Final report for FNE21-984
The purpose of this investigation is to determine whether there is a difference in the effect on the soil microbiology between solarization and occultation. This study was a randomized complete block experimental design with five blocks; two treatment plots and a control plot within each block. The control plot was roller crimped but un-tarped in any way, and two treatment blocks were the solarization and occultation treatments. Soil samples were taken weekly and tested for both plant-available nitrogen and soil respiration levels. The results showed no significant difference between solarization and occultation treatments. We gave two farm tours and discussed our process and results with 1 other farmer and several other agriculture related professionals.
The purpose of this investigation is to determine whether there is a difference in the effect on the soil microbiology between solarization and occultation, particularly in their application for the termination of a cover crop within a no-till system, and to characterize this difference over a growing season. Specifically this project will undertake the following tasks:
- Examine the effect of different tarping strategies on soil respiration and plant available nitrogen levels.
- Characterize this effect from the removal date of the tarps through the end of the growing season
These objectives will give us information on the health of the soil ecosystem and soil fertility, so that we can contextualize the soil health aspects of tarping in this application within a wider discussion of the potential costs and benefits of the Cover Crop Based No-till Management System (CCTNT) to be used in our outreach regarding this novel management system.
Cedar Circle Farm is in the initial stages of transitioning a large portion of its cultivated land into organic no-till management. It has become clear that 20 years of the heavy tillage that is necessary for traditional organic production has left our soils depleted of soil organic matter, and that management for soil health will be crucial to maintaining the remaining top soil and to building resilience to climate change into our farm landscape. As such, we are building a consideration for soil health into all of our management strategies. Organic No-till management has been shown to build the organic matter portion of the soil which is the foundation of a healthy, living soil; reducing erosion, increasing water holding capacity, and sequestering carbon.
A typical organic no-till vegetable management system, in general terms, requires the following steps. First a cover crop is planted, which is roller-crimped and tarped for termination. This is followed by a cash crop which is planted into this cover crop mulch. This system has been classified as what the SARE project “Expanding No-Till Vegetable Production through the Combination of High-residue Cover Crops and Solarizing Tarps” (SARE Project LNE18-371R) terms the cover crop-based tarped no-till (CCTNT) production system. Our own variation on the CCTNT system involves two successive plantings of cover crops, winter rye followed by a diverse mix of summer cover crops.
The tool within this management system that we want to examine more thoroughly is the use of tarps, particularly for cover crop termination. We use tarping to terminate the winter rye before we plant the diverse summer mix cover crop because it lends us flexibility in the termination date, completely terminates the rye, and eliminates the risk of the rye going to seed and contributing to the weed seed bank. When terminating the cover crop via tarping, there are two primary methods used by farmers, solarization and occultation. Solarization utilizes clear plastic to terminate the cover crop by raising the temperature underneath the tarps to a lethal level for pathogens and pests, or by conducting lethal levels of heat to the plants that are in direct contact with the tarps. Occultation utilizes black plastic to starve the plants of sunlight and limit the cover crop’s ability to photosynthesize. On our farm, in our small scale trials of the CCTNT system we have used both of these methods successfully to achieve a full termination of a roller crimped cover crop.
We have found however that there are a number of trade-offs made when deciding whether to use either solarization or occultation. Solarization can work quickly but if the timing of the tarping doesn’t coincide with clear skies and warm temperatures it can actually accelerate weed species growth (in particular perennial grasses) while not fully terminating the cover crop. Occultation is less risky but requires a much longer period of time to achieve the full termination and uses a far more expensive material. Particularly with regard to the effect on the soil biology we have noticed an accelerated decomposition of the cover-crop mulch when covered with black tarps and possibly more transplant stress when a crop is planted following termination via solarization.
In this project we are seeking to examine more closely the differing effects of tarping on soil health within this particular management system. To do this we will examine the soil respiration rates and plant available nitrogen, variables which have important significance to soil health and soil fertility, over the growing season after the removal of both solarization and occultation tarps. Looking at respiration will give us an indication of the effect of these tarps on the soil microbiology which is critical for building stable organic matter, and plant available nitrogen levels will inform us about the productivity of the soil following tarping over a roller crimped cover crop. These metrics will provide valuable information to the farm and others from a soil health perspective and with regard to nitrogen availability, a key factor in field productivity.
