1. Soil Systems Chapter 19

2. The Nature of Soil Soil, as the term is used in soil science (or pedology), is the uppermost layer of the land surface that plants use and depend on for nutrients, water, and physical support. In the Canadian system of soil classification, soil is defined as the naturally occurring, unconsolidated mineral or organic material (at least 10 cm thick) that occurs at the Earth’s surface and is capable of supporting plant growth.

3. The Nature of Soil In addition to mineral matter, soils also contain varying amounts of organic matter - biological origin and may be living or dead (living matter in the soil consists of plant roots and also many kinds of organisms, including microorganisms). Dead and partially decomposed organic matter is an important constituent of soils, and in most cases accounts for about 10 percent of the dry weight - finest organic debris forms humus and is important for soil fertility.

4. The Nature of Soil As well as solid materials, soils also contain gaseous and liquid components. The soil atmosphere consists of oxygen and other gases that diffuse from the air, together with gases, such as CO 2 and methane, that are derived from respiration and decomposition processes. Soil water contains dissolved substances, including plant nutrients, dissolved in solution.

5. Factors in Soil Formation Soils are a function of five main factors: parent material, topography, climate, biological factors, and time. The bulk of the material present is derived from disintegration of the underlying rock through a combination of physical and chemical weathering processes. Over time, weathering weakens, disintegrates, and breaks bedrock apart, forming a layer of regolith, or residual mineral matter (also transported materials, such as glacial tills, river alluvium, and wind transported loess may be deposited in sufficient thickness to act as parent material.

9. Soil Properties Soil Colour Soils that are black or dark brown in colour are invariably rich in humus; for this reason, the surface layers tend to be darker than the underlying subsoils. The colours of the subsoil are mainly determined by the presence and state of iron oxides. Red or yellow colours are indicative of good drainage and aeration, while greenishblue colours can indicate very moist, anaerobic conditions.

10. Soil Properties Soil Colour Munsell soil colour charts are used to standardize the assessment of soil colour. The charts are based on three components—hue (a specific colour), value (lightness and darkness), and chroma (colour intensity)—that are arranged in sets of colour chips.

12. Soil Properties Soil Texture Particles larger than 2 mm in diameter are generally considered inert, and only those that are less than 2 mm are used to determine soil texture. These smaller particles, collectively known as the fine earths, are separated into three classes according to size. Sand particles range from 2 to 0.05 mm in diameter, silt particles from 0.05 to 0.002 mm in diameter, and clay particles, which are less than 0.002 mm in diameter. Soil particles smaller than 0.001 mm diameter are called colloids.

14. Soil Properties Soil Texture Soil texture is based on the relative proportions of sand, silt, and clay present; each grade class is named according to the dominant particle sizes. For example, a silty soil is composed mainly of fine sand and silt; sandy soils contain mostly coarse sand. A loam soil contains from 28 to 50 percent silt, 7 to 27 percent clay, and less than 52 percent sand - various classes of loams are distinguished depending on which size fractions are dominant.

17. Soil Properties Soil Structure Soil structure refers to the way in which soil grains are grouped together into larger masses, called peds. These aggregates form as a result of cohesion between clay particles and between clays and larger particles. Organic materials and soil colloids also help bind particles together. The ease with which soil aggregates can be broken down is termed soil consistence.

18. Soil Properties Soil Structure A measure of pore space created by the textural and structural characteristic of the soil can be obtained by measuring the bulk density of the soil - represents the mass of dry soil per unit volume (g cm -3 ). This is distinguished from soil particle density that represents the mass of dry soil compacted to remove the voids, which eliminates the effects of texture and structure.

20. Soil Properties Soil Minerals Most soils contain both primary (compounds present in the parent rock) and secondary (formed through chemical alteration of primary minerals) minerals. Primary minerals include quartz and silicate minerals, such as feldspars and micas, which contain varying proportions of aluminum, calcium, sodium, iron, and magnesium. When primary minerals are exposed to air and water at or near the Earth’s surface, their chemical composition - mineral alteration process resulting in chemical weathering. Intensely weathered soils, such as those in the humid tropics, are almost entirely composed of resistant secondary minerals, especially oxides of iron (haematite) and aluminum (gibbsite) and kaolinite clay.

21. Soil Properties Soil Minerals The most important secondary minerals are the clay minerals. They form the majority of fine mineral particles in soils, and are essential to soil development and fertility because of their ability to hold plant nutrients. Several different types of clay minerals are found in soils, each with their own distinctive physical and chemical properties.

