Kale leaf nutrient density response to various mulches

Final report for FNE16-837

Project Type: Farmer
Funds awarded in 2016: $15,000.00
Projected End Date: 02/28/2018
Grant Recipient: Blackberry Meadows Farm
Region: Northeast
State: Pennsylvania
Project Leader:
Gregory Boulos
Blackberry Meadows Farm
Expand All

Project Information


The purpose of this study was to determine the nutritional impact of black plastic and two chipped slash mulch treatments on kale. During the study, researchers resolved some key practical questions involved with using chipped slash as a bi-annual mulch. In 2016, researchers spread mulch on three test plots and took brix readings and soil tests of the Kale crops planted in each plot. In 2017, researchers did not apply additional mulch, but tried to grow kale in the same plots without success to answer some additional questions left after the 2016 trials.

The results showed highest brix readings in the heavy mulch (7.07, n=64), with light mulch close behind (6.43, n=64) and the lowest in black plastic (5.49, n=71). The soil moisture was not factored into the nutritional uptake as no plots received supplemental irrigation, however, soil pH was monitored. Statistical analysis within each treatment was inconclusive since kale varieties differed within each planting cycle.

Several key lessons were learned from using slash mulch in annual vegetable fields: 1) Labor savings in using black plastic are “paid-for” in lower Brix and more dificult growing conditions, as well as offseason purchase and maintenance costs.  2) decompositional cycle of mulch needs more study to yeild a mushroom (winecap) crop and reliable method for turning it into soil organic matter by the end of year 2.

Results were shared with the Pennsylvania Association for Sustainable Agriculture, Womens Agriculture Network, a Local Beginning Farmers Group, the Pittsburgh local food community and local universities.


Black plastic mulch has been a core component of Blackberry Meadows Farm's strategy for weed suppression and moisture retention in the soil for the past 30 years. Removing this black plastic from the crops’ life cycle serves the organic farm’s goal of growing natural, nutrient dense food in a responsible and environmentally conscious method without reliance on chemicals. In 2010, the Rodale Institute received a SARE grant to launch a three-year study using cover crops in a no-till system (eg eliminate disposable black plastic).  They concluded that the required additional equipment and expenses of their tested alternatives to black plastic did NOT significantly lower production costs (Feeser et al., 2014).

In effort to eliminate black plastic from the crops’ lifecycle, yet continue growing nutrient dense crops, researchers investigated chipped slash as a mulch alternative. Black Plastic is inferior to organic mulches (typically straw) in soil moisture retention (Feeser et al., 2014) and does not increase soil organic matter, essential to sustaining the whole soil ecosystem (Sullivan, 1999). The results of a 2009 SARE study identified black plastic and fabric cloths as superior weed control methods and supporters of the best crop yields by weight, while wood shavings and grass clippings needed to be reapplied to maintain their ability to supress weeds (Thewis, 2011).

Wood chip from slash (tree branches and leaves, not from heartwood) represents an abundant resource in Pennsylvania, a waste product from the local arborists, and a potentially high source of organic matter as a soil amendment.  Surface applications of chipped slash mulches have previously been shown to have slow decomposition rates, require little nitrogen from the soil, and have a negligible effect on pH  (Sanchez et al, 2000; Allison & Klein, 1960).

Use of wood chips as surface mulch on ornamental domestic plants is ubiquitous in our society, as chips are marketed in designer colors and available nearly everywhere, including gas stations.  However, they are rarely if ever incorporated into vegetable cropping systems, which left the researchers with many questions about how they could be included in a way which could be a benefit to the farmer, while positively impacting the nutritional qualities of the crops.

Project Objectives:

The purpose of this study was to 1) determine the impact of black plastic mulch and various chipped slash mulch treatments on kale leaf nutrient content, and 2) resolve key practical questions involved with using chipped slash as a mulch, specifically, analyzing a methodology to minimize the nutrient leaching effect of decomposition during the cropping period.

Key practical questions:

  1. Does the decomposition process remove nitrogen and reduce nutritional content in the first months of application?
  2. Are there harmful effects of using wood chips in vegetable systems?
  3. How long does decomposition/efficacy take? can it be acclerated/prolonged to achieve symbiosis with the crop cycle?
  4. What are the labor variations in application/planting/cultivation/cleanup?
  5. Does the seasonal timing of the mulch application matter (weed suppressive efficacy v decomposition (nutrient leaching) cycle of chip mulch v crop cycle)?


