- Urban agriculture (UA) is defined as the production of food crops or livestock within urban areas. Despite its popularity in the United States, research into UA systems suffers from a general underrepresentation of commercial urban systems. As a result, urban growers often have unique technological needs that are unmet by research and extension. I worked with a particularly ubiquitous group of urban growers, home gardeners, to better understand the current status of urban agricultural soils. Specifically, this study had three parts. First, I documented the current extent of research and knowledge related to urban agricultural soils in the United States (Chapter 1). Second, I noted the characteristics of residential-scale vegetable gardens in Corvallis and Portland, Oregon, to better understand current growing conditions and needs (Chapter 2). Third, I characterized the biological, physical, and chemical characteristics of these same gardens (Chapter 3). Finally, I conclude with potential directions for further research (Chapter 4).
In Chapter 1, I reviewed the academic literature on urban soils and found research which directly analyzed urban agricultural soil to be lacking. Only 17 studies directly addressed the characteristics of urban agricultural soils in the United States. Heavy metals were the subject of the vast majority of these articles, with about half the studies investigating chemical fertility parameters, and even fewer examining biological and physical qualities of agriculturally productive urban soils. Nearly all studies were conducted in residential sites, which potentially limits data-driven urban agricultural policies focused on commercial urban agriculture as a means to supplement locally grown foods.
In order to better inform management recommendations, I recorded garden characteristics of trained urban food growers. In Chapter 2, I report on a survey of surveyed 27 residential food gardens (including two demonstration gardens) in two Pacific Northwest cities. All site managers were trained Oregon State University Extension Master Gardeners. I found 132 unique crops were tended across all gardens, and a variety of management approaches were used. The most noteworthy concern I noted from the site managers was a desire to reconcile the mechanics of crop rotation within a small production footprint.
In Chapter 3, I examined the composition of urban garden soils from those same 27 sites in Corvallis and Portland, Oregon. In addition to recording the physical, biological, chemical fertility, and heavy metal parameters of urban garden soils, I tested for differences between garden sites based upon bed-type (e.g. raised beds versus in-ground beds). Raised beds were significantly different than in-ground beds for nearly one-third of the soil parameters recorded. Further, the mean soil fertility values across all sites were 2-8x above the recommended range for one-third of the parameters examined. I believe excessive applications of organic matter to be the source of this nutrient excess. Excessive organic matter, annually added to small garden spaces, likely promotes soil nutrient imbalances. However, the message many urban growers are given is that adding organic matter to soils is good. My data suggests that urban growers need more nuanced recommendations which account for the unique constraints of small garden spaces. Further, the recommendation to build raised beds to avoid contamination did not hold in this investigation. The matter seems more complicated, and I suggest greater scrutiny be applied to discover the source of contaminated soils in raised beds.
In Chapter 4, I suggest how policy, training, laboratory procedures, and management goals can be adjusted in light of these findings. It seems that the excessive nutrient levels in raised beds is a waste of both economic and environmental resources, with the potential for nutrient leaching as well. I believe that a well-informed site manager can quickly alter the productive capacity of an urban soil. Researchers who wish to contribute to urban agriculture should search for alternative management options which confer the benefits of compost while balancing the varied nutrient content therein. This likely involves using alternative fertilizer sources as well as novel bulking agents which can build but not imbalance a newly productive soil.