Soil Density Calculations

Meghan Sindelar Author

06/22/2016 Added

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This recording captures the author working calculations to determine soil bulk density and soil particle density from collected data.

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[00:00:01.504]Hello.
[00:00:02.469]Dr. Meghan Sindelar here.
[00:00:04.235]This video is an example calculation of doing
[00:00:07.949]bulk density and particle density calculations
[00:00:11.327]for the soil physical properties.
[00:00:13.336]It is assumed that you are familiar with these concepts
[00:00:16.258]and just want to see an example of worked problems.
[00:00:21.008]Here are the formulas.
[00:00:23.275]All density is a mass per volume.
[00:00:28.408]So, you got the volume of this thing, weight it.
[00:00:36.462]Okay, so bulk density is the mass of solids in soil
[00:00:40.574]so air doesn't weigh much,
[00:00:42.546]water is variable so we don't use that,
[00:00:45.299]just mass of solids over the total or bulk volume of soil.
[00:00:53.339]Particle density is still the mass of solids
[00:00:56.846]but now we're only looking at the volume of solids
[00:00:59.919]or volume of particles.
[00:01:03.075]And so, particle density just tells us about
[00:01:05.048]the properties of the given solids.
[00:01:09.635]Like, how dense is this sand grain?
[00:01:12.043]And so, volume of solids is often determined by
[00:01:15.468]water displacement or really not determined at all.
[00:01:21.015]Okay, here's an example of some data
[00:01:22.919]you might collect in the field.
[00:01:25.341]So, use a tin of known volume,
[00:01:29.053]get some soil in it at field moisture, it's a certain weight,
[00:01:33.534]goes in the oven at 105 degrees for 24 hours.
[00:01:37.146]Now, we've got a lighter weight
[00:01:38.436]because the water's been driven off.
[00:01:40.667]Don't forget to subtract with the tin weight.
[00:01:43.694]Here are the dimensions of our tin
[00:01:45.674]so we can figure out the total volume of soil that we had.
[00:01:51.826]And then, in some cases,
[00:01:54.107]you might grind your dry soil sample,
[00:01:58.753]place it in water to get a displacement
[00:02:01.480]and that then is your volume of solids.
[00:02:05.268]So, if it displaces 57 milliliters of water,
[00:02:08.811]a milliliter is equal to a cubic centimeter,
[00:02:12.604]so our volume of solids is 57 centimeters cubed.
[00:02:23.672]Well, perhaps we took a sample from a location
[00:02:25.760]just to check if the density was ideal for our use.
[00:02:28.784]The different uses do call for different densities
[00:02:32.738]such as road construction or plant growth.
[00:02:36.708]Okay, so all of these worksheet pages
[00:02:39.150]will be relatively similar.
[00:02:41.957]Got the formula here.
[00:02:43.915]The snippet of data that we need over here.
[00:02:47.467]And let us begin.
[00:02:50.168]Bulk density is equal to the mass of solids
[00:02:52.759]over the volume total.
[00:02:55.366]Our mass of solids would be the dry soil,
[00:03:00.817]minus the weight of the tin.
[00:03:04.171]Our volume total is, in this case, 118 centimeters cubed.
[00:03:10.509]So, 183 minus 32 is 151.
[00:03:15.520]Over 118 is equal to 1.28 grams per centimeters cubed.
[00:03:24.884]So, a mass per volume,
[00:03:26.685]those are the appropriate units for a density.
[00:03:32.255]Is this a typical bulk density value?
[00:03:37.235]Yes. It is.
[00:03:38.303]So, let us assume we've done this correctly and move on.
[00:03:42.804]Particle density, remember now,
[00:03:44.954]is going to still be in the same mass of solids
[00:03:47.824]but then on the bottom, we'll use volume of solids.
[00:03:50.452]Though the top will be done the same. Right?
[00:03:53.518]183 minus 32 and now on the bottom,
[00:03:57.824]we've got our volume of solids, 57.
[00:04:09.570]2.65 grams per centimeter cubed.
[00:04:13.840]151 divided by 57.
[00:04:18.021]Like I said earlier, it's not too common
[00:04:19.817]to actually measure your volume of solids
[00:04:22.487]so instead we frequently assume a particle density
[00:04:26.003]rather than calculate a particle density.
[00:04:28.896]What is the assumed particle density for most soils?
[00:04:35.259]Yeah, 2.65.
[00:04:37.928]So, there is an example of calculating particle density
[00:04:42.221]and bulk density for a given data set.
[00:04:45.646]Remember, all densities are mass per volume
[00:04:48.461]and grams per cubic centimeter is the units typically used.

Soil Density Calculations

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