24. Soil Properties Soil Colloids Soil colloids consist of particles smaller than 0.001 mm. Inorganic colloids, which mainly include fine particles of clay and hydrous oxides, typically make up the majority of colloids in a soil. Organic colloids include soil humus, an amorphous substance derived from organic material that is resistant to decay.

25. Soil Properties Soil Colloids Soil colloids attract nutrients that are dissolved in the soil water. Colloid surfaces tend to be negatively charged because of their molecular structure, and thus attract and hold positively charged cations derived from the dissociation of chemical compounds. One important group of cations is the bases, which include plant nutrients such as calcium (Ca 2+ ), magnesium (Mg 2+ ), and potassium (K + ) - colloids retain these ions in the soil and also release them to plants.

26. Soil Properties Soil Acidity and Alkalinity In humid regions, the charges on the colloids are mainly dominated by Ca 2+, H +, and often Al 3+, resulting in acidic soils. As the soil becomes more acid, H + and Al 3+ become predominant. The cations Mg 2+, K +, and Na + are usually found in lesser amounts, while NH 4 + may be present in considerable quantities if the soil has been recently fertilized with ammonium fertilizers.

27. Soil Properties Soil Acidity and Alkalinity In semi-arid and arid regions, Ca 2+ usually dominates the cations, but Mg 2+ and Na + are often found in large quantities. H + and Al 3+ are usually present only in small concentrations.

28. Soil Properties Soil Acidity and Alkalinity The presence of H +, and to a lesser degree Al 3+, in the soils of humid regions makes the soil solution acidic. This can affect soil fertility and plant growth by altering the availability of the essential plant nutrients. The displacement of nutrient bases from the soil colloids in turn increases soil acidity (acidity or alkalinity of a soil is designated by its pH). Soil pH is important from an agricultural perspective because it not only affects the availability of plant nutrients, but also influences nutrient toxicity and microorganism activity.

32. Soil Properties Soil Moisture Precipitation that infiltrates and moistens the soil results in soil water recharge. Once the maximum capacity of a soil to hold water is reached, any additional water present in the pore spaces drains into the underlying substrate. When a soil has been saturated by water and then drains freely under gravity until no more water moves downward, the soil is said to be at field capacity (depends largely on the soil’s texture).

33. Soil Properties Soil Moisture Permanent wilting point is an agricultural term that approximates the water storage level below which plants can experience moisture stress. The wilting point also depends on soil texture, because fine particles hold water more tightly, making it difficult for plants to extract. The difference between the field capacity of a soil and its wilting point is the maximum available water capacity; this is greatest in loamy soils.

36. The Soil Water Balance The amount of water available at any given time is determined by the soil water balance, which includes the gain, loss, and storage of soil water. Water held in storage in the soil water zone is increased by recharge during precipitation, but decreased by use through evapotranspiration. Surplus water is disposed of by downward percolation to the groundwater zone or by overland flow.

37. The Soil Water Balance The rate at which water vapour returns to the atmosphere from the ground and its plant cover is called actual evapotranspiration (Ea – water use). Potential evapotranspiration (Ep – water need) represents the water vapour loss under ideal conditions. The difference between water use and water need is the soil water shortage, or deficit.

39. The Soil Water Balance A Simple Soil Water Budget The soil water budget consists of the amounts of water needed to satisfy each process in the soil water balance. All terms of the soil water budget are stated in centimetres of water depth: Precipitation, P Water need, Ep Water use, Ea Storage withdrawal, −G Storage recharge, +G Soil water shortage, D Water surplus, R

40. The Soil Water Balance A Simple Soil Water Budget Storage withdrawal (−G) is the difference between the water-use and precipitation. As storage withdrawal continues, it becomes increasingly difficult for plants to obtain soil water, and water need (Ep) eventually exceeds water use (Ea).

42. Soil Development Soil Horizons Most soils possess soil horizons — distinctive layers that differ in physical and chemical composition, organic content, or structure. Soil horizons usually develop through selective removal or accumulation of ions, colloids, and chemical compounds by water moving through the soil. A soil profile is the full set of horizons exposed in a soil pit that is excavated down to the parent material. A soil column exhibiting these same properties is referred to as a pedon; it too extends from the surface to a lower limit in regolith or bedrock.

45. Soil Development Soil Horizons The mineral horizons below the organic layers at the surface are distinguished as the A, B, and C horizons. The A horizon is the uppermost mineral horizon (usually rich in organic matter, consisting of numerous plant roots and humus that have washed down from the overlying organic horizons. In the lower parts clay particles and oxides of aluminum and iron, as well as plant nutrients, are generally removed by percolating water and leave behind grains of sand or coarse silt.

