Building Sustainability Metrics: Soil Testing on the Pollinator Boulevard

By Julia Prince

As an intern at BASE, I was brought on to help leverage support for the Dolores Street Pollinator Boulevard by communicating the project’s positive social and ecological impacts in the area. I came with some photography and writing skills under my belt but had never touched any Adobe programs. I was granted complete creative freedom and thrown into the fire, so to speak. After all, there is no better way to have a deep learning experience than to figure something out for yourself, from end-to-end . I grappled for a while over what to create that would move people to back this project. What I produced was a video, lots of photos, blog posts, and the visuals shown below. These visuals represent my research-based hypothesis that the sheet mulching and drought-tolerant, native plantings on the Pollinator Boulevard are having a positive, long-term impact on the ground which the new gardens inhabit, in terms of plant health, soil nutrient composition, structure, and fertility.

As we look toward developing the third median on Dolores Street, we are considering the metrics through which to quantifiably measure the social and environmental impacts the project is having on the surrounding area. Some of the methods we seek to employ are pollinator counts, post occupancy assessments, measurements of water retention, and soil tests.

At the end of March, we honed in on one of these measurements of impact and collected soil samples from Median 1, which was sheet mulched in August 2015 and planted in March 2016, from Median 2, which was sheet mulched in October 2016 and planted the day we collected the sample, and from Median 3, which we have yet to begin working on at all. We dug down about 6 inches and filled a small plastic baggy with soil from two points, at either end, of each median. We sent the samples into the Michigan State University lab for testing and received results well in line with the initial hypotheses conveyed in the diagrams below.

With concrete measurements of the changes being made for both people and the planet by installing the Pollinator Boulevard, we hope to galvanize engagement, educate the public, broaden our support network, and create an effective and replicable template for a community-driven, urban gardening, project.

This is the first of a series of images which depict my assumptions of how the soil is changed by the process of sheet mulching and the establishment of native, drought tolerant, plants. Separated down the middle by a Canary Island Date Palm, the iconic trees that line Dolores Street, it is a comprehensive, before and after view of the ground under the median. It shows on the left the sheet mulched and planted, Pollinator Boulevard median 1 and on the right, an untouched median like median 3. On the right, we see compacted, desiccated, nutrient leached soil, low in organic matter and microbial activity. In contrast, the left side is characterized by rejuvenation of the soil horizons and palm roots due to the accumulation of organic matter, increased microbial activity, greater availability and retention of nutrients, and improved soil structure.

My research revealed how all of these soil metrics–microbial activity, nutrient and mineral composition, organic matter content, and structure–are so interdependent on one another. It’s a bit of chicken before the egg when it comes to measuring soil quality. Essentially, soil metrics provide a tightly integrated, positively correlated, and closed feedback loop.

On the left side, I hypothesized that sheet mulching has assisted in the creation of humus, thus reinstating the nutrient rich, surface soil, or Horizon O. Deep rooted, perennial plantings also build humus down through Horizon A. Organic matter invites microbial activity which mineralizes nutrients, making them more available to plant roots. Thus, the roots of the plants are able to grow deeper and maintain their own health without the addition of fertilizers. Native root penetration and microbial activity breaks up compacted soil and leads to the formation of aggregates. As a result, the roots of the Canary Island Date Palms should be able to take on a more natural structure as they get a boost of available nutrients and water, and soil compaction is relieved. However, any drastic change to the palm roots could not be proven by a soil test alone and is thus still only a hypothesis.

What our soil test did prove true were the expected changes in soil organic matter. Organic matter is added to the soil by decaying organisms, like plants, microbes, and fungus. Our soil samples were dug from top Horizons O and A and showed an organic matter content of 32.4% in median 1, 13% in median 2, and median 3, in the absence of any Horizon O, contained only 9.3%. Furthermore, the soil type in median 1 was indicated to be “organic”, median 2 was “mineral, loam”, and median 3 was “mineral, sandy loam”. Sheet mulching and deep-rooted, perennial, plantings increased the organic matter content on median 1, while sheet mulching alone increased the organic matter content of median 2. This drastic increase in organic matter among the surface soil layers between median 3 to median 1 indicates positively correlated shifts in nutrient retention, microbial activity, soil structure, and plant health.

