The use of cover crops is an integral part of sustainable field cropping systems, with well-documented benefits to soil health, pathogen suppression, and sustaining crop yields. However, cover crops have been utilized to a lesser extent in year-round, intensive high tunnel cropping systems. This is due to both biophysical constraints in these systems, such as short windows for non-cash crops, as well as economic constraints, such as potential lost revenues from cash crops sacrificed by growing cover (non-cash) crops. This situation presents a set of trade-offs common to sustainable agriculture systems – balancing production with other ecosystem services enhanced by conservation practices that take lands temporarily out of cash crop production. High tunnel systems offer a compelling example of such trade-offs. These unheated greenhouses are passively heated and ventilated by physical manipulation of the plastic covering the structure. These highly productive, predominantly soil-based systems have been growing in popularity due to cost share programs and the growth of year-round market opportunities. In the Southeast, tunnels afford producers the ability to extend warm-season crops approximately one month earlier and later, and allow production of cool-season crops throughout the winter months. Although this may increase the availability of locally produced foods, the intensity of these production systems leave little time for integrating cover crops and/or fallow periods typically associated with sustainable agriculture practices. We approached this project from an interdisciplinary, systems-oriented perspective via three main objectives. Objective 1 was a survey of high tunnel growers in the southern region to identify current practices and potential temporal windows in which cover crops may be integrated. Objective 2 was focused on the trade-offs discussed above through evaluating the economic costs and ecosystem service benefits of cover crops in tomato high tunnel rotations. This multi-state field experiment integrated cover crop treatments into a spring-planted tomato production system on research farms in Kentucky and Tennessee. Ecosystem services evaluated include soil parameters associated with soil health, nitrogen leaching, weed seed bank management, and yields. Objective 3 was focused on identifying novel cover crops for high tunnels. Specifically, we evaluated a suite of warm- and cool-season cover crops for rapid biomass production, weed suppression, and other desirable traits identified by our participating farmers. Multi-state, on-farm trials were conducted with high tunnel growers in Georgia, Kentucky, and Tennessee. Project results have been communicated through numerous state and regional grower conferences, on-farm trials and many outreach activities. Additional outreach products (fact sheets, etc.) are in development and will be made available through the he multi-state Center for Crop Diversification (http://www.uky.edu/Ag/CCD/) and UT Organic and Sustainable Crop Production Program (http://organics.tennessee.edu). Results have been communicated to academic audiences through presentations at conferences, with two peer-reviewed articles in preparation.
Objective 1: Conduct a high tunnel grower survey to characterize high tunnel cropping systems and existing cover crop use in high tunnels in the Southern SARE Region through a high tunnel grower survey. This survey was designed to gather management practice data for high tunnels to be used to characterize management techniques regionally, establish baseline practices, and include market information that is in high demand in local and regional food systems including early and late season pricing for tunnel-produced crops. Survey responses were collected from 106 high tunnel producers through online (Survey Monkey) and print surveys from producers without internet access. The survey was conducted during Fall/Winter 2018 – 2019.
Objective 2: Evaluate the ecosystem services and economic costs of using cover crops in high tunnel crop rotations. A two-year, two-site study was conducted on the University of Kentucky Horticulture Research Farm and the University of Tennessee East Tennessee Research and Education Center. We incorporated winter cover crops into tomato production systems, and compared economic and ecological costs and benefits of cover crop treatments with a continuously cropped (year-round) tomato-lettuce rotation. The cover crop treatments included a cool season grass (winter wheat) alone, a cool season legume (crimson clover) alone, and a cool season grass-legume mixture (winter wheat and crimson clover) together. The ecosystem services evaluated included parameters associated with soil health, nitrogen leaching losses, weed seed bank dynamics, and yields. Economic parameters evaluated will include costs such as inputs (fertility, irrigation, water, seed, etc.), opportunity costs lost from not growing a cash crop, fertility (nitrogen) credits from the cover crop, labor requirements, and gross returns from yields.
Objective 3: Identifying novel cover crops that fit the unique “niche” of high tunnel production systems. This objective consisted of a two components, one research-focused and the other outreach-focused. The research component consisted of a two-year study to evaluate novel cover crop candidates at the UK Horticulture Research Farm conducted from Fall 2016 – Spring 2019, and b) On-farm trials with four participating farmers in Georgia, Kentucky, and Tennessee. Cover crops were selected for the novel cover crop trials to represent both traditional cover crops grown in the open field in the region, as well as some more unique accessions used in forages or being developed as novel new crops for the region. The novel trails consisted of evaluations of cool season and warm season cover crops, each evaluated for two years. The outreach component consisted of the University personnel working with the participating growers to identify key high tunnel management challenges they the farmers wanted to utilize cover crops to manage, including knot nematode suppression, compaction mitigation, and enhancing soil organic matter and overall soil health. Participating farmers in each state then trialed the selected cover crops on their farms.
Objective 1: Conduct a high tunnel grower survey to characterize high tunnel cropping systems and existing cover crop use in high tunnels in the Southern SARE Region through a high tunnel grower survey. The blended web- and paper-based survey consisted of 22 research questions focused management practices data, crop selection and timing, and marketing information that is in high demand in local and regional food systems including early and late season pricing for tunnel-produced crops. Surveys were distributed at producer conferences by staff from the UK Center for Crop Diversification, as well as via email to producer listservs in the region. The first 150 respondents to complete the survey were incentivized with a $20 check at the close of the survey period. Data were summarized by respondent demographics, characteristics of farms with high tunnels, marketing channels, and production practices and management challenges. Survey results are summarized in the Results and Discussion section below.
Objective 2: Evaluate the ecosystem services and economic costs of using cover crops in high tunnel crop rotations. This two-year, two-site study was conducted from Fall 2016 – Summer 2018 at the University of Kentucky Horticulture Research Farm (Lexington, KY) and the University of Tennessee East Tennessee Research and Education Center (Knoxville, TN). At both locations high tunnel air temperatures at 1-foot and 5-foot heights and soil temperatures at 6-inch depth were recorded continuously for the experiment. The experiment was a completely randomized block design with six replications. Three tunnels were used at each site, with each structure blocked along the length of the tunnel. Plots measured 3 feet wide by 30 feet long, allowing for two blocks per tunnel. Treatment plot assignments did not change across years to allow for more time for soil benefits from cover crops to accrue. Both sites were managed according to USDA National Organic Program (NOP) standards.