As we navigate the yield lag that occurs during a transition to no-till, we can use this information to attempt to minimize the amount of time it takes to increase our soil organic matter, and improve soil health and fertility. The more we can accelerate this transition period, the better we can help farms to better manage the risks associated with a transition to no-till management, and decide to take the leap.
Cedar Circle Farm & Education Center is located along the Connecticut river in the central part of the state of Vermont. The managers there have been managing roughly 45 acres of certified organic fruits & vegetables for over 20 years as well as providing annual & perennial flowers from 3 retail greenhouses and host an extensive "cut your own" flower garden. Cedar Circle has an on-site farm stand and cafe and continues to hosts many educational activities for both children & adults. The farm also attends 2 local farmers markets and does a small amount of wholesaling to local restaurants.
- - Technical Advisor
This study took place at Cedar Circle Farm in Field 3 of the home farm area (43.806973, -72.185862). Cedar Circle Farm and Education Center is located in southeast Vermont in climate zone 4b. The soils are a Heartland Silt Loam and the study site within the field has slopes from 0-3%. This soil is well drained and at last measurement has a soil organic content of 1.8%. This field has been planted with winter rye at a seeding rate of 250 lbs. per acre; it is this winter rye stand that will be terminated via tarps to analyze the impact of the different tarping strategies. The following is a generalized schedule for each stage of our CCTNT management system for strawberry production and shows where exactly within the system we want to examine with regard to these tarps, the stage of the process we are looking at is in bold:
- Year One
- Planted Winter Rye, variety (VNS), on September 15th 2020 at a rate of 250lbs per acre.
- Year Two
- Used roller crimper to roll down rye on June 7th 2021
- Applied silage tarps & clear plastic to plots on June 8th 2021
- Removed silage tarps & clear plastic from plots on July 1st 2021
- Seed diverse cover crop (Summer Mix) on July 5th
- October 6th, used roller crimper to knock down diverse cover crop (Summer Mix)
- Year One
- Year Three
- March/April – Additional round of tarping for weed suppression (if necessary)
- Early June – Strawberry Planting
- Year Three
It is the termination of the first round of cover crop in June of Year 2 that we are interested in for this study. The field in which the test plots were placed was prepped following this generalized plan.
This study was a randomized complete block experimental design with five blocks; two treatment plots and a control plot within each block. The control plot was roller crimped but un-tarped in any way, and two treatment blocks were left to be the solarization and occultation treatments. While the blocking here was done to control for field level variability, there was no known fertility gradient on this particular field. Soil conditions and slope are fairly uniform, and the growth patterns of the crops grown are uniform. The plot design is described further under results.
Solarization and Occultation Materials, Timing, and Duration
There are several considerations to be made with regard to the timing of the placement of the tarps. Because it is not possible to be precise to the day with the duration, starting, and end times for the purposes of this proposal. Typically a decision is made regarding when to terminate the cover crop which balances the weather necessary for solarization and the maturity of the rye stand. In this case we rolled the winter rye on June 7th 2021. Because the occultation does not require these specific weather conditions to be functional, we will align the starting date for the occultation plots to match the starting date of the solarization tarps. This was accomplished the day after rolling on June 8th 2021. Further because we are examining the solarization and occultation primarily for the utility as a cover crop termination method there cannot be a set time period for the tarps to remain on the plots, Rather, the tarps will be removed upon successful termination of the cover crop on all of the treatment plots. The clear plastic & tarps were removed from the plots approximately 3 weeks later on July 1st 2021.
The plastic that was used for solarization was used greenhouse plastic(6mm Polyethyline), and for occultation black silage tarps (6mm Polyethylene). To hold down the tarps we used 10 kg cinder blocks on the edges of the plastic.
Soil Testing Procedure
Soil samples were taken weekly and tested for both plant-available nitrogen and soil respiration levels. While testing for nitrate only gives us a limited look at the nitrogen dynamics because it is such a mobile nutrient , it is affordable, and if many tests are taken over time it can give insights into nutrient cycling and in particular nitrogen mineralization rates.
For the nitrate soil testing procedure we used the Hannah Instruments test kit for soil and irrigation water nitrates. This method utilizes a cadmium reduction to determine the concentration of nitrate ions in the sample. The resolution is 2 ppm and has a detection range between 0 and 60 ppm which well exceeds normal background rates for nitrate nitrogen concentrations.
The soil respiration tests were completed using the Solvita CO2 field test. In this method the soil sample was incubated over a period of 24 hours and the change in concentration of CO2 in the test jar was used to determine a respiration rate for the soil sample. To measure concentrations of CO2 a soil probe was left in the test jar which has a surface which is sensitive to changes in concentrations of CO2 and changes to a specific color corresponding to a specific concentration.
