APES Video Notes 4.3 - Soil Properties

Soil Properties and Fertility

Soil texture and other soil properties affect water movement, nutrient availability, and a soil’s ability to support plant growth. Texture is determined by the proportions of sand, silt, and clay in a sample.

Soil Texture and Water Movement

Weathering breaks rock into sand, silt, and clay particles. Because these particles differ in size, they influence soil characteristics. Most soils are mixtures rather than a single particle type.

• Porosity is the amount of pore space in soil.
• Permeability describes how readily water passes through soil.

These terms are related but not interchangeable. Clay generally has more total pore space than sand, but sand has larger, more connected pores, so it is more permeable.

Sandy soil drains quickly and tends to retain less water. Clay drains slowly and can become waterlogged, limiting oxygen to plant roots. Loam, a balanced mixture of about 40% sand, 40% silt, and 20% clay, is suitable for many plants because it provides both drainage and water retention.

Plants need a balance: water and oxygen must reach their roots, but water should not drain away too quickly or accumulate excessively.

Classifying Soil with a Texture Chart

1. Find the sand percentage on the chart’s bottom axis.
2. Follow the chart lines to locate where the sand and silt values correspond.
3. Read the clay percentage from the appropriate line.
4. Check that the sand, silt, and clay percentages add up to 100%.
5. Use the resulting point to identify the soil-texture class.

For example, a sample with 45% sand, 35% silt, and 20% clay is classified as loam.

Factors Affecting Soil Fertility

Factors that can improve nutrient availability include:

• Organic matter decomposing into humus, which releases nutrients and helps soil retain water.
• Clay particles, which can hold positively charged nutrients.
• Bases such as calcium carbonate, which help buffer soil and reduce excessive acidity.

Factors that can reduce fertility include:

• Excessive acidity, which can cause nutrients to leach out.
• Repeatedly growing the same crops, which removes nutrients.
• Failing to return organic matter to the soil.
• Topsoil erosion, repeated tillage, and compaction by agricultural machinery.

Factors that help soil retain moisture include:

• Good aeration and channels created by organisms such as earthworms.
• Adding compost, humus, or other organic matter, which can reduce evaporation.
• Maintaining plant roots, especially deep roots.

Compaction, topsoil erosion, and the loss of root structure can reduce the soil’s ability to hold water.

Soil Quality Tests

• Texture test: Place soil in a jar of water, let it settle overnight, and measure the layers of sand, silt, and clay. Use their proportions to classify the soil.
• Permeability test: Put a soil sample in a column, add a measured volume of water, and time how long the water takes to pass through. Compare the result with a control.
• pH test: Mix soil with water and use a pH strip to assess acidity. This matters because overly acidic soil can lose nutrients through leaching.
• Color test: Compare the soil with a color chart. Darker soil generally indicates more humus.
• Nutrient tests: Use chemical tests to check nutrients such as nitrogen compounds (ammonium or nitrates) and phosphorus compounds (phosphates).

The practical assignment asks students to describe one soil test and explain how its results could help a farmer decide whether the soil is suitable for a crop or needs adjustments.

Speaker or Source

Mr. SMES is the instructor and narrator, identified with the video’s channel, Jordan Dischinger-Smedes. No other speakers are featured.