Click linked name(s) to expand/collapse or show everyone's info
  • Patrick Drohan


Materials and methods:
Moving the mulch was labor intensive

Ramial wood chips were spread on four cultivated rows approximately 230’ long in 2016 and subsequently planted. Rows of black plastic were correspondingly prepared and planted. A local arborist was paid $50 for each load of fresh ramial wood chips delivered to the farm and dumped out in the vegetable field immediately adjacent to the rows. One dumptruck load (12+ yards) was sufficient to mulch one row, thus four loads of chips were purchased in 2016.

Researchers first tried using a tractor with a front loader bucket and a second person on the ground spreading mulch.  The tractor operation was difficult as a specific drive path needed to be maintained to avoid soil compaction in the beds and larger/inconsistent piles were applied than with wheel barrows, increasing the work for the "spreader".  This initial system proved to utilize as much time as three people on the ground using wheel barrows, and the wheel barrow application significantly reduced the amount of soil and ground debris in the mulch from the front loader operation.

Wheel barrowing the mulch

Thus, a deep wheel barrow was used to transport the chips from the main dump site to a particular spot in a row. Half of a row was light mulch (2-4" deep) and half was heavy mulch (4-6" deep). The light mulch treatment was established by spreading two wheelbarrows over 10’; the heavy mulch treatment by spreading three wheelbarrows over 10’. A hard rake was used to quickly shape the chips into a bed. All kale plants were started in trays in a greenhouse. Four rounds of trays, one round for each treatment set of light mulch, heavy mulch, and black plastic mulch, were started over the course of the growing season and transplanted once hardened-off and the weather permitted.

Round 1: Westlander kale transplanted May 23 in rows prepared May 22 (50% Maple, 50% Hemlock). Round 2: Westlander kale transplanted July 31 in rows prepared June 21 (90% Maple, 10% Pine). Round 3: Red Russian kale transplanted August 1 in rows prepared July 14 (100% Maple). Round 4: Nash’s kale transplanted August 31 in rows prepared August 21 (90% Maple, 10% Pine).


Planting into mulch was significantly more labor-intensive than planting into black plastic

Brix data was collected periodically in July, August, September and October. A particular group of kale plants were tested once they were of a size appropriate for a harvested leaf to be juiced. The last session of Brix data collection occurred in the middle of October. Frosts in late October catalyzed physiological changes in the kale plants that prevented juice from being obtained for Brix testing via the juicing method utilized throughout the study. Soil samples were collected in August, September, and October from all test plots.


Weeds encroached on the mulched rows in 2017

In 2017, no additional wood chips were added to the test beds.  Observations were made to investigate weed suppression of 2016 mulches, soil tilth and crop performance.  All test mulches contained weeds throughout the season. Most notably, taprooted weeds were more prevalent in mulched areas than in black plastic, while annual weeds (chickweed) was more prevalent in black plastic “holes” and were not in mulched areas at all.

Beds were replanted by weeding the beds, refreshing the mulch, moving the mulch (by hand) in a 2-3-2 planting pattern across a 3' wide bed, dibbling a hole, adding diluted liquid fertilizer to each hole, inserting the plug, and then replacing the mulch around the plant. 

Test beds were weeded in May and July 2017 prior to planting spring and fall kale crops. Weeds were predominantly thistle and wild lettuce and were weeded by hand to pull up as much root mass as possible.  A third weeding of all beds was made in August 2017.  There was not a significant difference in weeding labor hours per bed foot between the black plastic and wood chip treatments. The black plastic sustained significant damage from the earlier weeding process and 2016 usage, which may have contributed to increased weed pressure in the later season.

Preferred leaf "Juicer" for taking Brix readings

All chipped beds were refreshed with the use of a hard rake after weeding to reshape the beds and ensure homogeneous coverage of the remaining mulch.  Care was taken not to incorporate mulch into the soil strata. Observations of the mulch thickness and decomposition revealed a fungal strain of Stropharia rugosoannulata introduced by the researchers in June 2016 to be prevalent throughout the mulched beds.  Approximately 1 quart of fungal spawn was incorporated into the heavy mulch plot to help speed decomposition and produce a secondary crop.  The spawn spread into all mulched beds and bloomed in all treatments by July 2017.  Mulch was estimated to be about 2/3 the original thickness as applied in 2016, but was still sufficient to achieve coverage of the planting area, however weeds were not suppressed.

Handheld Refractometer. A drop or two of juice was placed on the prism.

Brix readings were taken by crushing the mature leaves of the kale in a Pampered Chef brand Garlic Press and dripping the plant juice onto a handheld Refractometer (Sper Scientific 0-32% Brix).  Over 60 data points were recorded per test plot to achieve statistically relevant results. Data was collected on three differerent occasions in 2016 from each of the test plots.

