What makes soil?
Soil is made very slowly over centuries by physical, chemical and biological processes. Physical processes (such as frost shattering) break up the rock. Rain water dissolves out some minerals. Micro-organisms living on, and later in, some geological material assist in the breakdown of rock and the weathering of minerals to release nutrients. Plants gradually colonise the surface and add their contribution, which the micro-organisms decompose to humus. Slowly this mixture becomes home to larger organisms such as earthworms which mix both humus and mineral particles together. Eventually, some kind of soil is formed.
Geology, or parent material as soil scientists like to describe it, means the initial starting material. In Wales this includes sedimentary rocks such as shale, sandstone or limestone; igneous rocks, such as granite, basalt and volcanic rocks; metamorphic rocks such as slate, schist and gneiss; and also large areas where the solid rocks are buried beneath glacial debris, recent river alluvium or wind-blown sand. All of these materials are very different and contribute to soil formation in different ways. The texture (particle-size distribution) of the soil is often inherited from the rock; for example, sandstones form sandy soils, shales form clay soils. The fertility of the soil is also affected by the chemical composition of the rocks and their rate of weathering. Limestone dissolves in mildly acidic rainwater to leave a residue of material to form soil which may still contain calcium carbonate (lime). Basalts and basic volcanic ashes contain more plant nutrients and weather faster and so produce more fertile soils than granites, rhyolites or acid volcanic ashes. The texture of river alluvium depends very much on the speed of flow of the river – slow-flowing rivers tend to deposit silt or clay while fast-flowing rivers remove small particles and leave behind sand or gravel. Glacial till (boulder clay) may contain a great assortment of material depending on the area the ice sheets have crossed on their way.
Landform interacts with climate and water movement. Sites higher up a mountain side are often colder and wetter than those in valleys. On steeper slopes, water is more likely to runoff the surface causing erosion, so infiltration and leaching through the profile will be less than expected. The landscape is dominated by Snowdonia but there are other areas above 500m with upland climates eg Cadair Idris and the Rhinogs. Coastal plateaux are extensive on Anglesey and Llyn. Rivers flowing to the east tend to develop broad valleys with wide floodplains whereas most of the valleys to the west are shorter and steeper with faster-flowing rivers.
All of Wales has been glaciated at some time, with impressive erosional features in the mountains. The glaciers stripped away soil and weathered rock from the uplands leaving many areas of almost bare rock, with mounds of debris known as moraine. In the lowlands, glaciers and their meltwater streams deposited a range of materials with a great variety of composition and texture which give rise to interesting but confused drainage patterns as sediments were laid down in an environment with very different drainage controls to those of the present day. This means that coarse sediments may be located in hollows with no drainage exit, leading to the formation of groundwater gleys or peat.
Changes in sea level dominate the morphology and soils of coastal areas. Recently formed coastal flats with sand dunes (terrestrial raw soils) and morfa or salt marsh (raw gley soils) are small but distinctive landscape features found all round the coast; notable among these are Harlech and Newborough. while submerged forests, sometimes with their soils preserved are found in many places. The change in relative sea level during the Ice Age is a complicated story and one that continues to the present day with the increasing concern over the greenhouse effect, rising sea levels and climate change.
Climate is a crucial factor in determining weathering and biological activity as well as the composition of the plant community growing on the soil. However, climate does not act in isolation. The coastal climate is mild but wet, while the mountains are cold and extremely wet.
The strong diurnal temperature changes in the mountains lead to freeze-thaw processes and on the very tops to near-glacial processes such as frost heave and stone sorting. The Carneddau, the Moelwyns and the Rhinogs have good examples of patterned ground including stone stripes and polygons indicating that periglacial (frost-assisted) climates are still with us. The lower temperatures on the higher ground mean that biological processes are much slower and so organic matter does not decompose, hence many of the soils have a peaty surface layer.
Rainfall is extremely variable over such a diverse landscape, with the Borderland being the driest (800-900mm), the coastal lowlands averaging some 1000mm and the uplands reaching 4300mm around Snowdon. High rainfall, especially on coarse-textured soils, is responsible for rapid leaching of lime and nutrients from soil materials leading to a very natural process of acidification. On less permeable materials, high rainfall produces waterlogging. The lower rainfall recorded on the coast is augmented, however, by sea mist which is common especially around the north and west coasts.
The moisture balance is of particular importance in understanding the potential use of soils. In winter, excess rainfall leads to leaching of nutrients while in summer a deficit of moisture can lead to drought for crops. For farming, this has mixed blessings. Crops do not go short of water, but access onto the land to cultivate or harvest, or to graze cattle, can be extremely limited. Dairy farming is common because there is enough moisture for good grass.
