Making the Most of Mulch: Strategic Systems for Small Organic Tomato Growers

Progress report for ONC18-044

Project Type: Partnership
Funds awarded in 2018: $28,394.00
Projected End Date: 12/31/2021
Grant Recipient: Indiana University
Region: North Central
State: Indiana
Project Coordinator:
Dr. Heather Reynolds
Indiana University
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Project Information


We tested combinations of cover crops (living white clover or residue of winter rye-hairy vetch) and leaf mulch for organic fresh-market tomato production on Indiana small farms. Organic mulches can suppress weeds while promoting soil carbon sequestration among other benefits. Mulches created from nitrogen-fixing cover crops, such as white clover and hairy vetch, are particularly attractive for nitrogen-demanding crops such as tomato. Yet reliability and convenience often turns farmers to plastic mulch, a fossil-fuel intensive product that generates substantial landfill waste, and helps retain but does not build soil nutrients or organic matter. Straw is another common mulch choice, but can contain weed seed, promote mold, and lead to crop nitrogen deficits.

To increase their viability for small, organic farmers, cover crop mulches must be optimized for organic contexts, where herbicide is not an option for controlling either incomplete weed suppression or competition from living cover crops. Combining white clover living cover crops or rye-vetch cover crop residues with in-row leaf mulch has potential to help small organic farmers realize the full potential of cover crops.

Results analyzed to date indicate that the farmer’s standard weed suppression method – black landscaping fabric, black plastic film, or straw – resulted in 28-95% greater salable tomato yield at all farms. However, when combined with leaf mulch strips, white clover living cover crops and/or rye-vetch cover crop residue consistently promoted the second highest salable yields at all farms. In addition, cover crop and leaf mulches promoted greater microbial biomass carbon and nitrogen at two farms, and soil carbon pools increased over the growing season at all farms, with greatest increases in the particulate organic matter (POM) pool due to increased plant inputs from cover crops and leaf mulches. 

Our conclusions thus far are that: (1) In-row leaf mulch strips show promise for mitigating cover crop challenges, but more work is needed to identify cover crop strategies that are economically viable, and (2) Cover crop mulches outperform plastic and straw mulches in terms of indicators of soil health and carbon storage. Video and other web resources, and reports designed for farmer and researcher audiences will maximize the reach of our results.

Project Objectives:
  1. Establish replicated mulch treatments and controls at three partner farms
  2. Collect and analyze soil, cover crop, weed, labor and crop data to assess optimal mulching strategies
  3. Share project rationale, methods and results with farmers and agricultural professionals via a video, extension-style technical report, IU Campus Farm webpage, Indiana Small Farms Conference poster presentation, and agronomic journal article


Click linked name(s) to expand
  • Adrienne Keller (Researcher)


Materials and methods:

Field preparation & initial data collection

Study fields at each of 3 farms (Strangerʼs Hill Organics, SHO; Blue Hour Farm, BHF; and Indiana University's Campus Farm, CF) were tilled to 4-inch depth in early October 2018. At BHF, raised beds were created after tilling.

Beds (2-4-ft x 22-ft) were laid out in a randomized block design at each farm, with corners of beds marked with survey flags color-coded for each treatment or control type. Treatments consisted of white clover living mulch (WC) or winter rye-hairy vetch residue (WRHV) with and without undecomposed leaf mulch (LM) applied in the central 12-inch planting strip. Controls consisted of leaf mulch by itself and the current standard practice at each farm (black plastic mulch at SHO, landscape fabric at BHF, straw at CF). Control methods were put in place just prior to planting beds in late May 2019.

Leaf mulch was collected from the Indiana University campus in November 2018 and stockpiled at CF until application to plots. Tomato seed was started in plug trays in early April 2019 for late-May transplanting to field plots. Each farmer started their own seedlings at each farm, using their chosen seed variety (Celebrity (F1) at CF and BHF, Mountain Fresh (F1) at SHO) and 32-cell trays filled with a soil mix made from OMRI-certified perlite, coarse vermiculite, peat moss, potting soil, bone meal and blood meal. For beds receiving leaf mulch, an ~12-inch wide, 6-inch deep layer of leaf mulch was applied down the center planting strip just prior to planting in late May 2019. To facilitate standardized application of leaf mulch, two guidelines running lengthwise down the planting strip were staked 12-inches apart, and nine four-gallon buckets of leaf mulch were distributed within each strip. 

