Earth science 4

Soil

Naturally occurring mixture of minerals, organic matter, water, and air that supports plant life.

Soil Physical Properties

The measurable characteristics of soil that affect its behavior and use.

Soil Texture

The proportion of sand, silt, and clay in a soil sample.

Sand

Largest soil particle; feels gritty and allows fast water drainage.

Silt

Medium-sized particle; smooth, flour-like texture that holds moderate water.

Clay

Smallest particle; sticky when wet, hard when dry, holds the most water.

Loam

Ideal soil mixture of sand, silt, and clay that supports plant growth.

Soil Structure

The arrangement of soil particles into aggregates or peds.

Granular Structure

Small, crumb-like peds common in topsoil; good for plant roots.

Blocky Structure

Irregular block-shaped peds common in subsoil.

Platy Structure

Thin, flat plates stacked horizontally that restrict water and root movement.

Prismatic Structure

Vertical, column-like peds commonly found in lower horizons.

Columnar Structure

Similar to prismatic but with rounded tops; common in arid soils.

Massive Structure

Soil with no visible peds; very dense and hard to break.

Single-Grained Structure

Loose soil with individual grains that don't stick together, like sand.

Bulk Density

Mass of dry soil per unit volume including air spaces.

High Bulk Density

Indicates compacted soil with low pore space and poor root growth.

Low Bulk Density

Indicates loose soil with high pore space and good aeration.

Particle Density

Mass of soil particles per unit volume excluding pore spaces (usually 2.65 g/cm³).

Porosity

The percentage of soil volume made up of pore spaces.

High Porosity

Contains many pores; allows good water and air movement.

Low Porosity

Fewer pores; slows water drainage and reduces aeration.

Soil Color

Indicator of mineral content, organic matter, and moisture.

Dark Soil

Color caused by high organic matter; holds nutrients well.

Red/Yellow Soil

Color caused by iron oxides; common in tropical climates.

Gray/Blue Soil

Color caused by poor drainage and anaerobic conditions.

Soil Temperature

Affects seed germination, microbial activity, and root growth.

Warm Soil

Increases biological activity and root development.

Cold Soil

Slows plant growth and reduces microbial decomposition.

Soil Consistency

How soil behaves when pressure is applied (sticky, soft, hard).

Friable Soil

Easily crumbles; best for root growth.

Sticky Soil

High clay content; hard to work with when wet.

Hard Soil

Strong, compacted soil that restricts roots.

Soil Moisture

Content of water present in the soil.

Field Capacity

Amount of water soil holds after excess water drains away.

Wilting Point

Soil moisture level where plants can no longer extract water.

Available Water Capacity

Water available for plant use between field capacity and wilting point.

Permeability

Rate at which water moves through soil.

High Permeability

Water moves quickly (sandy soils).

Low Permeability

Water moves slowly (clay soils).

Soil Horizon

Layers of soil with different characteristics.

O Horizon

Organic layer rich in decomposed plant material.

A Horizon

Topsoil containing minerals and organic matter.

B Horizon

Subsoil with accumulated clay and minerals.

C Horizon

Weathered parent material.

R Horizon

Solid bedrock layer.

Soil Profile

Vertical section showing all soil horizons.

Soil Fertility

Ability of soil to support plant growth through nutrients and structure.

Organic Matter

Decomposed plants and animals that improve soil structure and fertility.

Humus

Stable, dark organic matter that increases nutrient-holding capacity.

Soil pH

Measure of soil acidity or alkalinity.

Acidic Soil

Soil with pH below 7; can reduce nutrient availability.

Alkaline Soil

Soil with pH above 7; may limit nutrient absorption.

Neutral Soil

Soil with pH around 7; optimal for most plants.

Compaction

Pressing soil particles closer together, reducing pore space.

Causes of Compaction

Heavy machinery, foot traffic, and livestock.

Effects of Compaction

Reduces aeration, increases bulk density, restricts roots.

Soil Erosion

Removal of topsoil by wind or water.

Sheet Erosion

Thin layers of soil removed across a large area.

Rill Erosion

Small channels formed by running water.

Gully Erosion

Large channels formed by severe water flow.

Wind Erosion

Soil particles carried away by strong wind.

Soil Conservation

Practices to protect and maintain soil quality.

Mulching

Adding organic cover to reduce evaporation and erosion.

Contour Plowing

Plowing along slopes to reduce runoff.

Terracing

Creating steps on hillsides to slow water flow.

Cover Cropping

Planting crops to protect soil from erosion.

Crop Rotation

Changing crops yearly to maintain fertility.