Urban soils provide a variety of important ecosystem services that can help improve the quality of life of people living in cities (Hagan et al. 2010a and 2012). These soils, however, have not been studied as much as agricultural and forest soils, and remain poorly understood. In many cases, urban soils are assumed to be homogenous, heavily disturbed, and of low fertility (Pouyat et al. 2007). In fact, most soil surveys do not even describe urban soils and delineate them as blank areas on regional soil maps. Recent studies have shown, however, that the physical, chemical, and biological properties of urban soils are quite variable, with conditions ranging from highly modified to relatively natural. Despite this extreme range in variability, a few trends and patterns in soil properties are evident in urbanized landscapes/watersheds (Pouyat et al. 2007; Dobbs 2009; Hagan et al. 2010a and 2012). To sustainably manage urban areas and better assess the ecosystem services provided by urban soils, we must improve our understanding of patterns in soil properties and how they vary across urban and urbanizing landscapes.
To address this need and understand urban soils, we characterized two key soil properties—soil bulk density and organic matter content—in Miami-Dade County. We chose Miami-Dade because it is the most populous county in Florida (more than 3 million residents) and because it has urban and suburban areas of various ages and states of development. We used geographical information systems and geostatistics—a type of statistics that uses spatial data—to map and provide an overview of soil bulk density and organic matter content. We also showed how these two properties vary across different urban land uses. The study was part of a larger urban ecological assessment, in which we collected and analyzed the upper 10 centimeters of surface soil from 79 random sampling sites, across urban Miami-Dade County (Escobedo et al. 2010) (Figure 1). Property access difficulties and budget and time constraints prevented us from obtaining more site-specific information. Although our sample size was small, the information and maps from this publication will nevertheless provide a general overview and better approximation of urban soil properties than existing soil surveys. This information should be of use to land-use planners, water management districts, UF/IFAS Extension agents, municipalities, and citizens interested in understanding urban soils.
Soil bulk density, the mass of dry soil (g) per unit volume (cm3), is routinely used as a measure of soil compaction. It has many important implications for urban soils. For example, increases in soil bulk density can reduce the infiltration of water into the soil profile and increase runoff. This increased stormwater runoff can carry nutrients such as nitrogen to water bodies and can impair water quality (Gregory et al., 2006). Plant roots cannot penetrate compacted soil as freely as they would in non-compacted soil, which limits their access to water and nutrients present in sub-soil and inhibits their growth. Compacted soil requires more frequent applications of irrigation and fertilizer to sustain plant growth, which can increase runoff and nutrient levels in runoff. Generally, urban activities that disturb the soil, such as foot and vehicle traffic compact the soil and increase the bulk density. The presence of soil organic matter, which is considerably lighter than mineral soil, can help decrease bulk density. Heavily urbanized areas, therefore, typically have bulk densities greater than those of natural areas such as forests and wetlands. Vegetation cover, however, is not necessarily a good predictor of bulk density. Urban soils covered by turfgrass, for example, have higher bulk densities than most other urban land cover types (Hagan et al., 2010a and 2012). This is probably due to increased foot traffic and the fact that turfgrass is frequently planted on top of fill materials (i.e., sand) that have been intentionally compacted for construction and engineering purposes.
Soil bulk density in urbanized Miami-Dade County averaged 1.63 g/cm3, which is considerably greater than values reported from other cities in Florida as well as the 1.3 g/cm3 which is considered ideal for plant growth and good infiltration of water into the soil profile (Figure 2a). By comparison, soil bulk density in Tampa averaged 1.02 g/cm3 (Hagan et al. 2010b and 2012) and ranged from 1.01 to 1.52 g/cm3 in Gainesville (Gregory et al. 2006; Hagan et al. 2010c). Reported values for Miami-Dade were highly variable, ranging from 1.02 g/cm3 (low) to 3.2 g/cm3 (very high). In general, bulk density was highest in the southwestern part of urban Miami-Dade County, a predominantly agricultural zone where values approaching and exceeding 2.0 g/cm3 were common. Interestingly, the least compacted soils (less than 1.4 g/cm3 bulk density) were in residential, commercial, and undeveloped land uses (e.g. natural forests and Melaleuca quinquinervia stands) (Figure 2b). This pattern of low bulk density in urban land uses is similar to what was observed in Tampa (Hagan et al. 2010c). We believe that these low bulk density values in urban land uses might be due to (1) the inputs from vegetation and litter that resulted in greater organic matter (see next Organic Matter section) and (2) the fact that soils were sampled from the top 10 cm of the profile, where plant roots predominate and porosity and organic matter content are highest.
