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Publication #FOR 271

Florida's Urban Soils: Underfoot Yet Overlooked1

Donald Hagan, Cynnamon Dobbs, and Francisco Escobedo2

Approximately 90% of Florida residents—some 16.5 million inhabitants—live in urban areas. This ongoing urban development results in the conversion of agricultural lands, watersheds, and natural areas into a highly modified urban and suburban landscape. Florida's cities, too, have increased dramatically in size and density in recent decades.

The environmental effects of urbanization have, for the most part, been well documented. One area that has received considerably less attention, however, is urban soils. Indeed, alterations to soils represent one of the least obvious effects of urbanization, but these changes will have far-reaching consequences. While healthy soils are key to agricultural and forest productivity in rural areas, most urbanites (save the occasional backyard gardener, watershed manager, or soil scientist) are unaware of the many ecosystem functions provided by soils in urban watersheds and landscapes. Thus, a more complete understanding of the role of soils in the urban environment, as well as the effects of urbanization on soils, is essential if we are to sustainably manage the urban soil resource.

The objective of this fact sheet is to provide an overview of Florida's urban soils emphasizing their ecosystem services and sustainable management. We will use Miami-Dade County, Gainesville, and Tampa as examples, but this information should be of use to land-use planners, water management districts, Extension agents and municipalities throughout Florida and the Southeast.

Don't call it dirt: the role of soils in the urban green infrastructure

Urban soils at the "backyard scale" have been thoroughly reviewed (Shober and Toor 2009). At the "city scale" however, urban soils also provide numerous ecosystem services by virtue of their unique physical, chemical and biological properties. Urban soils are found in all urban and urbanizing areas including remnant natural and agricultural areas in cities. They are an essential component of the urban green infrastructure and watersheds. For example:

  • Urban soils are the growth medium for urban forests and vegetation including turfgrass, trees and shrubs, and other vegetation.

  • Urban soils are the site of biochemical processes that determine the fate and availability of plant nutrients. This "nutrient cycling" ability has ecological implications as well, since nutrients such as nitrogen and phosphorus are considered pollutants when they exceed certain levels in surface and groundwater.

  • Urban soils are the recipients—intentionally or by accident—of urban contaminants, including heavy metals, industrial chemicals, and human and animal waste. Depending on their clay and organic matter contents and level of aeration, soils might be able to retain and detoxify these contaminants, thereby preventing further contamination.

  • Urban soils allow for the infiltration of surface waters except when they are on an extreme slope, when they are severely compacted, or when they are covered by impervious surfaces. Infiltration reduces storm water runoff, soil erosion and offsite contamination, while allowing for groundwater recharge.

Improving our understanding of the urban soil resource

The physical, chemical and biological properties of an urban soil are what determine its suitability for a given use. The most important key to the sustainable management of urban soils, therefore, is an improved understanding of how these soil properties vary across an urban and urbanizing landscape. Since many urban soils in Florida are made up of fill brought from somewhere else, it is easy to assume that urban soils are homogenous, highly disturbed, or of low fertility (Pouyat et al. 2007). When the imported fill is sandy and has little to no organic matter this might be true. Most soil survey maps do not even describe urban soils, delineating them instead as blank areas on the landscape. However, recent studies in Florida have shown that soil properties are significantly affected by urbanization (seeTable 1), even across relatively small areas.

There are 5 factors—climate, organisms, topography, parent material, and time—that determine the development of soils. People are the organisms that most greatly influence urban soils through their construction activities and modifications of land use/cover. Soils may also vary spatially across the urban landscape/watershed due to differences in socioeconomics and/or time since urbanization, factors that are often tied to differences in land management practices. Most urban soils also have some natural variability due to their parent material, which in concert with human activities can have significant implications for soil properties. Topography is likely not a major soil forming factor in most Florida urban areas. Likewise, while climate is an important factor at the regional level, there is not enough climatic variability within a city to affect urban soil formation.

Parent material

Parent material refers to the geologic material from which soils were formed. This includes the underlying material or bedrock. When not obscured by the presence of a thick layer of fill material, an urban soil's parent material can tell us a lot about the geologic history of a site. In Florida, our parent materials are marine in nature and depending on location are typically sandy or clayey materials, or limestone. Individual urban areas often cover two or more parent material types, so they may differ depending on their location within a city.

Figure 1. 

Maps of urbanized areas in Alachua, Hillsborough and Miami-Dade County, FL showing dominant types of parent material.


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Parent material can have significant implications for soil fertility, pollutant retention capacity, drainage, and suitability for engineering purposes. Florida soils formed on sandy parent materials, for example, are typically more acidic (their pH is less than 7) and better drained than those formed over clays. Soils formed over clayey deposits are better at retaining soil nutrients, making them more fertile than sands. Assuming adequate drainage and aeration, clayey soils are also better than sands at retaining pollutants such as chemical wastes and heavy metals. South Florida soils that formed from porous limestone parent materials typically have higher pH values than other soils due to the alkaline properties of the parent material. A high pH can change soil fertility requirements for certain plants because soil nutrient availability is higher in pH-neutral or slightly basic soils. These parent materials are often very near the soil surface, and since they constitute the upper confining layer of the Floridan aquifer, there is often risk of groundwater contamination in these areas (Dobbs-Brown 2009).

