Ron Allen's soil and water pages
 
   
 
The Lowdown on Soils
 
Soil has its origins in geology, has been modified by physiographic and climatic processes, has developed a fauna and flora of its own, become adopted by the roots of trees and shrubs and herbs and even burrowed into by moles, rabbit, foxes and others.

But what is soil?
Soil is composed of rock fragments, mineral particles, water, air and bits of root, dead earthworms and the like, which together provide a material in which plants can grow. Thus, soil is the material in which plants root and obtain support, water and nutrients in solution. Soil is vital in maintaining our agricultural and natural ecosystems and differs from sediment in that it contains a biological component intimately mixed with the mineral matter. Soil of some kind covers most of the United Kingdom, except where it is replaced by water or concrete.

Different kinds of soil
There are many kinds of soil.
  • Semi-natural soils occur on many nature reserves and support (for example) ancient woodland, heathland and wetland.
  • Cultivated soils have been ploughed, drained, treated with fertilisers and herbicides and support crops.
  • Contaminated soils occur most widely on and around industrial sites and will be affected by mineral oils and a wide range of poisonous substances such as arsenic, lead and asbestos.
  • Buried soils occur at depth, such as below hillwash in valleys and under ancient burial mounds where they can tell us much about prehistory.
But what is soil like and where did it come from?
UK soils are typically about 1m to 1.5m deep, the depth to which most plants root.

Soil is made of a sequence of layers. In a lowland wood, you are likely to find a surface litter layer made of last year's dead leaves; below that a thin black layer of fully decomposed leaves or humus. Below that is a topsoil made of sand, silt or clay or a mixture of these and rich in dark organic matter. Below that will be a paler subsoil layer divided into blocks which shrink in summer and swell in winter and between which roots explore to find moisture and nutrients. Underlying this is the substrate made up of whatever geological material is below.

All soils comprise varying proportions of stones, sand, silt and clay and so we have stony, sandy, silty and clayey soils. Where peat has accumulated we have peaty soils. Sometimes these particles are directly related to the underlying solid geology, but more often they are related to thin layers of drift laid down during or since the Ice Ages. Some very old soils (palaeosols) were formed during the warm climates of the interglacial periods and retain characteristics of subtropical soils. Other, formed during cold glacial periods, contain structures formed by freeze-thaw cycles typical of Arctic areas.

Some permeable soils remain waterlogged for much of the year, others will be well drained and seldom wet. Soils with clayey layers at depth will become wet after winter rains and dry out during the late summer.

Heathland soils (known as podsols), are thought to have been derived from leaching following clearance, cultivation and cropping during prehistoric periods. These soils have a striking sequence of very pale mauve or white over black over orange layers.

Wetland soils have surface layers, at least, made of peat. In acid mires these peaty layers will be made of Sphagnum bog mosses, while elsewhere the peats may be of sedges and grasses.

Protecting semi-natural soils
While much is known about the wild creatures and plants protected in our reserves, very little is known about the range of natural soil conditions also so protected. As essential parts of our nature reserves, they deserve study and protection.

So why study soils?
The study of soil provides detailed information about how man has treated the environment over many thousands of years. Different soils record changing climates during and since the Ice Ages, changes resulting from woodland clearances by man, soil degradation and erosion from agriculture, metal contamination during the Industrial Revolution and most recently, changes in soil chemistry being brought about by acid rain. A study of today's soils will provide benchmarks against which future environmental conditions can be compared and change evaluated.


 
The Environmental Project Consulting Group
44A Winchester Road, Petersfield, Hampshire GU32 3PG
email: Ron Allen, tel: 01730 231019,
Copyright April 2005 Ron Allen
Geologist, Soil Scientist, Applied Ecologist, Hydro-ecologist, Chartered Environmentalist