The rotations evaluated consisted of three different cool season cover crops and a control (a cool season cash crop), followed by a warm season tomato crop, repeated for two years at each site. Cool season cover crop treatments were selected based on species in functional groups (grass or legume) known to perform well in the field in the study area. These included winter wheat (Triticum aestivum, ), crimson clover (Trifolium incarnatum), and these two grown in a mixture. A continuously cropped control including a cool season cash crop included romaine lettuce (Lactuca sativa) varieties, without a cool-season cover crop in the rotation. Tomato (Solanum lycopersicum) was selected as the warm season crop in all treatments, due to the long history of the use of high tunnels for early season production of this crop, and the continued economic importance and widespread production of tomatoes in high tunnels.
Two 0.25 m2 cover crop biomass samples were collected from each bed in spring of each year in each location. All vegetation was clipped at the soil surface and sorted into cover crops and weeds. The cover crop mixture was further sorted into clover and wheat fractions; weeds were also separated into main component species. Cover crop samples were dried at 65 °C, weighed, milled, and analyzed for carbon and nitrogen content for the components of each sample. The samples were the recombined and analyzed by near-infrared reflectance spectroscopy (NIRS) for predicted N release from the cover crop residue. Cover crops were terminated by mowing and incorporated with tillage by walk-behind tractors at both sites. Cover crops were allowed to decompose for two weeks after incorporation and prior to fertilization and bed preparation for the tomato crop. After two weeks, plots were fertilized with a pelletized organic fertilizer (Naturesafe 8-5-5, Cold Spring, KY), based on cover crop N credit estimates from vegetation samples taken prior to termination to bring the total fertilizer and cover crop N application to a rate of 100 lbs N/acre.
Tomato yield data were collected by weekly harvest from 10 plants per plot from fruits that were at minimum at the “breaker” ripening stage. Fruits were sorted by marketable (USDA #1, #2 and fancy) and unmarketable fruit. Marketable fruit were graded into Florida tomato grading classes, then counted and weighed by category. Unmarketable fruit were sorted by primary cause of unmarketability (e.g. too small, insect damage, yellow shoulder disorder, blossom end rot, etc.), and then counted and weighed by category. Fruit were harvested from June – July 2017 and 2018 (Fig. 1). Lettuce were harvested at maturity and were classified as marketable or unmarketable, then counted and weighed by category.
Soils were sampled semi-annually, at the transition between cool and warm season treatments. Three soil cores were collected per plot from the 0-15 cm and 15-30 cm depths, and bulked for a single analysis per plot. Soils were air dried and submitted to the University of Kentucky Regulatory Services Soil Testing Lab for routine soil analyses (pH, buffer pH, P, K, Ca, Mg, Zn, total percent N, total percent C and soluble salts. Active (permanganate oxidizable) carbon (POXC) assays and potentially mineralizable N (PMN, hot KCl method) were conducted in the project PI’s lab. Nitrogen leaching was assessed using ion exchange resin (IER) lysimeters utilizing a mixed bed ion exchange resin beads. At both locations, lysimeters were buried below undisturbed soil at a depth of 45-50 cm. Lysimeters were retrieved and new lysimeters deployed at each transition between warm and cool season treatments around the same time as soil sampling and planting. Once retrieved, IER lysimeters were disassembled and the resin beads extracted in 2M KCl and analyzed in the PI’s lab.
All statistical analyses were conducted using SAS Version 9.4 (Statistical Analysis System Version 9.4 for Windows; SAS Institute, Cary, NC, USA). Significance of main effects and interactions was assessed at level of alpha < 0.05. Tomato and lettuce yield data were analyzed using general linear mixed model (GLIMMIX) procedure. Winter weed biomass and soil data were analyzed using the MIXED procedure.
Objective 3: Identifying novel cover crops that fit the unique “niche” of high tunnel production systems – Novel cover crop evaluation. Novel cool season and warm season cover crops were selected based on the unique temporal and microclimate conditions of high tunnels occurring in each of these seasons. The accessions were evaluated at the UK Horticulture Research Farm using a randomized complete block design consists of 11 cover crop accessions in each season (warm and cool season experiments), plus a “weedy check” to evaluate background weed pressure. The study consisted of three replicates, each in a separate 30’ x 72’ high tunnel. Plots measured 5′ x 10′ and were randomly split to weeded and unweeded subplots. Weeded subplots are hand-weeded throughout the growing season to greatly reduce weed competition in order to determine growth and biomass potential under relatively ideal growing circumstances.
Biomass samples (1 ft2) were taken from each split plot, on a monthly basis for the cool season and bi-weekly during the more rapidly maturing warm season. Unweeded subplot samples were separated by cover crop and individual weed species, dried at 65 °C and weighed to determine cover crop and weed biomass. This allowed us to assess the ability for the cover crop to compete against weeds from establishment to maturity. Weeded subplots consisted of the pure cover crop sample, removed from competition with weeds. These samples were also dried at 65 °C and weighed. Based on input from our participating growers and common crop timing in high tunnels, a subset of cover crop samples from the weeded subplots were selected for nutrient analysis. These consisted of sample dates that corresponded to dates where the cover crops would need to be terminated for a subsequent crop (March 1 if preceding a warm season crop, Sept 1 if preceding a cool season crop). Sample dates that corresponded to the maximum biomass growers indicated they could tolerate, as well as final dates when cover crops flowered or set seed. Dried samples selected for nutrient analyses were milled and analyzed for carbon and nitrogen content and by near-infrared reflectance spectroscopy (NIRS) for predicted N release from the cover crop residue.
Temperature data from the high tunnels were used to determine if accessions could be modeled according to growing degree days. This would, theoretically, allow the results to be extrapolated throughout the region utilizing high tunnel temperature data for any particular site. The Southern SARE YSE Scholar conducted the initial modeling using linear regression modeling in Microsoft Excel utilizing the high tunnel air temperature data and weeded subplot cover crop biomass data. Additional analyses using the SAS REG procedure and additional model fitting will be conducted when preparing the data for publication.
Please note, due to a relatively large amount of data generated from the project we have summarized some key results and discussion points in the paragraphs below. Once published citations for each objective become available, we will update the final report. These works will contain more complete results and analyses than those presented here. The project team would like to acknowledge the analysis and writing assistance from supporting staff, including Mr. Alex Butler and Mr. Matthew Ernst (Objective 1) and Ms. Victoria Stanton (Objective 3).
