Developing Soil Fertility Systems for Urban Agriculture in the Upper Midwest

2012 Annual Report for FNC12-874

Project Type: Farmer/Rancher
Funds awarded in 2012: $7,239.00
Projected End Date: 12/31/2013
Region: North Central
State: Minnesota
Project Coordinator:
Alexander Liebman
Stone's Throw Urban Farm

Developing Soil Fertility Systems for Urban Agriculture in the Upper Midwest


I want to learn how integration of different cover cropping strategies in an urban farm system improves soil health while not conflicting too heavily with the farm’s production demands. Stone’s Throw Urban Farm rehabilitates vacant city lots to produce food for market and CSA production. Space is quite limited, forcing the farm to use available planting areas multiple times in one season for food production. A few sample crop rotations demonstrating multiple plantings in one season are:
1) salad mix (Apr. 15 until May 25), tomatoes transplanted directly into salad mix (May 25 – October 15)
2) beets (Apr. 15 – June 10), salad mix (July 6 – Aug. 15), spinach (Aug 17. – Oct. 1st)

These cropping strategies require a large amount of fertility and do not allow for the devotion of space or time to growing long-term cover crops. Therefore, it is important for market gardens operating on inherently poor soil quality to combine soil-building strategies with profitable crop production. This experiment attempts to identify how cover cropping integrates into an urban farming crop plan and how cover-cropping may improve soil quality.

I planted three types of cover crops: rye/vetch, oats/peas, and clover. Treatments were split between compost applied + cover crop, no compost + cover crop, and solely compost. Due to spatial constraints, clover was often planted without a compost + clover comparison. Preceding crops varied across lots.

Seeding rates were determined by studying MOFGA (Maine Organic Farmers and Gardeners Association) recommended seeding rates. I increased the rates ~ 30% for direct seeding due to chronic problems with poor germination in the urban setting due to low soil moisture and poor soil fertility.

CROP MOFGA (lbs/acre) STUF (lbs/acre) STUF (lbs/500 sq. ft.)

Rye 15-30 45 0.50
Vetch 50-60 75 0.86
Oats 15-30 45 0.50
Peas 50-75 85 1.00
Clover 9 to 20 30 0.33

I modified the lbs./500 sq. ft. seeding rates to match bed dimensions at all sites.

All direct seeding used an Earthway Seeder. Oats/peas and rye/vetch were mixed, placed into hopper, and seeded with seed plate #1002-22 (beets/swiss chard). Clover was planted with seed plate #1002-5 (radish-med/leek/asparagus/spinach). Existing tomatoes were intercropped, with placement of seed as close as possible to tomato plant and spreading away from plant throughout the bed.

At one site (Oliver) and two beds at Aldrich, it was necessary to broadcast seed due to crop type/density. Existing eggplants, peppers, melons, and summer squash did not allow for use of Earthway seeder due to potential damage of harvestable crops. Broadcast seeding rates were seeded at double the MOFGA seeding rate.

Objectives/Performance Targets

Entering the experiment in summer 2012, we had several main objectives. These were to:
– Establish cover cropping practice within existing farm system
– Measure soil bulk density for 2013 comparison
– Measure cover crop biomass for 2013 comparison
– Measure soil nitrogen, organic matter and pH for 2013 comparison
– Photo journal to track cover-crop growth
– Track costs/labor
– Host several outreach events to surrounding agricultural and community garden community


We were able to implement all of our objectives during the summer/fall of 2012.

All cover-cropping sites were planted during the first weeks of August, according to our plan. Sites where the cash crop had been removed were tilled. Sites with existing tomatoes, eggplants, or pepper stands were broadcast. Despite planting on schedule with a high seed density, cover crop germination and growth was extremely spotty. We believe this is due to two main factors. Lack of precipitation (no significant rainfall events in August and September 2012) impeded cover crop germination. In addition, in the intercropped cover-crop stands, soil disturbance due to harvest (cover crop was planted mid-August but we continued to harvest nightshades until late September thereby dragging bins across soil surface) may have disrupted growth and germination.

Soil samples were taken the first week of July and submitted to the University of Minnesota Research Analytical Laboratory for testing. We also sampled soil for bulk density measurements at that time. Bulk density soil samples were stored, dried for two days at 120 degrees Fahrenheit and measured on 10/21/12. Volumes were determined using a 100 ml graduated cylinder and density determined by dividing soil mass by soil volume.

Cover crop biomass was sampled 6 weeks after planting. We constructed a 1 foot x 1 foot square and cut all vegetation within square to measure cover crop growth in comparison to weed growth. We randomly sampled vegetation from two places per bed. Biomass was dried at 120 degrees Fahrenheit for two days and stored. Biomass was weighed on 10/25/12 at the University of Minnesota, Weed Ecology Lab of Professor Nick Jordan.

