Soil Properties: Physical and Chemical Processes

• This note will cover definitions, soil functions, soil forming factors and processes, parent material, weathering, soil composition, and soil sampling
• Chemical processes and properties will be discussed
• Physical processes and properties are outlined
• Biological properties and Soil Organic Matter (SOM) featured

Physical Processes and Properties

• Physical processes in soil include frost, salt crystallization, and insolation
• Physical weathering mechanisms are relevant
• Physical properties are texture, structure, density, porosity, consistency, temperature, color, aggregate stability, water/moisture content, and water flow
• These properties affect other processes like water cycling, biologic activity, pollutant fate, erosion, and degradation

Basic Volume and Mass Relations

Relate to soil components, including definitions and units like:

• Solid density (ρs = Ms/Vs)
• Bulk density (ρb = Ms/(Vs + Vw + Va) = Ms/Vt)
• Wet density (ρbw = (Ms + Mw)/Vs)
• Gravimetric water content (θg = Mw/Ms)
• Volumetric water content (θv = Vw/Vt = θgp♭)
• Degree of saturation (Se = Vw/(Va + Vw) = Vw/Vp)
• Soil porosity (φ = 1 - Vs/Vt = 1 - Pb/Ps)

Soil types vary in density and porosity:

• Sandy soil has particle density of 1.16 - 1.70 [g cm-3] and a porosity of .56 - .36 [cm3 cm-3]
• Silty soil has particle density of 1.17 - 1.63 [g cm-3] and a porosity of .56 - .38 [cm3 cm-3]
• Loamy soil has particle density of 1.20 - 2.00 [g cm-3] and a porosity of .55 - .30 [cm3 cm-3]
• Clayey soil has particle density of 0.93 - 1.72 [g cm-3] and a porosity of .65 - .35 [cm3 cm-3]
• Organic soils have particle density of 0.12 - 0.48 [g cm-3] and a porosity of .60 - .90 [cm3 cm-3]

Distribution of Solids and Pore Space

• Ideal soil has 5% organic matter, 45% mineral, 25% water, and 25% air
• Poorly Drained soil has 5% organic matter, 45% mineral, 45% water, and 5% air
• Compacted soil has 1% organic matter, 69% mineral, 25% water, and 5% air

Soil Texture

• Soil particles are categorized by size into sand, silt, and clay
• Relative percentages determine soil texture, such as clay loam (33.33% clay, loam, sand)
• Texture analysis involves methods like sieve, jar, pipette, and hydrometer
• Stones (>2mm) and fine soil (<2mm) are distinguished; fine soil determines soil type

Cumulative Particle Size Curve

• A graphical representation plots sieve passage (mass %) against particle diameter
• Used to calculate, showing the distribution of different particle sizes in a soil sample.

Degree of Non-Uniformity (U)

• Characterizes particle size distribution, with U = d60/d10
• d60 and d10 are particle diameters at which 60% and 10% of the soil are combined
• A smaller U indicates more uniform soil with a greater risk of siltation
• Glacial till has a U = 157.5
• River sand has a U = 4

Properties of Soil Structure

• Relates to soil particle grouping (sand, silt, clay, organic matter, fertilizers) into porous aggregates
• Pore quantity, distribution, and arrangement affect water holding capacity, infiltration, permeability, root penetration, and respiration
• Granular and aggregated structures have high permeability
• Blocky and columnar/prismatic structures have moderate permeability
• Platey and massive structures have low permeability

Soil Density

• Density is the mass-volume ratio (ρ = M/V) in g/cm³, indicating soil compaction
• Soil has three density types: solid, bulk, and wet
• Solid density (ρs = Ms/Vs) is soil particle density Bulk density (ρb = Ms/(Vs + Vw + Va) = Ms/Vt) is dry density
• Wet density (ρbw = (Ms + Mw)/Vs)
• Lower bulk density indicates lower weight and more pore space
• Higher bulk density indicates higher weight and less pore space

Soil Porosity

• Porosity is the pore space amount within a soil between mineral particles, filled with air or water, expressed as a percentage
• It indicates soil compaction; φ = (1 - (V/Vt))*100 or φ = (1 − (pb/ps))*100
• Lower bulk density indicates lower weight and more pore space.
• Higher bulk density indicates higher weight and less pore space.

Soil Consistency

• Describes the degree and kind of cohesion and adhesion between soil particles, related to the resistance of the soil to deform or rupture and depends on soil minerals and water content
• Includes descriptions of rupture resistance, stickiness, and plasticity.

