Solarization or Occultation? Optimizing Tarping for Soil Health and Productivity in No-Till Vegetable Production

Progress report for FNE21-984

Project Type: Farmer
Funds awarded in 2021: $5,875.00
Projected End Date: 03/31/2022
Grant Recipient: Cedar Circle Farm
Region: Northeast
State: Vermont
Project Leader:
Nic Cook
Cedar Circle Farm & Education Center, Inc
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Project Information

Project Objectives:

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:

 

  1. Examine the effect of different tarping strategies on soil respiration and plant available nitrogen levels.
  2. 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.

 

 

Introduction:

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 a 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 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.

 

Cooperators

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  • Rebecca Maden - Technical Advisor

Research

Materials and methods:

This study will take 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 Three
      • March/April – Additional round of tarping for weed suppression (if necessary)
      • Early June – Strawberry Planting

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.

Plot Design

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 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 will be provided in the supplemental materials.

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 Polyetheline), and for occultation black silage tarps (6mm Polyethelene). 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 test 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.

Statistical Analysis

We want to examine where there are 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 will use a series of one-way ANOVA tests (a lot of them) in a stepwise manner for each weekly sampling, and if necessary Tukey’s post-hoc test to determine exactly how the treatments differed. While there are more sophisticated ways of analyzing time series they are beyond the scope of this proposal.

 

Research results and discussion:

We have compiled results of the soil sampling, however we have not had the time to preform an analysis of the data at this time. We requested an extension and that request was granted. Thus we will have more time to review our results.    

Research conclusions:

None at this time.

Participation Summary
1 Farmer participating in research
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.