Artikel by Monica Bonu & Marian Stuiver
Abstract
In recent years, the growing significance of soil in urban development has become evident. However, soil faces significant pressure, especially due to pressure from urbanisation, climate change, and biodiversity loss. Despite available information on soil benefits, strategies to enhance soil health and quality in urban development remain limited. This article presents practical guidelines spanning the design, construction, and maintenance phases of urban development. These guidelines encompass comprehensive soil analysis, integrating soil systems, water buffering, water and air penetration, construction practices, nutrient cycling, underground preservation, and biodiversity promotion. The collective implementation of the strategies in this guideline will enhances urban soil health and optimise associated benefits.
From pressure to space
Soil is crucial for urban development, offering numerous benefits and functions. It provides habitat to diverse organisms and mediates hydrological and biogeochemical cycles (Pouyat et al., 2020). Soil filters nutrients and pollutants, provides stability to plants, structures, and infrastructures in urban areas. However, these functions and benefits are dependent on the interplay of diverse soil properties. Considering the impact of urban activities on soil properties is essential when handling urban soils.
Urban development activities, such as soil sealing, compaction, and soil transportation, interfere with soil functions and benefits. Soil sealing prevents the movement of air and water in the soil, hindering plant growth and organisms survival (Chen et al., 2016). Soil compaction from the heavy traffic of humans and machinery especially during construction further reduces air and water permeability in the soil. Although the extent of soil compaction is influenced by soil type and structure, compaction affects nutrient availability, support to structure and plants, and water retention in soil. Soil transportation, involving the removal and replacement of soil, leads to a loss of local diversity of plants and soil organisms.
One of the main drivers of poor soil quality in urban areas is the lack of comprehensive information about soil properties beyond strength and texture. This further widens the knowledge gap on urban soil. In the Netherlands, although there are geotechnical soundings and borehole sample profiles in urban areas (in the Basic Register of Subsoil2), urban areas remain a white spot on the Soil Map. Available samples contain limited information on a wide variety of soil properties (Woolderink et al., 2023). Also, urban soil health is a hot topic in current literature, yet the absence of specific guidelines remains a gap. To boost soil health and quality in urban areas and harness its vital functions, actionable guidelines for urban design and development are imperative. This article focuses on providing practical guidelines for urban development stakeholders. A crucial step is adopting a soil-inclusive approach, involving a thorough understanding of soil characteristics before starting design and construction activities. This ensures that appropriate measures are taken during these phases to improve soil quality and maximize urban soil's benefits.
Guidelines for soil inclusive development
To optimize soil functions and benefits in urban development, prioritizing soil quality and health is crucial. Healthy, biodiverse soil is essential for thriving urban environments, supporting green, nature-inclusive, and climate-resilient cities. Healthy soil is central to achieving diverse goals relating to urban biodiversity and climate adaptation of cities. Below are the eight on-mutually exclusive guidelines for enhancing urban soil health and quality.
Figure 1: Guidelines for soil inclusive urban development
Analysis
Prior to any urban development decisions, a comprehensive soil examination is essential. This entails a detailed assessment of the soil's physical, chemical, and biological properties that collectively shape soil quality and condition. A comprehensive analysis informs decisions regarding appropriate land use and development placement. This maximizes the soil's ability to provide a range of benefits without degrading its quality. Soil analysis should be viewed as an integral component of broader urban development and considered the cornerstone upon which all other development actions depend.
Connected soil systems
Soil, present on roofs, facades, and the ground around buildings, can enhance the local soil ecosystem and support recovery after disturbances when strategically integrated during development. Many new urban developments incorporate green roofs, facades, and gardens to for climate adaptation. However, these often use engineered or imported soils unsuited to local conditions. To address this, projects should reuse excavated soil within the project area. Also, new green spaces should also connect with existing green networks in the area. These approaches will benefit various soil functions, reduce establishment time, and minimize the need to import soil, fostering biotic and abiotic interactions that strengthen site-specific soil properties over time.
Buffer
Conventional urban development practices, which rapidly drain stormwater, can lead to drought and increased soil temperature. Urban development practices should prioritize the retention of stormwater in project areas. Surface water buffering plays a vital role in facilitating water infiltration into the soil. Thus, from project requirements to construction, development should minimize soil sealing. Practical solutions, such as constructing wadis/rain gardens, lowering road verges, and creating slight soil depressions in vegetated areas, can help retain rainwater for subsequent subsoil infiltration (Rijksdienst Voor ondernemend (RVO), n.d.)