Soil samples were taken prior to the application of the tarps to establish a baseline, and then weekly from that baseline through the last week of September. Soil samples were taken with a 1 inch soil corer to a depth of 12 inches. 5 cores will be taken randomly from each plot to make the subsample which will then be analyzed on the farm. The sampling area for the plots will be 5 feet from the perimeter of the tarps on each side to avoid any sort of edge effects within the plot.
Soil temperature and moisture data was recorded along with every sample taken, and climate data was recorded by the weather station on the farm. Minus the first sampling date in-which the soil temperature was not recorded.
We were unable to examine if there were significant differences between the time series of nitrogen availability and soil respiration levels. To determine both if, and equally importantly when and for how long, there are significant differences between the control and the two treatments we used a series of one-way ANOVA tests (a lot of them) in a stepwise manner for each weekly sampling. While there are more sophisticated ways of analyzing time series they were beyond the scope of this proposal.
The following chart represents the randomized plot design. Each plot was 50ft long and 10ft wide. Each plot was labeled with a corresponding plot number followed by the treatment label. Treatment labels were as follows: C=Solarization, B=Occultation & N=Control.
The following chart represents the avg. soil temperature while soil samples were collected. We used a digital soil temperature probe.
TEMPERATURE (degrees F)
The following chart represents the results of CO2 emissions from the soil samples. The soil respiration tests were completed using the Solvita CO2 field test. In this method the soil sample was incubated over a period of 24 hours and the change in concentration of CO2 in the test jar was used to determine a respiration rate for the soil sample. To measure concentrations of CO2 a soil probe was left in the test jar which has a surface which is sensitive to changes in concentrations of CO2 and changes to a specific color corresponding to a specific concentration.
CARBON DIOXIDE EMISSION (lbs/acre/day)
The following chart represents the recorded nitrate levels from the soil samples. For the nitrate soil testing procedure we used the Hannah Instruments test kit for soil and irrigation water nitrates. This method utilizes a cadmium reduction to determine the concentration of nitrate ions in the sample. The resolution is 2 ppm and has a detection range between 0 and 60 ppm which well exceeds normal background rates for nitrate nitrogen concentrations.
The following chart represents the recorded moisture in our soil samples.
MOISTURE (% moisture as a dry weight equivalent)
The following chart is recorded precipitation from the farm's weather station.
|7/1/2021||0.06 in||8/1/2021||0.87 in||9/1/2021||0.00 in|
|7/2/2021||1.23 in||8/2/2021||0.02 in||9/2/2021||0.64 in|
|7/3/2021||0.66 in||8/3/2021||0.00 in||9/3/2021||0.00 in|
|7/4/2021||0.01 in||8/4/2021||0.00 in||9/4/2021||0.00 in|
|7/5/2021||0.00 in||8/5/2021||0.00 in||9/5/2021||0.00 in|
|7/6/2021||0.05 in||8/6/2021||0.00 in||9/6/2021||0.11 in|
|7/7/2021||0.00 in||8/7/2021||0.00 in||9/7/2021||0.00 in|
|7/8/2021||0.47 in||8/8/2021||0.00 in||9/8/2021||0.74 in|
|7/9/2021||0.05 in||8/9/2021||0.00 in||9/9/2021||0.16 in|
|7/10/2021||0.00 in||8/10/2021||0.00 in||9/10/2021||0.00 in|
|7/11/2021||0.06 in||8/11/2021||0.00 in||9/11/2021||0.00 in|
|7/12/2021||0.14 in||8/12/2021||0.00 in||9/12/2021||0.00 in|
|7/13/2021||0.92 in||8/13/2021||0.00 in||9/13/2021||0.01 in|
|7/14/2021||0.33 in||8/14/2021||0.15 in||9/14/2021||0.00 in|
|7/15/2021||0.00 in||8/15/2021||0.00 in||9/15/2021||0.75 in|
|7/16/2021||0.00 in||8/16/2021||0.00 in||9/16/2021||0.01 in|
|7/17/2021||0.63 in||8/17/2021||0.01 in||9/17/2021||0.00 in|
|7/18/2021||0.76 in||8/18/2021||0.04 in||9/18/2021||0.11 in|
|7/19/2021||1.21 in||8/19/2021||0.95 in||9/19/2021||0.00 in|
|7/20/2021||1.42 in||8/20/2021||0.00 in||9/20/2021||0.00 in|
|7/21/2021||1.42 in||8/21/2021||0.00 in||9/21/2021||0.00 in|
|7/22/2021||0.00 in||8/22/2021||0.00 in||9/22/2021||0.04 in|
|7/23/2021||0.00 in||8/23/2021||0.19 in||9/23/2021||0.01 in|
|7/24/2021||0.00 in||8/24/2021||0.01 in||9/24/2021||0.16 in|
|7/25/2021||0.41 in||8/25/2021||0.00 in||9/25/2021||0.00 in|
|7/26/2021||0.06 in||8/26/2021||0.24 in||9/26/2021||0.00 in|
|7/27/2021||0.29 in||8/27/2021||0.01 in||9/27/2021||0.10 in|
|7/28/2021||0.03 in||8/28/2021||0.00 in||9/28/2021||0.01 in|
|7/29/2021||0.73 in||8/29/2021||0.00 in||9/29/2021||0.00 in|
