Making Sense of Particle-Size Distribution

(Note: Refer to the Learning Activity in the 1997 Supplement to the GLOBE Teacher's Guide)

The amount of each size particle (sand, silt, or clay) in the soil is called the particle-size distribution. Knowing the particle-size distribution of a soil sample helps to understand many soil properties such as how much water, heat, and nutrients the soil will hold, how fast water and heat will move through the soil, and what kind of structure, bulk density and consistence the soil will have.

Sand, silt, and clay are the three particle sizes of mineral material found in soils. The amount of each of these is called the "particle-size distribution" and the way it feels is called the "soil texture."

Sand is the largest sized particle, Silt is medium sized, and Clay is the smallest.

There is disagreement in the scientific community about the exact size ranges of sand and silt. For GLOBE, we will measure sand and silt based on two different size definitions.

1. US Department of Agriculture (USDA) which defines the size of:

sand = 2.0 mm - 0.05 mm silt = 0.05 - 0.002 mm

2. The International Soil Science Society (ISSS) which defines the size of:

sand = 2.0 mm - 0.02 mm silt = 0.05 - 0.002 mm

Clays are the smallest particles and are defined (by both organizations) as being smaller than 0.002 mm.

Particles greater than 2.0 mm (sand size) are called stones or gravels and are not considered to be soil material.