Objective 1: Results from the high tunnel producer survey.
Demographics of respondents. Nearly three-quarters (73%) of responding producers operated high tunnels in Kentucky, with another 11% from Georgia, Tennessee and Alabama, collectively. Males accounted for 59 percent of respondents, and females 41 percent. One-fourth of respondents were in the 35 to 44-year age range, and 19 percent were under 35; only one operator reported being less than 25 years old. More than one-third (37 percent) of operators were 55 or older. Growers were generally experienced with high tunnels, with half of the respondents indicating they have used high tunnels for 10 or more seasons.
Sales and Marketing Channels. Respondents were asked to estimate the percentage of their produce sales originating in the high tunnel. The average response was 46 percent of produce sold from the farm is grown in the high tunnel(s). There were 96 of 106 respondents that indicated their use of produce marketing channels before and after installing their high tunnels. Their responses provide insight into both the primary marketing channels of the responding producers and the expected impact of high tunnels upon their farm sales. The data indicate that producers tend to diversify market channels after installing high tunnels. The rates of respondents using community supported agriculture (CSA) and community farmers markets remained about the same after installing the high tunnels. Other market channels increased after high tunnel installation. The largest increase, by percentage of use, came in those selling produce at an on-farm retail market. This may indicate producers preferring to pursue a market channel with fewer transaction costs, like transportation and packing.
High tunnels helped respondents extend their produce growing and marketing season while diversifying the number of products offered. More than 90 percent of respondents said early- and late-season market activity “increased a lot” or “increased some” after installing their high tunnel. High tunnels also diversified product offerings, with 45 percent of respondents saying their variety of products marketed increased “some” after high tunnel installation and an additional 20 percent saying the variety increased “a lot.”
Those installing high tunnels also increased marketing to wholesale channels: groceries, restaurants, schools and other institutions, and auctions. The increase in serving schools and institutions is noteworthy, as an additional 10 percent of producers sold through those channels after installing their high tunnels. Producers were very optimistic about the school/institution market, with 94 percent of producers selling to schools expecting those sales volumes to increase. Interestingly, those using high tunnels also indicated optimism about the demand for local food in their area, with 24 percent expecting local food demand to increase significantly and 49 percent expecting demand to increase somewhat. Less than 10 percent said they expected any sort of decrease or had no good basis for knowing.
High tunnel production practices. Survey respondents indicated a mix of conventional and organic growing practices in their high tunnels. One-fifth said they used primarily conventional growing techniques, with an additional one-third incorporating some organic along with conventional methods. The most common response (36 percent) was that high tunnel production was according to organic standards but not certified. Ten percent of respondents indicated their high tunnels were certified organic. More than half of high tunnel operators said they used high tunnels for year around production always or sometimes.
Testing soil, rotating crops and scouting are the most regular production practices reported by high tunnel operators. Cover cropping, followed by solarization and thermal banking, were the next most frequent practices reported. Rainwater catchment systems were less frequent overall, but those using rainwater catchment were more likely to report regular use of rainwater catchment as a practice. Biofumigation was ranked as the least likely management practice by those surveyed.
High tunnel production challenges. Weed, insect and disease management were ranked as the most challenging production situations among a list of eight common management challenges (Figure 1). Soil fertility and space management were next most challenging, as evidenced by responses of “serious problem and difficult to manage” or “regular problem but readily managed.” Along with weed management, labor availability and salinity were most frequently ranked as serious problems difficult to manage. The least challenging area among this list was oversupply for current markets.
Areas of Biggest Questions. When asked to rank areas in which they had questions, more high tunnel operators said they were “in good shape” for crop selection than any other area. Cover crops were most likely to be rated as “not applicable” because of a significant number of high tunnel operators not using cover crops. High tunnel operators were slightly more likely to say they “need lots of help” in the areas of marketing, weed management, and soil fertility management. Insect pest management and disease management were most likely to be ranked as “making some progress.”
More high tunnel operators said peer-farm high tunnel demonstrations were “extremely helpful” than any other information source. Cooperative Extension programs were indicated as the resource most frequently used, also receiving favorable ratings. High tunnel operators also showed a penchant for accessing online resources and ranked online publications and YouTube or online videos to be useful. NRCS programs received high marks for being “extremely helpful,” likely reflecting resources available through cost share programs in Kentucky and other states. Other agency programs and trade association workshops were the resources respondents were least likely to draw upon for information about high tunnel production (Figure 2).
Objective 2: Results from the ecosystem services of cover crops in high tunnels experiment.
Climate Data. Temperatures in winter of 2018 were more extreme in 2018 than in 2017, with lower winter temperatures and higher early summer temperatures. The cooler winter temperatures led to less high tunnel cover crop growth at both sites in 2018, discussed below. Interestingly, our climate data show that minimum temperatures in high tunnels can be lower than the outside temperature, though with sun, they warm rapidly. This is consistent with other literature documenting high tunnel temperature dynamics, but it is somewhat counter to the conventional wisdom that high tunnels offer consistent cold protection above field temperatures.
Cover Crop Performance and Composition. In both Kentucky and Tennessee, total cover crop biomass was similar in all treatments, though more biomass was produced in 2016-17 compared to 2017-18. When averaged across all cover crop treatments, in both study years the total cover crop biomass was at least twice as great in Kentucky as Tennessee (2017: KY = 5.3 tons/ac, TN = 1.9 tons/ac; 2018: KY=3.8 tons/ac, TN = 1.4 tons/ac).
In Kentucky, weeds contributed a large component of the cover crop treatment biomass. There was greater weed biomass (p = 0.004) in the clover cover crop treatment, compared to the wheat and clover/wheat mixture (Figure 3). Wheat biomass in Kentucky was greater when winter wheat was grown alone compared to grown in with clover, likely due to the lower wheat seeding rate in the mixture. When averaged across both years, the wheat alone treatment had the greatest nitrogen content (141.6 lbs N/ac, p=0.0483) compared to the clover alone (115.6 lbs N/ac) or the clover and wheat mixture (115.1 lbs N/ac), although the means were not different from one another.
Overall, the clover was not a very effective weed competitor at the Kentucky site. This site is part of the “transition zone,” and falls within the southernmost region of many northern weed species, and the northernmost region of many southern weeds. Further, the site is located on fertile, prime agricultural soils in the state, and in nutrient-rich high tunnel soils. As such, legume cover crop species grown alone may not be the ideal choice for such soils, as their nitrogen fixing services may be outweighed by their lack of weed suppression.
