Chapter 4 Soil Properties PDF

SOILS210 Introduction to soils CHA P T ER 4 – S O I L A RCHI T EC TUR E AND P HYSI CAL P RO P ERTIES...

SOILS210 Introduction to soils CHA P T ER 4 – S O I L A RCHI T EC TUR E AND P HYSI CAL P RO P ERTIES Zubair Ahmed, PhD Keyano College Photo by Marie-France Jones.  Soil Colour Lecture Outline Soil Texture Soil Structure Please read Chapter 3 in your textbook. Elements of the Nature Soil Density and Properties of Soil. Brady and Weil. Soil Porosity Soil Engineering Properties 2 Soil Physical Properties Provides visual cues to what's going on in the soil. Important because: ▪ Influence soil function in ecosystem; ▪ Growth of plant species; ▪ Movement of water and dissolved nutrients. Also used to help in soil Photo by Marie-France Jones. classification (think Bf) and land use. 3  Soil Colour Three major factors influence the soil color: ▪ Organic matter content ▪ Water content ▪ Metal (Fe, Mn) oxides Munsell color charts are used to determine the soil color as a normalized standard. 4 Soil Colour Mussel Soil Colour Book Hue: The actual color (redness/yellowness) Value: light/dark (0 = black) Chroma: color intensity/brightness (0 = neutral gray) How would you name the sample? Chroma 10YR 4 /4 Hue Value 5  Oxidized iron Reduced iron (Weil and Brady, 2019) Soil Texture Description Size Range (cm) Coarse Boulder > 60 Fragments Stones 25 - 60 Cobbles 8 - 25 Gravel 0.2 - 8 Soil Separates Sand Very Coarse 2.0 - 1.0 Coarse 1.0 - 0.5 Medium 0.5 - 0.25 Fine 0.25 - 0.10 Very Fine 0.10 - 0.05 Silt 0.05 - 0.002 Clay < 0.002 Fine < 0.0002 7  Soil Texture Sand ▪ 0.05 - 2 mm particle size ▪ Gritty feeling between fingers ▪ Visible to naked eye ▪ Dominated by quartz therefore low nutrients ▪ Rapid drainage, promotes aeration but lacks ability to hold water ▪ Low surface area 8  Soil Texture Silt ▪ 0.002 - 0.04 mm particle size ▪ Not visible to naked eye ▪ Smooth and silky, flour like ▪ Good supply of nutrients due to faster weathering ▪ Retain water and has a lower porosity ▪ Susceptible to erosion by water and wind 9 Soil Texture Clay ▪ Smaller than 0.002 mm in size ▪ Extremely large surface area and can therefore adhere to more water and nutrients ▪ Sticky feel when wet ▪ Very low movement of air and water through a clay soil ▪ Shrink and swell properties 10 Soil Texture Surface Area ▪ When particle size decreases, specific surface area (cm2 g-1) and related properties increase greatly. ▪ Fine colloidal clay has about 10,000 times as much surface area as the same weight of medium-sized sand. ▪ Soil texture influences many other soil properties as a result of such fundamental surface phenomena as: → Sorption of water films, chemicals, and nutrients; → Weathering of minerals → Growth of microbes. 11 Soil Texture Surface Area (Weil and Brady, 2019) 12 (Weil and Brady, 2019) 13  Soil Texture Triangle Sand (%) + Silt (%) + Clay (%) = 100 (%) 45% What is the texture: 25% Sand - 15 Sand - 35 Sand - 60 Silt - 75 Silt - 35 Silt - 10 Clay - 10 Clay - 30 Clay - 30 30% 14 Soil Texture Triangle → Over long periods of time, pedologic processes such as illuviation, erosion, and mineral weathering can alter the textures of certain soil horizons. → However, management practices generally do not alter the textural class of a soil on a field scale. → Changing the texture of a given soil would require mixing it with another soil material of a different textural class. 