|7/30/2021||0.01 in||8/30/2021||0.28 in||9/30/2021||0.04 in|
|7/31/2021||0.00 in||8/31/2021||0.01 in|
The following charts represent our statistical analysis of our recorded soil CO2 data
The data collection process went accordingly. We made adjustments to sampling days depending on the weather as to not take soil sample when conditions where less desirable, too wet for example. There was one very interesting observation that was made during sample collection in the month of August. We began to notice very defined "crabgrass" growth in control and clear plastic treatment plots. There was no grass observed growing in the black tarp treatment plots.
This observation has led us to believe there may be some correlation between the use of black tarps and lack of "crabgrass" establishment in our cover crop mix. As a result of this observation we will continue to monitor for this condition as we move forward with our no-till strategies.
Our goal of this project was to examine where there are significant differences between the time series of nitrogen availability and soil respiration levels in relation to solarization or occultation. Because we were unable to record any nitrogen levels using the Hannah Instruments test kit this was not achievable. We did send out a couple of samples to a third party lab for verification of low nitrogen level results and indeed the nitrate levels were below the Hanna instruments detection abilities. We did run our CO2 emissions results through Prism's statistical software and it was determined that there is no significant difference in soil respiration levels between treatments. It was determined that more research is needed in this area to better determine the significant differences between nitrogen availability and soil respiration levels under the different treatments. I would suggest that this trial be replicated and data collection continue into year 3. Perhaps there would be some additional information to be captured from the additional decaying cover crop residue?
Education & Outreach Activities and Participation Summary
Cedar Circle Farm & Education Center hosted two farm tours during Open Farm Week in 2021. the tours were held on Tuesday, August 10th, & Thursday, August 12th at 6:30pm.The tour featured our no-till strategies and the SARE funded project was showcased during both tours. We are planning to share the results of this project on our website for all to learn.
This project has made me more interested & aware of the effects of solarization & occultation on pre-existing weed species present in our soil. Through this process I have made connections with other researchers & farmers that are beginning to make these same connections and I am looking forward to learning more about the possible relationship between solarization & occultation on weed species.
Due to our profound observation relating to "crabgrass" and the use of black tarps, we will be more likely to choose the use of the black tarp for occultation over the clear plastic for solarization for effective "crabgrass" management. We recognize that we don't fully comprehend what in "fact" is occurring to cause the suppressing the "crabgrass" however we have decided it is a more effective short term weed suppression tool.
I believe this study accomplished what it was set out to do. Although the results from the nitrogen testing were non, there was some assumption that may be the case. Nitrogen has traditionally been challenging to replace in the form of cover crops in a no-till system, and trying to understand its relationship with microbial activity even more so (for us anyway). I plan to speak to the differences I observed between the treatment plots, and talk plainly about our results, making it clear that we feel there is room for more research into this topic. If I were to replicate this study again I would simply like to continue the soil testing regime into the following year (Year 3) of this trial. Understanding that improved nutrient cycling in no-till systems may often take a long time to develop I believe we would be able to capture useful nitrogen information in the third year of our no-till system from the additional cover crop. This information will certainly be helpful for farmers who would like to adopt a larger no-till strategy. I would also incorporate into this trial some kind of observational weed monitoring between treatments. I would like to figure out what effect the solarization or occultation treatment has on other weeds besides grasses. Again, I believe farmers would benefit the most from this type of information.