Land Use/Land Cover

As part of the typical urbanization process, forests are converted to pasture, pastures to low-density urban developments, and low-density developments to high density developments. The end result is a highly modified urban core covered largely by impervious surfaces. Another common effect of urbanization is that the subsurface soil is brought to the surface and deposited over surface soils, or the soil surface is removed, with drastic changes to soil properties.

With increasing human modification, urban soils in general become more compacted and less acidic, the latter likely being due to the "liming" effect of concrete, which is more prevalent in heavily developed urban areas. On average forest soils in Gainesville, Miami-Dade County, and Tampa had a pH near 6 while urban soils had a pH closer to 7 (Dobbs-Brown, 2009). Soil nutrient concentrations are variable, ranging from very low in transportation right-of-ways to very high in some residential areas. Because of these factors, the suitability of urban soils for green infrastructure purposes depends on their location within the city, as well as the current land use. In general, vegetated areas like parks and urban forests, with the exception of lawns, are less compacted than areas where the soil surface is exposed (Dobbs-Brown 2009). These areas, therefore, can serve as important infiltration and filtration zones, especially in high density urban areas where much of the soil is covered by impervious surfaces. However, average bulk density values for the three Florida cities were near or below normal (1.3 g/cm3), which suggests that soil compaction is not a major issue (Seitz and Escobedo 2008, Shober and Denny 2010). Nonetheless, isolated areas with excessively compacted soils (> 2.0 g/cm3) were found.

Figure 2. 

Average soil bulk density (a measure of compaction) for Gainesville, Tampa and Miami-Dade County, FL.


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Time since urbanization

Time since urbanization is another factor that can affect the properties of urban soils. Two different sites, for example, could have the same degree of urbanization, but one site could be much older than the other. One explanation is that older urban areas have had more time for soils to "recover" from urbanization impacts like compaction, nutrient depletion/enrichment (Scharenbroch et al., 2005). "Legacy" factors including long-term contamination affect a soil's ability to perform certain functions.

Based on entirely random samples, soil physical properties in Tampa, Miami-Dade, and Gainesville, did not appear to be affected by time since urbanization. However, soil nutrient concentrations, which typically are depleted in recently urbanized areas, appear to stabilize or recover to pre-disturbance levels after 60 years. Phosphorus, for example, tends to accumulate in some soils and is found at the highest levels in older urbanizations. All of these factors suggest that urban soils in Florida become more suitable for plant growth as they age. Phosphorus is essential for plant growth, but high phosphorus levels in older areas of a city and excess phosphorus fertilization are not necessarily beneficial, since they can increase the risk of surface and groundwater contamination. Contamination by heavy metals is also of concern. Older soils often contain traces of consumer products like paints and insecticides that are no longer commercially available. Fortunately for the example cities, heavy metal contents are far below EPA maximum allowable concentrations, and do not appear to be affected by time since urbanization (Dobbs-Brown 2009).

Figure 3. 

Effect of time since urbanization on Mehlich-3 soil Phosphorus for Gainesville, Tampa and Miami-Dade County, FL.


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Socioeconomics

Urban and urbanizing areas are diverse in terms of socioeconomics and property values. Accordingly, these differences are likely associated with different land management practices and site history. In Gainesville, Tampa, and Miami-Dade, socioeconomic factors do appear to affect the characteristics of urban soils (Dobbs-Brown 2009). In general, soil compaction and fertility increase with increasing property value and household income. The soil compaction difference is small and difficult to explain, but the increase in fertility is a larger difference, and one more easily explained. It is likely due to higher rates of fertilizer use in higher income areas. This suggests that these soils are better suited for plant growth, but it also highlights the potential for offsite contamination in these areas. In some cases, heavy metal (lead and zinc) contents are greater in higher income areas (but remain well below EPA maximums). Since metals are not typically used as soil amendments, this trend is likely a reflection of a previous land use, rather than the current one. Note, also that at low levels, zinc is an important plant micronutrient and values might be reflecting the type of soil laboratory analyses used (Dobbs-Brown 2009).

Figure 4. 

Comparison of Mehlich-3 soil zinc content for low and high property values in Tampa and Miami-Dade County, FL. NOTE: The EPA recommended maximum content for zinc in soil is 2300 mg/kg


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Figure 5. 

Comparison of Mehlich-3 soil lead content for low and high property values in Tampa and Miami-Dade County, FL. NOTE: The EPA recommended maximum content for lead in soil is 400 mg/kg.


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Conclusions

Urban soils are often managed at the garden or "backyard" level. They provide a variety of ecosystem services at the city and watershed scale, too, but these remain largely overlooked. It is increasingly clear, however, that urban soil types are complex and that soil quality is affected by factors such as parent material, land use/land cover, time since urbanization and socioeconomics. Improving our understanding of these and other factors is a first step for interpreting soil surveys, planning effective urban land use, identifying best watershed management practices, improving decision making and sustainably managing the urban soil resource.

