Unit 2 - Soil Texture

Chapter 1 -  Soil Texture & Mechanical Analysis

Soil Texture

Soil texture is the single most important physical property of the soil.

{feel the soil}

Soil texture is a basic physical property of the soil. When studying the formation of soil particles, we indicated that the glaciated parent materials were of varied size components. Till is a mixture of clay, rocks, sand and silt. The size of outwash particles is dependent on the speed of the moving water; fast water deposits sands and gravels while slow water deposits clays and silts. Soil is made up of different size of separates and collectively these are called Soil Texture

The texture of the soil is dependent on the mixture of the different particle size separates (soil separates). From largest to smallest the soil separates are :

Stones and cobbles are bigger than 64 mm (diameter)
Gravel is from 2 mm to 64 mm
Sand is from .05 to 2 mm
Silt is from .002 to .05 mm
Clay is less than .002 mm.

Relative Size

Essentially all of these soil separates, except for clay, are made in the soil by the physical breakdown of rocks and minerals.

Clay is different because clay is a chemical recombination of the base elements of silicon, aluminum, and oxygen into a silicate clay mineral. Clay minerals will be studied later in the Chapter onClay Minerals. In this section we will learn how to determine soil texture--that is, the proportion of sand, silt and clay or what is the percentage of the different soil separates

Soil texture is important because it affects and is related to several soil properties such as soil structure, aeration, water holding capacity, nutrient storage, water movement, and bearing strength.

Borrow pits are areas where the sandy soil is mined for sand and gravel. This sandy outwash material becomes a valuable construction material. This kame is almost completely removed by a sand mining company.

Large soil separates can be separated from the finer ones by using a 2 mm sieve. sieve gravelThe amount by volume of gravels and cobbles can be visually estimated to determine the percentage by volume. If the soil has >15% gravel, a "gravelly" modifier is used for the textural class name (for example, "gravelly sandy loam"). Soils with more than 15% gravel will have different characteristics and be less productive than non-gravelly soils.

The student in the soil pit is determining soil texture by feel but needed to use a sieve to separate the gravel (> 2mm) from the soil particles. Note the close up of the sieve here.Sieve for Gravel

Once the gravel has been removed from the fine soil particles (sand, silt, and clay) we can accurately determine their proportion.. Clay particles are so small that individually they cannot be seen with the naked eye. However, in terms of their influence on the soil, they are the most active separate. In Minnesota, we find soils with the highest clay content in areas with lacustrine parent materials .  

  The Nemadji River Valley (south of Duluth, Mn.) is noted for the clay soils (Ontonogon Series) that easily slump into the river where the slopes are steep; thus, the river is very red with clay sediments. The sediments are lacustrine deposits from Glacial Lake Duluth. For more information go to Basin Plan or Land Use Impacts

Mechanical Analysis

The determination of soil texture is called particle size analysis or mechanical analysis. Determining the texture in the laboratory uses a basic principle of sedimentation called "Stokes Law".

Stokes Law states that the speed or velocity with which particles settle out of a liquid medium is dependent on a constant factor (K) and the radius of the particles. Or, the bigger the particle, the faster it will fall out of suspension. K is composed of the factors:

g=acceleration of gravity
d1=density of particle
d2=density of liquid
µ=viscosity of liquid

v=Kr², and K=2g(d1-d2) ÷ µ

A visual concept of stokes law={short description of image}From:Juma, N. G. The Pedosphere and its Dynamics: Soil Texture, Structure & Color, 3.2.3 Determination of Soil Textural Classes [Online]. (1998, October 19). Available HTTP: http://www.soils.rr.ualberta.ca/pedosphere/content/section03/page02_03.cfm [June 10, 1999].

In order for the procedure to work, we must make sure all the particles are separated from each other. Calgon, which contains sodium ions, is added to the soil before the soil is mechanically stirred on a malt shaker. Calgon acts as a dispersing agent to separate all the clay particles. This also breaks apart the soil aggregates.

Mechanical Analysis Equipment

Next we transfer the solution to the settling cylinder. After a certain period of time, in this case 40 seconds, the largest particles in the soil (the sand) will have settled out of the soil/water mixture and will be on the bottom. The instrument we use to determine the amount of sand is a hydrometer - often called a Bouyoucos hydrometer after the scientist who invented it.

The hydrometer is not going to measure sand, except indirectly. What it is going to measure is the density of the liquid in the cylinder, which, if all the sand has settled out, the water in the cylinder is only going to have silt and clay in it. So, the 40 second reading is the reading of "grams of silt and clay in suspension".

The two hour reading reads grams of clay and the percent sand is determined by subtraction from the total amount of soil used (50g). As the temperature of the liquid increases, the soil particles fall faster than if the liquid were colder. The hydrometer is calibrated at 20 o C, so we need to add 0.4 g/L for each degree above 20 oC or subtract 0.4 g/L for each degree below 20 o C.

In order to stir the soil, we will use a rod with a disk on the end. This can be lifted up and down in the cylinder, but not too vigorously. As the rod is lifted out, notice the time. After about 10 seconds, insert the hydrometer slowly--be careful with the hydrometers, they are fragile and expensive. Use your finger to keep the hydrometer from bouncing. Read the hydrometer at the bottom of the meniscus.{short description of image}

If we wait until all of the particles have settled out, the cylinder would look like this. However, this may take a few days. It has been determined that after a wait of two hours, approximately all the silt has settled out, leaving the clay in suspension.
The 2-hour reading, therefore, reads grams of clay in suspension and is available on the card in the laboratory.

Temperature of the solution can be taken before or after you have completed your 40-second reading.

Math Summary --40 second reading=33 --2 hr reading=18. Sample weight=50 grams.
Determine % sand: subtract -33 from 50=17 and 17/50 x 100=34% sand
% clay=18/50 x 100=36% clay;
and % silt=100 - (34 + 36)=30% silt

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