Soil Porosity Calculations

Meghan Sindelar Author

08/09/2016 Added

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This recording captures the author working calculations to determine soil porosity from collected data.

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[00:00:02.015]Hello, all.
[00:00:03.927]Dr. Megan Sindelar here to give you
[00:00:05.552]a sample calculation on porosity.
[00:00:08.495]This video is for those who have a general idea
[00:00:11.206]of what porosity is and just want to see a worked problem.
[00:00:17.512]Here is our data set that we are working with
[00:00:21.423]for these soil physical properties calculations.
[00:00:25.012]From this data, we have already calculated
[00:00:27.445]that the bulk density is 1.28 gram per cubic centimeter
[00:00:31.635]and the particle density is 2.65 gram per cubic centimeter.
[00:00:38.929]This is the actual formula for porosity.
[00:00:43.424]Porosity is the percent of the soil that is pores
[00:00:48.115]versus the percent that is solids.
[00:00:51.278]It tells us nothing about the size of those pores
[00:00:53.719]or what's in those pores, if it's all air, all water.
[00:00:56.868]It's just the percent of the total soil that is pores.
[00:01:03.017]However, it is awfully difficult to measure
[00:01:07.996]the volume of pores, so, instead, more commonly,
[00:01:12.704]you would have volume total minus
[00:01:15.624]volume of solids over volume total.
[00:01:21.682]Which, you might algebraically notice,
[00:01:25.472]would be volume total over volume total minus
[00:01:28.410]volume of solids over volume total,
[00:01:32.501]or one minus volume of solids over volume total.
[00:01:39.524]That's a little bit more practical of an equation.
[00:01:44.968]Still, we don't often actually measure volume of solids.
[00:01:50.635]Where's the other place we would use volume of solids?
[00:01:53.276]To calculate particle density.
[00:01:55.157]Often, we use an assumed particle density of 2.65.
[00:02:03.610]Particle density is the mass of solids over
[00:02:11.033]the volume of solids.
[00:02:14.017]If we multiply both sides by volume of solids, that cancels.
[00:02:17.909]Then we would divide both sides by particle density.
[00:02:23.960]Our volume of solids equals the mass of solids
[00:02:27.996]over the particle density.
[00:02:31.062]We can kind of stick that in over there.
[00:02:34.434]Similarly, bulk density is the mass of solids
[00:02:40.837]over volume total.
[00:02:42.634]How convenient is that?
[00:02:44.811]That can be rewritten as volume total is
[00:02:49.228]mass of solids over bulk density.
[00:02:55.688]Where are we at here, in our equation?
[00:02:58.492]Let's take this part and substitute in here.
[00:03:03.598]We've got one minus mass of solids over particle density,
[00:03:10.483]over, I'll take this part and substitute here,
[00:03:15.684]mass of solids over bulk density.
[00:03:20.101]I don't know how good some of you are at algebra,
[00:03:22.665]but when you are dividing two fractions, what's on the
[00:03:26.363]bottom of the bottom loops around to the top.
[00:03:29.065]If you were dividing two fractions, in theory,
[00:03:31.929]what you do is flip the second one upside down
[00:03:35.927]and then multiply.
[00:03:37.193]We've got MS over PD,
[00:03:42.514]divided by MS over BD,
[00:03:45.686]so in reality, you would write that as MS/PD times
[00:03:50.878]bulk density over MS.
[00:03:53.970]These guys now cancel, and we're left with bulk density
[00:03:58.650]over particle density.
[00:04:03.039]The two tops cancel.
[00:04:04.496]The bottom of the bottom loops around to the top,
[00:04:06.815]or you can look at this stuff down here.
[00:04:08.852]One minus bulk density over particle density
[00:04:15.205]is a more realistic formula for doing
[00:04:17.665]a porosity calculation.
[00:04:20.135]This is probably the formula that you want to learn,
[00:04:23.785]and you want to plan to use in the future.
[00:04:27.759]Our porosity, our data here, we've got one minus
[00:04:32.852]1.28 grams per cubic centimeter over
[00:04:39.666]2.65 grams per cubic centimeter, which, you remember,
[00:04:45.230]if you don't have particle density value,
[00:04:47.695]you can assume 2.65.
[00:04:51.560]That's one minus 0.48,
[00:04:58.302]or 0.52, or 52%.
[00:05:06.551]This soil is 52% pores.
[00:05:14.497]Now, given an ideal bulk density, around what percent
[00:05:20.598]pores would we expect a soil to have?
[00:05:27.147]Fifty, yes.
[00:05:28.839]A soil should be about half solids and half pores.
[00:05:31.941]This is a healthy bulk density, and a healthy porosity.
[00:05:38.219]There we go, this is just an example of calculating porosity
[00:05:41.327]from bulk density and particle density.

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Soil Porosity Calculations

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