Potential of Cover Crop Influence on Water Repellency and the Sustainability of Southern U.S. Soils

Progress report for GS23-283

Project Type: Graduate Student
Funds awarded in 2023: $12,042.00
Projected End Date: 08/31/2025
Grant Recipient: Clemson University
Region: Southern
State: South Carolina
Graduate Student:
Major Professor:
Dr. Dara Park
Clemson University
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Project Information


Integrating cover crops into a crop rotation improves soil properties, including increasing soil organic carbon and soil structure, promoting nutrient cycling and credit to the subsequent cash crop, and reducing soil loss via surface runoff and erosion. Cover crops affect soil water content during cover crop and subsequent cash crop growth. There is little information on the effects of cover crop residues on soil water repellency (SWR). Southern USA surface soils are susceptible to developing water repellency due to their sand-dominated textures and lack of organic matter. Water repellency affects soil hydrology, altering water infiltration and distribution uniformity throughout the soil. Results include water and chemical loss via deep percolation and via surface runoff and soil erosion. Cash crops become water-stressed and cannot efficiently utilize nutrients and chemicals, impacting quality and yield. Producers must increase inputs to compensate, reducing sustainability of the field, surrounding environment, and the producer’s livelihood. Hotter temperatures, short intense droughts and rising CO2 levels are factors that are directly and secondarily associated with increasing SWR presence. This project aims to identify if residues from five common Southern USA cover crops include water repellent compounds, their short-term soil impacts, identify the potential for longer term impacts considering climate change, and educate a broad range of stakeholders on SWR and which cover crops to utilize. We will disseminate results via articles, webinars, and workshops. The results of this project will help Southern USA producers a make educated decisions on which cover crop to use for their field soils.

Project Objectives:

OBJ 1. Identify if the composition of fresh biomass, as well as residue and the soil under commonly used cover crops in the Southern USA before the cash crop is planted. This includes determining compounds that potentially contribute to SWR (cellulose, hemicellulose, lignin, and suberin).  Importance: Identifying the presence and concentration of these compounds in cover crops can allow us to determine if they could potentially cause SWR. Typically, cash crops are planted four to eight weeks after cover crop termination. Most SWR compounds are recalcitrant and thus do not degrade quickly. They may result in longer residues on the surface. Some SWR compounds may be partially degraded and migrate into the soil, where they may either add to the aggregation of coarse sands and or eventually build up and contribute to SWR. We hypothesize that cover crops with higher amounts of hydrophobic components will be more reluctant to decomposition, and therefore will be more likely to contribute to the occurrence and development of SWR within a field.  

OBJ 2. Identify baseline SWR and how quickly it can affect two related soil properties: infiltration rates and aggregate stability. Importance: research shows that SWR development is thought to occur primarily over the long term, as the SWR plant compounds “build up” in the soil. Short-term characterization of SWR and its effects are poorly understood and previously minimally investigated. We hypothesize that cover crops with more hydrophobic plant components will induce SWR and influence short-term soil-water dynamics (infiltration rates and aggregate stability).

OBJ 3. Deliver outreach and extension of project results to stakeholders and underrepresented groups. Importance: This objective contributes to sustainable agriculture as it helps to educate others on SWR and the influence SWR has on different factors, including water use efficiency, runoff, and nutrient efficiency. Often research is conducted, and results are published in peer-reviewed scientific journals and scientific conferences. Yet, there is a minimal translation of the science to practical application. Stakeholders’ change in knowledge of SWR and how (if) commonly used cover crops may impact its occurrence can result in a change in practice, thus increasing the sustainability of soil and water natural resources and stakeholder livelihood. 


Materials and methods:

Objective 1: Identify if the composition of fresh cover crop biomass before cash crop planting and the underlying soil chemistry from commonly used cover crops in the Southern USA consists of compounds that potentially contribute to SWR, including suberin, lignin, cellulose, hemicellulose, and carbohydrates.

Activity 1: Establish cover crops experiments. Two simultaneously conducted field experiments, each organized in a randomized complete block design will have three replicates of five treatments of one factor (cover crop), resulting in each experiment having an n=15. Field experiments have been completed in fall /winter 2021/2022 and 2022-2023 Thus there will be four field site years. Field experiments are essential to capture variability due to climate and soils and provide validity to the findings.