46. Soil Development Soil Horizons The B horizon receives the clay particles, aluminum and iron sequioxides, as well as any organic matter that has washed down from the A horizon. The filling of natural spaces with clays and sesquioxides makes the B horizon dense and hard. Beneath the B horizon is the C horizon, which consists of the parent mineral matter of the soil — the weathered regolith. Below the regolith lies unaltered bedrock or accumulated sediments.

47. Soil Development Soil Forming Processes Soils develop in response to four types of soil-forming processes. Soil enrichment processes add material to the soil (humification). The second class of soil-forming processes consists of those that transport material out of the soil column – removal occurs when surface erosion carries sediment away from the soil’s uppermost layer. Another important process is leaching, in which percolating water dissolves soil materials and moves them below the soil profile or into the groundwater - moist climates - water movement leaches calcium carbonate (CaCO 3 ) from the entire soil profile in a process called decalcification.

48. Soil Development Soil Forming Processes The third class of soil-forming processes involves translocation, in which materials are moved within the soil body, usually from one horizon to another (calcification, salinization). Two translocation processes that operate simultaneously are eluviation and illuviation. Eluviation consists of the downward transport of fine particles, particularly clays and colloids, from the uppermost part of the soil. Illuviation is the accumulation of materials that are brought downward, normally from the Ae horizon to the B horizon - materials that accumulate may be clay particles, humus, or iron and aluminum sesquioxides.

49. Soil Development Global Scope of Soils Soil classification systems group soils according to their inherent properties. One such global classification system, called the World Reference Base for Soil Resources (WRB), was developed by the United Nations Food and Agriculture Organization (FAO) with the support of the UN Environment Programme (UNEP) and the International Society of Soil Science.

50. Soil Development Global Scope of Soils Soil classification systems typically use the soil order as the highest-level grouping. In the Canadian system, the next subdivision is the great group, while the American system uses the term suborder. Soil orders and great groups are often distinguished by the presence of a diagnostic horizon. Each diagnostic horizon has some unique combination of physical properties (such as colour, structure, and texture) or chemical properties (for example, an abundance of calcium). Two basic types of diagnostic horizons are used. A diagnostic horizon that forms at the surface is called an epipedon; one that forms by processes occurring at various depths in the soil is simply referred to as a subsurface horizon.

51. Soil Development Soils of Canada The Canadian system emphasizes young soils of cold regions in more detail than do other systems. the Canadian System of Soil Classification (CSSC) uses a system of classes based on the properties of the soils themselves, rather than interpretations of various uses of the soils. Classes are based on generalized properties of real, not idealized, soils. soils grouped under a single soil order are considered the product of a similar set of dominant soil-forming processes resulting from broadly similar climatic conditions. The Canadian System of Soil Classification includes 10 soil orders: hernozemic, Brunisolic, Luvisolic, Podzolic, Cryosolic, Gleysolic, Organic, Solonetzic, Regosolic, and Vertisolic.

54. Soil Development Soils of Canada Chernozemic soils form in areas that have cold winters, hot summers, and low precipitation that quickly evaporates in the summer heat - range in colour from black to various shades of brown and grey, depending on how much organic matter has been incorporated into the profile (deepest and darkest soils have developed under tallgrass prairie in more easterly regions of the Prairies). Brunisolic soils typically lack the degree of horizon development found in many of the other soil orders – generally associated with coniferous or deciduous forests and occur in a wide range of climates - especially in cooler, drier regions where mean annual temperatures are near 0 °C and precipitation is less than 700 mm.

55. Soil Development Soils of Canada Luvisolic soils are characteristic of forested regions, underlain by loamy tills derived from glacially eroded sedimentary parent materials - rich in calcium and magnesium or fine lacustrine sediments – well - to imperfectly drained soils (mostly occur in the central to northern interior plains under deciduous, mixed, and coniferous forest). Podzolic soils typically form under coniferous forests in cool, moist regions where abundant precipitation causes pronounced leaching - well-developed where parent materials, such as granite or sandstone, are rich in quartz - also found on sandy glacio-fluvial deposits.

56. Soil Development Soils of Canada Cryosolic soils occupy much of the northern third of Canada, where permafrost remains close to the surface of both mineral and organic deposits - predominate in arctic tundra north of the tree line, but are also common in the open subarctic forests and extend into the boreal forest, especially in some organic materials - occasionally found in alpine areas of mountainous regions. Gleysolic soils have features indicating periodic or prolonged saturation, and are usually associated with either a high groundwater table for part of the year, or temporary saturation above a relatively impermeable layer - particularly abundant in the low-lying river basins, but commonly occur in patches among other soils in the landscape.

