SOIL The Exciting World Beneath Our Feet. J. Kenneth Torrance Professor Emeritus Geography and Environmental Studies Carleton University Learning in Retirement, Jan 12 Feb 9, 2015
Outline 1 Intro; Minerals; Weathering 2 Soil description; Physical properties 3 Clays minerals; Chemical aspects 4 Soil fertility; Soil ecology 5 Soil profile development and Quick clay
Winklerprins and Weisenborn (2004)
Winklerprins and Weisenborn (2004)
Gelisol Histosol Spodosol Vertisol Alfisol Inceptisol Mollisol Aridisol Ultisol Oxisol
Soil means different things to different people Engineer: unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks, regardless of whether or not they contain an admixture of organic constituents. Geologist, biologists, agricultural sciencists, some geographers: that portion of Earth s surface that supports plant life. Pedologist, some geographers: the unconsolidated surface material in which chemical and physical changes have taken place, normally as a result of an agent or agents introduced from the surface Farmer, gardener: that portion of the Earth s surface in which plants grow and which they cultivate (generally the top 15 to 20 cm)
WHAT IS SOIL? (Ecological Soil Scientist perspective) Soil is the life-sustaining, biologically active, porous and structured medium at the Earth s surface, formed of inorganic and organic particles, air, water and living organisms. Benno P. Warkentin, 2007
Other Aspects Soils are dynamic, three-dimensional bodies Soils constitute a living ecosystem Soils are a dynamic part of the Earth s geomorphological cycle
Functions of soil Support the growth of higher plants Principal factor controlling the fate of water in the hydrological system Recycling (Nature s system) Organism habitat Building material and engineering medium
The plant perspective Essential elements Macronutrients: Elements required in large quantities From the air: carbon, hydrogen, oxygen From the soil: nitrogen, phosphorus, potassium, calcium, magnesium and sulfur Micronutrients: elements required in small quantities From the soil: iron, manganese, boron, molybdenum, copper, zinc, chlorine, cobalt and nickel Plants are 94 and 99.5 % carbon, hydrogen and oxygen obtained from the air, and 0.5-6 % material obtained from soil solids
Soil components Mineral matter Organic matter (living and dead) Water (soil solution) Air
Average Composition of the Earth s Crust Element Weight % Volume % Element Weight % O 44.60 >90 H 0.14 Si 27.72 P 0.12 Al 8.13 Mn 0.10 Fe 5.00 F 0.08 Ca 3.63 S 0.05 Na 2.83 Cl 0.04 K 2.59 C 0.03 Mg 2.09 The rest 0.41 Ti 0.44
Partial Periodic Table of the Elements I A II A III A IV A V A VI A VII A VIII A H He Li Be B C N O F Ne Na Mg III VI B VII B VIII Al Si P S Cl Ar K Ca --- Mn Fe
Minerals A mineral is a naturally occurring crystalline material with a definite chemical composition and definite physical and chemical properties. Primary Minerals: crystallized from molten rock during the original rockforming period Secondary minerals: formed by the alteration of pre-existing minerals, usually accompanied by the loss of some soluble elements
Common primary minerals in soil Quartz - SiO 2 (roughly equi-dimensional) Feldspars - KAlSi 3 O 8 (or Na and Ca variants) (roughly equi-dimensional) Mica - K 2 (Si 6 Al 2 )Al 4 O 20 (OH) 4 (sheet minerals) Amphiboles and Pyroxenes (chain minerals) (only in young soils) All of these minerals are silicates; the latter 3 are aluminosilicates
Secondary minerals Iron oxides and hydroxides [hematite, goethite, ferrihydrite (poorly crystalline)]: some present in most soils Aluminum hydroxide [gibbsite/bauxite]: generally restricted to soils of humid tropical regions Carbonates [calcite and dolomite]: common in soils of arid and semi-arid regions, and in soils for which limestone is the parent rock Clay minerals [extremely important - more later]
Physical Weathering Decreases the size of the mineral particles but has no effect on the mineralogy Agents: Rapid temperature change Freeze-thaw cycles Wet-dry cycles Abrasion in running water and by wind-borne particle Glacial grinding (very effective)
Chemical weathering Chemical weathering changes the mineralogy of the soil Agents Solution Oxidation/reduction Hydration/dehydration Hydrolysis Acid weathering processes Biological agents Vulnerability of aluminosilicates to chemical weathering increases with a) increasing number of elements in the mineral, b) increasing symmetry if composition is the same, c) if iron is present in the crystal structure
General sequence of soil development 1. Fragmentation 2. Chemical weathering 3. Organic matter accumulation 4. Fe-oxide coatings on sand particles 5. Clay mineral formation 6. Downward migration of very fine clay 7. Leaching of basic elements with accompanying acidification 8. Development of strong acidity 9. Breakdown of clay minerals Mineral resistance to chemical weathering (increasing order) Mineral Gypsum secondary Calcite secondary Amphibole/pyroxene primary Micas primary Feldspars primary Quartz (clay sized) primary Illite secondary clay Montmorillonite secondary clay Kaolinite secondary clay Gibbsite secondary oxide Hematite secondary oxide Anatase (TiO2) primary
SOIL The Exciting World Under Our Feet. Physical properties of soil Texture Structure Particle density Bulk density Pore space Water relations Plasticity Soil tilth is the term used in publications aimed at the general population to refer to the physical condition of the soil.