We kept a photo-journal of cover-crop growth at our sites. This will be continued in late winter and early spring as cover-crop growth resumes (rye/vetch and clover stands) or decomposes (oats/peas).

I tracked labor hours spent on the project to estimate cost of planting and maintaining the cover crop. This may be the hardest element of this experiment to calculate as I recorded all hours spent cover cropping in 2012, despite the fact that many of these hours were spent implementing the grant-specific portions of the cover-cropping experiment. I did not discern between time spent planting and time spent recording soil biomass. This was an oversight but I will still be able to roughly estimate the cost to integrate cover crops into our farm plan.

The cover-cropping experiment increased the farm’s contact with the surrounding sustainable agriculture community. A few examples are:

– Anne Pfeiffer, Community and Regional Food Systems, University of Wisconsin-Madison, toured the farm for an afternoon. She is also researching how cover crop systems may improve the fertility of urban farm/market garden systems. She is researching if spring-planted cover crops followed by late summer cash crops may be an effective means of improving soil fertility. She is also measuring yield differences in intercropped systems. We are planning to share insights as we gain more clarity regarding the outcomes of our experiments.

– University of Minnesota class, Ecology of Managed Ecosystems, taught by Professor Nick Jordan, visited the farm twice. About 60 students total walked through our farm plots in two separate groups. This was an excellent forum to display our intention in designing a cover-cropping system and showing the growth differences across treatments.

– I spoke at a University of Minnesota, What’s Up in Sustainable Agriculture (WUSA) event in mid-September. This informal 1-hour lecture period takes place over the lunch hour. About 10 students attended this session. The people attending my talk ranged from first-year agronomy students to student-run Cornercopia Organic Farm participants to graduate students studying corn/soy-cropping systems.

I organized but was unable to host a guided afternoon discussion regarding my experiment and general strategies for improving soil fertility and increasing vegetable production on depleted urban soils. I intend to reschedule this event during January/February 2013.

Impacts and Contributions/Outcomes

Future Work

We made several insights into how to better integrate cover cropping into our farming system. We also have several ideas how to improve soil fertility in manners different from fall-planted cover crops.

A major improvement of the experiment would include the comparison of fall-planted and spring-planted cover crops. Anne Pfeiffer mentioned she was studying the integrated use of clovers in cucurbit production. This is an attractive option for several reasons including nitrogen fixation, full-season ground cover, and competition against weeds pressure.

No-till trials would also be valuable, as this type of management could significantly improve soil quality. However, this might not be practical as we depend so heavily upon small-seeded crops, including carrots, beets, salad mix, and arugula.

Overhead irrigation may also improve cover cropping success, especially in intercropped systems where water demands by the cash crop are already high. However, this may be difficult or impossible due to the drip-tape set-up in summer nightshades. Also, tomato quality generally suffers if exposed to too much overhead water. Mini-sprinklers located near ground level and activated at key points during germination and initial growth could increase cover crop growth.

As our plot sizes are small, the farm is space-limited, and it is located near large sites of organic waste, mulching may be a more appropriate method of providing ground cover and increasing soil organic matter while not compromising cash crop space. Current regulations in Minnesota prohibit the unlicensed hauling of vegetable waste, thereby complicating the production of on-site compost. City regulations also limit the size available for compost production. However, abundant waste streams exist that could be harnessed with relatively little hassle. These might include, human hair, leaves, and coffee grounds. I am intrigued by the ability of the cover crop to perform above-ground and below-ground roles, increasing soil organic matter and microbiological activity while also physically limiting the potential of erosion. In the future I would like to develop a more comprehensive experiment comparing mulching and cover-cropping.

During spring/summer 2013 we will finish the experiment. We will take soil samples in the same beds sampled in summer 2012. We will compare soil nutrient levels in 2013 to levels in 2012. We will also compare any changes in soil bulk density. These measurements and statistical comparisons will be completed by the end of summer 2013. Biomass measurements will also be taken in spring 2013 to measure regrowth of rye/vetch and clover and final growth of oats/peas. Decomposition and ease of planting will be documented by continuing the photo journal. Outreach events will continue in 2013 to review the findings of our experiment. I hope to have more conclusive evidence regarding the role of cover-cropping to improve soil conditions in an urban agriculture setting by the end of the experiment.


Nate Watters

[email protected]
Stone's Throw Urban Farm
3406 Garfield Ave S.
Minneapolis, MN 55408
Office Phone: 4135528872