Physical Properties of Soil

Minerals in Soil

There are three categories of soil:  sand, silt, and clay. 

This classification is based on the size of the soil particle.  Particles of sand are visible to the naked eye.  Silt must be magnified at lease 10x in order to see the particles.  And clay particles require an electron microscope to be seen.

Below are standards for the size limits of the three types of soils as set forth by the U.S. Department of Agriculture (USDA):

Soil Type

Particle Diameter (mm.)

Surface Area for 1 gr (sq. cm.)

Very Coarse Sand

2.0-1.0

11

Coarse Sand

1.0-0.5

23

Medium Sand

0.5-0.25

45

Fine Sand

0.25-0.10

91

Very Fine Sand

0.10-0.05

227

Silt

0.05-0.002

454

Clay

< 0.002

8,000,000

 

It is important to note that the size of soil mineral particles and the corresponding amount of surface area are inversely proportional.  In other words, the smaller the particle, the more surface area.  This is significant because much of nutrients and water in soil are held on the surface of soil particles.  The particle surface is also where important chemical reactions occur.

Soil particle size

Sand and silt are composed mostly of primary minerals like quartz, mica, feldspar, hornblende, and augite.  These primary mineral crystalline structures contain secondary minerals like iron, sodium, calcium, potassium, and magnesium which are released as the primary minerals break down as they weather.  Clay is composed primarily of these secondary minerals released during weathering.

Soil Texture

Soil texture is determined by the proportions of sand, silt, and clay in each sample.  This physical characteristic of soil is very important in determining tilth, or the ability of the soil to support plant growth, as well as its ability to hold water and nutrients.

The following triangle illustrates the 12 basic soil textural classes and their corresponding percentages of sand, silt, and clay:

Soil texture pyramid

Soils that are 85+% sand with small percentages of silt and clay are called “sand”.  Such soils are often called “light” soils.  Soils that are 40+% clay are called “clay” and are often called “heavy” soils.  When soils contain roughly the same percentage of sand, silt, and clay, they are called “loam” soils.

The 12 basic soil textural classes can be further categorized as coarse soils, medium soils, and fine soils.  Sandy soils like sand and loamy sand are categorized as coarse.  Clay soils like clay, sandy clay, and silty clay are categorized as fine.  All loamy soil textures (sandy loam, loam, silt loam, silt, clay loam, sandy clay loam, and silty clay loam) are categorized as medium.

These different textural classes have varying abilities to hold water and nutrients:

  • Coarse classes like sand have large pore spaces between particles allowing water and nutrients to percolate or drain quickly through the pores. 
  • Medium classes like silt have smaller pores than sand, so they retain more water and nutrients than course soils. 
  • Fine classes like clay have even smaller pores than medium classes, so they retain even more water and nutrients than medium loamy soils and coarse sands.

The best soils to use for gardening are loam, sandy loam and silt loam soil with 5-10% of the soil composition being organic matter.  These soils retain an adequate amount of water and nutrients while still allowing water to move into and through the soil.

Soil Structure

Soil particles in sand occur as individual particles.  For all other soils, particles clump together to become aggregates or granules.  Soil structure refers to how these aggregates are arranged and organized.  The relationship between air and water in the soil depends heavily on its structure.

Soil structure of more coarse soils can generally be improved by adding organic matter.  Doing so promotes aggregation or clumping while improving water and nutrient retention.

You should avoid digging in soil that is too wet or extremely dry as this can damage soil structure.  The soil is too wet if it sticks to your shovel when digging.