To a geologist or an engineer, soil is not only a medium which is built upon but also a medium to be built with. Stones, bricks and sand are three but widely used components of a building obtained from earth’s crust. Although, built structures generally fall short of providing nutrients necessary for life to thrive. For instance, manufacturing process of brick requires clay to be burnt – volatilising elements like potassium, nitrogen and phosphorus critical to support lifeforms. Not surprisingly then, our understanding within ‘human-dominated systems’ has remained of soil that is built upon or “restricted to the surface horizons”.
Collage by author: tiers of vegetation
on built surfaces above ground level
Biological growth can still occur on the surfaces of buildings such as walls, cornices and roofs – without coming in contact with the earth’s crust. Such growth can originate from very low-depth of soil-matter or even thin layers of dirt deposited on built structures. I refer to this growth-medium on buildings as “Film-Soils” in this article. If we return to the most basic definition of soil, as supporter of plant life – one can find ample potential with Film-Soils on buildings to render services similar to other soils.
Architects of the Film-Soil: living and non-living agencies
In urban areas, layers of soil supporting plant life can be formed on built surfaces through deposition of organic matter by wind and rain; or by withering of building material itself. Soil is often found deposited on roof-tops, chajjas and rough surfaces that has the ability to hold moisture and particles. Biological growth along pipes and walls can easily be explained with plants having their roots travel through the cracks of walls retaining moisture, or plumbing joints that leak water flowing through them.
My collection of instances from cities in India and France captures a variety of plants adapted to grow on buildings. Common plants found were stranglers like banyan and fig, a wide variety of ferns, climbers, grasses and moss. Interestingly one can observe growth on buildings to encompass all layers of plant growth – ground cover, shrubs and trees (see collage). The vegetation have become homes to birds, insects and rodents who play a fair share of role in shaping biological activity and nutrient cycles on roof tops.
The wind carries eroded soil as well as organic matter such as seeds, leaves and insects; to deposit on building surfaces as layers of dirt and litter. Moisture from rain (or even water from an occasionally overflowing overhead tank) helps decompose organic materials to compose a layer of Film-Soil. Organisms like insects, slugs, birds and rodents that reach building heights by climbing walls or staggered levels also contribute to creating a nutrient layer of soil on roof tops through eating habits, excretion and biological decay. The patch of tall grass plant growing on roof of an old market building in Bengaluru can be credited to carrier of seeds like pigeons perched right next to it and wind that is causing the plant to sway (see photo below).
Photo: A tall grass plant growing on a locked-down rooftop, is swayed
by gentle breeze while skies darken as precursor of rain, Bengaluru
In another glimpse of the same market building, a dense bed of plants grow uniformly. Appearing no less charming than a garden under regular human-care, the wild growth occurs on a roof that has remained locked down for years (see photo below). Agricultural and horticultural produce sold around the building offer abundant supply of seeds as and nutrients to be transposed by our above mentioned architects – both living and non-living.
Broadly the decomposed organic matter that forms soil is known as detritus. Micro-organisms or detritivores help break the compounds and remineralise soil. Slugs and insects are common detritivores that can scale walls. Growing fungi like mushrooms and mould are part evidence of the decomposition process.
Photo: The ‘wild’ rooftop garden growing
on the old market building at Bengaluru
Cultural and ecological services of the Film-Soil
Thin layers of soil are made available on buildings through anthropogenic and natural processes. Two benefits - ecological and cultural - of Film-Soils are briefly highlighted below:
Film-Soil formation on roof helps support a multi-tiered arrangement of biodiversity. One of my favourite examples remains to be something I spotted in a peri-urban locality of Bengaluru (see photo below). An abandoned structure that was perhaps part cowshed and part storeroom, now finds a dense bed of grass growing on top. A tree that towers over the roof makes it rather obvious, that falling leaves, insects and bird droppings from the tree make up for a nutrient base on the roof. This is a typical setting found in various parts of the city. But what is unique about this structure is that it also has a giant termite hill growing beneath the shed – offering two distinguishable layers of biodiversity. It forms a novel arrangement given grasses seldom grow on termite hills naturally.
Photo: Unique arrangement of grass bed on top of a termite hill,
made possible by formation of ‘Film-Soil’ on the trabeated structure
Film-Soils, can contribute to place making and add to aesthetic charm of cities. Narrow lanes allow a compact building layout which is helpful to beat the heat of tropical climate; but leaves little space for neighbourhood greening. Below is a sequence showing a refreshing patch of moss and fern in an otherwise dark lane of Kolkata. The dramatic growth has chosen its spot: contrasting green over grey, yet in harmony with the sloping terrain of courtyards. While the ferns and other flowering shrubs roots themselves in the cracks of the cemented slope of the service duct – the moss growth requires no additional soil to grow. Indeed, mosses can grow on very thin layer of dirt and can draw nutrients from air and moisture. One might then realise that the discolouration of walls from exposure to heavy rain and moisture, otherwise has at least one useful connection.
Image sequence: moss and ferns growing on a service channel in old city of Calcutta
Landscapes created through biological growth on buildings can compare in beauty with its more-natural counterparts. I owe the credit of the image from near les canaux de Valence that falls in department of Drôme to my friend Léa Peyron living in France (see photo below). Ivy and plants growing upon the abandoned neighbourhood in Valence provide evidence that even incidental biological growth can continue to survive on buildings naturally.
Les canaux de Valence (Drôme). Photo gracieuseté de Léa Peyron
Film-Soils to play a significant role in urban areas
Film-Soils reflect a definition by depth of growth-medium for plants – as opposed to categorisation by composition of soils. Film-Soils are not necessarily human modified or human transported and can develop naturally on human-made structures. Soil formation on building surfaces offer unique opportunity to engage with a growth environment traditionally overlooked by soil-scientists, engineers and architects alike. Thin layers of soils or ‘Film-Soils’ occurring on built surfaces are generally looked at as dirt; but can offer highly productive growth-medium for plants and other non-human lifeforms above the earth’s crust. In addition, Film-Soils help produce visually pleasing ecological landscapes in dense urban areas. Clearly Film-Soils are a significant part of the urban environment, whose complete meanings are waiting to be unravelled by – quoting Dr Krzywoszynska’s phrase for soil scientists from Rediscovering Soils workshop as – “Adventurers of Soils”.
About the author:
Amartya Deb received MA in Cities and Global Development as Allan and Nesta Ferguson Scholar at The University of Sheffield. He is a member of Soil Care Network.
Springman S., Davison L., (2000). Soil Description and Classification. University of the West of England, Bristol.
Sindelar M., (2015). Soils Support Buildings/Infrastructure. Soil Science Society of America.
DeBano L. F., (1990). The Effect of Fire on Soil Properties. Paper presented at the Symposium on Management and Productivity of Western-Montane Forest Soils, Boise.
Highlights of the12th Edition of Keys to Soil Taxonomy report published on 2014 by USDA is presented by Randy L Riddle of Natural Resources Conservation Service (NRCS) where Human-Altered / Human Transported Soil Criteria and Classification is discussed in terms of landforms, artefacts and manufactured layers beneath the earth’s crust. This reflects my claim of scientists focussing on soil that is ‘built upon’.
IUSS Working Group WRB (2006). World Reference Base for Soil Resources 2006. World Soil Resources Reports No. 103. FAO, Rome.
Wigglesworth S., (2018). The Difference between Soil and Dirt is a blog piece offering gardening advice on New England Today website. The article explains dirt as, “void of any beneficial nutrients and microbes that healthy plants need”.