The cover crop treatments will be a fallow plot to serve as the control, cereal ryegrass (Secale cereale), crimson clover (Trifolium incarnatum), red clover (Trifolium pratense), and daikon radish (Raphanus sativus). Cover crop selection is based on the Southern Cover Crop Council’s Cover Crop Resource Guide and farmer preference. Plots will be seeded at rates following Managing Cover Crops Profitably (2012). Prior to planting, soil samples will be taken for nutrient provisions. The 2.5 m2 plots will be located at the Calhoun Research Station on a Toccoa soil series. The taxonomic class is Coarse-loamy, mixed, active, nonacid, thermic Typic Udifluvents.

Activity 2: Terminate cover crops. Cover crops will be terminated by winter mortality, or when they reach 80% flowering or 2 weeks before subsequent cash crop is to be planted. Termination via hand crimping using pliers will mimic roller crimping. 

Activity 3: Identify water repellent compounds (cellulose, hemicellulose, lignin, and suberin) in fresh cover crop biomass. After termination, above plant material and root tips will be sampled from a 0.25 m2 quadrant within each plot, prepared, and sent to the University of Georgia’s Agricultural and Environmental Service Lab for compound analysis.

Activity 4. Maintain plots. The remaining plant material will be terminated and spread evenly over the remainder of each plot surface.

Activity 5: Conduct statistical analysis. Data will be checked for normality (Levene’s test) and homogeneous variance (Shapiro-Wilks test) assumptions. Non-normal data will be transformed. If data is normal, a two-way ANOVA will test “experiment” as a random factor. If significant, a one-way ANOVA will be conducted for each experiment; otherwise, data will be pooled and proceed with the analysis of comparing treatment least-squares means using Fisher’s Protected Least Significant Difference Test. 

Activity 6: Interpret results to begin developing recommendations. We will determine if the fresh biomass and residues of the cover crops (above and belowground) include water repellent compounds and if the cover crop residues have degraded and translocated into the soil and include water repellent compounds. If these compounds persist within the soil, SWR could develop under certain climatic conditions and subsequently impact soil and water natural resources and farmer livelihood sustainability.


Objective 2. Identify baseline SWR and how quickly it can affect two related soil properties: infiltration rates and aggregate stability.

Activity 1: Document baseline and short-term SWR (Dekker et al., 2009 and Alvarez et al., 2016), infiltration rates (NRCS Soil Quality Test Kit Guide (1999)), and aggregate stability (NRCS Soil Quality Test Kit Guide) in the native soil before cover crops are planted, mid experiment, at time of CC termination, and at time of cash crop planting. In addition, SWR will be measured every two weeks once cover crops are established. All measurements will be taken three times per physical replicate. Field experiments have been completed in fall /winter 2021/2022 and 2022-2023 in SWR was collected. Thus, there will be four field site years for SWR data.

Activity 2: Conduct statistical analysis. Data will follow the same procedure outlined in Objective 1, Activity 4. Regression analysis will identify the impact of biomass and compound presence on SWR, infiltration rates, and aggregate stability. 

Activity 3: Interpret results to finish developing recommendations. We will determine how the cover crop residues (and SWR compounds) impact water repellency throughout the surface soil, infiltration rates, and rootzone aggregate stability. If short-term impacts are present, cover crops to avoid can be recommended. This will increase crop sustainability by maximizing water use and nutrient use efficiency of crops, natural resource sustainability, and farmer sustainability.

Objective 3: Outreach and extension of project results to stakeholders including underrepresented groups.

Activity 1: Develop stakeholder-oriented articles regarding SWR and results. A short introduction on SWR and results will be translated into a producer-friendly Land-Grant Press (LGP) article and an article for the Minority Landowner Magazine. The LGP publication will be submitted for inclusion on the Southern Cover Crop Resource Guide website. Easily accessible, user-friendly versions will assist in a change in knowledge and practice increasing agricultural sustainability.

