Overview of Soil Types and Particle Size Distribution

 

 

 

 

 

 

 

 

 

 

 

The ASTM defines Particle size distribution, also known as a soil’s gradation, as the proportions by dry mass of a soil distributed over specified particle-size ranges. It is used to classify soils for engineering and agricultural purposes, since particle size influences the rate at which water or other fluid moves through a soil and has significant effects on its horticultural properties.

A soil’s particle size distribution is determined using a procedure called a sieve analysis, where a soil is passed through a series of standard sieve sizes to determine what percent passes each size. The results are then charted and analyzed to determine the percentages of sand, silt, and clay present.

Soil particles are categorized into different ranges, as follows:

*Boulders: Stones greater than 200mm in diameter

*Cobbles: Stones ranging from 60-200mm in diameter

*Gravel: Some overlap with the largest sand particles, ranging from 2-60mm

*Although these standard ranges are defined for classification purposes, boulders, cobbles, and gravel are typically too large to be factored into a standard soil texture analysis.

Sand: Particle sizes from 0.05-2mm

Silt: Finer particles, ranging from 0.002-0.05mm

Clay: The smallest soil particle type, with diameters less than 0.002mm

Based on the particle size proportions detected during the sieve analysis, a soil will be classified according to its texture, as outlined in this chart:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sand is formed primarily of small broken-down rock particles, and is a critical component of soil drainage and overall structure. The angular or spherical faces of these particles, which occur naturally when larger stones break down into smaller pieces, tend to not fit together tightly even under load or pressure – this makes sand very good at resisting compaction, and very well-suited for soils used in high-traffic areas. The soils found beneath golf course greens and fairways, for instance, are often 90% sand or greater.

In general, the greater the proportion of fine particles in a soil (silt and clay) the more slowly a soil will drain water. The greater density with which the smaller particles pack together creates smaller gaps (called pores) between them, which results in slower drainage due to molecular adhesion between the water and soil particles it is moving through. By comparison, larger particles and aggregates found in sands and gravels contain much larger pores between the particles, so water will move much more freely.

Silt and clay particles contribute to making a soil fertile, as they tend to retain nutrients at a higher rate and buffer changes to a soil’s pH through their cation exchange capacity. Although they do not drain quickly, this moisture retention is helpful for plants in moderation. It is important that a soil contain a balance of different particle sizes depending on what the soil is designed to do.