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Soils ece cree vsesecesscsssseseeseereeee® ‘To many people who do not live on the land, soil appears Soil formation to be an inert, uniform, dark-brown coloured, uninteresting . rn i 7 : e The first stage in the'formation of soil is the material in which plants happen to grow. In fact little could accumulation of a layer of loose, broken, be further from the truth.’ . unconsolidated parent material known as rego- Brian Knapp, Soil Processes, i679 lith. Regolith may be derived from either the - in situ weathering of bedrock (i.e. the parent or Soil forms the thin surface layer of the Earth’s underlying rock) or from material that has been crust. It can be defined as the unconsolidated transported from elsewhere and deposited, e.g. mineral and organic material on the Earth’s as alluvium, glacial drift, loess or volcanic ash. surface, often characterised by horizons or layers ‘The second stage, the formation of true soil (Figure 10.5), that serves as a hatural medium for oT topsoil, results from the addition of water, the growth of plants and therefore the support of gases (air), living organisms (biota) and decafed animal life on land. It has been subjected to, and organic matter (humus). shows the effects of, genetic and environmental Pedologists have identified five main factors of: climate (including water and tempera- factors involved in soil formation (Figure 10.1). ture), macro- and micro-organisms, relief and the As all of these are closely interconnected and underlying parent rock (Figure 10.1). It develops interdependent, their relationship may be over a period of time through the interaction of summarised as follows: several physical, chemical, biological and mor- soil = f (parent material + climate + Figure 10.1 phological properties and characteristics. topography + organisms + time) ; The study of soil, its origins and characteris- where: f= function of. Fac affecing the tics (pedology) is a science in itself. Parent material When a soil develops from an underlying rock, its supply of minerals is largely dependent on that rock. The minerals are susceptible to dif- ferent rates and processes of weathering — see the example of granite, Figure 10.2. Parent material contributes to control of the depth, texture, drainage (permeability) and quality (nutrient permeability mineral content weathering precipitation temperature content) of a soil and also influences its colour. organic matter altitude a oe nutrient cycle/recyclin« aspect In most of Britain, parent material is the major ye ycling P ij mixing and aeration slope angle factor in determining the soil type, e.g. lime- stone, granite or, most commonly, drift. usually: mainly affected by sands and (crimp cane bae quartz (echarder’ mineral) ——>-S physical (mechanical) ———> silts left as aay shallowersoil a gests fewer nutrients | granite usually: finer texture mica mainly affected clays : | (softer minerals). ——— by,chemical > leftas (secondary poor diainage feldspar ‘weatherin iG minerals) deeper soil . aS more nutrients 1 ciencad eR RwCNauraremrEemetenies Figure 10.2 260 Soils The influence of a parent rock — granite — on soil formation Climate Climate determines the type of soil at a global. scale. The distribution of world soil types corre- sponds closely to patterns of climate and vegeta- tion. Climate affects the tate of weathering of the parent rock, with the most rapid breakdown being in hot, humid environments. Climate also affects the amount of humus (organic material) in the soil. The amount is a balance between the input and output, the input and output being a funce- tion of the effects of temperature and moisture on biological activity. One might expect tropical rainforest soils to have more humus than tundra soils because of the greater mass of vegetation. However, it is possible for some tundra soils to have more humus accumulation due to a lower output, and some tropical rainforest soils to have less because of greater humus breakdown. Rainfall totals and intensity are afso impor- tant. Where rainfall is heavy, the downward movement of water through the soil transperts mineral salts (i.e. soluble minerals) with it, a process known as leaching. Where rainfall is light or where evapotranspiration exceeds pre- cipitation, water and mineral salts may be drawn upwards towards the surface by the process of capillary action. Temperatures determine the length of the growing season and affect the supply of humus. The speed of vegetation decay is fastest in hot, wet climates as temperatures also influence (i) the activity and number of soil organisms and (ii) the rate of evaporation, i.e. whether