Degrading soil health, biodiversity, and soil resilience are major challenges to sustainable crop production under changing climates (Freckman and Virginia, 1997). Excessive tillage, inappropriate crop rotations, monocultures, excessive grazing or crop residue removal have degraded soils in the Southern U.S. crop production systems. Recent extreme climatic events that drastically affected the South have exacerbated soil degradation in this region. Soil health is strongly related with beneficial soil and ecosystem functions including water storage, decomposition, and nutrient cycling; and responds sensitively to land management practices and climate. According to the Organic Farming Research Foundation survey among farmers (National Organic Research Agenda, 2016), 42 percent of respondents in the Southern US demanded more research to identify cultural practices to improve soil health that would improve the resilience of production systems to extreme weather. Recently, much attention is being focused on cover cropping as a sustainable practice to achieve these goals.
Reducing weed pressure on crops without affecting soil health through intense and frequent tillage and cultivation practices is a major challenge in sustainable crop production. With the increasing interest toward organic grain production in the Southern U.S., fall cover crops could be explored as a sustainable practice that reduce weed pressure, as well as improving soil health and cropping system intensity and diversity. Even though the multiple functions make cover crops a whole-farm system approach for improving sustainability and profitability of cropping systems, few producers in the South have included cover crops as part of their cropping systems. A major reason being lack of knowledge regarding the suitable cover crops for their locality that lead to improved use of resources and soil health. In addition, farmers often ask, “Why should I plant a cover crop that uses up all my water?”
To address this issue, we conducted a study (2016-2017 and 2017-2018 field seasons) to evaluate the common fall cover crops in South Carolina for water use with the financial support of the Southern SARE On-Farm Research Grant program. In this preliminary research, we evaluated seven cover crop treatments (rye, crimson clover, rye/crimson clover mixture, oat/ radish mixture, crimson clover/turnip mixture, oats/wheat/crimson clover/radish/turnip mixture, and Austrian winter peas/rye/wheat/crimson clover/hairy vetch mixture. First year results indicated that none of the cover crop treatments depleted soil water more than a fallow control. We identified certain single cover crop species and mixtures that utilize water efficiently and that would actually increase soil moisture for the following cash crop.
In the current proposed study, we will determine the effect of four selected fall cover crops that conserve soil water for the following cash crop, based on the results from the Southern SARE-funded preliminary research, on soil health, including addition of organic matter and weed suppression, and their impact on the performance of the following cash crop in an on-farm trial.
We will evaluate cover crops belonging to brassica, grass and legume groups, as individual species and in mixtures for their benefits to soil health and weed suppression.
We will measure soil health parameters and weed growth during the cover crop season and/or subsequent cash crop (soybean) season. The cover crop treatments will be compared with that of two control treatments: fallow with herbicide control (weed-free) and fallow without herbicide control (with weeds).
We anticipate that the best cover crop(s), which are already found as water savers in the preliminary research, will also retain maximum amount of soil nutrients, increase organic matter, improve soil health, and suppress weed growth for the subsequent soybean crop. Measurement of soybean biomass and seed yield will determine whether/how cover crops have improved the performance of the following cash crop.
The proposed project involves the farmer cooperator in each step of the planning and implementation processes, and the study will be conducted on his farm (Millam farm, Anderson County, SC). The on-farm trials will be conducted in the fall and winter of 2018-2019 and 2019-2020 to evaluate cover crops for soil health and weed suppression, and their impact on the performance of the following cash crop. Four cover crop treatments (rye; oat and radish (mixture of two); oats, wheat, crimson clover, radish, and turnip (mixture of five; NRCS recommendation), Austrian winter peas, rye, wheat, crimson clover, and hairy vetch (mixture of five; recommended by Adams Brisco Seed Vendor)] that conserve soil water for the following cash crop (based on the preliminary research funded by the Southern SARE) will be tested in the proposed study. The control treatments will be a weed-free fallow (fallow with herbicide application) and a weedy fallow (fallow without herbicide application). Cover crops will be planted (6 m x 6 m plots) at recommended seeding rates using a seed drill (as in the preliminary study). No fertilizer will be added to the cover crop plots. Cover crops will be terminated by herbicide application (preferred method by our farmer cooperator) prior to flowering or two weeks prior to planting of the following cash crop (whichever happens first) in order to prevent seed production and the potential to become a future weed. Soybean will be planted using a no-till planter two weeks after cover crop termination. Soybean crop culture will follow farmer’s standard practices, and no herbicides, fertilizers, and irrigation will be provided.
Experimental design for the field trial will be a randomized complete block with 5 replications. Analysis of variance will be performed using the MIXED procedure in SAS.
Percent weed cover will be estimated through the point intercept method to assess weed presence in field plots (Elzinga et al., 2001). Briefly, weed cover is measured by point intercept based on the number of “hits” on the weed species out of the total number of points measured along a transect line, and expressed as a %. The measurement will be started one month after planting of cover crops, and will be continued at monthly intervals until cover crop termination and at biweekly intervals during the cash crop season.
Cover crop biomass: Just before termination, cover crop biomass will be hand harvested from 1 m2 area within each plot to determine dry weight.
Cash crop (Soybean) performance: Soybean biomass will be estimated from 1 m rows during the vegetative growth period of the crop (1 month after planting) and at maturity from each plot. Seed yield will be estimated at maturity.
