Research Article by Christy Bryar
Soil is everywhere yet its importance is often overlooked. When healthy, soil has the capacity to capture and clean water, transfer pollutants and make nutrients. It underpins our buildings and produces our food, it is the foundation of life and a critical resource. Its connection to ecological sustainability is paramount yet its fragility and health are often not considered. A better understanding of soil can inform meaningful sustainable design.
Soil is a living system of sand, silt, clay, organic matter, air, water and micro organisms. Minute particles of sand and silt are held together by the clay and organic matter to form aggregates. Between the aggregates are spaces called pores, which allow air and water to travel through and plants, fungi, bacteria and micro organisms to grow. If compacted, contaminated, disturbed or eroded its ability to support life is greatly diminished.
Soil is broadly described under two main areas: structure and composition. Soil structure is the arrangement of aggregates and pores. Good soil structure has adequate spaces around the aggregates to allow water and air to penetrate the soil and drain easily yet compact enough to hold enough moisture for plants to grow.
Soil composition is the proportions of clay, sand, gravel and silt in the soil. The composition of materials in the soil gives it its texture. There are 19 categorized textures, ranging from sandy through loam and heavy clay. It is the composition of soil that determines the type of foundations that will be required to carry the load of our buildings to stable ground. Just like foundations, different plants are more or less suited to different compositions and structures.
For architects, landscape architects and urban designers, understanding soil is a critical part of the design research. By understanding and assessing levels of compaction, water infiltration, percolation, natural surface flows, salinity, pH levels and contaminants appropriate decisions during the design process can be made to enhance the productivity and success of a project and minimise potential costs from negative impacts, such as flooding, erosion and silt run off. In order to achieve sustainable outcomes designers must consider the long term maintenance, productivity, and viability of the site.
Healthy soil needs to be protected through careful design and managed construction. Aside from determining suitable soils for planting, understanding the natural systems of a site can also inform decisions relating to storm water management, for example identifying the type and location of any hard landscaping and pavings so as to not impede the natural percolation of storm water through soil. Likewise, minimising compaction has implications on material choices, delivery and construction methods, excavation and staging of construction. A thorough understanding of the site’s soil also allows the designer to identify zones to protect during construction; in the same way you might specify the protection of significant vegetation
Remembering that disturbed soil has less ability to sustain life, it is important to understand that not all soils are the same. Transferring soil from another site is disturbed soil and placing it over damaged soil simply masks the problem, resulting in higher maintenance, less productive and water intensive sites. Where possible the existing soil should be re-used and plant selection and design decisions be made appropriate to the soil type and quality. Not all soil is immediately healthy. If original soil cannot be used it should be rejuvenated with organic matter. Soil should only be replaced as a last resort.
Understanding soil on a minute level is one side of the design consideration. It is also critical to understand it from a large scale perspective, by assessing it topographically. Researching the history of the site and understanding its natural system will inform a more connected response that will work as part of the system, rather than against it. In an urban setting the natural systems may be extensively disturbed however they will still exist. Was the site a floodplain, grassland, lowland, volcanic plain, does it have hidden or ephemeral streams? Even the tabula rasa has a history.
Challenges and Opportunities
While there is selective information on soil in relation to architecture and design, there is extensive information on assessing and remediation of soil condition in relation to agricultural grazing land in Australia. This information would serve as a starting point to understanding the impact of soil condition on growing patterns and could be adapted and expanded on to cover a wider variety of environments.
In addition, architects already engage soil engineers to carry out soil tests to determine the appropriate footings for a building. It would be simple to request additional tests for soil condition, such as pH levels, bacterial and fungal activity, density and nitrogen levels and to discuss the results with the technician to better understand their implications, constraints, or benefits on the design.