Soil Composition & Properties (College Board AP® Environmental Science): Study Guide

Soil water holding capacity

What is soil water holding capacity?

• Soil water holding capacity refers to the total amount of water that soil can retain and make available to plants

• Affects plant growth, land productivity, and soil fertility

Factors influencing water holding capacity

Soil texture

• Sandy soils: Low water retention due to large particle size and high drainage

• Clay soils: High water retention but poor drainage due to small particles

• Loamy soils: Balanced water retention and drainage, ideal for agriculture

Organic matter

• Increases soil’s ability to hold moisture by improving structure

• Example: Soils rich in humus retain more water and nutrients

Soil compaction

• Compacted soil reduces pore space, decreasing water infiltration and retention

• Example: Overgrazed land often has poor water-holding capacity due to soil compaction

Soil particle size & composition

What is soil particle size & composition?

• Soil particle size refers to the relative proportions of sand, silt, and clay in soil

• Soil composition affects soil texture, porosity, permeability, and fertility

Types of soil particles

Sand

• Largest particles with high permeability and low water retention

• Drains quickly, making it prone to drought conditions

• Example: Sandy soils are common in deserts and coastal regions

Silt

• Medium-sized particles that retain moisture better than sand

• Smooth texture, often found in floodplains and river valleys

• Example: Silt-rich soils are highly fertile and used for agriculture

Clay

• Smallest particles with low permeability and high water retention

• Drains slowly but holds nutrients well

• Example: Clay soils are found in wetland areas and often require aeration for crop growth

Importance of soil composition

• Determines water movement and aeration in soil

• Affects plant nutrient availability and soil structure

Testing soil properties

Why test soil properties?

• Soil testing helps determine chemical, physical, and biological characteristics

• Provides information for irrigation, fertilizer application, and land management

Methods of testing soil properties

Chemical testing

• pH testing: Determines soil acidity or alkalinity

  • pH can affect nutrient availability

• Nutrient analysis: Measures levels of nitrogen (N), phosphorus (P), and potassium (K)

• Salinity testing: Identifies salt concentration, which can impact plant growth

Physical testing

• Permeability test: Measures how quickly water drains through soil

• Bulk density test: Assesses soil compaction and porosity

• Soil texture test: Determines sand, silt, and clay proportions

  • One type of soil texture test involves the following steps:

    • The laboratory test involves mixing soil with water and allowing it to settle into distinct layers

    • This method provides a clear visual representation of the proportions of sand, silt and clay

    • Any large debris like rocks, roots, or organic matter, are first removed from the sample

    • The sample is added to a transparent container

    • Water is added and the container is shaken vigorously

    • The container is left on a flat surface and left undisturbed (e.g. for 24 hours)

    • Silt settles first, then clay, and finally sand

    • The thickness of these layers can be measured to determine their proportions

  

Biological testing

• Microbial activity test: Evaluates the presence of beneficial microbes for soil health

• Organic matter test: Measures decomposed plant and animal material, influencing fertility

Soil texture triangles

What is a soil texture triangle?

• A soil texture triangle is a diagram used to classify soil types based on their proportion of sand, silt, and clay

• Helps determine soil properties such as drainage, aeration, and fertility

How to use a soil texture triangle

• The three side of the triangle represent the proportions of the different soil particle sizes: sand, silt, and clay

• Locate the percentage of sand, silt, and clay on the diagram

• Follow the lines corresponding to each percentage until they intersect

• The intersection point determines the soil classification (e.g., loam, sandy clay, silty clay)

Importance of soil texture classification

• Affects water retention and drainage:

  • Sandy soils drain quickly, while clay-rich soils retain more water

• Influences agricultural productivity:

  • Loamy soils are ideal for farming due to balanced properties

• Determines soil management strategies:

  • Helps farmers and environmentalists choose appropriate irrigation and fertilization techniques