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Name: Katherine
Status: Student
Age: N/A
Location: N/A
Country: N/A
Date: October 12, 2002


Question:
How do you determine the size of the pores of different types of soil?



Replies:
Katherine,

A simple model of the soil will give you a good approximation. First you have to know the soil type and it's average particle diameter.

These are generally:
very coarse sand 1.5 mm
coarse sand 0.75 mm
fine sand 0.175 mm
very fine sand 0.075 mm
silt 0.02 mm
clay 0.0015 mm

Most soils are not composed of just one of these mineral types and usually have a substantial amount of organic matter also. Coarse textured soils (like sand) actually have less total pore space (although the pore size is greater) than fine textured soils (like clay).

Knowing the soil type and the percentage of sand, clay, and silt, you can determine an average particle size and therefore an average pore size. You can find a good set of these in a triangular diagram (Figure 1) at the Washington State University web site. Go to www.wsu.edu. In the search box on that page type in pnw0475 and click on the yellow "Site" dot. Then click on the first link (PNW0475 Soil Water Monitoring and Measurement), not the second link, which is an abstract. Scroll down to Figure 1. From the triangular diagram you can estimate how much of a soil type is sand and clay, with the remaining percentage being silt. For instance, clay loam is about 35% clay and 32% sand on average, meaning that the residual of 33% is silt. Multiply the fine sand (typical in clay soils) particle diameter (0.175) by 0.32, the clay particle diameter (.0015) by 0.35, and the silt particle diameter (0.02) by 0.33; add the resultant numbers and you get an average particle diameter of 0.063 mm. This might actually be an overestimate, as fine particles (such as clay) can fill in in between large particles like sand. For sandy loam the same calculation gives an average diameter of 0.12 mm.

Assume that each particle is accompanied by one pore space. Then go to Table 1, which is three pages past the triangular diagram on the web page, and find the porosity (% of soil volume occupied by pores and not occupied by soil particles) of the soil that you are interested in. Clay loam is 49% and sandy loam is 43%. In clay loam the pore space (49%) makes up almost the same amount of volume as the volume of soil particles (51%); multiply the ratio 49/51 times the 0.063 mm particle diameter to get the average pore size of 0.061 mm. For sandy loam, multiply the ratio of 43% porosity and 57% soil particles times the 0.12 mm particle diameter to give an average pore size of 0.09 mm. Remember that these are averages and that because of the mixed composition of most soils, the range of pore sizes can easily be a factor of two to ten, depending on the soil type.

As soil particle size increases, the pore size increases, although not proportionately to the increase in particle size. Therefore, as soil particle size increases, the porosity decreases. Soil with large diameter particles (coarse texture) can contain less water than soil with small diameter particles (fine texture).

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


Addendum - March 2009
Determining the soil water content and porosity is not too difficult, just a little time-consuming.

We use aluminum or steel tins about 4 inches in diameter and 3 inches deep to take or hold a soil sample. Any can that size, such as a tuna can or a green bean can will work, depending on how thick a layer of soil you want to collect. Cans with too big a diameter are hard to push into the soil. We usually use a special tool to core the soil, but you don't really need that.

First, remove the paper from the outside of each can. Then use a permanent marker to mark a different number on each can bottom. Then weigh each can and record their weights (it's best to use grams as the unit of weight). Use a good balance scale like the electronic ones that are found in a Jr. High or Sr. High school science room.

Remember to adjust the zero on the scale before weighing anything.

Now you can take your sample cans outside.

You can sample the same depth of soil with each can by marking a ring around the outside of the can at the same distance from the can top.

If the soil is not too dry, you can push a can into the soil to the depth that you're interested in (turning it as you push helps), wiggle it around and then pull it back out. After some practice you'll be able to do this so that the soil will come out about even with the can top. Don't push the can so far into the soil so that the soil smashes up against the bottom of the can, because that will compact the soil and make the sample unusable for calculating porosity.

Cover the can with a plastic lid or plastic wrap to keep water from evaporating from the soil. It's best to use at least 3 can samples to take three soil samples, as you would be surprised how different three samples from only a few feet apart can be.

Next, measure the depth of the hole (in centimeters, cm) that you made in the soil with each can and record the depth for the can. Determine the volume of soil that you have in each can by calculating the area of the inside of the top of the can from it's radius (in cm) squared, times pi (3.1416) times the depth of the soil. Do that and record the volume of soil sampled with each can in cm cubed.

Weigh each soil can (without the cover on it) and record this.

Subtract the weight of the can itself to get the "wet" weight of the soil.

Put the soil cans and soil into an oven set at at least 150 degrees F for 24 hours (your Mom will love that if you have to do this at home). 24 hours is the minimum amount of time required to make sure that all of the water has evaporated from the soil in the can.

After 24 hours, carefully remove each soil can and weigh each one (with the soil in it). If you don't have a scale at home, you can cover the can (tightly) as soon as it cools off enough and take the cans to school to weigh them.

Record the weight of each soil can.

Subtract the weight of the can itself to get the "dry" weight of the soil.

Now we're ready to perform the calculations. "sm" below is soil moisture, which can be calculated in reference to volume or weight (the latter is called gravimetric soil moisture). "wet weight" and "dry weight" refer to the weights of the soil when wet and dry. "volume" refers to the volume of soil in the can. The density of water is 1 g per cm cubed, so by dividing by volume in cm cubed, you are actually dividing by grams of water. "bd" is the bulk density, which is used to help determine the "soil type". "p" is the porosity.

sm (by vol.) = [(wet weight - dry weight) / volume](100) in units of % by volume

sm (by wgt.) = [(wet weight - dry weight) / dry weight](100) in units of % by dry weight

bd = (dry weight) / volume in units of g cm-3

p = 99.67 - 37.45(bd) in units of %

Information on bulk density and soil type can be found at

http://cru.cahe.wsu.edu/CEPublications/pnw0475/pnw0475.html

On the fourth page you'll find the soil texture triangle which show the effect of clay and sand on soil porosity and on page 8 is a table of soil textures (soil type) and bulk density that you can use to determine the type of soil that you sampled.

David Cook



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