The Tennessee site did not have the weed pressure seen in Kentucky, and the clover performed well relative to weeds. This is perhaps due to the relatively lower fertility, clay soils at the Tennessee site, in which legume cover crops can be effective competitors in low weed pressure situations. The clover biomass was greater when grown alone than when grown in mixture in both years (Figure 3). When grown alone, the wheat biomass was similar between the two years, although the wheat biomass in the mixture was greater in 2017 compared to 2018. Weed biomass in Tennessee was not affected by treatment, year, nor the interaction of the two. In Tennessee cover crop N content was affected by treatment (p=0.0189), with the clover alone having a higher N content (58.1 lbs N/ac) versus the wheat alone (43.7 lbs N/ac), and the clover + wheat (53.2 lbs N/ac), which did not differ significantly from either species grown alone.
Soil Quality. In Kentucky, we saw no significant effects of cover crop treatment on soil parameters, either through main effects or interactions. Nearly all soil parameters, except phosphorus and zinc, varied by depth, which is expected due to the stratification of nutrients through soil layers. All soil parameters, except for zinc, changed significantly over the course of the experiment. For reasons that need further explanation, many soil parameters had levels that increased from year 1 to year 2, then decreased from year 2 to year 3. This pattern occurred in many of the soil parameters, and was a similar trend in the significant date by depth interactions observed for soil P, Zn, soluble salt, and total carbon and nitrogen levels (data not shown). However, these changes did not vary by treatment, and may have been a function of regional climate patterns or variability due to production practices, as similar trends were seen in Tennessee (discussed below).
In Tennessee we observed cover crop treatment effects on phosphorus, pH and soluble salt levels. Phosphorus were greater levels were greater in the continuously cropped (62 lbs/ac) and clover+wheat (53 lbs/ac) treatment, though the wheat and clover alone (49 lbs/ac each) treatments were not significantly less than the clover+wheat mixture. Potassium and magnesium and zinc levels all declined within each treatment throughout the course of the experiment, but did not vary between treatments within each year. As such, no one treatment significantly buffered or drove the fertility declines seen in the experiment.
Several soil parameters (phosphorus, calcium, magnesium, zinc, soluble salts, and total soil carbon and nitrogen) had significant date by depth interactions. The trends for phosphorus and calcium were similar, with values varying by depth, with lower values in the deeper layers and higher values in the shallow layers throughout the experiment. However, within each layer, levels did not differ from starting levels by the end of the experiment. Magnesium levels varied by depth and did not differ at either depth between year 1 and year 2; however, in year 3 Mg levels dropped and did not vary by depth. Zinc levels also varied by depth throughout the experiment, and declined from year 1 to year 2, but did not change from year 2 to year 3. Soluble salts varied by depth throughout. They did not differ significantly from start to finish in the surface layer, but were reduced by year 3 in the deeper soil layer. However, none of these levels are problematic for tomato or lettuce production. Similarly, total soil carbon did not differ in the surface layer from start to finish, but was slightly reduced in the lower soil sampling depth by the end of the experiment. Soil pH varied by depth in 2016, but by the end of the experiment did not differ significantly within or between soil sampling depths. pH in the soil surface layer dropped from 5.9 – 5.6 during the three-year study, but was not unexpected due to the lack of application of lime or other pH altering amendments during the experiment.
Of particular note to soil quality issues in high tunnels, we observed a decline in soluble salts throughout the experiment at both sites. Soluble salts can be production-limiting to certain crops, and are prone to building up in high tunnels due to lack of flushing rains under the plastic cover. Soil K levels also decreased significantly every year at each sampling depth, concomitant with the decline in soluble salts. This decline in K was likely the source of the tomato ripening disorder (yellow shoulder) observed in the final year of the experiment at both sites.
Nitrate Leaching. We did not see significant treatment or seasonal effects on nitrate leaching losses in the ion exchange resin lysimeters. Overall, leaching losses were quite low, and were under 5 lbs NO3/ac at each sampling date. This could reflect very little free water percolating through the soil depths where the lysimeters were located, which was largely below the rooting zone (20 inches). This is probable, as soil conditions were fairly dry at each excavation/deployment. This would indicate that as a conservation practice, high tunnels effectively reduce nitrate leaching in soils under the structures, irrespective of the crops and cover crops used in this experiment. These low levels may also be due, in small part, to incomplete de-sorbtion of nitrate ions from the ion exchange resin used in the resin lysimeters. The resin inside the lysimeters was only extracted with a single round of KCl solvent, and additional ions may have been released with additional rinses.
Tomato Yield. Marketable and total tomato yields (by weight) did not differ among treatments in Kentucky or Tennessee in either year, with the exception of total yield in Tennessee in 2017 which was lowest for the wheat treatment and similar for the other three treatments (data not shown). Total marketable yields were higher in Kentucky than in Tennessee when averaged across both years (p<0.0001, Kentucky = 64.2 lbs/100 ft2, Tennessee = 39.3 lbs/100 ft2), and yields across both sites were greater in 2017 than in 2018 (p<0.0001, 2017 = 80.9 lbs/100 ft2, 2018 = 23.8 lbs/100 ft2). High levels of the tomato ripening disorder yellow shoulder (YSD) were observed at both sites, especially in 2018 (data not shown). Prevalence of YSD is related to soil nutrient imbalances, and is common in high tunnel tomato production. We discuss the relationship between yield and soil quality in the section below.
Correlating Relationships Between Cover Crops, Yields, and Soil Quality. Although we did not see cover crop treatment effects on tomato yield or soil quality through analysis of variance, we did find correlations between these attributes. Tomato yields were strongly positively correlated with total cover crop biomass (rs = 0.71, p<0.0001) and the wheat cover crop biomass (rs = 0.63, p=0.0027) in Kentucky, and moderately correlated with these variable (rs = 0.43, p=0.0089; rs = 0.46, p=0.0246, respectively) at Tennessee. In Kentucky yields were also moderately correlated with the nitrogen content in the cover crop (rs = 0.56, p=0.0012). These data indicate that there is a positive relationship between cover crop biomass and yield, and further, that yields are not positively or strongly correlated with legume cover crop biomass.