Management recommendations for Florida's urban soils (with suggested readings and cited literature)–

  • Understand the urban soil properties listed in Table 1 in your area and how they will influence your management decisions.

Bouma, J., P. S. C. Rao, and R. B. Brown. 2003. Soil as a Porous Medium: Basics of Soil-Water Relationships – Part 1. EDIS Publication SL 37. http://edis.ifas.ufl.edu/ss108.

Shober A. L. and G. C. Denny. 2008. Soil pH and the Home Landscape or Garden. EDIS Publication SL 256. http://edis.ifas.ufl.edu/ss480.

Shober, A. L., and G. S.Toor. Soils and Fertilizers for Master Gardeners: Urban Soils and their Management Issues. EDIS Publication SL 276. http://edis.ifas.ufl.edu/mg456.

  • Conduct soil tests to evaluate the retention capacity of soils that are contaminated, or could become contaminated, with metals and organic or chemical wastes.

Mylavarapu, R. S. 2009. UF/IFAS Extension Soil Testing Laboratory (ESTL) Analytical Procedures and Training Manual. EDIS Publication 1248. http://edis.ifas.ufl.edu/ss312.

  • Conduct soil tests before adding fertilizers and soil amendments.

Shober, A. L., and R. S. Mylavarapu. 2009. Soil Sampling and Testing for the Home Landcape or Vegetable Garden. EDIS Publication SL 281. http://edis.ifas.ufl.edu/ss494.

  • Preserve urban forests and restore pervious surfaces such as natural and vegetated areas.

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.

Seitz, J. and F. Escobedo. 2008. Urban Forests in Florida: Trees Control Stormwater Runoff and Improve Water Quality. EDIS Publication FOR 184. http://edis.ifas.ufl.edu/fr239.

Escobedo, F, S. Varela, C. Staudhammer, and B. Thompson. Southern Escambia County Florida's urban forests. EDIS Publication FOR 231. http://edis.ifas.ufl.edu/fr293.

Dobbs-Brown, C. 2009. An index of Gainesville's urban forest ecosystem services and goods. Master's Thesis, University of Florida.

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 Sci. Soc. Am. J. 71, 1010–1019.

  • Avoid management practices that disturb soil structure, cause soil compaction, reduce soil organic matter and decrease water infiltration.

Shober, A. L., and G. C. Denny. 2010. Soil Compaction in the Urban Landscape. EDIS Publication SL 317. http://edis.ifas.ufl.edu/ss529.

Scharenbroch B. C., J. E. Lloyd, and J. L. Johnson-Maynard. 2005. Distinguishing urban soils with physical, chemical, and biological properties. Pedobiologia 49:283–296

Glossary of Key Terms and Concepts

Bulk density – a measure of soil compaction. Soils typically have bulk density values of around 1.3 g/cm3, but values can be higher or lower depending on the degree of compaction. Highly compacted soils are less pervious and less suitable for plant growth.

Ecosystem services – Ecological processes and functions from natural and semi-natural ecosystems that are important for human well-being.

Impervious surface – one that does not allow for the infiltration of water. Pavement, concrete, or buildings, for instance.

Infiltration – the process by which surface water enters the soil.

Nutrient cycling – the process in which nutrients move and undergo chemical transformations in an ecosystem.

Pervious surface – One that permits the infiltration of water. Bare soil, or vegetation, for instance.

pH – a measure of acidity on a scale of 0–14 (0 being the most acidic and 14 being the most alkaline). Soil pH is an important consideration as it has implications for a soil's ability to support plants and provide other ecosystem services.

Tables

Table 1. 

Summary of some key soil properties and how they are affected by urbanization.

Soil property

Importance

Effect of urbanization

pH

Plants have an optimum pH range for ideal growing conditions. Nutrient cycling processes are also pH dependent.

Increases (soils become more alkaline)

Bulk density

A measure of compaction. Highly compacted soils inhibit plant growth and surface water infiltration.

Increases (soils become more compacted)

Total porosity

The inverse of bulk density. Porosity affects water retention capacity and aeration.

Decreases (porosity decreases with increasing compaction)

Nutrient cycling

Soil nutrients are essential for plant growth and ecosystem processes, but some become pollutants at high concentrations.

Variable (depends on land use, management, and time since urbanization)

Soil metals

Some metals are plant-essential micronutrients, but most are pollutants at high concentrations.

Increases (metals tend to accumulate over time)

Organic matter content

Improves soil structure, water retention capacity and nutrient availability.

Variable (depends on type and amount of vegetation and management)

Pervious surface area

Pervious surfaces allow for the infiltration of surface waters.

Variable (pavement and building structures increase impervious surface area)

Footnotes

1.

This document is FOR 271, one of a series of the School of Forest Resources and Conservation Department, UF/IFAS Extension. Original publication date July 2010. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Donald Hagan, PhD graduate student, School of Forest Resources and Conservation; Cynnamon Dobbs, MS graduate, School of Forest Resources and Conservation; and Francisco Escobedo, assistant professor, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University Florida.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.