4.3 APES

Soil Composition and Properties

Soil is composed of three main particle types: sand, silt, and clay, arranged from largest to smallest.
Soil texture is defined by the percentage of these particles, always summing to 100%. For example, a soil with 40% sand, 40% silt, and 20% clay is classified as loam.
Sand has larger pores, allowing for easier air and water movement, while clay has smaller pores, making it more difficult for air and water to penetrate.
Porosity refers to the amount of pore space in the soil; more sand results in higher porosity, while more clay results in lower porosity.
The relationship between porosity and permeability is positive; higher porosity leads to higher permeability, facilitating water drainage.
Example: A soil texture chart can help determine the composition of soil based on its particle size.

To use the soil texture chart, start at the bottom with the sand percentage and move horizontally to where it meets the silt line, then go straight over to the clay line.
Ensure that the total percentage of sand, silt, and clay equals 100%.
Example practice: A blue circle on the chart indicates 30% sand, 20% silt, and 50% clay.
Understanding soil texture is crucial for predicting water retention and drainage capabilities.
Loam is considered ideal for plant growth due to its balanced texture.
Soil texture influences the soil's ability to support plant life and its overall fertility.

Porosity is the measure of pore space in soil; more sand leads to higher porosity, while more clay leads to lower porosity.
Permeability indicates how easily water drains through soil; higher porosity correlates with higher permeability.
Water holding capacity refers to the soil's ability to retain water; more porous soils typically have lower water holding capacity.
There is an inverse relationship between porosity/permeability and water holding capacity.
Ideal soil for plant growth balances these properties, with loam being the most favorable.
Excessively sandy soils drain too quickly, while clay-heavy soils can retain too much water, leading to root suffocation.

Soil fertility is defined as the ability of soil to support plant growth, influenced by nutrient availability. Key nutrients include nitrogen (N), phosphorus (P), potassium (K+), magnesium (Mg2+), calcium (Ca+), and sodium (Na+).
Organic matter and humus play a critical role in nutrient release and retention, enhancing soil fertility.
Decomposer activity is essential for recycling nutrients back into the soil.
Clay particles have a negative charge, which helps bind positively charged nutrients, making them available for plant uptake.
Factors that can decrease soil nutrients include acid leaching, excessive rainfall, and intensive farming practices.
Topsoil erosion can significantly reduce nutrient levels, impacting soil fertility.

Soil must retain adequate water for plant growth without becoming waterlogged.
Factors that increase water holding capacity include aerated soil, organic matter, and clay content.
Compacted soil, often caused by machinery or livestock, can decrease water holding capacity.
Root structure, particularly from native plants, can enhance water retention in the soil.
Excessive topsoil erosion and sand loss can lead to reduced water availability for plants.
Understanding the balance of water retention and drainage is crucial for maintaining healthy soil.

Soil texture can be tested by letting soil settle in a jar of water, measuring the three layers formed (sand, silt, clay). This indicates the soil's porosity and permeability.
Permeability is assessed by timing how long it takes for water to drain through a column of soil; optimal drainage is crucial for root health.
Soil pH is measured using pH strips to determine acidity or alkalinity, which affects nutrient availability.
Color comparison with a soil color chart can indicate humus content; darker soils typically have more nutrients and moisture.
Nutrient levels can be measured for ammonium, nitrate, or phosphate, with higher levels correlating to better plant growth.
Low nutrient levels may indicate acidic conditions or depletion due to poor management practices.