leaching or capillary action is dominant. Topography (relief) As the height of the land increases, so too do amounts of precipitation, cloud cover and wind, while temperatures and the length of the growing season both decrease. Aspect is an important local factor in mid-latitudes (page 212), with south-facing slopes in the . northern hemisphere being warmer and drier than those facing north. The angle of slope affects drainage and soil depth. Greater mois- ture flows and the increased effect of gravity on steeper slopes can accelerate mass movement and the risk of soil erosion. Soils on steep slopes are likely to be thin, poorly developed and rela- tively dry. The more gentle the slope, the slower the rate of movement of water through the soil and the greater the likelihood of waterlogging and the formation of peat on plateau-like sur- faces at the top of the slope (Figure 10.3). There is little risk of soil erosion but the increased rate of weathering, due to the extra water, and the receipt of material moved downslope, tend to produce deep soils at the foot ofthe slope. A catena is where soils are related to the topog- raphy of a hillside and is a sequence of soil types down a slope. The catena (Figure 10.3) is described in more detail on page 276. Organisms (biota) Plants, micro-organisms such as bacteria and fungi, and animals all interact in the nutrient cycle (page 300). Plants take up mineral nutri- ents from the soil and return them to it after they die. This recycling of plant nutrients (Figure 12.7) is achieved by the activity of micro-organisms, which assist in nitrogen fixa- tion (page 268) and the decomposition and decay of dead vegetation. At the same time, macro-organisms, which include worms and ter- mites, mix and aerate the soil. Human activity is increasingly affecting soil development through the addition of fertiliser, the breaking up of hori- zons by ploughing, draining or irrigating land, and by unwittingly accelerating or deliberately controlling soil erosion. flatter upland (plateau): hill peat develops on colder, waterlogged, acidic soils shedding (degrading or eluviation) zone:sheds water (well drained), soil, organic and mineral matter Acatena: the relationship between soil type and slope (not drawn to scale) | 1 ' I 1 1 1 transfer (translocation) zone: rapid movement of water, soils soil infiltration m il Hebel eacibe and minerals giving a thinner, less acidic, drier soil f fe receiving (accumulation or i | aes surface illuviation) zone: receives i » runoff water, soil, organic and | ‘ mineral matter; valley peat il | bedrock develops on waterlogged throUBhflow or gleyed soil | ——— > movement of water | The‘open soll system @ All horizons need not always be present. iw The depth of soil and of each horizon vary at different sites. Local conditions produce soils with characteristic horizons differing from the basic A, B, C pattern: for examiple, a waterloggéd soil, having a shortage of oxygen, develops a gleyed (G) horizon (page 275). ae. The soil system Figure 10.6 is a model showing the soil as an open system where materials and energy are gained and lost at its boundaries. The system comprises inputs, stores, outputs and recycling or feedback loops (Eramework 3, page 45). Inputs include: ™ water from the atmosphere or throughflow from higher up the slope @ gascs from the atmosphere and the respiration of soil animals and plants | mincral nutrients from weathered parent mate- tial, which are needed as plant food § organic matter and nutrients from decaying plants and animals, and > @ solar energy and heat. Outputs include: = water lost to the atmosphere through evapotranspiration @ nutrients lost through leaching and through- flow, and '® loss of soil particles through soil creep and erosion. Recycling Plants, in order to live, take up nutrients from the soil (page 268). Some of the nutrients may be stored until: im either the vegetation sheds its leaves (during the autumn in Britain), or = the plants die and, over time, decompose due to the activity of micro-organisms (biota, page 268). inputs into the water soil system 4 outputs from the soil system ) recycling (air) leaching and throughflow gases solar energy These two processes release the stored nutrients, allowing them to be returned to the soil ready for future use — the so-called nutrient (or humus) cycle. Soil properties The four major components of soil — water, air, mineral and organic matter (Figure 10.4) — are all closely interlinked. The resultant interrelationships produce a series of ‘properties’, ten of which are ~ listed and described below. mineral (inorganic) matter ‘texture structure organic matter (including humus) moisture . air organisms (biota) nutrients acidity (pH value) 10 temperature. It is necessary to understarid the workings of these properties to appreciate how a particular soil can best be managed. CHONANERWNHH 1 Mineral (inorganic) matter As shown in Figure 10.2, soil minerals are obtained mainly by the weathering of parent rock. Weathering is the major process by which nutrients, essential for plant growth, are released. Primary minerals are minerals that were present in the original parent material and which remain unaltered from their original state. They are present throughout the soil-forming process, mainly because they are insoluble, e.g. quartz. Secondary minerals are produced by weathering reactions and are therefore produced within the soil. They include oxides and hydroxides of primary minerals (e.g. iron) which result from the exposure to air and water (page 40). nutrients taken up by plant roots soil creep and erosion SS SES arr 1000 analysis ! aries may vary ee 90ks) 90, 10 i A 4 d A 80, hs th (read in this ys: % silt am aa P adte nee 30 (read in this xy % clay fee os direction) 60, 40 ‘ m 12 aN 50 a 50 loam 5 EY 0 10 70 80 9 90 8 400 “CMs Ou 20) 10 ) {read in this direction) Yday silt. | sand a 65 18 We | b 35 59 6 (ie 27 7 56 gure 10.9 il texture analysis: euseofa angular graph The importance of texture ‘As texture controls the size and spacing of soil pores, it directly affects the soil water content, water flow and extent of aeration. Clay soils tend to hold more water and are less well drained and aerated than sandy soils (page 267). Texture also controls the availability and retention of nutrients within the soil. Nutrients stick to — i.e. are adsorbed onto - clay particles and are less easily leached by infiltration or throughflow than in sandy soils (page 268). Plant roots can penetrate coarser soils more easily than finer soils, and ‘lighter’ sandy soils are easier to plough for arable farming than ‘heavier’ clays. Texture greatly influences soil structure. How does texture affect farming? The following comments are generalised as it must be remembered that soils vary enormously. Sandy soils, being well drained and aerated, are easy to cultivate and permit crop roots (¢.g. carrots) to penetrate. However, they are vulner- able to drought, mainly because, due to their rel- atively large particle size (Figure 8.2a), they lack the micropores that would retain moisture (page 267) and partly because they usually contain limited amounts of organic matter. They also need considerable amounts of fertiliser because nutrients and organic matter are often leached out and not replaced. Silty soils also tend to lack mineral and organic nutrients. The smaller pore size means that more moisture is retained than in sands but heavy rain tends to ‘seal’ or cement the surface, increasing the risk of sheetwash and erosion. Clay soils tend to contain high levels of nutrient and organic matter but they are difficult to plough and, after heavy rain and due to their small particle size (Figure 8.2b) which. helps to retain water (page 267), are prone to waterlogging and may become gleyed (pages 272 and 275). Plant roots find difficulty in penetration. Clays expand when wet, shrink when dry and take the longest time to warm up. > The ideal soil for agriculture'is a loam (Figures 10.8 and 10.9). This has sufficient clay (20 per cent) to hold moisture and retain nutri- ents; sufficient sand (40 per cent) to prevent water- logging, to be well aerated arid to be light enough to work; and sufficient silt (40 per cent) to act as an adhesive, holding the sand and clay together. A loam is likely to be least susceptible to erosion. 3 Soil structure It is the aggregation of individual particles that gives the soil its structure. In undisturbed soils, these aggregates form different shapes known as peds. It is the shape and alignment of the peds which, combined with particle size/texture, determine the size and number of the pore spaces through which water, air, roots and soil organisms can pass. The size, shape, location and suggested agricultural value of each of the six ped types are given in Figure 10.10. It should be noted, however, that some soils may be structureless (¢.g- sands), some may have more than one ped structure (Figure 10.11), and most are likely to have a dis- tinctive ped in each horizon. It is accepted that soils with a good crumb structure give the highest agricultural yield, are more resistant to erosion and develop best under grasses — which is why fallow should be included in a farming crop rota- tion. Sandy soils have the weakest structures as they lack the clays, organic content and secretions of organisms