A grounding in soil
Blair McKenzie, Environmental and Biochemical Sciences, James Hutton Institute
One of the defining images of my childhood was the July 1969 photo of Neil Armstrong standing on the “lunar soil”. While that may define my age it also planted a seed for what was to become my career as a soil scientist. It does so because it raises the question “what is soil”? When I attended university and studied soil science my enlightened lecturers spoke of the soil minerals being more than simply ground rock fragments. They were assemblages formed from weathering that included interaction with water and gases, notably oxygen, from the atmosphere. Given that my undergraduate university days were so long ago I was even more fortunate to be taught, in part, by soil biologists. They held a view that soil without life was simply not soil at all, because it was unable to fulfil many of the important functions (or services) that we expect our soils to deliver – such things as supporting food and fibre production; storing, filtering and buffering water and gases; cycling of nutrients; and providing a gene pool and habitat. Even on the most extreme soils on earth, those of the ultra-dry soils of the Antarctic valleys life has been found in the form of viruses. So the question remains – what is soil? If it must have life and have minerals influenced by water can the lunar surface with neither water nor life be considered soil? To continue the theme can the Martian surface which on current understanding has evolved with water and an atmosphere, but without life, be considered soil? What of all the planets - the well-known and the newly-discovered; is there soil on their surfaces? As a soil scientist I am loath to confine my interests to one small planet.
It is interesting that the UN in the declaration of 2013 declared 2015 to be International Year of Soils –plural. Unlike botany or zoology where individuals are discrete and can be neatly separated to genus and species, soil is a continuum. If a landscape is inundated e.g. by sea-level rise, does the soil cease to be soil simply because it is underwater? But there are some boundaries – few would argue the soils of the Britain are continuous with those of mainland Europe. However within e.g. mainland Britain, at whatever scale, any soil has continuity with adjacent soil. To map and classify soil, fuzzy boundaries are needed.
The line of thinking above sets out the basis for the question on soil science final examinations: Soil Science or soil sciences – discuss. A common separation is that soil is the mineral fraction less than 2mm. Anything larger is gravel and falls into the domain of geologists. Many of the typical analyses of soil used by farmers and gardeners such as pH, nutrient concentration are done in laboratories that accept the 2mm demarcation. However looking at soil profiles in the field particles, be they mineral or organic; do not neatly separate on the basis of size. The function delivered by these soils e.g. drainage and the through-flow of water, are strongly influenced by the particles greater than 2 mm in diameter.
As with many disciplines dealing with the natural world e.g. geology, ecology, geography are we simply using more fundamental understanding and applying that to a particular range of case studies (here the understanding of soil) or does soil science have something more to offer? Is the end point greater than the sum of the parts? In his masterpiece “Growth of the Soil” Nobel Prize-winning author Knut Hamsun explores the interaction between the book’s main character and nature through the connection with soil, fertility and food production. Hence words such as cultivate have meaning not only in terms of preparing soil for agriculture but also more generally as improve or develop. Many indigenous communities have a particular affinity or connection with the land. In this case “land” is more than soil type, vegetation, geomorphology or climate. It is greater than the sum of the parts. There is an element of spirituality involved. A modern and more commercial example is in the French concept of “terroir”, or sense of place. When a particular set of characters; soil, climate, geography, aspect etc. interact with plant genetics and crop management to deliver a product of special quality. The products from these can be protected by legislation as “protected designation of origin” and others are prevented from using particular names.
So while rigorous definition may restrict “soil” to a thin mantle, with defined particle size, produced by a limited set of weathering processes, those who frequently use, study, play or in other ways deal with soil know that to truly understand and appreciate it requires more than reductionist science and that if treated with respect its nature is to deliver lasting functions on which humanity depends.
This article was originally published in the Summer 2015 issue of The Geographer, the magazine of the Royal Scottish Geographical Society, and has been reproduced by kind permission of the RSGS.
