Friday, 21 July 2017
Soil has a life story. It is born and grows old.
In the beginning, when volcanic action or movements of the Earth’s crust expose a new land surface, there is only bare rock. Between 1963 and 1967 a series of volcanic eruptions on the seabed 32 kilometres south of Iceland thrust a new island above the surface. Icelanders called it Surtsey and today it is a World Heritage Site where biologists monitor the arrival and establishment of living organisms. Its 141 hectares currently support 60 species of vascular plants, 75 species of bryophytes, 71 of lichens, and 24 of fungi, as well as 335 species of invertebrates, and 89 species of birds have visited it. No people live there, so it is a kind of living laboratory where scientists can observe the natural compilation of a habitable environment, based on the production of soil.
Bare rock expands in the summer sunshine and contracts in the cold of winter. Expansion and contraction produce cracks and the minerals in rock expand and contract by different amounts, causing additional fractures. Warm sunshine causes the rock surface to expand more than the underlying rock, detaching the surface layer, which gradually flakes away. This is called exfoliation or onion-skin weathering. Rain fills the cracks and in high latitudes it freezes in winter. Water expands as it freezes with enough force to shatter rock, breaking away particles that wash out when the ice melts. Wind and rain detach and remove fragments of rock and roll and grind loose rocks against each other and against the solid rock. The rock wears away, loose fragments accumulate, and these, too, gradually become rounded and smaller until huge boulders are reduced to tiny grains. The process is called mechanical weathering. And it is not all.
The crevices are small, but large enough for bacteria, carried in the air and falling randomly everywhere, to find shelter from wind and shade from direct sunlight. Rainwater also penetrates and natural rainwater is weak carbonic acid because it contains dissolved carbon dioxide. The acid reacts with minerals in the rock, slowly dissolving parts of the rock and becoming a solution that contains substances on which the bacteria can subsist. This is chemical weathering.
The colonies of bacteria secrete their own protection in the form of a jelly-like coating, and as the rock crevices deepen and widen the bacteria expand into them. Bacterial wastes also accumulate and react chemically with the rock. After a time, layers of organic material develop. They are thin, but provide purchase for lichens. With more organic material, mosses are able to establish themselves and some time later the first vascular plants appear. Plants have roots, which penetrate the crevices in search of moisture and nutrients, and in doing so they excrete substances that feed microorganisms, accelerating chemical weathering.
A surface layer of mineral particles is called a regolith. When organic material becomes mixed with the mineral particles, the mixture becomes soil.
All soil is derived from rock and since the process of its formation has a beginning, soil must have a life story. The soil must age and eventually become very old, and the rate at which this happens depends on the climate. In high latitudes, where winters are long and cold and summers short and cool, the process is very slow. In deserts, where aridity inhibits organic activity, the process may never begin and the regolith remains regolith. Near the equator, in contrast, high temperatures and abundant rainfall accelerate the process greatly. So the age of a soil has nothing to do with the time that has elapsed since it first formed, but describes only its stage of development.
Larger plants arrive, with roots that penetrate deeper and allow rainwater—weak carbonic acid—to percolate further into the bedrock, extending the weathering process downward. Dead leaves, fallen twigs and later branches, the remains of annual plants, animal waste, and dead animals form a surface layer that provides food for a large and diverse population of organisms and, little by little, the soil grows deeper. The nutrients the organisms remove from the soil for their sustenance return to the soil through the processes of decomposition, so nutrients are recycled in the soil, though not completely.
As rainwater drains through the soil it removes materials in suspension from the upper layers and transports them to lower layers. This process is called eluviation and it accompanies the process of illuviation, in which compounds are deposited in layers of soil either from above or by being washed in laterally. Weathering continues to release compounds that enter the soil solution and these also move away, to lower layers or adjacent soils. That process is called leaching.
In time, the vertical movement of materials produces distinct layers, called soil horizons. These are illustrated in the above diagram, which is a vertical section cut through the soil from the surface to the underlying bedrock. The bedrock supplies the parent material from which the mineral component of the soil is derived. There are many horizons in a fully matured soil, although few soils exhibit all of them, grouped into four principal categories labelled by letters. The O horizons consist of organic debris, some of it partly decomposed. The A horizons are the topsoil, made from mineral particles mixed with organic material that has moved downward by eluviation and leaching. The B horizons form the subsoil in which compounds draining from above accumulate. The parent material, of partly weathered rock, forms the C horizon, and the underlying bedrock forms the R horizon.
At first the young soil has only a thin covering of organic detritus above undifferentiated mineral particles, comprising horizons O and C, but as more plants become established and animals arrive to feed on them, the soil deepens and matures, with the development of more horizons. All the time, though, the soil is losing compounds by leaching and eluviation and as the soil starts to show signs of ageing, the contrast between horizons A and B becomes extreme, as the A horizon is depleted of substances that accumulate in the B horizon. Plants then begin to suffer as the soil becomes less fertile, and with further ageing the fertility continues to decline. The more aggressive plant species that were previously dominant begin to fail as the rich supply of nutrients they demand dwindles. That allows the vegetation to become more diverse as species with more modest requirements flourish in the old soil. Finally, only the least soluble nutrients remain accessible to plants, and the only plants are those with shallow roots that live on nutrients they obtain from recycling organic matter. The soil is then senile and continues to deteriorate until at last it is fully weathered. All its nutrients are then gone and its fertility is extremely low.
Types of soil
Gley soils are often waterlogged for prolonged periods. This leads to the chemical reduction of iron compounds, which then move downward. Their horizons are often grey in colour and often with rust-red mottling. The process of acquiring these features is known in Britain as gleying and in the United States as gleyzation.
Lithomorphic soils are very shallow, with an organic horizon lying above bedrock.
Brown soils are well drained with no sign of gleying in the uppermost 40 centimetres, but with differences in mottling and lessivage—the downward movement of soil particles, especially clay—below 40 centimetres forming the basis for subdivisions. These are the most fertile soils, highly prized for agriculture and horticulture.
Pelosols are clay soils that crack when dry.
Podzolic soils form where rainfall is abundant and the mineral particles migrate readily. The soil is usually sandy and acid, often with a rust-coloured B horizon rich in iron oxides and aluminium oxides.
Man-made soils result from human activities and include soils resulting from the reclamation of mine and quarry wastes.
Peat soils are rich in organic matter in a layer at least 40 centimetres thick. The United States Department of Agriculture and the National Cooperative Soil Survey have developed a classification known as the US Soil Taxonomy that covers the world and is widely used. It divides soils into 12 orders and each of the orders into suborders, great groups, subgroups, families, and series. The map opposite shows the global distribution of the 12 orders of the US Soil Taxonomy.