Basics of Environmental Science

108 / Basics of Environmental Science

it proceeds more slowly than in moist ones, for example. Yet the process is remorseless. At widely

varying speeds it dismantles mountains.

It does not proceed far before living organisms accelerate it: respiration and the decomposition of

plant remains are the main source of the carbon dioxide engaged in subsurface carbonation. The

chemical changes release compounds useful to organisms in soluble forms they can absorb, and their

metabolic wastes and dead cells add to the stock of reactive compounds as well as providing sustenance

to still more organisms. Bacteria are usually the first to arrive, forming colonies in sheltered cracks,

invisible to the naked eye. Lichens often follow, composite organisms comprising a fungus and alga

or cyanobacterium. The fungus obtains water and mineral nutrients from the rock, the alga or

cyanobacterium supplies carbohydrates that it photosynthesizes and oxygen as a by-product of

photosynthesis. Each partner supplies the other and the fungus protects them both from drying out

and provides firm attachment to the rock, which it grips tightly with filaments that grow into the

tiniest crevices. This remarkable partnership allows lichens to flourish where no plant could survive.

Organic material, derived from wastes and the decay of dead cells, accumulates beneath the lichen,

mixing with the mineral particles and accelerating chemical reactions. This mixture is better at

absorbing and holding water, and in time there is enough of it to provide anchorage and nutrients for

plants. Mosses may arrive and small herbs may root themselves in the deeper cracks.

As the layer of mixed organic and mineral material thickens, some of it begins to be washed to

deeper levels, a few centimetres below the surface. The material is starting to form two distinct

layers: an upper layer from which soluble compounds and particles are being washed (the technical

term is ‘leached’) and a lower layer in which they are accumulating. This is the first stage in the

formation of soil.

From this point, vegetation becomes part of the developing soil and contributes greatly to its

formation. Plant roots penetrate the material and when they decay leave channels that assist aeration

and drainage. Dead plant material contributes fresh organic matter to the surface, which decays to

release compounds that drain into the soil. In detail, however, this process can vary widely over a

small area, in large part because of the efficiency with which the soil drains and the depth of the

water table below the surface. If the soil is derived from similar mineral particles all the way

down a slope, a hydrologic sequence may occur (CRUICKSHANK, 1972, pp. 47–48), illustrated in

Figure 3.9 Soil drainage