Different people mean different things when they use the word “soil”. To civil engineers it is unconsolidated loose material that isn’t rock – so geological deposits like estuary mud, lake clays, boulder clay, loess (wind-blown silty deposits) and sand dunes might be included. To gardeners and farmers it is, perhaps the top 40 cm or so of the ground, the bit that you dig, and sink fence-posts into. A grave digger is traditionally interested in the top 2 metres! For purposes of soil study we are close to the grave digger. Soil scientists and surveyors who map soil types usually study the surface of the earth to about 1.2 metres, although in most parts of the world, as we shall see, soil can be much deeper than this. In practice, many soils are quite shallow, as they are developed in hard rocks, but we need a cut-off point. 1.2 metres is a practical depth, as health and safety legislation in several countries rightly insists on shoring and hard hats once you get deeper than this. Much survey work is done with a hand auger – a screw tool like a wood auger that brings up samples. These are usually one to 1.2 metres long. So soil can be defined as the physically altered top 1.2 m of the earth’s crust. We will see that the physical and chemical alterations of the earth's surface at these depths are very significant, especially for the growth of plants.
We can all recognise topsoil when we see it. It is a dark crumbly looking material that readily breaks up in the hand, and leaves your fingers dirty. Most people, if asked, will say that it is a mixture of mineral particles and organic matter from decomposed plants. Moist topsoil shaken in the hand will break up into lumps of various sizes, but these lumps are obviously made of some kinds of mixed materials stuck together, and will break down further if we grind them between our fingers, or with a pestle and mortar. Those who study soil have agreed to consider the basic mineral particles in four categories, depending on their size. The finest particles are known as clay defined as having a diameter of less than 2 micrometres (m, a micrometre is one thousandth of a millimetre. Just for comparison, a red blood cell is about 7m in diameter). Clay does not easily separate into individual particles as it is very cohesive, to study it we have to disperse it in water using chemicals that break the bonds between particles. The next size of particle is called silt and has a diameter of 2 to 60m. Particles larger than this can be seen with the naked eye, and are known as sand sometimes divided into fine, medium and coarse sand. Sand, silt and clay contitute what is known as the “fine earth”. Particles larger than 2000m (2 mm) are included with stones which vary from fine grit, or gravel to large boulders.
Soil samples, of course, are never composed of just one of these particle-sizes. They contain mixtures of sand silt and clay, and this mixture is conventionally called the texture or “particle size-class” of the soil. Stones also occur in most soils, but they are considered seperately, and do not constitute part of the particle size-class. The word “loam” has come to be used for a good mixture containing enough clay to provide nutrients and enough silt and sand to give good drainage and water-holding properties to the soil. Thus soil scientists refer to sandy loams, silt loams, silty clay loams and clay loams depending on the proportions of these particle-sizes in the mix. Soils with more than 35% clay-size particles are called clay, whilst those with more than 80% sand are called loamy sands and sands. There is more information on this available from the Wikipedia entry on “soil” - see http://en.wikipedia.org/wiki/Soil . Although rather technical in parts, this is, in general, a useful source of further information on most aspects of this course.
The dark colour of topsoil is caused by finely divided organic matter, or humus. This is the stuff that sticks to your hands and makes them dirty. In fact, it sometimes reacts chemically with fluids on and in the skin, staining the surface cells. (Soil scientists know this well, as it takes ages to clean hands that have been dealing with humus-rich soils). We are also likely to see some recognisable fragments of plants – pieces of stem, leaf, flower fruit, root etc. in various states of preservation, and often some animals like earthworms, ants, millipedes, slugs or insect larvae. A magnifying hand lens will reveal more detail, including ways in which the humus is distributed through the other materials, and smaller soil fauna like springtails. The architecture of the soil becomes evident, with living and dead roots, often forming dense mats near the surface, and leaving impressions in the form of pores. The fauna also create pores, whilst the casts, or droppings of earthworms and other animals may be visible. Sometimes there are chemical concretions that have precipitated out in the soil; small nodules of manganese and iron for example, or larger fragments of ironpan – we will discuss this later in relation to waterlogged and upland soils, and to the laterite soils of tropical areas.
We begin to get the feeling that this is pretty special and complex stuff. The feeling is right! Chemists have spent centuries prying into the chemical components of soil, and are still some way from identifying all the molecules that make it up. This is not so surprising when we consider the origin of the materials in soil. The minerals are derived from geological deposits that were there before soil formed. These, in themselves, are chemically and physically complicated, having been created ultimately from the earth’s mantle in volcanic activity that heated and pressurised the rocks near the earth’s surface. Although clay particles are minute, their molecules are very complex and varied, depending on the chemical origin of the rocks from which they are derived. As well as being chemically versatile, the molecules in clay have complicated three dimensional structures that affect the ways in which mineral elements attach to them, and they play an important role in the storage and distribution of important plant nutrients. They also affect the acidity or alkalinity of the soil. This important characteristic has a fundamental effect on what will grow in the soil. The rocks from which the soil particles (sand, silt, clay and stones) derive may be acidic in themselves.
