Soils and Hydrology

What is Soil?

Soil is “the unconsolidated mineral or organic matter on the surface of the Earth that shows the effects of climate, organisms, relief, parent material, and time.”

Soil-Forming Factors

Climate, organisms, relief, parent material, time.

Four Components of Soil

1. Rocks/minerals

2. Decomposing organic matter

3. Living organisms

4. Pore space (air + water)

Rock Definition

A cohesive aggregate of one or more minerals.

Igneous Rock

Formed from cooling magma/lava (e.g., granite)

Sedimentary Rock

Formed from particles deposited in water and lithified (e.g., shale)

Metamorphic Rock

Formed from existing rocks undergo heat/pressure, causing recrystallization (e.g., marble)

Mineral Definition

A naturally occurring inorganic solid with a definite chemical composition and uniform atomic structure.

Example Mineral: Quartz

Made of silicon + oxygen (SiO₄).

Weathering

Breakdown of rocks through physical disintegration or chemical decomposition.

Soil Organic Matter (SOM)

Carbon‑containing organic substances formed through aerobic decomposition. ~50% carbon.

Labile Organic Matter

Rapid turnover; available to plants and microbes.

Humus

Stable SOM; slow turnover; improves soil structure

Living Soil Organisms

Micro (<0.1 mm), meso (0.1–2 mm), macro (>2 mm).

Examples of Soil Microorganisms

Bacteria, fungi, protozoa, nematodes.

Soil Pores

Macropores = drainage & aeration

Micropores = water storage.

Unsaturated Soil

Contains both air and water in pore spaces

Saturated Soil

All pores filled with water

Sand Particle Size

2.00 - 5.00 mm

Silt Particle Size

0.05 - 0.002 mm

Clay Particle Size

<0.002 mm

Sand Feel

Gritty

Silt Feel

Smooth/floury

Clay Feel

Sticky

Texture Definition

Proportion of sand, silt, and clay in a soil.

Sand Key Properties

Low water‑holding, good aeration, rapid drainage, low nutrients, high leaching.

Silt Key Properties

Medium water‑holding, moderate aeration, slow–medium drainage, high erosion risk.

Clay Key Properties

High water‑holding, poor aeration, very slow drainage, high nutrient storage, high shrink–swell.

Ribbon Test Purpose

Determines soil texture by feel (ball formation, ribbon length, shine, gritty sound).

Relative Size: Sand vs Silt vs Clay

Sand = 50–2,000 µm; silt = 2–50 µm; clay <2 µm.

SOM Environmental Benefits

Less erosion, more infiltration, more groundwater recharge, pollutant adsorption, CO₂ sequestration.

Primary Producers in Soil Food Web

Plants, algae, lichens, bacteria.

Primary Consumers in Soil Food Web

Shredders, root‑feeding nematodes.

Secondary Consumers in Soil Food Web

Protozoa, mites, microbial‑feeding nematodes.

Higher‑Level Consumers

Beetles, spiders, centipedes, birds, mammals.

Five soil forming factors

parent material, climate, biota, topography, time

Parent Material

The geological material from which soil develops

Residual parent material

soil formed in place from weathering of underlying rock

Glacial parent material

material deposited by glaciers or glacial meltwater

Ablation till

coarse, poorly sorted sediment dropped from melting glacier ice

Basal (lodgement) till

fine, compact sediment deposited beneath a glacier

Glacio-fluvial deposits

sediments sorted by flowing glacial meltwater

Glacio-lacustrine deposits

fine, layered sediments deposited in glacial lakes

Glacio-marine deposits

sediments deposited in ocean environments by glacial meltwater

Alluvial parent material

material deposited by rivers and streams

marine parent material

fine sediments accumulated in ancient seas

Lacustrine parent material

well-sorted fine sediments deposited in lakebeds

Eolian parent material

wind-deposited sediments such as loess, ash, or sand

Colluvial parent material

poorly sorted sediments moved downslope by gravity

organic parent material

accumulated plant material in poorly drained environments

what does soil texture describe

the percentage of sand, silt, and clay in a soil

which soil separate has the highest water-holding capacity

clay

which soil separate has the fastest drainage

sand

which soil separate has the best aeration

sand

which soil separate decomposes organic matter the fastest

sand

which soil separate has the highest shrink-swell potential

clay

which soil separate has the highest pollutant leaching potential

sand

which soil separate stores plant nutrients best

clay

what determines which sediment size water can transport

flow velocity relative to particle settling velocity

What is the first step in texture-by-fell analysis?

moisten and knead soil into a putty-like consistency

if soil cannot form a stable ball, what texture is it?

sand

if soil forms a ball but cannot form a ribbon, what texture is it?

loamy sand

what indicates a sandy loam during ribbon testing?

ribbon forms but is shorter than 2.5 cm

what indicates a clay loam during ribbon testing

ribbon is longer than 2.5 cm but not shiny

what indicates a clay soil during ribbon testing

shiny ribbon longer than 5 cm

what does a gritty feel during ribbon testing indicate

high sand content

what does a silky smooth feel indicate

high silt content

what does a sticky, plastic feel indicate

high clay content

why does clay hold more water than sand?

clay contains micropores inside aggregates that retain water