Chapter 8

Formation of soils

Figure 1 Types of rock from which many soils have formed; silver granite (left), rose (bottom), sandstone (top), slate (right)

Origins of soil

Our soils are mainly derived from the rocks that lie at the surface of the Earth. The great variation in this material goes some way to explain the range of soils with all the commensurate complications for those trying to achieve the best plant performance whether for production, display or to deliver excellent sports surfaces. However, the differences in parent material are only part of the story as the soils, once formed, undergo significant changes over time according to where they are situated in terms of the climate to which they are exposed but also the interrelated vegetation, topography, drainage conditions.

 The rocks at the surface now differ greatly from the original crust that formed on a ball of molten rock minerals which solidified. The least dense rocks floated on the top so when they cooled a surface layer of granite with basalt below it was created. Since then it has had a long and turbulent history during which time the crust has frequently fractured, crumpled, lifted and fallen. Furthermore, more molten material has been pushed up from below through the breaks in the crust and in volcanoes. The many types of rocks that lie at the very surface are the materials in which our soils form and develop. Many are also used in horticulture for building rock gardens, walls, paving, statuary and other garden features.

There are three main rock groups – igneous, sedimentary and metamorphic (see Figure 1).

Types of rock

Igneous rocks are those formed from the Earth’s molten material. All other rock types, as well as soil, are ultimately derived from them. When examined closely, most igneous rocks can be seen to be a mixture of crystals. Granite is one of the most common and contains crystals of quartz, which are white and shiny; felspars, grey or pink; and micas, shiny black (Figure 1). Many of these crystalline materials have a limited use in landscaping as formal structures rather than in the construction of rock gardens; they are more commonly used in monuments and building facades where the larger grained igneous rocks (‘porphyry’) are sought after.

The surfaces of rocks are exposed to weathering leading to their disintegration. The main agents of chemical weathering are rain (carbonic acid resulting from the mixing of carbon dioxide and water) and oxidation. As granite is weathered (‘rotted’), the felspars are converted to kaolinite (one of the many forms of clay) and soluble potassium, a plant nutrient. Similarly, the mica present is chemically changed to form another type of clay and yield soluble minerals. While the many types of clay retain much of the potassium, sodium, calcium, etc., the soluble material is carried by water to the sea, making the sea ‘salty’. The quartz content is inert so remains unchanged, but is simply released from the disintegrating granite; it then becomes subject to erosion and is usually physically ground up to eventually become the sand grains in soils.

There are hundreds of other types of igneous rocks including basalt, gabbro, diorite, andesite, tuff, pumice all of which disintegrate in a similar way to yield a range of different clays and soluble materials.

Sedimentary rock is derived from accumulated fragments of rock such as the huge deposits of sand in deserts. Many have been formed in the sea or lakes to which agents of erosion such rivers and the wind carry the weathered rock particles. Layers of sediment build up and, under pressure and slow chemical change, eventually become rock strata such as sandstone, limestone, mudstone, greywacke and shale.

Sandstones vary enormously in colour – browns, yellow, red and pink, as well as black, grey and white – and in texture and hardness depending on the nature of the source material (mainly quartz and feldspars) and the conditions in which they were laid down (e.g. hot dry desert or in water). There is usually strong indication of having been deposited in layers (strata).

Limestones (see Figure 2). Organisms which die in the sea and accumulate on the seabed may become one of many types of limestone which contain at least half its content as calcium carbonate. The shells of the organisms are often quite evident, but some are derived from algae. Strata (layers) are less obvious in limestones generally. Some are almost pure calcium carbonate whilst others have significant amounts of other material present to provide a range of colours. There are also limestones that are made up of weathered limestone (‘recycled’ calcium carbonate) where shell is not evident or derived from calcium carbonate in solution.

• Chalk is a soft limestone mainly white or grey, but sometimes stained with iron oxide (rust) or other impurities. Usually weak or no strata seen in the stone used to build rock gardens.
• Fossiliferous limestone is made up of clearly visible fossils usually with little strength; it comes in a variety of colours according to the shell and the matrix present,
• Oolitic limestones are composed of ‘oolites’; small easily seen spheres (looking like fish eggs) formed by the concentric precipitation of calcium carbonate on a fragment of shell or sand grain. Usually a warm yellow/buff colour.
• Limestone shales in which there is significant amount of mudstone giving rise to a wide range of colours (including many browns, blue and reddish) and hardness. There are black limestones that are sought after for paving but they do have a tendency to fade unless treated.
• Dolomite comprises calcium magnesium carbonate rather than calcium carbonate and an example of a crystalline limestone. Dolomitic limestone (or ‘magnesian limestone’) is a less pure form of dolomite. Both can be used to provide magnesium as well as calcium to supply the soil. Usually has a yellow colouring.
• Tufa is a soft, very porous limestone produced by precipitation from calcium-laden waters.

1. chalk



2. oolitic limestone



3. blue lias



4. tufa



5. fossiliferous limestone

    Figure 4 Types of limestone

In subsequent earth movements much of the sedimentary rock has been raised up above sea level and weathered again and again.

Metamorphic rock is formed from igneous or sedimentary rocks. The extreme pressures and temperatures associated with movements and fracturing in the Earth’s crust or the effect of huge depths of rock on underlying strata over very long periods have altered the original material. Slate is formed from shale, quartzite from sandstone, and marble from limestone in many colours from white through browns to black. Metamorphic rock tends to be more resistant to weathering than the original rock.

Weathering

From the moment that rocks are formed they are subject to weathering especially when exposed to the atmosphere. This breakdown of the rocks is brought about by the effect of physical, chemical and biological factors.

