3.2 Soil Texture, Mechanics, and Revegetation

Soil Particle Sizes, Mechanical Properties, and Settling Velocity

Under the USDA textural classification system, only the fine-earth mineral fraction passing a $2.0\text{ mm}$ sieve is used to determine soil texture. Particles larger than $2.0\text{ mm}$—including gravel ($2.0\text{ to }75\text{ mm}$), cobbles ($75\text{ to }250\text{ mm}$), and stones/boulders ($> 250\text{ mm}$)—are classified as coarse rock fragments that modify the texture name (e.g., gravelly sandy loam) and help armor the soil surface against raindrop splash.

The three USDA fine-earth mineral separates are:

1. Sand ($0.05\text{ to }2.0\text{ mm}$): Rounded to angular quartz and primary mineral grains visible to the naked eye; feels gritty between the fingers. Because sand grains are relatively large, they pack with large interconnected macropores, providing high permeability and rapid infiltration ($> 0.8\text{ to }6.0+\text{ in/hr}$) but very low specific surface area, low water-holding capacity, and zero cohesion or plasticity.
2. Silt ($0.002\text{ to }0.05\text{ mm}$): Microscopic mineral particles that feel smooth and floury when dry and silky (like talcum powder, not sticky) when wet. Silt pores are predominantly medium-sized mesopores that hold abundant plant-available capillary water, but silt grains are predominantly quartz-like fragments that possess little to no electro-chemical cohesion or plasticity.
3. Clay ($< 0.002\text{ mm}$, or $< 2\text{ }\mu\text{m}$): Microscopic, plate-like crystalline secondary aluminosilicate minerals (e.g., kaolinite, illite, and swelling smectite/montmorillonite). Clay feels sticky when wet, rolls into long flexible ribbons between the thumb and forefinger, and bakes into hard clods when dry. Clay has an enormous specific surface area (hundreds of square meters per gram) and net negative electrical charges that attract nutrient cations (Cation Exchange Capacity, CEC) and water films, creating strong cohesion and plasticity but microscopic micropores with very slow infiltration ($< 0.05\text{ to }0.15\text{ in/hr}$).

Atterberg Limits, Compaction, and Stokes' Law

• Cohesion and Atterberg Limits: In soil mechanics, fine-grained soils transition from solid to semi-solid to plastic to liquid states as moisture content rises. The Plastic Limit ($PL$) is the moisture content at which a soil thread begins to crumble at $1/8\text{-inch}$ diameter, the Liquid Limit ($LL$) is the moisture content at which soil flows under standard mechanical jarring, and the Plasticity Index ($PI = LL - PL$) measures the moisture range over which the soil behaves plastically. Clays have high $PI$ values and strong inter-particle cohesion; sands and clean silts are non-plastic ($PI \approx 0$).
• Compaction and Bulk Density: Bulk density is the dry weight of soil per unit total volume ($\text{g/cm}^3$). Undisturbed topsoil typically has a bulk density of $1.1\text{ to }1.4\text{ g/cm}^3$ with roughly $50\%$ pore space. Structural compaction by sheepsfoot rollers and heavy haul trucks crushes macropores, driving bulk density to $1.65\text{ to }1.90+\text{ g/cm}^3$. Compacted soil sheds rainfall almost like pavement and physically blocks seedling root penetration unless ripped or subsoiled prior to stabilization.
• Stokes' Law and Settling Velocity: Once soil particles are detached and entrained in stormwater, their gravitational settling velocity ($v_s$) in quiescent water is governed by Stokes' Law, which demonstrates that settling velocity is directly proportional to the square of the particle diameter ($d^2$):

$$v_s = \frac{g (\rho_s - \rho_w) d^2}{18 \mu}$$

Because $v_s$ scales with $d^2$, reducing particle diameter by a factor of $100$ (from $0.2\text{ mm}$ fine sand to $0.002\text{ mm}$ clay) reduces settling velocity by a factor of $10,000$. Coarse and medium sands settle out in seconds at the toe of a slope or inside a sediment trap, medium silts require hours of hydraulic residence time in a baffled skimming basin, and colloidal clays ($< 0.002\text{ mm}$) stay suspended for days or weeks due to mutual negative charge repulsion and Brownian motion unless coagulated or flocculated (e.g., with anionic polyacrylamide, PAM).