The effect of low soil potassium levels on yields in the second year of the experiment has been discussed above. However, the correlation data suggest a slightly more nuanced story. In Kentucky, yields were weakly positively correlated with soil K content (rs = 0.36, p=0.0223 in the 0-6” layer, (rs = 0.36, p= 0.0244 in the 6-12” layer) and were moderately positively correlated in Tennessee (rs = 0.54, p=0.0223 in the 0-6” layer, rs = 0.55, p<0.0001 in the 6-12” layer). Yields were moderately negatively correlated with magnesium content in Kentucky (rs = -0.59, p<0.0001 in the 0-6” layer, rs = 0.49, p=0.0014 in the 6-12” layer) and very weakly negatively correlated in Tennessee (rs = -0.15, p=0.3253 in the 0-6” layer, rs = -0.14, p=0.3395 in the 6-12” layer). However, in both locations yield was more strongly correlated with the “Hartz ratio,” a measurement of the ratio of exchangeable potassium and magnesium content (millequivalents of soil exchangeable K/√Mg). Yields were strongly correlated with Hartz ratio in Kentucky (rs = 63.15, p<0.0001 in the 0-6” layer, rs = 0.64, p<0.0001 in the 6-12” layer), and moderately correlated in Tennessee (rs = 0.57, p<0.0001 in the 0-6” layer, rs = 0.56, p<0.0001 in the 6-12” layer). Finally, in Tennessee, yields were also moderately positively correlated with active carbon (rs = 0.56, p<0.0001 in the 0-6” layer, rs = 0.51, p=0.0002 in the 6-12” layer), indicating a positive relationship between labile carbon and yields on these heavy clay soils.
As such, these data indicate that the declining yields observed between the two years are more than simply a function of low soil K levels. In Kentucky, they indicate a fertilizer salt imbalance rather than an overall decline in fertility may be more of the driving factor in yield and quality reductions. In Tennessee, yields were moderately correlated with K content, Hartz ratio and active carbon (POXC). This indicates that yield increases are correlated with increasing soil carbon content and increasing K fertility on these heavily-weathered clay soils.
Economic Trade-Offs of Winter Crops and Cover Crops. In Tennessee and Kentucky, two lettuce crops were harvested in the 2016-2017 winter season, and one crop in 2017-2018. Gross profits ranged from $2.06 – $2.99 per square foot for each harvest, but harvest quality and yields were variable from year-to-year and between sites. For example, almost half of the first lettuce harvest in Tennessee was unmarketable, largely due to bolting and tip burn (data not shown). In Kentucky, the second harvests in both years were significantly less than the first harvests, and of lower quality. Thus, although cover crops do come with both the direct costs of the cover crop practice, they also come with opportunity costs associated with forgoing a winter cash crop. However, the profit and predictability of the winter cash crop depends upon weather and viable direct markets in winter.
Our results to do not indicate a strong effect of specific cover crop direct ecosystem service benefits that would come with input cost reductions or other production cost savings. However, correlation data do indicate that cover crops in general, and nutrient scavenging cover crops in particular, may confer yield and other soil health benefits in high tunnels. Additional research is needed to improve our understanding of nutrient cycling processes when utilizing legume versus non-legume cover crops in high tunnels, as these unique environments are often more nutrient-rich and subject to different factors driving decomposition (e.g. temperature and moisture conditions) than the open field. However, the results of this experiment do indicate that cover crops may play a different and unique role in high tunnel crop rotations and that cool seasons cover crops do not negatively impact warm season crop yields.
Objective 3. Results from the novel cover crop for high tunnel evaluation.
Evaluating Cover Crop Accessions. Cover crop evaluated included accessions that are known to perform well in the southern region in the field, as well as some new accessions that may confer benefits to high tunnel management for our region. At the time of this writing, biomass data for the weed-free cover crop subplots have been analyzed, and are presented in Figures 4 and 5. The majority of the warm season cover crops we evaluated were considered viable high tunnel cover crops for the Kentucky site where this evaluation occurred, with the exception of the brassica cover crops, which are discussed below. Cover crop biomass was analyzed within functional group (brassica, legume, grass, or other), with functional groups compared to one another.
Warm season grasses and the sesame cover crops had the greatest biomass each year, but as a group, grass biomass did not differ from summer legumes in 2017. In summer 2018, sesame did not differ from summer legumes. Biomass in 2018 was less than in 2017 in all accessions, likely due warm summer weather which made germination challenging. Within the legume group, the sunn hemp varieties had the greatest biomass, although differences were not significant in 2018. The varieties of sunn hemp evaluated included those with reported nematocidal effects (AU golden), as well more widely available varieties. We found all sunn hemp varieties to be fast growing and viable for high tunnels. However, the fibrous nature of the stalks as well as the quantity of biomass could make termination by mowing challenging if a producer is not mindful of the limits of their equipment. The two varieties of cowpea were selected to provide producers management options with forage peas that are popular field cover crops in the region. Iron clay cowpea is a reliable summer cover crop throughout the region and is reported to have some nematocidal effects. Chinese red cowpea is a lesser known forage crop, but is later maturing and has a more vining growth habit that ‘iron clay.’ Both cowpea varieties produced similar amounts of biomass each year with similar management requirements, and decomposed readily when incorporated (Figure 4).
The grasses selected included two millet crops (German and Japanese), which were fast growing, high biomass cover crops. The German millet was later to mature than Japanese millet, offering some options for cover crop rotation length before these covers set prolific seed at maturity. Teosinte, a lesser known forage crop more widely recognized as the wild progenitor of modern-day corn, was also evaluated. It consistently produced great amounts of biomass and could be used as an alternative to sorghum-sudangrass. Sesame was also evaluated, as it is known to be a non-host to pest nematodes and as a member of the Pedaliaceae family, would also offer some variation in botanical family in crop rotations, as no commercial vegetable crops are closely related. Although sesame was slower in growth than some of the accessions, it produced a weed suppressing canopy and the copious biomass was easily managed with mowing even at maturity, offering a high biomass and novel alternative summer cover crop.
Winter grass cover crops consistently produced the greatest quantity of biomass, though in 2018-2019 biomass did not differ between grass and brassica cover crops. Within the grasses evaluated, cereal rye consistently produced the greatest biomass, though it did not differ significantly from triticale in 2017-2018. All of the grasses were deemed viable as they reliably overwintered, with the exception of festulolium, which experienced significant tip burn and stunting in winter 2017-2018. Further, as a very fine-structured grass, it was a weak competitor with winter annual weeds (data not shown). In general, we found that grass cover crops that performed well in the field in Kentucky to be viable as cool season high tunnel cover crops. The legumes selected represented a variety of less winter-hardy alternatives to the medium red clover typically grown in as a field cover crop in the Upper south. The clovers demonstrated great variability year-to-year, and experienced significant winter kill in winter 2017-2018 .