Soil organic matter is made up of decomposed and partially decomposed parts of organisms, mainly plants. Like bulk density, organic matter has many important implications for urban soil quality. First, organic matter promotes plant growth since it is a storehouse of essential plant nutrients like nitrogen and phosphorus, especially in soils that are nutrient poor (e.g. coarse-textured or sandy) and need regular inputs of fertilizer. Second, organic matter is light and porous, which improves soil structure. The more organic matter in the soil, the more water it can hold. Organic matter in the soil will make water available to plants longer between rains and irrigation sessions, reducing the need to irrigate. The increased water-holding capacity of high-organic-matter soil also means less stormwater runoff. Soil organic matter also increases the ability of soil to retain and/or degrade pollutants, thereby reducing the likelihood of losses and offsite water contamination. Finally, because organic matter has about 50% carbon, a soil with a high organic matter can store more atmospheric carbon dioxide and thereby help mitigate climate change (Escobedo et al. 2010). Since organic matter is primarily of plant origin, it tends to accumulate in areas with high tree, shrub or grass cover, such as natural areas, or in poorly drained areas like wetlands, where organic materials decompose more slowly (Hagan et al. 2012). More information about the importance of soil organic matter and how to build organic matter content in soils can be found in https://edis.ifas.ufl.edu/mg454.
Soil organic matter content in urban Miami-Dade County averaged 9.5% of total soil weight, which is quite high compared to other Florida cities. For example, soil organic matter content in Tampa and Gainesville averaged 4.8% and 3.8%, respectively (Hagan et al. 2010a and 2012, https://edis.ifas.ufl.edu/ss536). Like bulk density, organic matter content was highly variable, ranging from 0.7% to 36.5% in Miami-Dade (Figure 3a). In general, soil organic matter was highest in recreational and residential land uses and lowest in forested and agricultural land uses (Figure 3b). Some areas in Miami-Dade with low bulk density (e.g. older residential areas with high tree cover and recreational areas like parks and golf courses) were also very high in organic matter. Agricultural soils in the southwestern part of the urban area were especially low in organic matter, as were densely urbanized commercial and residential areas in the northeastern part of the county.
Urban soils, as indicated by soil bulk density and organic matter content, are highly variable across the urbanized areas of Miami-Dade County. This suggests that Miami-Dade's urban soils are diverse and complex, and that they are affected by numerous human and environmental factors. Improving our understanding of the properties of urban soils is an essential first step to correctly interpreting soil surveys, improving urban land use planning decisions, mitigating the effects of climate change, sustainably managing our urban soil resources, and promoting the ecosystem services they provide. The more we know about urban soil properties, the more effectively we can manage soil quality and improve the quality of life of people living in urban and urbanizing areas.
We recommend several management practices for maintaining and enhancing urban soil quality:
Learn to recognize the physical properties such as bulk density and chemical properties such as pH and organic matter of urban soils in your community. Table 1 in Hagan et al. (2010a and 2012) provides detailed information on soil properties that can be affected by urban development and how they will influence your management objectives.
Avoid or minimize management practices that disturb soil structure, cause soil compaction, reduce or remove soil organic matter, and decrease water infiltration.
Conduct analyses to evaluate the soil contamination levels and determine the optimum retention (holding) capacity of polluted soils or soils that could become polluted with metals, organic or chemical wastes. For example, see Banger et al. (2010) for soil contamination with polycyclic aromatic hydrocarbons in different land uses in Miami.
Avoid excess irrigation and conduct soil tests before adding fertilizers and soil amendments.
Preserve urban forests and maintain pervious surfaces such as natural and vegetated areas, particularly near waterways (Hagan et al. 2012).
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Dobbs, C. 2009. An Index of Gainesville's Urban Forest Ecosystem Services and Goods. University of Florida MS Thesis.
Escobedo, F., S. Varela, M. Zhao, J. Wagner, and W. Zipperer. 2010. Analyzing the efficacy of subtropical urban forests in offsetting carbon emissions from cities. Environmental Science and Policy, 13:362–372.
Gregory J. H., M. D. Dukes, P. H. Jones, and G. L. Miller. 2006. Effect of urban soil compaction on infiltration rate. Journal of Soil and Water Conservation, 61: 117–124.
Hagan, D. L., C. Dobbs, and F. Escobedo, F. 2010a. Florida's urban soils: Underfoot yet overlooked. Gainesville: University of Florida Institute of Food and Agricultural Sciences.
Hagan et al 2010b should be: Hagan D., F. Escobedo, G. Toor, C. Dobbs, and M. Andreu. 2010b. Key soil physical and chemical properties in an urban and urbanizing landscape in Tampa. SL 324. Gainesville: University of Florida Institute of Food and Agricultural Sciences.
Hagan D., F. Escobedo, and Z. Szantoi. 2010c. Urban soils in Gainesville: key physical and chemical properties across an urban and urbanizing environment. Gainesville: University of Florida Institute of Food and Agricultural Sciences.
Hagan, D., Dobbs, C., Timilsina, N., Escobedo, F., Toor, G., Andreu, M. 2012. Anthropogenic effects on the physical and chemical properties of subtropical coastal urban soils. Soil Use and Management, 28: 78–88.
Pouyat, R. V., I. D. Yesilonis, J. Russell-Anelli, and N. K. Neerchal. 2007. Soil chemical and physical properties that differentiate urban land-use and cover types. Soil Science Society of America Journal, 71: 1010–1019.