57. Soil Development Soils of Canada Organic soils are often associated with poorly drained depressions and so are saturated with water for prolonged periods - can also form under cool, wet climatic conditions, which in combination with high acidity and nutrient deficiency, restrict microbiological decomposition. Most Solonetzic soils are associated with a vegetation cover of grasses and forbs - primarily associated with glacial tills derived from sedimentary rocks – distinguished by their high sodium content

59. Soil Development Soils of Canada Regosolic soils develop on unconsolidated parent materials, such as dune sands and alluvium. They have weakly developed profiles - B horizons are absent - due to the youthfulness of the parent material, as is the case for recently deposited alluvium, or instability, as occurs in colluvium on slopes subject to mass wasting. Soils of the Vertisolic order occur in parent materials rich in montmorillonite clays that expand greatly when wet and then shrink excessively when dry - Shrinking and swelling is strong enough that horizons characteristic of other soil orders have either been prevented from forming or have been severely disrupted.

62. Soil Development Soils of the World The soils of the world present a much broader range of soil types and conditions than those found in Canada. At the highest level, the CSCS system recognizes three groups of soil orders. The largest group includes seven orders with well-developed horizons or fully weathered minerals. A second group includes a single soil order that is rich in organic matter. The last group includes three soil orders with poorly developed or no horizons.

64. Soil Development Soils of the World Oxisols develop on stable land areas in equatorial, tropical, and subtropical regions with large water surpluses – principally associated with rainforests and occur throughout the wet equatorial climate zone in Africa, South America, and Asia. Ultisols are similar to the Oxisols in appearance and environment of origin - reddish to yellowish in colour have a subsurface horizon of illuviated clay, which is not found in the oxisols.

65. Soil Development Soils of the World Vertisols are typically black in colour and have a high clay content - predominant clay mineral is montmorillonite, which shrinks and swells with seasonal changes in soil water content - wide, deep vertical cracks develop in the soil during the dry season. Alfisols are characterized by a pale-coloured eluviated A horizon that is low in bases, clay minerals, and sesquioxides - these materials are concentrated by illuviation in the B horizon where the clay holds bases, such as calcium and magnesium (base status of the Alfisols, therefore, is generally quite high).

67. Soil Development Soils of the World Spodosols are formed in the cold boreal climate beneath coniferous forest - distinguished by their acidic, reddish-orange B horizons - called the spodic horizon and is composed of organo- aluminum and iron compounds brought downward by eluviation (intensive leaching produces a conspicuous, ash-grey sandy horizon in the upper part of the soil profile, with a dark organic layer at the soil surface). Histosols have a high organic matter content in a thick, dark upper layer (peat or muck) - formed in shallow lakes and ponds by accumulation of partially decayed plant matter.

68. Soil Development Soils of the World Entisols are mineral soils that lack distinct horizons - soils in the sense that they support plants, and may be found in any climate under various types of vegetation. Inceptisols are soils with horizons that are weakly developed, usually because the soil is relatively young – occur quite extensively in the same regions as Ultisols and Oxisols.

69. Soil Development Soils of the World Andisols are soils in which more than half of the parent mineral matter is volcanic ash - fertile soils and, in moist climates which support a dense natural vegetation cover. Mollisols are soils associated with the semi-arid and subhumid midlatitude grasslands - have a thick, dark brown to black surface horizon with a loose, granular structure. Calcium is abundant in the A and B horizons, which together with other plant nutrients gives these soils a high base status - some of the most naturally fertile soils and now produce most of the world’s commercial grain crop.

70. Soil Development Soils of the World Aridisols, the soils of desert regions, are dry for long time periods - since the climate supports only very sparse vegetation, humus is lacking and the soil colour ranges from pale grey to pale red. Soil horizons are weakly developed, but there may be important subsurface horizons of accumulated calcium carbonate and other soluble salts, especially sodium.

71. Soil Development Soils of the World Tundra soils are formed mainly of primary minerals, ranging in size from silt to clay, that have been broken down by frost and glacial action - layers of peat are often present between mineral layers - Beneath the tundra soil is perennially frozen ground (permafrost).

72. A Look Ahead Soil horizons and properties can also vary markedly over short distances. This not only affects crop production, but is an important factor controlling natural vegetation patterns and ecosystem processes. This is discussed in the three chapters that follow.