Infiltration
Healthy soil relies on water and air penetration through soil pores, which is mostly overlooked in traditional development projects. Typical developments practice seal surfaces, reduce greenery, and compact soil. These inhibit root growth, oxygen levels, and increasing CO2. Urban development should prioritize water infiltration and aeration by desisting from sealing existing surfaces and increasing vegetation in new projects area (Adobati & Garda, 2020). Solutions include permeable walkways, unsealed spaces around trees, rain gardens, and perforated brick for high infiltration (Craul, 1992). Perforated brick is noted for its high infiltration capacity (Craul, 1992) and should be prioritised where sealing cannot completely be avoided. Achieving a balance between sealed, partially paved, and unsealed areas is vital for healthy soil in urban areas.
Soil inclusive real estate
In urban development, construction has a significant impact on soil health, often resulting in reduced pore space, elevated soil temperatures, and a loss of soil organisms due to compaction, soil mismatch, and modifications (Craul, 1992). To mitigate these effects, construction should adopt practices that minimize soil disturbances, including short-term on-site soil storage, scheduling excavations considering soil type and condition, and use lighter machinery for subsoil work. Furthermore, construction practices should promote the reuse of site-specific soil in various areas, such as roofs and gardens. This approach ensures the rapid establishment of local soil organisms. This also prevents the introduction of non-native species which can negatively impact microbial communities, nutrient cycles, and plant health.
Nutrient cycling
Organic matter is crucial for soil development, serving as a binding agent and supporting soil organisms. However, many urban soils lack organic content due to design, construction, and maintenance choices that impede decomposition. Soil sealing and the removal of organic matter for aesthetic reasons deplete nutrients, hampering the nutrient-cycling process and the survival of organisms that facilitate such processes (Craul, 1992). To enhance urban soil health, design and construction should prioritize nutrient cycling by preserving organic matter, especially leaves. Strategies to achieve this include promoting tree understory growth over concrete, minimizing surface sealing in gardens, and reducing the complete removal of organic elements for aesthetics.
No disturbances
Underground infrastructure in urban development can significantly impact soil health through disturbances and changing conditions. For example, underground water pipes can raise soil temperatures, and maintenance work can disrupt the soil ecosystem. To address these issues, development projects should cluster underground infrastructure components to minimize soil disturbance during construction and maintenance. Additionally, pipes that transport substances with temperature fluctuations like water and sewage, should be insulated to prevent excessive heat transfer to the soil. This proactive approach aids in maintaining soil health and biodiversity by preventing soil overheating and supporting the overall soil ecosystem.
Diverse vegetation
Development projects should actively promote the use of native and diverse vegetation. In alignment with the previous guidelines, both new and existing development activities should incorporate vegetated surfaces equipped with native and diverse plant varieties. While vegetated surfaces shield the soil from direct sunlight and help retain water, native and diverse vegetation is beneficial for local biodiversity.
Wrap up
Urban development practices often degrade both topsoil and subsoil quality in urban areas, resulting in the loss of the associated benefits provided by soil processes. In this article, we have presented eight guidelines to optimize soil health throughout the design, construction, and maintenance phases. These guidelines acknowledge soil as the essential foundation for urban development. Implementing these strategies effectively will improve soil health and quality. It would also contribute to the achievement of city goals and visions for enhancing biodiversity and climate resilience.
References
Rijksdienst Voor ondernemend (RVO). (n.d.). Measures for climate adaptation and nature inclusion in the built environment. https://infographics.rvo.nl/klimaatadaptatie/maatregelen/#mii-rvokam-bouwen-en-renoveren
Craul, P. J. (1992). Urban soil in Landscape design. John Wiley & Sons.
Adobati, F., & Garda, E. (2020). Soil releasing as key to rethink water spaces in urban planning. City, Territory and Architecture, 7(1). https://doi.org/10.1186/s40410-020-00117-8
Chen, Y., Wang, X., Jiang, B., Yang, N., & Li, L. (2016). Pavement induced soil warming accelerates leaf budburst of ash trees. Urban Forestry & Urban Greening, 16, 36-42. https://doi.org/10.1016/j.ufug.2016.01.014
Pouyat, R. V., Day, S. D., Sally, B., Kirsten, S., Richard E., S., Katalin, S., Tara L. E., T., & Ian D., Y. (2020). Urban Soil. In P. Richard V., P.-D. Deborah S., P.-W. Toral, & G. Linda H. (Eds.), Forest and Rangeland Soils of the United States Under Changing Conditions: A Comprehensive Science Synthesis. Springer Nature Switzerland AG. https://doi.org/https://doi.org/10.1007/978-3-030-45216-2
Woolderink, H. A. G., Dill, S. N. T., Teuling, C., & Stuiver, M. (2023). Bodem als basis voor stadsinrichting. Stadswerk, 46-49. https://edepot.wur.nl/639326
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