15 Soil Texture Feel Test (Weil and Brady, 2019) A moist soil sample is rubbed between the thumb and forefingers and squeezed out to make a “ribbon”, (a) the gritty, no cohesive appearance and short ribbon of a sandy loam Ribbon Characteristic with about 15% clay, (b) the Dominant No Short Ribbon Medium Long Ribbon smooth, dull appearance and Wet Feel Ribbon (< 2.5 cm) Ribbon (> 5 cm) Smooth Silt Silt Loam Silty Clay Silty Clay crumbly ribbon characteristic of Loam a silt loam, (c) the smooth, Neither Loam Clay Loam Clay Sandy clay shiny appearance and long, Gritty Sand Sandy Loam Sandy Clay loam flexible ribbon of a clay. 16 Soil Texture Particle Determination ▪ Separation into size groups can be accomplished by shaking the particles through standard sieves that are graded in size to separate the coarse fragments and sand separates, permitting the silt and clay fractions to pass through. ▪ Silt and clay are usually determined using the sedimentation method due to their binding qualities. Generally makes it difficult to properly measure with sieves. 17 Soil Texture Sedimentation A sedimentation procedure is usually used to determine the amounts of silt and clay. The principle involved: Because soil particles are more dense than water, they tend to sink, settling at a velocity that is proportional to their size. → In other words, the bigger they are, the faster they fall. 18  Soil Texture Sedimentation The equation that describes this relationship is referred to as Stokes’ law: 2 (𝐷𝑝 − 𝐷𝑤 ) 2 𝑉= 𝑔 𝑟 9 𝑛 V= velocity of the fall (cm/sec) g = acceleration of gravity ( cm/sec2) Dp = particle density (g/cm3) Dw = density of the fluid (g/cm3) n = viscosity of the fluid r = radius of the sphere (cm) (Weil and Brady, 2019) 19 Recap! What are the three defining characteristics of soil colour? - Hue - Value - Chroma Which soil texture has a higher surface area? 20 Soil Structure ▪ The term soil structure relates to the arrangement of sand, silt, clay, and organic particles in soils. ▪ The particles become aggregated together due to various forces and at different scales to form distinct structural units called peds or aggregates. ▪ When a mass of soil is excavated and gently broken (Weil and Brady, 2019) apart, it tends to break into peds along natural zones of weakness. 22 Soil Structure ▪ Many types or shapes of peds occur in soils, often within different horizons of a particular soil profile. → Some soils may exhibit a single-grained structural condition in which particles are not aggregated. ▪ At the opposite extreme, some soils (such as certain clay sediments) occur as large, cohesive masses of material and are described as exhibiting a massive structural condition. 23 Soil Structure Grade ▪ Weak ▪ Moderate ▪ Strong Kind / Shape ▪ Granular ▪ Angular Blocky ▪ Subangular Blocky ▪ Prismatic ▪ Platy Size ▪ Fine ▪ Medium ▪ Coarse 24 Soil Structure Spheroidal Granular Crumb Plate-like Platy Block-like Angular Block Subangular Block Prism-like Columnar Prismatic (Weil and Brady, 2019) 25 Soil Structure 26 Soil Structure Aggregates Processes Influencing Aggregate Formation and Stability in Soils: → Flocculation of Clays and the Role of Adsorbed Cations → Volume Changes in Clayey Materials → Activities of Soil Organisms → Influence of Organic Matter → Influence of Tillage → Influence of Iron/Aluminum Oxides 27 Soil Structure Aggregates Levels of aggregation (Weil and Brady, 2019) 28 Soil Structure Aggregates Process of clumping together. Flocculation of Clays and the Role of Adsorbed Cations: ▪ Di- and trivalent cations (Fe3+ Ca2+) create strong bonds between clay sheets. ▪ Monovalent cation (Na+) can cause clay to repel. (Weil and Brady, 2019) 29 Soil Structure Aggregates Volume Changes in Clayey Materials: ▪ Swelling and shrinking can cause fissures which can then increase over time and create zones of weakness. ▪ Can break off pieces and created defined peds. 30 Soil Structure Aggregates Activities of Organism ▪ Burrowing animals (worms, termites) ▪ Roots and fungal Hyphae (mycorrhizae) ▪ Organic glues (bacteria and fungi) 31 Soil Structure Aggregates Influence of Organic Matter ▪ much more stable with high OM ▪ low-organic-matter soil aggregates fall apart when they are wetted (Weil and Brady, 2019) 32 Soil Structure Aggregates Influence of Organic Matter (Courtesy USDA Natural Resources Conservation Service) 33 Soil Structure Aggregates Influence of Tillage ▪ Breaks clods into aggregates ▪ Incorporates OM from surface ▪ Tilled wet can crush/smear the soil aggregates 34 Soil Structure Aggregates Influence of Iron/Aluminum Oxides ▪ Acts as cementation agents, which helps soil from breaking down. ▪ Tropic soils have very large amount of Fe in soils due to high moisture. https://www.britannica.com/science/Ferralsol 35 Recap! What is flocculation? What soil properties influence it? What is the role of organic matter in soil aggregation? 