Our participating growers were specifically interested in using cover crops to suppress plant parasitic nematodes. Certain brassica cover crops have been used as biofumigants for nematode suppression through naturally occurring isothiocyanates in the cover crops. These crops are typically grown in the cool season, but it is known that these compounds aiding in plant parasitic nematode suppression are produced in higher quantity under warm conditions. As such, we evaluated brassica cover crops as both warm and cool season cover crops. Although growth was much faster in the warm season, low germination due to high soil temperatures as well as flea beetle insect pest pressure limited the viability of the brassica covers evaluated during the warm season. The cool season brassicas evaluated had much greater biomass and limited pest issues compared to those grown in the summer, and performed particularly well during the milder winter 2018-2019 season (Figure 5).
In general, cool season cover crops required much of the winter and early spring to produce appreciable biomass. For the accessions evaluated in this trial, this would require the producer to forgo a cool season cash crop. In the upper south when winter conditions may limit winter production, this may be an option. However, for much of our region high tunnel production occurs throughout the winter months, and thus summer cover crops may be the most economically viable option for producers. Summer covers offer their own challenges, but most obtained appreciable biomass in 30-45 days during mid-summer.
At the time of this writing, all plant material data have been collected and statistical analyses are underway. Additional data on cover crop weed suppression is currently being analyzed, including weed community dynamics in each accession as well as cover crop biomass and quality with unchecked weed pressure. Cover crop quality data, including carbon and nitrogen content and potentially available nitrogen from NIRS protein analysis are also being analyzed. We have one peer-reviewed manuscript in preparation for this work, as well extension/outreach deliverables, described below.
Developing Growth Models for High Tunnel Cover Crops. We have completed some initial modeling of cover crop growth with thermal units (growing degree days) at the time of this writing. Analysis of the accessions from the first year of each of the cool season and warm season data have shown that the warm season covers are good candidates for simple GDD regression modeling approaches, and several accessions (‘Tropic Sun’ sunn hemp, Japanese millet, and Chinese red pea, in particular) have high R2 values with simple Base 50 GDD models. The cool season covers do not fit into simple GDD models and will require additional model refinement. This is likely due to significant light limitation during winter months at the Kentucky site where these were evaluated. GDD models do not account for light limitation, and assume temperature is the only growth limiting factor. In fact, the daily light integral (DLI) in central Kentucky from late November – late January is growth limiting for many crops. DLI is further reduced in crops grown in high tunnels, in which each layer of polyethylene plastic reduces light transmission by approximately 10%. We plan to continue with these activities as we complete the analyses needed to complete the research manuscript for this objective. Although we do not envision these models to be sufficient to be a stand-alone paper, they provide excellent preliminary data to do region-wide work on evaluating these novel cover crops for high tunnel producers and will be used in future grant proposal activity.
Novel Cover Crop Outreach. As a result of this project, and specifically the involvement of our Southern SARE Young Scholar Enhancement award, we were able to connect this work with the Southern Cover Crop Council. This was an exciting development of the study tour that we took with our YSE scholar, and will provide a home for outreach materials we develop for particular varieties of cover crops. As we complete our outreach deliverables, we are working on general Extension fact sheets regarding general cover crop considerations for high tunnels, as well as fact sheets on selections for cool season and warm season covers. We plan to format these for use on the Southern Cover Crops Council website. Our SARE YSE scholar (Alexandra Tracy) created a draft (Table 1) for warm season cover crop selections based on experimental data and observations from this project, as well as from the literature when supplemental information was needed.
The primary educational approach used in this project consisted of co-production of knowledge and team building with our project participants. This included our project team listening and learning about our participating farmer’s production systems and tailoring our experimental design and on-farm trials to their systems. Farmers provided ongoing feedback about what was going well and refinements needed. University personnel provided ongoing technical support and shared research results and reporting on an ongoing basis. Similarly, our YSE scholar was an integral part of the project, adding the preliminary modeling efforts and stimulating relationship building with the Southern Cover Crops council and other outreach packaging efforts. Secondarily, the project team generated a number of educational talks and demonstrations, as seen in our education and outreach activities summary.
Educational & Outreach Activities
The activities above are listed in detail, including the estimated number of participants at the bottom of this section. Briefly, education and outreach activities included on-farm trials with participating growers in Georgia, Kentucky, and Tennessee, a host of field days throughout Kentucky related to cover cropping in high tunnels, and outreach talks at local, state and regional meetings. In the paragraphs below we describe these activities, as well as outreach deliverables in progress at the time of this writing.
On farm demonstrations. Farmers in each state worked with project PI’s to select cover crops for on-farm trials based on the timing that best fit into their rotation (warm season or cool season) and management goals for increasing the sustainability of their high tunnel systems (soil health, nitrogen credits, and nematocidal effects). In each trial temperature data were collected from data loggers, and biomass data were taken to help validate growing degree day models for the covers being development. Management observations from the producers were also noted, and helped shape recommendations on irrigation and seeding to improve stand establishment, species selection for management goals, and method and timing for cover crop termination.
In Georgia, both participating farmers after spring crops were removed and prior to fall crop establishment would best fit their rotation. Farmers completed one cool-season trial Winter 2016-2017 and warm season trials in Summer 2017 and 2018. Georgia farmers were provided with customized reports on their cover crop nutrient composition, potential nitrogen release from NIR protein modeling, and other management observations by Georgia PI’s. The Georgia producers emphasized the importance of a short window for cover crop production (~60 days for growth and decomposition) between the completion of the spring season and bed preparation for fall crops. They also stated their goals for including cover crops in their rotation for increasing soil organic matter and aid in management of plant parasitic nematodes.
Participating producers in Tennessee and Kentucky preferred cool season cover crops for their high tunnels. These producers used their high tunnels throughout the summer for warm season crop production, and had less developed winter markets, allowing space for a cool season cover crop in their rotations. They were interested in nematode suppression (Tennessee) and alleviation of soil compaction (Kentucky) using cover crops. The participating farmers in Tennessee and Kentucky completed their cool season cover crop trials in 2017-2018.