Research results and discussion:

Heavy mulch had highest Brix (7.07), Light mulch had less (6.43) and Black Mulch had the least (5.49).  These results support reserachers' hypothesis that the heavy chip mulch would increase the nutritional density of the crop. 

Over 60 data points per crop demonstrated a significant difference in Brix under different mulches.

Key practical questions:

  1. Does the decomposition process remove nitrogen and reduce nutritional content in the first months of application? No.  There was no evidence of any crop problems in Year 1.  There were major crop failures in 2017 (year 2) which need further investigation, since soil test comparison did not indicate significant changes in soil chemistry. 
  2. Are there harmful effects of using wood chips in vegetable systems? Yes. The mulch decompositional cycle must be understood to avoid risks of extended period of recovery. Researchers used fungal spawn to speed up the decomposition, and were sucessful to some degree since several large blooms occurred under the kale crops in the mulched rows (fungus has no love for black plastic). It is uncertain if the Winecap had a correlation to the higher brix found in the mulch treatments.

    Using mushroom spawn during mulch application could yield a commercially significant volume of Winecap Mushrooms over two years.

  3. (a) How long does decomposition/efficacy take?  Decomposition takes well over two
    Kale under wood chip mulch was harvestable through November, a result of higher Brix.

    seasons if left on the surface, inactively tended, however efficicacy (all mulches had the same weed suppression abilities) is limited to one season.  They all failed in season two.  (b) can it be acclerated/prolonged to achieve symbiosis with the crop cycle? Unknown.  Research hasn't proceeded long enough to demonstrate whether the mulch treatments were significantly more decomposed when innoculated with fungus.  Nor is there any certainty as to whether an active surface compost and covercrop treatment would accelerate the decomposition. Because of the labor required to maintain the mulch, researchers aren't convinced that it's worth the labor to try to salvage or continue building the mulch layer up each year, but rather to try to compost the wood chips in situ every few seasons, preceeding a fallow rotation. 
  4. What are the labor variations in application/planting/cultivation/cleanup? Ignoring equipment costs in money and time for procurement and maintenance, the application of mulch (mulch layer v wheel barrowing mulch/rake) and the planting (water wheel v hand dibble) were significantly faster in the black plastic treatment.  For cultivation, the treatments were about equal, but in clean-up, the mulch won (pulling up/hauling trash v nothing).   Consider black plastic usage requires a drafting force larger than the skilled farmer, but very little additional human labor except during planting.  Whereas wood chip mulch requires 4 times the unskilled human labor with no drafting requirements for implementation.
    Mulching AFTER planting could save significant labor

    Using a wheel barrow is significantly cleaner (eg less weedy) and more efficient than mulch applied with a tractor bucket.  Thus, farmers could apply the mulch anytime in the

    kale life cycle.    An additional labor savings could be gained by applying the wood chips after the first cultivation, when kale plants are "knee high" and this could eliminate the labor of trying to plant into the mulch by hand.  

  5. Does the seasonal timing of the mulch application matter (weed suppressive efficacy v decomposition (nutrient leaching) cycle of chip mulch v crop cycle)? Yes.  The ideal timing would be to till fields as usual and plant mechanically in the spring to eliminate the labor of trying to plant into the mulch.  After the crop has reached "knee high", heavily cultivate (weeded plant material become a green manure and weed seedbank is depleted) and then apply innoculated heavy wood chip mulch on the crop.  This research hasn't proceeded long enough to know what happens after the decomposition process in year 3.  Wood chips should be incorporated after a single crop season. A host of problems emerged trying to use the same mulch treatment in a second season. Intense and rapid early season weed pressure in all beds (the second season black plastic seemed to accelerate weed growth by warming the soil. Aggressive summer weeds followed. Second season mulch thickness did not retain moisture.  Soil compaction seemed present under the wood chips while the black plastic remained somewhat powdery.
Research conclusions:

None of the test plots were irrigated throughout the experiment.  One conclusion is that the mulch was significantly better at retaining soil moisture than black plastic during the first year (determined by the look and feel of the soil, as well as plant performance) thus created a better habitat for microbial interactions (including symbiotic soil bateria and fungi) enhancing both plant growth and nutritional uptake.  Researchers were unable to secure an accurate set of data to verify the specific differences in soil from drying soil samples prior to shipment.

Hot weather started in the second half of May 2016 and continued through the summer, creating growing conditions that were not ideal for kale. The Red Russian variety of kale, planted in early August, was similar in growth rate and final size across all mulch treatments, but for all other varieties of kale the plants in black plastic grew slower and never achieved the same size as that of the plants in the wood chips. Out of the four rounds of kale planted in black plastic, only the Red Russian kale grew to a size sufficient for profitable harvesting.

