Soil Composition and Properties

Soil Composition and Properties

Overview of Soil Composition

• The discussion begins with an overview of soil composition, highlighting that under ideal conditions, soil consists of 45% inorganic minerals, 25% air, 25% water, and 5% organic matter.

Importance of Soil Properties

• The speaker emphasizes the uniqueness of each soil type due to various physical and chemical properties. Biological properties are acknowledged but not deeply discussed.

Interaction Between Soil Properties

• Chemical and physical properties influence biological properties in soil ecosystems. Healthy interactions among these properties contribute to fertile soils suitable for agriculture.

Particle Sizes in Soil Composition

• Soil is composed of three particle sizes: sand (largest), silt (medium), and clay (smallest). These sizes affect both the physical texture and some chemical characteristics of the soil.

Understanding Soil Texture

• Most soils are a mixture of sand, silt, and clay rather than pure forms. The term "LOM" refers to a mix containing all three particle sizes, which is crucial for determining soil texture.

Determining Soil Texture

Practical Application: Identifying Clay Percentage

• An example problem illustrates how to determine clay percentage when given percentages for silt (55%) and sand (25%). The intersection point on a triangle diagram indicates the clay percentage as 20%.

Conducting a Soil Texture Jar Test

• A method called the soil texture jar test is described for determining proportions of sand, silt, and clay by mixing soil samples with water and borax. After settling over two weeks, layers can be measured.

Calculating Percentages from Measurements

• In an example calculation using total height measurements from the jar test results in approximately 60% sand content based on its proportion relative to total height.

Interconnectedness of Soil Properties

Porosity vs. Permeability

• The relationship between soil texture affects porosity (the number of empty spaces between particles), which in turn influences permeability (the ability to transmit water).

Misconceptions About Porosity

• Common misconceptions about porosity are addressed; while sand has large pores, it actually has lower overall porosity compared to other types like clay or silt due to fewer total pore spaces.

Understanding Soil Properties and Their Impacts on Water Dynamics

Pore Structure and Permeability

• The soil has high porosity due to numerous tiny pores, but they are not interconnected, affecting water movement.

• Infiltration and permeability refer to how quickly water enters and drains through soil; this will be tested in a lab experiment with sand, silt, and clay.

• Sand is expected to drain the fastest due to its larger, interconnected pores, while clay will have the slowest drainage rate.

Compaction Effects on Soil

• Compaction from heavy machinery on farms reduces porosity and permeability of soil, leading to poor water flow.

• Roots struggle to penetrate compacted soil, resulting in dry conditions as water cannot permeate effectively. This can lead to waterlogging where excess water sits atop the surface.

Water Holding Capacity

• Water holding capacity refers to how much moisture soil retains against gravity; it is inversely related to permeability. Sand has the lowest capacity while clay holds the most water due to its small pore size.

• Clay's ability to retain moisture makes it useful for pottery but also leads to low permeability issues when wet.

Soil Color and Fertility

• Darker soils typically contain more humus (partially decomposed organic material), enhancing nutrient recycling and overall fertility of the soil. This creates a positive feedback loop promoting biodiversity and plant growth.

Cation Exchange Capacity (CEC)

• CEC is crucial for nutrient retention in soils; it measures the ability of soil particles to hold or exchange positively charged ions (cations) essential for living organisms like magnesium, calcium, potassium, etc.

• Clay has a greater surface area than sand despite both having similar volumes; this allows clay better nutrient retention capabilities due to higher electro negativity compared with sand which primarily consists of uncharged quartz particles.

Understanding Soil Chemistry and Cation Exchange Capacity

The Role of Charge in Soil Particles

• Clay particles are predominantly negatively charged, attracting positively charged ions (cations) which cling to them.

• Humus, also negatively charged, contributes to the soil's cation exchange capacity (CEC), enhancing nutrient retention.

Sandy vs. Clay Soils

• Sandy soils have a lower cation exchange capacity due to their lack of charge, leading to nutrient leaching during rainfall.

• In contrast, clay-rich soils retain nutrients better because the negative charge allows cations to stick, preventing leaching and making nutrients available for plant absorption.

Understanding Soil pH

• Soil pH measures hydrogen ion concentration; higher hydrogen ion levels indicate more acidic conditions and lower pH values.

• The original parent material and rainfall influence soil pH levels significantly. A visual representation shows how cations adhere to negatively charged soil particles under normal conditions.

Effects of Acidic Soil on Nutrient Availability

• Lowering soil pH increases hydrogen ions that occupy space on soil particles, reducing fertility by displacing other essential cations. This can lead to aluminum toxicity at very low pH levels (5.5 or below).

• Aluminum hydroxide dissociates in acidic conditions, mobilizing aluminum which can block root nutrient uptake and precipitate with phosphorus, rendering it unavailable for plants. This results in multiple issues for plant health including loss of exchange sites and nutrient blockage.

Addressing Acidic Soils

• Common causes of soil acidity include acid rain from fossil fuel combustion and synthetic fertilizers; these factors will be discussed further in later units.

• Farmers can amend acidic soils by applying lime (calcium carbonate), which helps raise the pH back towards neutral levels for improved crop viability.

Practical Applications of Different Soil Types

• For landfill bases where leachate containment is critical, clay is preferred due to its compact structure that minimizes permeability and prevents contamination of groundwater supplies.

• Sand is ideal for athletic turf as its large pores allow high permeability ensuring effective drainage after rainfall, keeping surfaces playable for sports activities like soccer.

• Loam is best suited for vegetable farming as it combines properties from sand, silt, and clay providing balanced drainage while retaining necessary nutrients effectively for plant growth.

Soil Composition and Its Impact on Drainage

Importance of Soil Particle Size

• The presence of various particle sizes in the soil contributes to effective drainage, as larger particles facilitate water movement.

Cation Exchange Capacity (CEC)

• The soil's good CEC is attributed to the inclusion of clay, which is known for its ability to retain nutrients effectively.

Overall Soil Quality

• The combination of good drainage and nutrient retention makes this soil type reliable for supporting water and air needs essential for plant growth.