The visual may or may not be an over-dramatization of the differences in soil and root health between the medians and to prove that would require further investigation. However, the test results, which are restricted to only the surface soil layers, do support my hypotheses in regards to the A Horizons. With proven organic matter increases, we can qualitatively infer positively correlated growth in the other soil metrics. As originally hypothesized, median 1, represented by the left side of the visual, has a soil Horizon A that is, “[A] Combination of organic and inorganic matter…[where] A high concentration of roots and soil microorganisms lead to the development of humus and soil aggregates, plus facility of air/ water exchange within this horizon and on to lower ones. Abundant microbial activity mineralizes organic nutrients to support root growth and plant/tree health.” In turn, median 3, represented by the right side of the visual, has a Horizon A characterized by, “Extremely shallow roots [that] lead to low amounts of organic matter and microbial activity, causing minimal formation of soil aggregates. In the absence of macro-pores, air and water permeation are restricted, further exacerbating the decline of microbial activity. With low instances of organic nutrient mineralization, plants and trees suffer.”

The visual to the right represents, more specifically, the improved soil structure which should result from increased organic matter. The right side, once again, represents median 3 and the left side represents median 1. Soil compaction, brought on by years of shallow-rooted, turf grass plantings and human activity on the surface of the medians, should be reduced from median 3 to median 1 as a result of the Pollinator Boulevard’s sheet mulching, perennial plantings, and subsequent increases in organic matter and microbial activity. More organic matter, due to the building of humus that leaches down into Horizon A, invites microbial activity. The movement and waste of these micro-organisms contributes to the formation of soil aggregates. Thus macro-pores are created, allowing water molecules and nutrients to penetrate the soil. More moisture and organic matter molecules invite even greater concentrations of microbes, which metabolize the nutrients present in the organic matter, making them more available to plant roots. The movement of water through the soil also allows nutrients to travel to deeper horizons and root zones. Increased nutrient availability makes for healthy plants and a sustained source of material to be decayed into organic matter. Thus, the cycle continues.

This last visual on the right specifically shows how microbial activity is effected by increased levels of soil organic matter. The top image represents the stifled soil food web of median 3, while the lower image represents the prosperous soil food web of median 1. Median 3 has low organic matter content and that which exists is being metabolized by algae and mosses. Thus at the first trophic level, organic matter feeders are dominated by bacteria and some disease fungi. Mycorrhizal fungi are not present due to a low concentration of roots. Insect pests dominate the consumption of unprotected shoots and roots and there are few earthworms. Median 1 has high organic matter content metabolized by algae, P-bacteria, and lichen. The first trophic level is dominated by organic matter feeders like saprophytic fungi,  accompanied by bacteria and acetinomyces. High levels of mycorrhizal fungi support an abundance of roots and keep insect pests in check. There are more earthworms present, as well. Without beneficial fungi, median 3’s second trophic level consumers are dominated by bacterial feeders like nematodes, protozoa, and rotifera. There is an absence of fungal feeder and few white worms to feed on insect pests. Median 1’s second trophic level is dominated by fungal feeders like springtails, mold mites, beetle mites, and feather winged beetles. There are beneficial levels of bacterial feeders and white worms keep insect pests in check. At the third trophic level, median 3 has a low density of consumers while median 1 has a beneficial level.

I hope these diagrams give the viewer a closer look at the changes that take place in the soil when organic matter content differentiates,  enlightening the intricacies, delicate balance and interwoven nature of the Earth beneath our feet. Like a Jenga tower, you can only take so many pieces out before things start to lean over and the whole thing collapses. Alas, we rebuild.