Rupture Resistance

• A part of soil consistency
• Wet consistency: measures the strength of moist soil to withstand applied stress, rated from loose to extremely firm
• Dry consistency: measures the strength of air-dried soil to withstand applied stress, rated from loose to extremely hard

Soil Stickiness

• Is the wet consistency
• Press a small amount of wet soil between thumb and forefinger and rate stickiness as non-sticky, slightly sticky, sticky, or very sticky

Soil Plasticity

• Is the wet consistency
• Roll a small amount of wet soil to form a 3 mm thick wire and rate plasticity as non-plastic, slightly plastic, plastic, or very plastic

Measuring Soil Consistency

• Done with Atterberg Limits which correspond to the moisture point where a soil sample changes consistencies
• Liquid consistency is fluid or liquid mud
• Plastic consistency allows kneading and molding
• Semi-solid consistency means no more kneading is possible

Atterberg Limits

• Liquid Limit (LL) is moisture content that shifts soil between liquid and plastic states
• Plastic Limit (PL) is moisture content that shifts soil between plastic and semi-solid states

Soil Temperature

• Controls growth of biological systems and influences physical, chemical, and microbiological processes
• Affected by season, management practices, and land cover
• Mouldboard plough (till=MP); Ridge till (RT); No till (NT)

Soil Color

• Influenced by soil mineral composition, water, and organic content
• Calcium (white), iron (reddish), and humus (dark brown to black) are key influences
• Determined using the Munsell soil color system, including hue, value, and chroma. -hue: dominate/basic color -value: color degree -chroma: color intensity/saturation

Aggregate Stability

• Measures soil aggregates' ability to resist breaking apart when exposed to external forces like water/wind erosion, shrinking/swelling, and tillage
• Stable aggregates increase soil productivity
• Formation results from primary soil particle interactions and cementation

Water/Moisture Content

• Shown in Soil pores
• Capillary rise occurs as described in the equations h = (2 cos(γ)σιν) / (PlugR)
• Variables: -h = elevation of the liquid (m) -γ = the contact angle between the liquid and the capillary tube (radians) -σιυ = surface tension of the liquid (N/m) -Plυ = density of the liquid (kg/m3) -g = acceleration of gravity (m/s2) -R = radius of the capillary tube (m)
• Capillaries with the smallest radii lead to the largest capillary rise

Soil Water Retention Curve

• Describes relationship between soil water content and matric potential
• Includes gravitational, capillary, and hygroscopic H₂O
• Air capacity (AC), Field capacity (FC), and Wilting Point (WP) can be determined using pF graph

Concept of Potentials

• Soil water has an assigned energy content or hydraulic potential (J m⁻³ or Pa)
• Indicates work needed to bring water into the soil
• Soil water potential is expressed as pressure (Ψτ = energy/volume in Pa) or as a water column (H = energy/mass in m)

Total Potential

• Total potential (hydraulic potential) is sum of individual potentials: Ψt = Ψz + Ψm + Ψo + Ψp, including gravitational (Ψz), matric (Ψm), osmotic (Ψo), and hydrostatic (Ψp) potentials. Osmotic potential: the energy between the water and the soil water Hydrostatic potential: the potential energy of the water to the pressure exherted

Water Flow/Movement

• Soil water moves by capillary action, from high to low potential areas, due to hydraulic tension gradient
• Hydraulic conductivity dictates water flow speed
• Saturated soils have all pores water-filled, follow Darcy's Law qw = -Ks ΔΗ / Az
• Unsaturated soils have Buckingham-Darcy's Law apply: qw = -K(0) ΔΗ / Az Factors affecting it are: dimension, saturated conductivity, the gradient
• Ks is saturated hydraulic conductivity
• K(0) is unsaturated hydraulic conductivity
• Is the hydraulic gradient
• Dimensionless with is in m
• Important for water storage

Ability to Facilitate Water Flow

• Saturated hydraulic conductivity is water transmitter fully, hence, Ks conductivity is the max water transfer value.
• Key factors: -Texture: Coarse more Ks than fine Ks -Structure: Well-aggregated have stable pore network to have higher Ks than those without -PS D: size impacts continuity of pores -Organic matters: content can improve soil structure, increasing Ks -Compaction: reductions in porosity -Temperature: increases viscosity -Moisture content: decreases, K(0) decreases exponentially -Matric Potential: decreases, K(0) decreases