36 Soil Density Soil particle density, Dp, is defined as the mass per unit volume of soil solids (in contrast to the volume of the soil, which would also include spaces between particles). Particle density is not affected by pore space => not related to particle size or to the arrangement of particles (soil structure). Bulk density, Db, is defined as the mass of a unit volume of dry soil (this volume includes both solids and pores). 37 Soil Density Particle Density = Soil weight (g) / volume of solid (cm3) Volume = 4 x (2 cm x 1 cm x 1 cm) = 8 cm3 No Pores Weight = 21.6 g 1 cm Density = 21.6 g / 8 cm3 = 2.7 g/cm3 1 cm 2 cm 38  Soil Density Bulk Density = Soil weight (g) / Total volume (cm3) Volume = πr2h =3.14 x (1.5)2 x 2.5 = 17.7 cm3 3 cm Weight = 21.6 g Density = 21.6 g / 17.7 cm3 = 1.22 g/cm3 2.5 cm Column of soil, consisting of soil and pores. Same soil, different volume. 39 Soil Density Calculating bulk density, Db for a field sample: Volume = 1 m3 (solids + pores) Weight = 1.25 tonnes Density = Weight of oven dried soil 1.25 Volume of soil tonnes (solids + pores) = 1.25 t / 1 m3 or 1.25 g/cm3 How many g / t? How many cm3 / m3? 1,000,000 40 Soil Density Calculating particle density, Dp for a field sample: Volume = 0.5 m3 (solids only) Weight = 1.25 tonnes (solids only) Pores Density = Weight of oven dried soil Volume of Solids 1.25 tonnes = 1.25 t / 0.5 m3 = 2.5 g/cm3 Assuming 50% pore space 41 Soil Density Increases in bulk density usually indicate a poorer environment for root growth, reduced aeration, reduced water infiltration. Factors affecting soil density: → Soil texture → Soil profile depth 42 Soil Density Texture (Weil and Brady, 2019) 43 Soil Density Texture (Weil and Brady, 2019) 44 Soil Density Depth Singh et al. (2005) 45 Soil Density Management that affects bulk density: → Trafficability → Urban Soils → Agriculture How can we mitigate the effects of each on soil density? 46 Soil Density Strength Root growth depends on the soil density. They grow by pushing into pores. Soil strength can cause issue for plant growth. It’s the ability of the soil to resist penetration. Increase bulk density. → Strength decreases with moisture. Why? 47 Soil Porosity 48 Soil Porosity Calculating for soil porosity: Solve for Ws Db = Ws / Vs + Vp Ws = Db (Vs + Vp) Db = bulk density (g/cm3) Dp = particle density (g/cm3) Dp = Ws / Vs Ws = Dp x Vs Ws = weight of soil (t) Vs = volume of solids (cm3) Vp = volume of pores (cm3) Vs + Vp = total soil volume Db (Vs + Vp) = Dp x Vs Vs/(Vs + Vp) x 100% = solid space Db /Dp = Vs/(Vs + Vp) solid space = Db /Dp x 100 solid space + pore space = 100% pore space = 100% - (Db /Dp x 100) 49 Soil Porosity Calculating for soil porosity: pore space = 100% - (Db /Dp x 100) Db = 1.25 (g/cm3) Dp = 2.65 (g/cm3) 15%, strength high when dry, low when wet) ▪ Electrostatic force between clay platelets. ▪ Noncohesive Soils (Sandy) ▪ Collapsible Soils (thixotropy = liquification with earthquakes, blasts) ▪ https://www.youtube.com/watch?v=zOM2raS4zdQ Soil Compression (Weil and Brady, 2019) Atterberg Limits ▪ Plastic limit: point at which soil reaches a malleable, plastic mass, trafficability. ▪ Liquid limit: point at which soil reaches a liquid stage to cause mass flow. Atterberg Limits https://www.youtube.com/watch?v=2na_RCUpykM