Soil health analyses
Soil nutrient content, organic matter, pH, and bulk density will be estimated through analyses of soil samples taken at the time of cover crop termination, at soybean planting, and 1 month after soybean planting. Soil samples will be collected from each of the 30 plots (4 cover crops + 2 controls and 5 replications), and analyzed for standard soil test parameters (extractable P, K, Ca, Mg, Zn, Mn, Cu, B, and Na), organic matter, pH, and bulk density at the Clemson Agricultural Service Laboratory.
Time series release of nutrients will be quantified using resin bags buried in the field (modified from Lajtha, 1988; Crews et al., 1995). Nutrient accumulation in resin bags depends on N mineralization, ion form, competition for nutrients by microbes and plants, and water movement in the soil (Binkely, 1984). Since these are the same parameters that govern plant nutrient uptake, resin bags provide a useful index of plant nutrient availability (Binkley and Vitousek, 1989, Fisher and Whitford, 1995). Both anion exchange (Dowex 1-X8) and cation exchange (Dowex 50W-X8) resin bags will be made by placing 4 g of the resin in 50 cm2 nylon bags of 86 mesh size. The resin bags will be buried at a depth of 15 cm in a horizontal orientation with minimum disturbance to surface soil. Six resin bags of each kind (anion and cation) will be randomly placed in each plot. Resin bags will be analyzed for nutrient accumulation at the time of cover crop termination, at soybean planting, and 1 month after soybean planting. Upon collection, the bags will be rinsed with deionized water. Anion bags will be extracted with 0.5 M HCl and analyzed for PO4 and NO3. Cation bags will be desorbed with 2 M KCl and analyzed for NH4. The nutrients will be quantified using an autoanalyzer.
Soil enzyme activity. The potential activity of carbon and nitrogen mineralizing enzymes- β-1,4-glucosidase (BG, EC 184.108.40.206), and β-1,4-N-acetylglucosaminidase (NAG, EC 220.127.116.11) will be quantified using 4-methylumbelliferyl-β-D-glucopyranoside, 4-methylumbelliferyl-N-acetyl-β-D-glucosaminide, respectively as compound-specific substrates. The enzyme activities will be measured in 1 g soil with 150 ml of 50 mM acetate buffer (pH 5.5) in a 96-well plate (Ex-355nm, Em-450nm; Triebwasser et al., 2012). The activity of peroxidase (PER, EC 18.104.22.168) and phenol oxidase (POX, EC 22.214.171.124) that catalyze the oxidation of plant phenolic compounds in the above soil slurry will be estimated using tetramethylbenzidine as substrate (Triebwasser-Freese et al., 2015), and absorbance at 450 nm will be measured using a spectrophotometer. Soil enzyme activity will be measured at the time of cover crop termination, at soybean planting, and one month after soybean planting.
Educational & Outreach Activities
A training workshop for organic farmers, extension agents, and other agriculture professionals will be organized during the cash crop season in 2020. In addition to an email list of over 2500 SC farmers, announcements of the training workshop will be sent to farmers, sustainable agriculture program coordinators and also to key NGOs in neighboring states of GA and NC. The training will be promoted by the Clemson Sustainable Agriculture Program directed by Dr. Zehnder, with a 16 year history of providing statewide training to address critical stakeholder needs (Program workshops typically attract ≥ 40 participants per event, and are often attended by farmers from neighboring states of GA and NC, and occasionally from TN and VA). The objectives of the workshop will be to discuss results of the field trials and to showcase field plots that demonstrate cover cropping for sustainable crop production. Training will include classroom instruction and experiential learning in the field. Training handouts will be prepared including seed sources and field trial results. The trainers/speakers at the workshop will include the PI, co-investigators, farmer cooperator and additional regional experts on cover cropping and soil health who have previously participated as instructors for Clemson Sustainable Agriculture Program training events. Short videos recorded during field trials demonstrating planting, biomass cover, weed-suppressive effects, and termination of cover crops will be shown during the training workshop and posted on eXtension.org (part of the U.S. Cooperative Extension System) to reach a wider audience. Dr. Zehnder has numerous publications on eXtension.
An article on cover cropping for weed control, soil health, and soil moisture conservation, based on the results of the proposed project, will be developed and submitted to eXtension.org for publication, along with the brief video clips from the field trials. An original research paper will be prepared and submitted to a peer-reviewed journal for publication.
PI and/or co-investigators will present research results at field days and agricultural meetings including Southern Sustainable Agriculture Working Group (SAWG) Conference, which involve significant involvement of farmers. Therefore, these would be great opportunities to reach out to a large group of farmers. Southern SAWG bring researchers and students interested in sustainable crop production from the southern US. Therefore, this would be a great venue to disseminate the research results to researchers from the southern region, who would be interested in region-specific research.
Our extension agent cooperator, Chris Talley is enthusiastic to disseminate the research results to as many producers as possible through personal communication. Our farmer cooperator, Daniel Millam serves on the Anderson County Farm Bureau Board of directors and he can reach out to other producers through personal communication.
The project includes involvement of a graduate student for the implementation of the project. The PI, co-investigators, and the farmer cooperator will work with the student and train him for cover crop culture and data collection, analysis, and interpretation. We anticipate that this training will help grow a new ‘sustainable agriculture’ professional.