Activity 2: Offer a webinar to stakeholders regarding SWR fundamentals and research results. Stakeholder-oriented presentations and on-field SWR testing can be provided to commodity boards, SC Certified Crop Advisors, and or stakeholders at field days. Giving results in real-time in user-friendly versions will facilitate better discussion and assist in a change in knowledge and practice, increasing agricultural sustainability. A Qualtrics survey will be distributed to document knowledge changes and anticipated changes in practice. 

Activity 3: Offer workshops that provide a more in-depth SWR review, including how SWR can be determined and the role of cover crops in deterring and or promoting SWR. These workshops will be offered to Clemson University and South Carolina State University extension agents and South Carolina Natural Resource Conservation Service employees. A stakeholder-appropriate poster will be presented at the 2024American Society of Agronomy (ASA), Crop Science Society of America (CSSA), and Soil Science Society of America (SSSA) international annual meeting in San Antonio, TX.  The poster on SWR that was part of the 2023 Southern Regional Cooperative Soil Survey Conference received a lot of interest. A Qualtrics survey will document post workshop interest, knowledge changes, and anticipated changes in practice. This knowledge can assist extension agents, NRCS employees, and other stakeholders convey cover crop benefits, issues, and selection to farmers, increasing agricultural sustainability.

Research results and discussion:

April 2024 Report

There are 16 deliverables throughout the duration of this project (Figure 1).  The following have been completed:

  • Plots were prepared on September 5th, 2023, mapped, and flagged on September 8th, 2023. (Deliverable #1)
  • Cover crops were planted by hand in a way to mimic broad casting on September 13th, 2023. (Deliverable #2)
  • Pre-experiment tests including collecting soil samples for aggregate stability and soil analysis were taken on September 8th. The plots were too wet to measure infiltration and soil water repellency on September 8th. Soil infiltration and soil water repellency were measured on September 15th. (Deliverable #3)
  • Plots were maintained and weeds were hand-picked from plots weekly following planting. (Deliverable #4)
  • November 7th, 2023, radish, and mustard were harvested as they were affected by frost and starting to show signs of winter kill. Fresh biomass was weighed in the field (Fresh weight) and returned to the lab where the biomass was dried in the oven. (Deliverable #5)
  • December 5th, 2023, Collected 4-week-old residue samples of radish and mustard. (Deliverable #5)
  • December 13th, 2023 took mid-experiment measurements for soil water repellency, and infiltration. Also took soil samples for aggregate stability. (Deliverable #6,7, & 8)
  • March 12th- 14th 2024 harvested and measurements for biomass, soil water repellency, and infiltration. Also took soil samples for aggregate stability and soil analysis. (Deliverable 5, 6, 7, & 8)
  • March 19th, 2024, contacted stakeholders and informed them of the project and upcoming workshops. (Deliverable #13)

Other task completed:

  • September 14th, 2023, meet and interviewed student from MANNRS on September 14th, 2023 for part time undergraduate position.
  • Student hired for part time position on September 25th, 2023.
  • November 6th, 2023 took first observations of percent coverage and measured volumetric water content, and soil temperature.
  • November 20th, 2023 percent coverage, soil volumetric water content, soil temperature, and soil water repellency measurements were taken in the plots. These measurements were taken every other week from here on out.
  • March 2024 - contacted stakeholder groups via email to introduce project and gauge interest in webinars/workshops (Deliverable #13).


  • Collect 4-week-old biomass samples and prep for analysis. (Deliverable #5)
  • Conduct infiltration test at time of crop planting. (Deliverable #6)
  • Collect soil samples for aggregate stability test at time of crop planting. (Deliverable #7)
  • Conduct soil water repellency test at time of crop planting. (Deliverable #8)
  • Prepare biomass samples for analysis. (Deliverable #9)

Figure 1: https://docs.google.com/document/d/16yOInYdfajKtF0NO-FSoJeH_pqe8r4oWZEh79FIt83M/edit?usp=sharing

Participation Summary

Educational & Outreach Activities

1 Webinars / talks / presentations

Participation Summary:

Education/outreach description:

Dr. Park and graduate student Payton Davis gave a talk at a MANNRS meeting on September 5th, 2023 about the project and topics related to cover cropping and soil water repellency. Additionally, stakeholders have been contacted and informed about the project and upcoming workshops. In April, Davis will present a poster at a farmer field day on preliminary results of the study. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.