needed to cause the individual par- ticles to aggregate. A crumb structure is ideal as it provides the optimum balance between air, water and nutrients. Soils 265 Size of structure (mm) crumb, gE5. granular 1-5 platy 1-10 blocky 10-75 prismatic 20-100 columnar 20-100 Figure 10.10 Different soil structures Figure 10.11 Differences in peds (after Courtney and Trudgill) 266 = Soils ac ey) a Sse mae) porous x =) non-porous aH ee topsoil c on topsoil 2 o Description of peds Shape of peds small individual particles'similar to breadcrumbs; porous. ~ i © small individual particles; usually non-porous vertical axis much shorter than =, horizontal, ike overlapping plates; rae restrict flow of water eS 7) irregular shape with horizontal and vertical axes about equal; may be rounded or angular but dpsely fitting vertical axis much larger than horizontal; angular caps and sides ~ tocolumns vertical axis much larger than horizontal; rounded caps and sides tocolumns 4 Organic matter Organic matter, which includes humus, is derived mainly from decaying plants and animals, or from the secretions of living organisms. Fallen leaves and decaying grasses and roots are the main source of organic matter. Soil organisms, such as bacteria and fungi, break down the organic matter and, depending on the nature of the soil-forming processes (Figure 10.17), help develop up to three distinct organic layers at the surface of the soil profile (Figure 10.5): mi Hives) 30cm. e A ote Location (horizon: texture) and formation Ahorizon: loam soil; formed by action of soil fauna (e.g. earthworms, mites and termites), high content of fibrous roots (grasses) and excretion of micro-organisms Ahorizon: clay soil; formation as for crumb structure Bhorizon: silts and clays; formed by contraction by tree roots, especially when trees (e.g. Scots pine) sway in wind. Also due toice ens, and compaction due to farm machinery Bhhorizon:clay-loam sols; formation Agricultural value the most productive; well aerated and drained — good forroots fairly productive; problems with drainage and aeration the least productive; hinders water and air movement; restricts roots productive; usually well associated with wetting—drying drained and aerated and freeze—thaw processes Band C horizons: often limestones usually quite productive: or clays; formation associated withwetting~ formed by wetting, and drying; drying and freeze-thaw processes adequate water movement and root development Band C horizons; alkaline soils; quite productive (if water formation associated with available) accumulation of sodium 1. Lor leaf litter layer: plant remains are still visible. F ot fermentation (decomposition) layer: decay, which biochemically involves yeast, is most rapid, although some plant remains are still visible. Hor humus layer: primarily organic in nature where, following decomposition, all recog- nisable plant and animal remains have been broken down into a black, slimy, amorphous organic material. Wherever soil biological activity is low (due to one or a combination of acidity, low tempera- tures, wetness or the difficulty in decomposing organic matter), soil organism activity is greatly reduced or absent. As the litter layer cannot be mixed into the soil, then organic horizons build up to give the distinct L, F and H layers of a mor. Where soil organisms are active, they will readily mix the litter into the soil, dispersing it throughout the A horizon where it decomposes into an A horizon rich in humus — the mull layet. Where organic material and mineral matter do mix, mainly due to earthworm activity, the result is the clay-humus complex (page 268). The clay-humus complex is essential for a fertile soil as it provides it with a high water- and nutrient holding capacity and, by binding particles together, helps reduce the risk of erosion. Availabilty foil mo hygroscopic water drought Humus gives the soil a black or dark-brown colour, The highest amounts‘are found ity the chernozems, or black‘earths (page 327), of thé North American Prairies, Russian Steppés and Argentinean Pampas. In tropical rainforests, heavy: rainfall and high biological activity cause the rapid decomposition of organic matter which releases nutrients ready for their uptake and storage by Plants (Figures 10.6 and 11.29c) or, if the forest is cleared, for leaching out of the system. In drier cli- mates there may be insufficient vegetation to give an adequate supply. 