Reserachers found that using a tractor front loader bucket to move chips after they were dumped at the farm increased the likelihood that dirt and plant material was incorporated into the chips prior to spreading on the rows. Dumping the chips adjacent to the vegetable rows at delivery and filling wheelbarrows with shovels helped keep the chips clean.

Participation Summary
3 Farmers participating in research

Education & Outreach Activities and Participation Summary

1 Consultations
1 On-farm demonstrations
7 Tours
3 Webinars / talks / presentations

Participation Summary:

42 Farmers participated
10 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

January 24, 2017, Greg and Jen hosted a New Farmer Study Circle at Blackberry Meadows farm and described the research project and results to over 25 attendees, all of whom were farmers and exension educators.

March 9, 2017, Greg gave a presentation of the reseach to a Food-Conscious.org event including approximately 75 guests, including an additional 3 agricultural professionals, extension educators and service providers and 5 farmers from Pittsburgh.  As a result of this talk, Greg performed a consultation with a local investment group (Blue Angel) on the possibilities of investing in nutritionally dense food production.

Summer 2017, Greg and Jen gave tours and discussed the proejct results to 7 different farmer colleagues at the farm.

August 14, 2017 Greg and Jen hosted the Soil Science graduate students from Penn State University and provided an on-farm demonstration of the project in collaboration with Patrick Drohan, their Technical Advisor.  15 students and faculty participated in the day.

October 14, 2017, Greg spoke to a group of 80+ students, agricultural professionals, and fellow farmers at Chatham University's Food and Climate Change Conference.  An additional 4 agricultural professionals and service educators were present as well as 8 additional farmers.

October 22, 2017 Jen had scheduled an Informational Field Walk through PA Women's Agriculture Network and discussed the project with 4 farmers during the scheduling of the event.  The event was cancelled due to lack of registrations.


Learning Outcomes

4 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Results shared amongst the two farmers at Blackberry Meadows (Greg Boulos and Jennifer Montgomery), Jeff Lackey (Whiteoak Farm) and Chad Christopher (Peas and Quiet Farm) generated awareness of how conditions differ under decomposing mulches and black plastic. Things these farmers are now considering are acidification from decompositional bacteria/fungi, nutrient development under different mulches, media temperature and bed dessication.

Project Outcomes

2 Farmers changed or adopted a practice
Project outcomes:

Since the researchers observed an increase in Brix in year One, Greg Boulos and Jen Montgomery are working to develop a method to utilize more wood chips as mulch on Blackberry Meadows Farm. While wood chip mulch was labor intensive, including application, planting, cultivating, etc, they still actively discuss and are trying to develop a way to easily incorporate this mulch into their farming system.

Assessment of Project Approach and Areas of Further Study:

Researchers looked at crop nutritional density, but there are a mriad of factors which play a role. Primarily it is the microbial interactions of the decompositional bacteria and fungus with the beneficial microbs that contribute to nutritional uptake in the kale plants. Additional research could be done in the following areas:

Crop cycling: timing the crop/mulch/innoculations to maximize 1. weed suppression, 2. nutritional uptake during crop maturity, 3. rapid post season decomposition of the mulch

Source/type of mulch on nutritional uptake: while researchers recorded the types of mulch used per planting, they didn't have significant data from each plot to determine which types of wood are better than others.

Consistent crops of kale varieties across the spring summer and fall planting periods: this would allow for statistical analysis of maximizing the nutritional uptake during the production season and decompositional cycle of the mulch.  Due to specific seedling production  (and crop failures) kale was able to be planted consistently across treatments, but not during each planting period.

Specific soil microbial interactions under mulch vs black plastic were not considered.  Researchers did not have access to a microscope powerful enough to observe and monitor microbial changes.  Also the use of innoculations of decompositional bacteria, perhaps using a compost tea brewer or "seeded" from an older composting mulch pile could accelerate the decomposition of the chips in year 2 and return the rows to productive status by year 3.

The problem of Kale survival in 2017 could have been biological, from perhaps both microscopic decompositional soil microbes in combination with rodent/rabbit predation.

Researchers purchased a handheld infrared specrtometer (SCIO) as part of this project but were unable to achieve a correlation between the Brix data and the spectragraph generated by the device.  They also could not correlate the spectragraph with any specific nutrients during the course of this grant.  Since the meter was backordered until late fall 2016 (after the bulk of the data had ben collected).  This type of device would have been far superior in data collection time, as well as usable information, if tissue samples were concurrently analysed to specifically investigate elements and minerals present in the Heavy Mulch kale as compared to the black plastic kale.

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.