Natural Vegetation plays an important part in soil development due to the interaction of plant roots which extract water and nutrients, with the fall of leaves, and other organic debris enriching the soil in various ways. Different plant communities have varying effects on the soil. Deciduous trees have deep root systems drawing nutrients from depth and so tend to counteract leaching. Their leaves decompose readily to recycle nutrients and add humus to the soil. A wide range of organisms live in the soil beneath deciduous trees. Coniferous trees, on the other hand, lead to acidification, partly by locking up nutrients in the fallen needles that take years to decompose, and partly due to the organic acids, tannins and polyphenols that are washed off the bark and needles by rainfall and thus cause podsolisation. Soils under conifers therefore accumulate litter and peaty layers that are too acid for earthworms and many bacteria to survive in, which in turn reinforces the system as the decomposers are not present in the soil. Heather moorland affects soils in a similar way to conifers. Grassland recycles nutrients rather like deciduous woodland, but with shallower root systems, leaching may become more prevalent and the soils gradually lose nutrients and become more acid.
Woodland used to be more extensive than at present in the lowlands, but the demands for timber and fuel, especially during the little-publicised industrial activity of iron smelting, during Medieval times, have reduced the extent of the natural forests.
Most of Wales is farmland, much of it rough grazing for sheep or beef cattle in the uplands with improved grassland for dairying in the lowlands. Vast areas of brown podsolic soils in Mid-Wales have been improved with lime and fertilisers and now form rich sheep grazing pastures. Sheep grazing prevents the regeneration of trees and heather, so even the rough grassland we see today is really not natural at all. More recently, the intensity of sheep grazing within woodland has prevented any regeneration, reducing woodland to scrub which may be overrun by bracken on the better soils.
Arable farming is largely restricted to the lowlands and along the broad river valleys, though Anglesey was recorded in Roman times as the “granary of Wales.” Barley is now a major crop, having replaced oats, while some wheat is grown. Most crops are grown for livestock feed. Silage has largely replaced hay. The trend to silage together with the general intensification of agricultural methods, has led to greater use of fertiliser, and more careful management of acidity, hence soils are becoming less acid and more fertile in the lowland zone. Drainage schemes have been installed to improve the land. In the uplands, there has been little change that will affect soils directly, though any increase in livestock numbers would entail more fertiliser use and more animal manure would be produced. One of the greatest risks with livestock in wet areas is erosion of poached or damaged swards.
Increased use of fertiliser generally leads to a decrease in species diversity as native plants are out-competed by grass species. Low input or integrated farming systems aim to use fertilisers only to replace nutrients removed from the soil, avoiding any major changes in fertility. Organic farming aims to recycle nutrients wherever possible and prohibits the import of artificial fertilisers. The most important aspect of organic farming methods is the fundamental philosophy of long-term soil management, rather than short-term crop management, by encouraging the biological features of the soil such as humus and nutrient cycling. These farming systems are less likely to lead to problems with the soil, or leaching of nutrients or pesticides to the environment.
This century has seen extensive tree planting in the uplands, principally of Sitka spruce and Douglas fir, which has changed the character of these areas, and is having a significant impact on the soils beneath.. Forestry has replaced stock rearing in the less favoured grazing areas and conifer plantations now cover about 10% of the country, much of it now ready for felling. Plantations of conifers will develop the acidification trend mentioned above, especially on soils which are already acid and low in nutrients, though increasingly this natural trend (known as podsolisation) is exacerbated by the entrapment of atmospheric pollution (acid rain). Ground preparation for tree planting used to include ploughing and drainage which often acted as channels for erosion, but modern forest practice limits the gradient of ditches and prefers mounding to ploughing. The use of brash mats when harvesting also minimises the risk of compaction or erosion along vehicle tracks. Modern planting policy also keeps conifers away from water courses, and encourages the use of broad-leaved deciduous trees in sensitive areas.
Talk of Ice Ages and changing natural vegetation or land use automatically introduces the concept of time into this discussion. The lowlands of Caernarvonshire were probably free of ice by about 14,000 years ago, but some small glaciers survived in the mountains as recently as 10,000 years ago. The development of peat bogs, which trap and preserve pollen, allows scientists to reconstruct the changes in vegetation, and thus to infer changes of climate. A pattern emerges of oscillations between periods of wetter and drier, warmer and colder phases than at present. This resulted in continued melting of ice elsewhere throughout the northern hemisphere which in turn caused significant rises in sea level from an all-time low of maybe 100m below present levels some 100,000 years ago, to possibly only 40m below about 10,000 years ago. From 10,000 to about 8,000 years ago there was a dramatic rise to almost present levels – no doubt the legends of drowned lands in Cardigan and Conwy bays relate to this period of post-glacial sea level rise. What is certainly important is the changing nature of drainage in the coastal regions as sea level rose, and the movement of sediment onshore by wind and waves to form the morfa which skirt the coast.
Soils take considerable time to reach maturity, and almost certainly most of the soils we see are still adjusting to changes. Welsh soils are young by world-wide standards. However, that philosophical view leads us nowhere if we are to attempt to describe, classify and map the soils of Wales. We need to examine the variation in soil characteristics at the present day and use that to segregate soils of different properties and land use potential.