An additional on farm demonstration was requested by agriculture professional (Cooperative Extension and Soil and Water Conservation District staff) in Jefferson County, Kentucky. These professionals support a horticulture therapy program at a women’s residential substance abuse treatment center (The Healing Place, Louisville, KY). The high tunnel on the site had experienced significant compaction, and there was a desire by the staff to involve the residents in cover cropping and soil health-building practices as part of their horticulture curriculum on site. The demonstration included a mix of summer cover crops selected for biomass production and drought resistance. This was considered a highly successful field demonstration, as in addition to the cover crops being successfully managed, it prompted project staff to consider how to best utilize the tunnel for the rotating population of residents and set up a structure with permanent raised beds to avoid these problems in the future. Kentucky staff from this project provided additional consultation and walk-behind tractor equipment to train SWCD staff and another local charity supporting refugee farmers on utilizing walk-behind tractors for cover crop management in high tunnels. Both groups are planning on writing this equipment into grants to help facilitate cover crop adoption on their other urban agriculture sites.
Field days and tours. Of the tours listed, three are tours of the research site at the University of Kentucky Horticulture Research Farm. These include two tours for visiting Master Gardener groups in Kentucky and a Twilight Tour for the general public. It also includes a tour for visiting scholars funded by a USDA OREI grant focused on cover crops in high tunnels that PI Jacobsen is involved with. In the OREI project, Kentucky is the southernmost state in the tri-state collaboration. In this SSARE R&E project, Kentucky is the northernmost state in the tri-state collaboration. This tour provided valuable comparisons for all participants. The final tour was a study tour with the SSARE James Harrison Hill YSE Scholar (Alexandra Tracy), Kentucky research staff and undergraduate students to visit collaborators in Tennessee and Georgia and help provide the students working on the project a regional perspective, particularly informed by visiting with participating growers and Extension co-PI’s on the project.
Talks and presentations (including classes). Talks and presentations included invited presentations at local, state and regional conferences. Populations reached through these presentations included agriculture professionals, including service providers such as Natural Resource Conservation Service agents, Cooperative Extension Agents, and university research and extension personnel. Farmers reached include specialty crop producers, farmers interested in diversifying into specialty crop production due to high tunnels, as well as beginning farmers. Master gardeners and the general public were also reached through farm tours and presentations. University of Kentucky students were reached through lectures incorporating the aims and results of this project in the classroom as well as through field laboratory activities.
Description of Activities.
Curricula, fact sheets, or educational tools (2)
- Jacobsen, K.L. 2018. High Tunnel Cool Season Crops and Cover Crops Guide. Field Day Handout, 2 pages.
- Jacobsen, K.L. Cover Crops for High Tunnels Comparison Chart. Presentation handout, 3 pages.
Journal articles (0)
- We have two journal articles in preparation for Objectives 1 and 2.
On-farm demonstrations (5)
- Summer 2019. Warm season cover crops for improving soil physical properties. The Healing Place, Louisville, KY.
- Summer 2017 and 2018. Warm season cover crops for organic matter management and nematode suppression. Woodland Gardens, Winterville, GA.
- Summer 2017 and 2018. Warm season cover crops for organic matter management and nematode suppression. Crystal Organic Farm, Newborn, GA.
- Fall 2017 – Winter 2018. Cool season cover crops for improving soil physical properties. For Pete’s Sake Farm, Lexington, KY.
- Fall 2017 – Winter 2018. Cool season cover crops for diversified vegetable rotations. Hines Valley Farm, Lenior City, TN.
(Total of 4 producer participants, 5 agriculture professionals not including PI’s)
- Jacobsen, K.L. UK Horticulture Research Farm Tour, including SSARE Cover Crops Under Cover Project Overview. Calloway and Caldwell County KY Master Gardeners. May 30, 2017. Lexington, KY.
- Jacobsen, K.L. UK Horticulture Research Farm Twilight Tour, including SSARE Cover Crops Under Cover Project Overview. July 26, 2018. Lexington, KY.
- Jacobsen, K.L., A. Wszelaki, J. Gaskin, J. Sutton, A. Tracy, and R. Lark. UK SSARE Project Team and YSE Regional Site Tour. July 20 – 23, 2018. Sites from Lexington, KY to Athens, GA.
- Jacobsen, K.L. UK Horticulture Research Farm Tour, including SSARE Cover Crops Under Cover Project Overview. OREI Project Team “A Multi-Regional Approach for Sustained Soil Health in Organic High Tunnels: Nutrient Management, Economics, and Educational Programming”, J. Grossman Univ. of Minnesota, PI. May 30, 2019. Lexington, KY.
- Wszelaki, A. ETREC-Organic Crops Unit Tour, including SSARE Cover Crops Under Cover Project Overview. SERA 45 group. October 5, 2018. Knoxville, TN.
- Wszelaki, A. ETREC-Organic Crops Unit Tour, including SSARE Cover Crops Under Cover Project Overview. International Agriculture Contingent from Guatemala and Laos. Knoxville, TN.
- Wszelaki, A. ETREC-Organic Crops Unit Tour, including SSARE Cover Crops Under Cover Project Overview. National Association of County Agricultural Agents Horticultural Tour. Knoxville, TN.
(Total of 105 attendees, including 49 agriculture professionals and 10 producers)
Webinars, Talks and Presentations (12)
- Jacobsen, K.L. “Cover Crops in High Tunnel Vegetable Systems.” Southern Cover Crops Council. July 16, 2019. Auburn, AL. (75 attendees, including ~ 20 producers and ~ 20 agriculture professionals)
- Wszelaki, A. “Cover Crops Under Cover: Finding Time to Cover Crop in Your High Tunnels.” Indiana Horticulture Congress. February 14, 2019. Indianapolis, IN. (25 attendees)
- Jacobsen, K.L. “Organic Soil Management of High Tunnels.” Southern Sustainable Agriculture Working Group. January 25, 2019. Little Rock, AR. (100 attendees, including ~ 50 producers)
- Wszelaki, A. “Managing Cover Crops to Maximize Yield in High Tunnels.” Southeast Regional Fruit and Vegetable Conference. January 10, 2019. Savannah, GA. (30 producer attendees)
- Wszelaki, A. “Finding Time to Cover Crop in Your High Tunnels.” Kentucky Fruit and Vegetable Growers’ Association. January 8, 2019. Lexington, KY. (56 attendees, including ~ 40 producers)
- Wszelaki, A. Upper Cumberland Beginning Farmer Series. Cookeville, TN. (30 attendees)
- Wszelaki, A. “Managing Cover Crops to Maximize Yields in High Tunnels.” NRCS Organic and Small Farm Training. (35 attendees)
- Jacobsen, K.L. “Soil Conservation Practices for High Tunnels: Cover Crops, Conservation Tillage and Movable Tunnels.” Muhlenburg County NRCS and local producers funded by NRCS EQIP high tunnel grants in western Kentucky. May 23, 2018. (18 attendees, including 15 producers and 3 agriculture professionals.)