5 Soilmoisture Soil moisture is important because it affects the upward and downward movement of water and nutrients. It helps in the development of horizons; + it supplies water for living planté and organisms; it provides a solvent for plant nutrients; it influences soil temperature; and it determines the.incidence of erosion. The amount of water in a soil at a given time can be expressed as: WeR-E+T+D) (input) - (outputs) where: W= water in the soil = proportional to rainfall/precipitation transpiration evaporation drainage. Drainage depends on the balance between the water retention capacity (water storage in a soil) and the infiltration rate. This is controlled by porosity and permeability which in turn is con- trolled by the soil’s texture and structure. It has already been shown how texture and structure affect the size and distribution of pore spaces. Clays have numerous small pores (micropores) which can retain water for long periods, giving it a high water retention capacity, but which also restrict { wilting capillary water increasingly dry infiltration rates (page 59). Sands have fewer but much larger macropores which permit water to pass through more quickly (a rapid infiltration rate), but have a low water retention capacity. A loam provides a more balanced supply of water, in the micropores, and air, in the macropores. ‘The presence of moisture in the soil does not necessarily mean that it is available for plant use, Plants growing in clays may still suffer from water stress even though clay has a high water-holding capacity. Soil water can be classified according to the tension at which it is held. Following a heavy storm or a lengthy episode of rain or snowmelt, all the pore spaces may be filled, with the result that the soil becomes saturated. When infiltration ceases, water with a low surface tension drains away rapidly under gravity. This is called gravitational or free water which is available to plants when the soil is wet, but unavailable when water has drained away. Once this excess water has drained away, the remaining moisture that the soil can hold is said to be its field capacity (Figures 3.3 and 10.12). Moisture at field capacity is held either as hygroscopic water or as capillary water. Hygroscopic water is always present, unless the soil becomes completely dry, but is unavailable for plant use. It is found as a thin film around the soil particles to which it sticks due to the strength of its surface tension, Capillary water is attracted to, and forms a film around, the hygroscopic water, but has a lower cohesive strength. It is capillary water - that is freely available to plant roots. However, this water can be lost to the soil by evapotranspiration. When a plant loses more water through transpira- tion than it can take up through its roots it is said to suffer water stress and it begins to wilt. At wilting point, photosynthesis (page 295) is reduced but, provided water can be obtained relatively soon or if the plant is adapted to drought conditions, this need not be fatal. Figure 10.12 shows the different water-holding characteristics of soil. field capacity Gravitational water : saturated increasingly wet : \ i increases — eo ees» decreases capillary water available for plant roots; lost by evapotranspiration 1 \ hygroscopic water i Unavailable for plant ! \ \ plant roots; lost by gravity Toots; always present i 1 sis f | gravitational water unavailable for i ' at tions adsorbed on clay-humus irticle from weathering of parent ck and decay of organic matter 114 ss of change irtney and ale showing yand increasingly acid <= clay particle ) hydrogen ions released from vegetation As well as providing nutrients for plant roots, the cation exchange releases hydrogen whi¢h in turn increases acidity in the soil (see below). Acidity accelerates weathering of parent rock, releasing more minerals to replace those used by plants or lost through leaching. The cation exchange capacity (CEC) is a measure of the ability of a soil to retain cations for plant use. Soils with a low CEC, such as sands, are less able to keep essen- tial plant nutrients than those with a high CEC, like clays and humus; consequently they are less fertile. 9 Acidity (pH) As mentioned in the previous section, soil contains positively charged hydrogen cations. Acidity or alkalinity is a measure of the degree of concentration of these cations. It is measured on the pH scale (Figure 10.15), which is loga- rithmic (compare the Richter scale, Figure 1.3). This means that a reading of 6 is 10 times more acidic than a reading of 7 (which is neutral), and 100 times more acidic than one of 8 (which is alkaline). Most British soils are slightly acidic, neutral released hydrogen cations increase soil acidity and weathering of parent material ===) increasingly alkaline equal charge nutrients of cations absorbed by exchanged plant root although in upland Britain acidity increases as