Soil Composition & Properties
The Settling Test (Jar Test): This physical analysis involves mixing soil with water in a transparent container and allowing it to settle over time. Because particles settle at different rates based on size (sand first, then silt, then clay), the resulting layers can be measured to calculate the percentage of each component.
Soil Texture Triangle: This graphical tool allows scientists to classify soil into one of 12 textural classes (e.g., Loam, Sandy Clay) by finding the intersection of the three particle percentages. It is essential that the sum of the three percentages always equals .
Chemical and Biological Testing: pH testing is used to determine the acidity or alkalinity of the soil, which directly affects the solubility and availability of essential nutrients like Nitrogen (N) and Phosphorus (P). Microbial activity tests assess the health of the soil ecosystem by measuring the presence of decomposers.
Sand vs. Clay: Sand provides excellent aeration and drainage but poor nutrient retention, making it prone to drought. In contrast, clay holds water and nutrients exceptionally well but suffers from poor drainage and can become waterlogged easily.
Loam: Loam is considered the ideal agricultural soil because it contains a balanced mixture of sand, silt, and clay. This balance provides the drainage of sand, the nutrient capacity of clay, and the moisture retention of silt.
| Property | Sand | Silt | Clay |
|---|---|---|---|
| Particle Size | Large ( to mm) | Medium ( to mm) | Small ( mm) |
| Permeability | High | Moderate | Low |
| Water Retention | Low | Moderate | High |
The 100% Rule: When using a soil texture triangle, always verify that the percentages provided for sand, silt, and clay add up to exactly . If they do not, there is an error in the data or the interpretation.
Reading the Triangle: Remember that each axis on the texture triangle is read in a specific direction (usually following the tilt of the tick marks). A common mistake is reading the numbers horizontally for all three sides; instead, follow the angled lines provided on the graph.
Predicting Drainage: If an exam question describes a soil with high clay content, you should immediately associate it with low permeability, high risk of waterlogging, and high nutrient cation exchange capacity.