- Jacobsen, K.L. 2018. “Soil Health in High Tunnels.” Midwest Organic and Sustainable Education Service (MOSES). February 23, 2018. LaCrosse, WI. (50 attendees, including ~ 25 producers)
- Wszelaki, A. 2018. “Finding a Time and Place for Cover Crops in Tunnels.” Pick Tennessee Conference. February 16, 2018. Chattanooga, TN. (20 attendees)
- Jacobsen, K.L. “Good Soil Under Cover.” Organic Association of Kentucky Annual Conference. March 3, 2017. Shephardsville, KY (25 attendees, including 10 producers)
- Jacobsen, K.L. “High Tunnels: Structures, Sites and Soils.” Kentucky District 1 NRCS Agent Training. December 12, 2017. Hopkinsville, KY. (15 agriculture professional attendees)
(Total of 479 attendees, including 38 agriculture professionals and 190 producers)
Workshop/Field days (4)
- Jacobsen, K. 2018. “High Tunnel Rotations and Tools for Fall Production.” High Tunnel Field Day at Magney Legacy Ridge Farm sponsored by the Organic Association of Kentucky. November 2, 2018. Princeton, KY. (20 attendees, including 10 producers, 1 agriculture professional.)
- Jacobsen, K. “High Tunnel Rotations and Tools for Fall Production.” High Tunnel Field Day at Good Thymes Farm sponsored by the Organic Association of Kentucky. September 6, 2018. Williamsburg, KY. (39 attendees, including 10 producers, 7 agriculture professionals.)
- Wszelaki, A. “Cover Crops Under Cover.” UT Steak and Potatoes Field Day. August 7, 2018. (72 attendees.)
- Wszelaki, A. “Incorporating Cover Crops into High Tunnel Rotations.” University of Tennessee Organic Crops Field Tour. October 26, 2017. Knoxville, TN. (50 attendees.)
(Total of 181 attendees, including 7 agriculture professionals and 20 producers)
Farmers participating in on-farm demonstrations reported increased skills at incorporating cover crops into their high tunnel crop rotations and in the management skills needed to effectively manage cover crops and their decomposition.
Farmers participating in on-farm demonstrations reported an improved knowledge of the role of cover crops in nutrient cycling and soil fertility.
High tunnels offer producers opportunities to diversify and expand markets and enterprises and contribute to community food systems in unique ways. These include extending the market season, particularly for direct-marketed specialty crops, a critical gap in many local food systems. Higher total yields and higher rates of marketable crops compared to the open field decrease on-farm food waste and increase farmer net returns. Further, reductions in soil slash and increased control over soil moisture may ease the transition to organic or lower-input systems as well. For these reasons, as well as a federal cost share program through the NRCS (EQIP High Tunnel Systems Initiative) have led to increased adoption of these important tools for expanding the production of nutritious vegetables and fruits in our region. Producers in the Southeast have been utilizing high tunnels for decades, and through their experiences we also see the challenges of maintaining productivity in these structures. This project focuses on evaluating the trade-offs of one of the most common sustainable agriculture production practices used in fields around the country – cover cropping.
We approached this project from a highly interdisciplinary perspective with a team of natural and social agricultural scientists – and with a lot of input from our participating farmers. We wanted to better understand the challenges and opportunities for incorporating conservation practices like cover crops into high tunnels (Objective 1), put cover crops to the test in common high tunnel crop rotations in our region (Objective 2), and identify some exciting new practices for producers wanting to incorporate cover crops into their tunnels (Objective 3). Although we are still in the process of analyzing all of our data and packaging our story and broader impacts, we have gleaned some key take-away messages on the agricultural sustainability of this work.
First, we affirmed the widespread value of high tunnels to producers and their ongoing challenges and opportunities. Our experienced producers indicated the soil fertility, weed management and cover crop selection were all areas that they could use more assistance. Further, their most valuable learning resources come from peer-farm demonstrations and online resources (from Cooperative Extension and otherwise). As we move forward promoting conservation practices for highly intensive systems, capturing the stories of producers successfully incorporating these practices and packaging them for online platforms will be critical in expanding adoptions. This includes articulating both the benefits and the trade-offs of practices like cover cropping so producers can make informed decisions for their bottom line.
Secondly, from our evaluation of the ecosystem services of cover crops in high tunnel tomato rotations, we learned that there is less of an effect of cover crop species selection soil properties and yields, but rather an overall effect of cover crop biomass. Very much in line with the advice given in SARE’s Managing Cover Crops Profitably, selecting the right cover crop(s) depends on the cover crop niche specific to the high tunnel environment. We found that in high fertility sites with a lot of weed pressure that legumes may not be our best choice. Though we do see some ecosystem service benefits from nitrogen credits, we may actually see more nutrients recycled in the system by selecting a grass cover crop that can effectively compete against weeds and scavenge the available nutrients in these fertile soils. However, on sites with less weed pressure and decreased fertility, legumes or mixtures incorporating legumes can be very productive. In addition to weed pressure and soil fertility dynamics, there are many other factors that are important in defining a high tunnel cover crop niche, including very tight timing constraints, extreme temperatures, and ease of management (Figure 6). We did not find that the ecosystem service conveyed by the cover crops in this study amounted input replacement costs in the short term. However, we did find correlations to yield benefits that may contribute to economic profitability in the long term, particularly if the cover crops can be incorporated in windows of low productivity or profitability for producers.
Finally, this project contributes to increased tools in the cover crop toolbox for high tunnel growers with some expanded cover crop options that have been evaluated for their growth, weed competitiveness, and nutrient quality. Completing our outreach activities related to the novel cover crops aspect of this project will be important in providing producers information on cover crops that may help with difficult management issues in high tunnel production systems, which will incentivize cover crop adoption and contribute to the sustained productivity of high tunnels in our region.