Chemical weathering is mainly brought about by the action of carbonic acid that is produced whenever carbon dioxide and water mix, as in rainfall. Some rock minerals dissolve and are washed away. Others are altered by various chemical reactions. All but the inert parts of rock are eventually decomposed and the rock crumbles as new minerals are formed and soluble material is released. Oxidation is particularly important in the formation of iron oxides, which give soils their red and yellow (when aerobic), or blue and grey colours (in anaerobic conditions).

Physical or mechanical weathering processes break the rock into smaller and smaller particles without changing the chemical character of the minerals. However, the chemical changes to minerals in rock that do occur are more rapid as the fragments become smaller because more surface area is exposed. This physical weathering occurs on exposed rock surfaces but, unlike chemical weathering, has less effect on rocks protected by layers of soil.

The main agents of physical weathering are frost, heat, water, wind and glaciers.

In temperate zones, frost is a major weathering agent. Water enters porous rocks or percolates into cracks in the rock and expands on freezing. The pressures created shatter the rock and, as the water melts, a new surface is exposed to weathering. Moving water in streams and rivers carries rock (‘load’); this abrades the rocks with which they make contact. The faster the water moves, the heavier the load leading to disproportionally more wear; a river in spate does the most damage as large boulders are bounced along the river bed.

Where there are glaciers the rock is worn away by the ‘scrubbing-brush’ effect of a huge mass of ice loaded with stones and boulders bearing down on the underlying rock and plucking more large pieces from the valley sides. The rock debris is carried until the ice melts when it is deposited.

In hot climates the rock surface can become much hotter than the underlying layers. The strains created by the different amounts of expansion and the alternate expansion and contraction cause fragments of rock to flake off the surface; this is sometimes known as the ‘onion skin’ effect. Moving wind carries fragments of rock that rub against other rocks and rock fragments, wearing them down (‘sandblasting’). As with moving water, the load increases as the windspeed goes up.

Biological weathering is attributable to organisms such as mosses, ferns and flowering plants which fragment rock by both chemical and physical means, e.g. they produce carbon dioxide which, in conjunction with water, forms carbonic acid; roots penetrate cracks in the rock and, as they grow thicker, they exert pressure which further opens up the cracks. Quarrying and mining activities lead to ‘spoil’ that becomes the raw material of soils.

Formation of natural soil profiles

Sedentary soils form in the material gradually weathered from the underlying rock. This can be seen easily in soils that have formed over rock where it comes close to the surface e.g. over shales, chalk, and sandstone such as the rich red soils overlying the Old Red Sandstone rocks that occur in many parts of Britain and Ireland. A hole dug in such a soil shows the gradual transition from unweathered rock to an organic matter rich topsoil.

Transported soils are those that form in eroded materials that have been carried from sites of weathering sometimes many hundreds of miles away from where deposition has occurred. They can be recognized by the definite boundary between the eroded material and the underlying rock and its associated rock fragments. Where more than one soil material has been transported to the site, as in many river valleys, several distinct layers can be seen. How they are moved depends on where the loose material lies:

• Gravity affects anything on a slope. On steep sides, e.g. cliffs, particles fall and accumulate at the bottom to form heaps of rock called ‘scree’.
• Rainsplash moves particles down the gentler slopes. As a result, surface soil is slowly removed from higher ground and accumulates at the bottom of slopes. This means that soils on slopes tend to be shallow, whereas at the bottom deep, transported soils develop, known as colluvial soils.
• Glaciers carry vast quantities of rock downhill and deposit their load at the ‘snout’ or ‘terminal moraine’. Of more significance in Britain and Ireland is the enormous load that was left behind when the glaciers retreated after the last Ice Age (10 000 years ago). This is known as ‘till’ or ‘boulder clay’ (it comprises boulders down to clay-sized particles).
• Running water moves material which eventually settles out according to particle size. Coarse sands are deposited first then the finer sands where the river widens and slows down. The finer silts are laid down in the slow-moving waters. The river valley bottoms become covered with material (alluvium) in which alluvial soils develop. Most clay particles, the very smallest fraction, do not settle out until they are in the sea.
• Wind removes dry sands and silts that are not ‘bound in’ to the soil. These particles are deposited where the wind slows down, often when the particles contact the water surfaces of seas, estuaries or lakes. Soils that develop from wind-blown deposits are known as ‘loess’ or ‘brick-earth’. Over many years, lakes become completely filled with these deposits: many that developed in Britain and Ireland since the last Ice Age proved to be ideal for market gardens (fertile and flat), then ideal for airfields (flat).

Many transported soils are in accumulations of particles all dropped according to their size, ‘sorted’, by wind (loess) or water (alluvium) as it slows down. These sorted soils usually provide excellent rooting conditions for horticultural crops. They tend to be deep, loose and open, making them easy to cultivate and ideal for root growth. However, those that have a high silt or fine sand content may be prone to compaction and can be more difficult to manage.

Soils derived from glacial material is made up of ‘unsorted’ particles which make it more difficult to produce a good rooting environment e.g. till (also known as ‘boulder clay’ as it is made up of varying quantities of the full range of particle sizes); when used in production horticulture these tend to be put down to longer term crops e.g. orchards and soft fruit areas where the cultivation is limited to plant establishment.

The nature of new soil (regosols) is largely determined by the parent material, i.e. the rock minerals from which they are formed. However, they continue to undergo changes under the influence of climate, organisms (vegetation and soil fauna), topography and drainage at the site. These interact over time to give rise to characteristic soil profiles in different parts of the world.

The soils that develop can be described in terms of the characteristics of the different horizons (layers) that make up the soil profile.