To assess baseline soil conditions just prior to seeding cover crops, six soil cores evenly spaced throughout each bed were taken to 8-inch depth and combined into a ziplock bag to create one bulked sample per bed. Soil samples were held on ice until transport to Indiana University's Biology Department, where they were processed for soil pH, total soil carbon and nitrogen, microbial biomass carbon and nitrogen, inorganic nitrogen and nitrogen mineralization rate, particulate organic matter, and mineral associated organic matter using standard protocols.  

WC and WRHV cover crops were seeded into beds October 6-8, 2018 at seeding rates of 15 lbs/acre (WC) and 60/25 lbs/acre (WR/HV). Seed for each bed was mixed well with soil from the bed, hand broadcast, and raked in to 1/2-inch (WC) or 1.5-inch (WRHV) depth with an iron rake. Due to either winter kill (BHF and CF) or initial poor germination (SHO), white clover treatments were reseeded at all farms in mid-March 2019. Reseeding produced good cover by late April 2019 at all farms. Rye-vetch treatments established well at all farms from the single seeding in October 2018. An ~one-two inch thick rye-vetch residue was created by flail mowing in late May 2019.

Percent cover of cover crops, weeds and litter/bare ground for all treatment and control beds was estimated from three, ~ 10-inch square quadrats evenly spaced along the length of each bed in late May 2019 and early July 2019. As another measure of percent cover, percent of photosynthetically active radiation (PAR) was measured in each treatment and control bed using a ~ 3-foot long Decagon Sunfleck Ceptometer held across at ground level across the bed width at three evenly spaced points along the bed length. Soil moisture and soil temperature were also measured to ~2-inch depth at three points evenly spaced along the length of each bed in late May, late June and early August 2019, using a Dynamax SM150T soil moisture probe and an Escali digital kitchen thermometer. 

Farmers at all 3 farms were interviewed at the end of October 2018 to explore their pre-project tomato growing practices, current and planned use of cover crops and mulches on their farm, sense of the environmental/economic/social benefits of cover crops and mulches, attitudes toward climate change, awareness and implementation of carbon farming, and sense of satisfaction with organic farming and weed management.

Tomato planting, bed maintenance and data collection

Tomato seedlings were planted into treatment and control beds in late May 2019. Before planting, drip tape was laid down along the center of the planting strip and, for white clover plots lacking leaf mulch strips, a six-inch width within the planting strip was wheel-hoed to remove white clover cover. Ten seedlings per bed were planted at CF and SHO. Due to limited seedling establishment, nine seedlings per bed were planted at BHF. Within the planting row, seedlings were spaced two-feet apart. Planting holes were made using a stainless steel dibble. A one-time application of 1/4 cup of OMRI-certified Nature-Safe 8-5-5 fertilizer was made to each planting hole before dropping in the seedling plug. At CF, tomatoes were stunted due to poor soil conditions, and an application of foliar fish emulsion was made to tomatoes weekly starting in early July 2019. Tomatoes at all farms were irrigated to soil saturation at time of planting and whenever farmers deemed necessary due to dry conditions. Irrigation was done on a whole-field basis. 

Beds were maintained by in-row hand weeding (all farms) and mowing (CF, SHO) or weed whipping (BHF raised beds) to four-inch height as needed. Maintenance inputs (time, gas) were recorded on data sheets for later conversion to dollar amounts. Tomatoes were trellised starting in early June 2019, using four hardwood stakes evenly spaced along the planting strip and strung with baling twine. 

Relatively minor insect herbivory and little to no disease was observed at any of the three farms. Little to no mammalian herbivory was observed at CF or BHF. Mammalian herbivores were a challenge at Stranger's Hill Organics, the most rural site. Seedlings were bent and sometimes pulled from planting holes within a few days after planting, resulting in one-three tomato plants missing from just over half of the beds by the end of June 2019. Furthermore, obvious deer predation (heavily browsed stems) occurred in just over one third of beds at the end of June 2019. OMRI-certified deer repellent (Messinas® Deer Stopper® Ready-to-Use Repellent Spray) was applied to tomato stakes and around the perimeter of the experimental area during the first three weeks of July 2019, until deer fencing was installed. The deer fencing was constructed from extra trellis stakes installed around the perimeter of the experimental area and strung with three rows of baling twine, creating an approximately five-foot high fence. Strips of white flagging were tied onto the twine at intervals. This low-cost fencing strategy was effective, and no further deer herbivory was observed. 

