The word ‘indicator’ makes it sound as if plants can be read as signs of soil conditions, when in fact, it is more like a starting point for a more complicated analysis that (in theory at least) takes a range of factors into consideration, including not only the other conditions at the site, but also the relative quantity and vigour of the plants, and the presence of other species. It is also true that soil conditions can vary dramatically in a small area (if polluting or fertilising substances are present, if animals have been defecating there, and so on). The discovery of indicator plants in a given location thus often yields contradictory ‘results’. For example, on our site visits to date, we have observed ragwort (Senecio jacobea), which is an indicator of poor soil, together with nettle and ivy, which are indicators of fertile soil. In practice, indicator plants rarely provide evidence of soil conditions; rather, they pose questions: might the soil be more fertile here than over there? What could lead the soil to be locally basic within an otherwise acidic site?
I prepared a mini guide to indicator plant species for use in the mobile lab. This includes a short list of plant species that are common in Glasgow, and which have a specific and reliable relationship to different kinds of soil (e.g., acidic, basic, fertile, infertile etc.). This was an interesting but surprisingly time consuming task: many of the plants that are strong indicators of soil conditions are only rarely present in the city; at the same time, certain soil conditions (e.g., dampness) are so widespread in Glasgow as to be irrelevant (since they do not distinguish between different sites). Finally, in April, many plants are still at an early stage in their growth cycle (or not even visible), which means that they can only really indicate something to experts. All this underscores the fact that what can be ‘read’ about urban soil through plants is both geographically specific and highly subjective.
Though it is important not to expect too much, the search for indicator plants is a pleasant way to get to know your local park, garden or vacant lot, and a good way to begin to see the inextricable and complex relations between plants and soil. Plants simply do not grow where the soil cannot support them. In this light, the surprising abundance and diversity of vegetation in Glasgow is worth thinking and talking about. What more could we be doing here?
Erin Despard, 14 April, 2016