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Reading the USDA Soil Textural Classification Triangle

The USDA Soil Textural Triangle partitions mineral soils into 12 textural classes based on the weight percentages of sand, silt, and clay, which must always total $100\%$:

$$\%\text{Sand} + \%\text{Silt} + \%\text{Clay} = 100\%$$

Step-by-Step Plotting Procedure and Worked Examples

Each side of the equilateral triangle serves as the zero-to-100 scale for one separate, and its grid lines angle parallel to the previous counter-clockwise axis:

1. Locate $\%\text{Sand}$ along the bottom axis (reading from $0\%$ on the right to $100\%$ on the left) and trace the grid line diagonally up and to the left (parallel to the right-hand Silt axis).
2. Locate $\%\text{Clay}$ along the left axis (reading from $0\%$ at the bottom to $100\%$ at the top) and trace the grid line horizontally to the right (parallel to the bottom Sand axis).
3. Locate $\%\text{Silt}$ along the right axis (reading from $0\%$ at the top to $100\%$ at the bottom) and trace the grid line diagonally down and to the left (parallel to the left-hand Clay axis) to confirm the three lines intersect at a single point.

• Worked Example 1 (Classic Loam): A geotechnical laboratory report shows a topsoil sample contains $40\%\text{ sand}$, $40\%\text{ silt}$, and $20\%\text{ clay}$ ($40\% + 40\% + 20\% = 100\%$). Tracing the $40\%$ sand line up-left and the $20\%$ clay line horizontally right intersects squarely on the $40\%$ silt line inside the Loam compartment. Notice that a Loam does not contain equal thirds ($33.3\%$ each)—because clay has such a massive surface area, only $20\%$ clay is required to balance $40\%$ sand and $40\%$ silt, whereas a $33\%\text{ sand} / 33\%\text{ silt} / 34\%\text{ clay}$ soil plots in the Clay Loam class.
• Worked Example 2 (High-Erodibility Cut Slope): A cut-slope sample contains $15\%\text{ sand}$, $70\%\text{ silt}$, and $15\%\text{ clay}$. Tracing $15\%$ clay horizontally right and $70\%$ silt down-left intersects inside the Silt Loam class—alerting the inspector to severe splash crusting, rapid rill incision, and high $K$-factor erosion potential.

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Soil Structure, Organic Matter, Soil pH, and Fertility (N-P-K)

Beyond particle-size texture (which cannot be altered without importing new soil), four modifiable soil properties govern infiltration and vegetative stabilization:

• Soil Structure: The arrangement of sand, silt, and clay particles into secondary aggregates (peds). Granular (crumb) structure in A horizons maximizes porosity and seedbed contact. Blocky and prismatic structures occur in B horizons. Platy structure (horizontal overlapping plates caused by equipment compaction) and massive structureless subsoils block vertical percolation and root growth.
• Organic Matter: Increasing organic matter via compost incorporation or topsoil salvage binds silt and clay into water-stable aggregates, lowers bulk density, lowers the RUSLE $K$ factor, boosts water-holding capacity, and supports soil microbes.
• Soil pH and Chemical Amendments: Soil pH measures active acidity or alkalinity on a logarithmic scale from $0\text{ to }14$ ($7.0$ is neutral). Most erosion-control grasses and legumes require an optimal pH of $6.0\text{ to }7.0$ to absorb macronutrients:
  • Acidic Soils ($\text{pH} < 5.5$): Common in leached humid subsoils or pyritic rock cuts; soluble aluminum becomes toxic to roots and phosphorus is locked up in insoluble forms. Remediated by incorporating agricultural limestone (calcium/magnesium carbonate).
  • Alkaline ($\text{pH} > 7.8$) and Sodic Soils: Common in arid and semi-arid basins. High pH is lowered using elemental sulfur. Sodic/dispersive clays (high exchangeable sodium, $Na^+$, which causes clay aggregates to deflocculate and pipe) are remediated with gypsum (calcium sulfate, $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$), where $Ca^{2+}$ displaces dispersive $Na^+$ on clay micelles and restores flocculated soil structure.
• Soil Fertility (N-P-K): Fertilizer labels state three weight percentages in order—Nitrogen ($\text{N}$) for rapid vegetative shoot and leaf blade growth, Phosphorus ($\text{P}_2\text{O}_5$) for seedling root development and establishment, and Potassium ($\text{K}_2\text{O}$) for cellular strength, drought tolerance, and winter hardiness.