Tomatoes were harvested from late July 2019 through late September 2019, once or twice weekly depending on yield. Harvesting did not start until mid August 2019 at SHO, as tomato plants were set back due to the heavy herbivory experienced at that farm. All tomatoes were harvested that had any amount of red color developing. Both the total and saleable yield from each replicate bed were weighed to the nearest pound. Saleable yield varied slightly by farm. At CF, saleability criteria included no splitting, no end rot, and less than or equal to one blemish. At BHF and SHO, saleability was defined as no end rot and 30% or less blemishes.

Soil variables were remeasured in each bed at time of tomato planting in late May (i.e., after final establishment of all cover crop treatments) and at final harvest in early September. Soil sampling and measurement protocols were followed as described above for the initial soil sampling. 

Research results and discussion:

Soil Measurements

Initial soil measurements indicate that BHF soils were ~ twice as rich in available nitrogen pools as soils at CF and SHO (Table 1). Soil pH was comparable at BHF and SHO, while CF had more acidic pH (Table 1).

  Blue Hour Farm (BH) Campus Farm (CF) Stranger's Hill Organics (SHO)
Soil Variable Mean SE Mean SE Mean SE
Inorganic N (NH4+NO3, lbs/acre) 48.60 2.04 25.80 1.64 26.00 3.14
Net mineralization (lbs/acre/day) 1.52 0.05 2.40 0.09 1.31 1.22
Net  nitrification (lbs/acre/day) 3.66 0.11 3.06 0.10 2.42 0.20
pH 6.66 0.03 5.75 0.03 6.70 0.02

At two farms (CF and SHO), living & residue mulches with or without in-row leaf mulch resulted in significantly higher soil microbial biomass carbon and nitrogen compared to the farmer’s standard weed suppression method – black plastic film or straw. Preliminary analyses indicate that percent carbon in particulate and mineral associated organic matter pools increased over the growing season, with greater increases in particulate organic matter pools due to increased plant inputs from cover crops and leaf mulches.  

Tomato Yield

Results analyzed to date indicate that the farmer’s standard weed suppression method – black landscaping fabric, black plastic film, or straw – resulted in 28-95% greater salable tomato yield at all farms. However, when combined with leaf mulch strips, white clover living cover crops and/or rye-vetch cover crop residue consistently promoted the second highest salable yields at all farms. Analyses of net yield (e.g. taking into account time and fuel inputs) and other data (e.g. weed suppression) are in progress.

Farmer Perspectives

In pre-project interviews, farmers discussed using a variety of practices for tomato production. Black plastic was acknowledged as a common, successful method for mulching/ground cover. All farmers noted that leaf mulching was beneficial for moisture retention and reducing plastic waste, but required well-advanced planning to ensure access to leaf mulch 7-9 months after the autumn season. Farmers discussed challenges from climate change including the need for earlier irrigation (with rainfall in May and June less frequent in recent years) and some interest in carbon farming. Two of the farms are quite small in landholdings and carbon farming at a useful scale may not be feasible. Two of the farmers use traditional tillage techniques in combination with black plastic (and sometimes leaves) to combat weed pressures, while one farmer is more accepting of weeds and minimizes soil cultivation across the board.

Post-experiment interviews with farmers have been conducted and are in the analysis phase.


Participation Summary
3 Farmers participating in research

Educational & Outreach Activities

1 Webinars / talks / presentations

Participation Summary:

1 Farmers
Education/outreach description:

We presented a poster of results-to-date at the 2020 Indiana Small Farm Conference. Two faculty researchers, one farm manager, and two Indiana University students attended.

We planted cover crop demonstration beds at Indiana University's Campus Farm, adding crimson clover to the demonstration beds along with the original white clover and rye-vetch cover crops. We had initiated planning for a July 2020 demonstration field day, but all university summer scheduling was cancelled due to COVID-19. In lieu of the field day event, we adapted outreach plans to include video footage of the demonstration plots for a podcast that can be shared with farmers. We are running the demonstration beds again this spring for this purpose. We are also still planning to produce a technical report, webpage for the university's Campus Farm website, and an academic journal article. 

Learning Outcomes

Key changes:
  • Nothing to report yet

Project Outcomes

Project outcomes:

Nothing to report yet.

Success stories:

Nothing to report yet.


Nothing to report yet.

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.