the heavier rainfall leaches out elements such as calcium faster than they can be replaced by weathering, Acid soils therefore tend to need con- stant liming if they are to be farmed successfully. A slightly acid soil is the optimum for farming in Britain as this helps to release secondary min- erals. However, if a soil becomes too acidic it releases iron and aluminium which, in excess, may become toxic and poisonous to plants and organisms. Increased acidity makes organic matter more soluble and therefore vulnerable to leaching; and it discourages living organisms, thus reducing the rate of breakdown of plant litter and so is a factor in the formation of peat. In ateas where there is a balance between pre- cipitation and evapotranspiration, soils are often neutral, as in the American Prairies (page 327); while in areas with a water deficiency, as in deserts (page 323), soils are more alkaline. 10 Soiltemperature Incoming radiation can be absorbed, reflected or scattered by the Earth’s surface (Figure 9.4). The topsoil, especially if vegeta- tion cover is limited, heats up more rapidly than the subsoil during ithmic) the daytime and loses heat more em 2 : BAe : Gigehe s Sa eatabed Wabealnlicr len tie) 2 tks rapidly at night. A ‘warm’, moist battery lemon vinegar 1 distilled, ammonia caustic soil will have greater biota activity, acid juice | water 1 soda | recorded acic ilibrit Feand Al released \ 1 clean rain i ith atmospheric CO,) 2.2 lowest giving a more rapid breakdown of organic matter; it will be more likely to contain nutrients because the chemical weathering of the parent material will be faster; and seeds will germinate more readily in it than in a ‘cold’, dry soil. Soils 269 Figure 10.17 Soil-forming processes 4 (i) Humification and (ii) cheluviation 3. Organic sorting/reorganisation processes () Eluviationand —- (li) Leaching illuviation (page 262) (v) Calcification _ (iv) Gleying ier ; [1], Weathering: parent rock * (i), , hydroylis 5 i (pages 42-43) fiv) reduction ay a Processes of soil formation Numerous processes are involved in the formation of soil and the creation of the profiles, structures and other features described above. Soil-forming processes depend on all the five factors described on pages 260-262. Some of the more important processes are shown in Figure 10.17. 1 Weathering As described on page 263 and in Figure 10.2, weathering leaves primary minerals as residues and produces secondary minerals as well as deter- mining the rates of release of nutrients and the soil depth, texture and drainage. In systems terms, this means that minerals are released as inputs into the soil system from the bedrock store and transferred into the soil store (Figure 10.6). 2 Humification and cheluviation Humification is the process by which organic matter is decomposed to form humus (page 266) — a task performed by soil organisms. Humification is most active either in the H horizon of the soil profile (Figure 10.5) where it can result in mull (pH 5.5 to 6.5), or in the upper A horizon where it can. produce mor (pH 3.5 to 4.5) (page 266). Moder (pH 4.5 to S.5) is transitional between the mor and mull (page 262). As organic matter decomposes, it releases nutrients and organic acids. These acids, known, as chelating agents, attack clays and other min- erals, mainly in the A horizon, releasing iron and aluminium. The chelating agents then combine (ii) Podsolisation (vi) Salinisation (ii) hydration (iii) oxidation “> % (v) solution é with the cations of the iron and aluminium to form organic-metal compounds known as chelates. Chelates are soluble and are readily transported downwards through the soil profile — the process of cheluviation. The iron and alu- minium may be deposited in the lower profile as they become less soluble in the slightly higher pH levels found there (Figure 10.5). 3 Organic sorting Several processes operate within the soil to re- Organise mineral and organic matter into horizons, and to contribute to the aggregation of particles and the formation of peds. 4 Translocation of soil materials Translocation is the movement of soil compo- nents in any form (solution, suspension, or by animals) or direction (downward, upward). Tt usually takes place in association with soil moisture. In Britain, there is: = usually a soil moisture budget surplus due to an annual excess of precipitation over evapotranspiration (water balance - Figure 3.3) = locally, an increase in soil moisture due to poor drainage. The increase in soil moisture, resulting from these two factors, can lead to: = either the translocation processes of leaching and podsolisation, or @ gleying associated with areas of poor drainage. 271 