The organic soil constituents are even more complex, consisting of the breakdown products of plants mixed with substances excreted by animals and micro-organisms. Some of this material is readily recognisable as fragments of plant tissue, but most of it is broken down – either by passing through the digestive tracts of animals, or by being incorporated into microbial material. This humus is found in particular parts of the soil – especially in the topsoil, where it glues fragments of soil together, and coats the surfaces of the resulting structural units, which are known as peds. If you shake some garden soil in your hand it will usually break into breadcrumb shaped fragments a few mm to a couple of cm in diameter. These are the peds. Their surfaces are often coated with humus, whilst pores that run through them may also contain enhanced amounts of this material.
This leads us on to the physical structure of soil. Peds are a unique feature of the material, and can be used to identify ancient soil deposits, for example below the archaeological “occupation layers” formed from the debris of human lives – the remains of buildings and the accumulated junk that people have discarded over the centuries – or even millennia. At the base of these layers it is usual to find soil that has been preserved by the overlying layers. Below Mediaeval and Roman layers in York U.K. , for example, I have seen topsoils with peds and the remains of earthworm casts still identifiable after 2000 years.
Topsoil crumb structures in grassland soil
Soil is never homogeneous - the solid mineral parts are mixed with humus, air and water to form a complex system in which the "whole is more than the sum of the parts". The mixture of mineral particles giving the texture is itself organised into the larger, compound peds - soil structural units. Subsoils often form into blocky or prismatic peds, which may have angular or more rounded surfaces. These peds form by wetting and drying of the soil, by the binding and secreting actions of roots and by the activities of the soil animals. They often have a zonation of organic matter content, with most of the OM coating the surfaces of the peds. The ped surfaces may be coated with other materials such as iron and manganese oxides, or clay that has been re-distributed by water flowing through the soil pores and over the ped surfaces.
Prismatic peds with clay coatings in subsoil
Porosity is a subdivision of structure - pores and fissures occur between peds, but also within the ped structures. They form through root penetration and decay, and through the activities of small boring animals like nematodes and pot worms. Larger earthworm channels extend from the surface to as much as 1.5 m. depth.
So far I have been talking mainly about the topsoil. This is the dark brown, black or grey layer extending about 20 to 30 cm below the surface. Under wild (natural or semi-natural) vegetation it contains a dense mat of roots from the surface to about 10 cm down. In grassland it is this root mat that can be scraped up as turf and used for laying lawns. Many people regard the topsoil layer as the soil, and don’t think too much about what lies below it. However, if you look at soil sections exposed in road sides, quarries and so on, you will notice that the layer immediately below the topsoil, often called the subsoil, still has some characteristics which distinguish it from the material from which it is developed.
Roadside section – U.K.
It is often a brownish colour, contrasting with the material below. Closer inspection will show that it still contains roots and soil animals, and the pores, fissures and peds that we saw in the topsoil. These are all soil characteristics and can be used to differentiate soil from non soil. The material from which the soil is developed is called the “parent material” (see box 1). It is conventional to call the organic matter-enriched topsoil, the A horizon, whilst the distinctive layer below, which is not as dark and has fewer roots is called the B horizon. The B horizon is of particular importance as its depth, structure and wetness are critical to plant growth, and to the way the land can be managed. The colour and structure of the horizons below the topsoil give us vital information about the character and potential of the soil, and are used to classify the soil. The relatively unaltered parent material below this is called the C horizon. The C horizon may be solid rock, much weathered rock, or a variety of unconsolidated deposits like sands, silts, clays etc. In temperate regions the rock surfaces were often strongly altered by freezing and thawing during the period just after the glaciers retreated. During this period too, large deposits of boulder clay were deposited from the glaciers leaving a material (properly called till) that contains chaotic mixtures of sand, silt, clay and stones, including large boulders.
- Write your own definition of soil, and describe how it is used in your local area. What do you think are the most important factors for maintaining or improving the quality and sustainability of your local soils?
- What are the best opportunities for examining soil in your area? Are there good roadside exposures, or other places where soil profiles are visible?
- If you have access to the world-wide web, look up some soil characteristics like soil structure, soil profile etc. What do you think of the results? Are they in agreement with what you have learnt so far, or do they create confusion?