Urban Soils Update, May 2018

Urban agriculture has received a lot of attention over the past decade, as more folks are looking to localize their food supply, reduce food miles, and/or exert greater control over their food. Urban agriculture, however, brings a distinct set of challenges from farm systems in more rural regions. For example, urban farms tend to be relatively small and diverse (which can make it challenging to rotate crops), and are often close to neighborhoods and housing developments (which may make urban farms more prone to nuisance complaints). Urban farmers tend to be younger and to have less experience in agriculture, compared to rural farmers, and in need to high levels of technical assistance from Extension and other providers (Oberholtzer et al. 2014). However, many of the resources that Extension has to offer are focused on traditional growers, rather than new urban farmers.

Our lab group wanted to examine an issue that is specific to urban growers, and for which we could find very little information: urban agricultural soils. Soil scientists have prioritized research on urban agricultural soils as a key priority for the 21st century (Adewopo et al. 2014). Yet for his thesis work, Mykl Nelson could only find 17 academic papers that looked at urban agricultural soils in the United States. Most of these studies focused on

residential-scale or community-scale urban agriculture (in home or community gardens). Only one paper looked at soils on an urban farm.

Still, residential- and community-scale gardening is an important type of urban agriculture. In Portland, a conservative count of 3,000 home gardens collectively covers more than 20 acres of land (McClintock et al. 2013). In Chicago, residential food gardens cover 29 acres of land, and represent 89% of all urban agriculture (Taylor and Lovell 2012). In Madison, WI, more than 45,000 food gardens cover more than 121 acres of land (Smith et al. 2013).

For Mykl’s thesis, he looked at urban soils from 27 Master Gardener-tended gardens, in Portland and Corvallis, OR. Even though all gardens were tended by OSU Extension trained Master Gardeners, they were incredibly diverse: 74 different annual crops, and 58 different perennial crops were grown across these gardens. Unique crops included kalettes, papalo, thistle, savory, paw paw, quince, sea berry, and service berry, among others.

In terms of the soils, Mykl found that soils were within the recommended range for physical parameters, such as bulk density, wet aggregate stability, and soil compaction. However, home garden soils tended to be over-enriched in soil organic matter. Growers generally aim to foster soils that are between 3-6% organic matter. However, Mykl’s tested soils were on average 13% organic matter! Raised beds were on average 15% organic matter. In ground beds were a bit better: 10% organic matter, on average. So to put this another way, Master Gardener vegetable garden soils had 2-5X the recommended level of organic matter for productive agricultural soils. We suspect that Master Gardeners were annually adding organic matter to their soils, without necessarily knowing the baseline levels in their soils. Adding more organic matter, without knowing where you’re starting from, encourages over-applications.

Does that matter? Afterall, for years, we have been preaching that if you have sub-par soils, ‘just add organic matter’. Biological activity in these soils was great! But, the excess in organic matter promoted excess in several soil nutrients. Garden soils were over-enriched in phosphorus (mean phosphorus across all gardens was 2-3X recommended levels. Potassium in some gardens was 5X recommended levels! Gardens were over-enriched in magnesium and manganese, too. Nutrient excess was worse in raised beds, compared to in-ground gardens.

Unexpectedly, Mylk found the highest lead levels in raised beds. Often, we tell gardeners to grow their food in raised beds, to avoid heavy metal contaminants. Why would there be high lead in raised beds, if we weren’t finding elevated lead levels in nearby in-ground beds? We suspect that the lead might be coming in from compost waste that can be purchased on the retail market. If a compost product makes no nutritional claim, then it is exempt from analysis and contamination limits.

We can’t wait to finalize this work for publication. In the meantime, I wanted to share a brief update on this work.

Mykl will be defending his thesis on May 31st. We’re trying to arrange an online broadcast of the public portion of his thesis defense (1pm-2pm, May 31st). I will update this post, if we are able to get an online link for his presentation.