no treatment differences for MBN (Figure 2.2 and 2.6). Microbial biomass carbon was greatest in Y2 at St. Paul for both SM and OM treatments under AWP (p=0.023, Figure 2.2) and appears to be driven by spring termination (p=0.0024, Figure 2.5).
Bell pepper yield
Bell pepper marketable and unmarketable yield was measured to understand productivity benefits and tradeoffs associated with winter legume cover crops and is reported as mean annual kg⋅plant-1. Marketable yield was affected by fall planting and spring termination time factors. Several interactions also affected marketable yield, including, site and fall planting time; year and fall planting time; site and spring termination time; year and spring termination time; and a three-way interaction between cover crop species, fall planting time, and spring termination time. In Y1 in St. Paul, late terminated cover crop plots (and subsequent late planting time of cash crop) did not negatively impact marketable cash crop yield and produced the greatest marketable yields across cover crop species (p=0.006 for AWP, p=0.01 for HV). However, this trend was reversed in the following years at both sites, where marketable pepper yield decreased by as much as 40% with an earlier cover crop planting time (Table 2.13) and a later cover crop termination time. In Lamberton, there were no combined treatment factor differences in Y1. Still, single-factor differences for fall planting time and spring termination time suggest marketable yield was highest when the cover crop production period was cut short in fall or spring. In Lamberton Y3, the shortest cover crop production period (OA) produced the greatest marketable yield for both cover crop species (p=2.8 e-05 for AWP, p=0.01 for HV).
The unmarketable yield was affected by year, fall planting, spring termination time, and an interaction between year and fall planting (Table 2.5). Cover crop treatment affected unmarketable cash crop yield in three cases: Y1 at Lamberton for each species (p=0.03 for AWP, p=0.01 for HV) and Y2 at St. Paul for AWP (p=0.01). In all cases, the longest cover crop production period (SM) produced the least unmarketable cash crop yield. In the case of Lamberton Y1, the unmarketable yield decreased with delayed cover crop spring termination time (p=0.002 for AWP, p=0.005 for HV) when peppers were planted later, but fall planting time had no impact (p=0.2 for AWP, p=0.9 for HV). However, in St. Paul Y2, unmarketable cash crop yield appeared to be impacted by termination and planting time factors. In almost all cases, unmarketable yield increased when the cash crop season was extended by a shorter cover crop season.
Educational & Outreach Activities
Participation Summary:
So far I have had the opportunity to interface with many new, emerging and well-established farmers in the form of field days, online webinars, and a farmer-focused conference. I have also had the opportunity to consult with farmers on their journey to implementing cover crops in their high tunnel systems. In January 2024 I gave an online presentation of my work at the Wisconsin Organic Vegetable Production Conference with over 30 participants. In February 2024 I presented my research and had in-person contact with dozens of farmers at the Marbleized Organic Farming Conference. In March 2024 I gave a joint presented webinar on cover cropping practices in high tunnels at the Route 1 Emerging Farmers Institute to over 40 farmer participants. I also had the opportunity to work with three working farms over the 2023-24 winter season on a cover crop pilot trial, in which farmers experimented with planting cover crops in their high tunnels. Participants reported at the beginning, midway and end of the trial on performance of cover crops.
Project Outcomes
There are many environmental benefits to cover cropping, however our work has shown that more time may be needed to see results in arid-like environments such as high tunnels. This is important to know, because farmers may have to supplement short term with other practices like compost application. Long-term the use of cover cropping may help farmers economically as organic matter is accumulated and reliance on costly compost is reduced.
My knowledge and attitude about sustainable agriculture has become more complex. I now understand the challenges of implementing cover crops in working farm operations comes with many considerations. Blanket solutions do not work for all farms, thus regionally-focused research and guidelines are essential. I have also learned that there is a need for more economic assistance in these practices if we want to see real change. In some cases, cover cropping may be more beneficial to the environment, but no the pocketbook. Many people who want to try these practices are not able to take on the risk due to lack of funding through government cost share programs. Farmers want to try these practices, but there are still many barriers other than lack of knowledge that keep them from implementing them.