Soil texture The relative proportions, by weight, of the particle size fractions (sand, silt and clay) that are < 2mm in equivalent spherical diameter. Also called particle size distribution. Soil texture is determined by sedimentation from suspension. The rate of sedimentation of particles is related to their density, diameter and shape. The diameters are calculated from falling times, assuming that the particles are spheres hence, equivalent spherical diameters The size fractions are: Sand from 2 to 0.05 mm in diameter individual particles can be seen with the naked eye and felt by the fingers Silt from 0.05 to 0.002 mm in diameter not visible by naked eye, but visible under the light microscope. Not felt individually by fingers, but will scratch fingernails and grit between the teeth. Clay - < 0.002 mm (2 microns) visible with the electron microscope If I was allowed to know only one physical property of a soil, I would want to know its texture.
Soil texture triangle Twelve soil textural classes are recognized, based on the percentages of sand, silt and clay present (see figure). Sands: particles act individually (do not stick together). Soils dominated by them: have large pores, low water holding capacity, drain rapidly, and have low fertility. Silts: may form aggregates. Soils dominated by them: have medium size pores, high water holding capacity, drain at moderate rate and have moderate fertility. Clays: have a high surface area per unit weight. Soils dominated by them: form aggregates, have small pores, high water holding capacity. If dominated by phyllosilicate clay minerals, they drain slowly and are sticky, plastic, and tend to be fertile; if dominated by oxide clay minerals, they drain more rapidly and tend to be non-sticky, non-plastic and of low fertility. Soil texture is not easily altered; it can change over long periods in intense chemical weathering environments.
Soil structure Soil structure refers to the grouping of soil particles into aggregates ( peds to the soil classification people). It refers to the spatial organization of the soil particles, and controls the size distribution of pores and the nature of the pore spaces. Types of soil structure A) Structureless single grain: particles act individually - massive: soil act as one irregular mass B) Spheroidal rounded granular aggregates < 10 mm diameter (also known as crumb structure) C) Blocky angular: sharp edges and corners - subangular: rounded edges and corners D) Prism-like columnar: rounded tops -- prismatic: flat tops E) Platy - have horizontal structures (rare) Soil texture quite strongly influences the soil structure, but not vice-versa.
Factors influenced by soil structure moisture holding capacity movement of air and water into and through soils plant availability of soil water root penetration soil microorganism activity heat transfer ease of cultivation seed germination and more
Management of soil structure Soil structure, the spatial arrangement of the soil particles, is changing on an almost continuous basis. Changes for the worse are very easy to accomplish, whereas changes for the better require effort and diligence. For example, compaction is easy to cause and makes a soil more dense, thereby decreasing the amount of pore space and the size of individual pore spaces. The soil becomes harder for roots, water and air to penetrate, harder to cultivate, and less productive. Reducing the bulk density, and retaining the lower bulk density, requires good management and persistence.
Factors affecting soil structure Climate related: wetting and drying freezing and thawing Biological activity: plant roots microorganism excretions soil animals insects, worms, etc. organic residues Management: cultivation (not too much, or when soil is too wet) crop rotation liming fertilizers (when necessary for fertility reasons)
Other physical parameters Particle density: Silicate dominated soils have particle densities between 2.65 and 2.70 gcm -3. The silicate and carbonate minerals all have densities in or close to this range. Particle density does not change, except with major production of iron oxides by extreme weathering. Bulk density: The weight per unit volume of the whole soil, including the pore space. Normal ranges: sand dominated = 1.2-1.8 gcm -3 ; clays and silts = 1.0-1.5 gcm -3. Bulk density is constantly changing. Porosity: The percent of the soil volume that is pore space. Subject to almost constant change. Macropores: Drain under the force of gravity. Micropores: Retain water against the force of gravity. Water content: normally presented on a dry weight basis the weight of water as a percentage of the dry soil weight. Percent saturation: The percent of the pore space that is occupied by water.