# Soil compaction: misguided explorations by a dense airhead



## dwaugh (Aug 25, 2020)

The recent discussions regarding liquid aeration piqued my interest in what compaction and aeration really are (or what they mean from a practical standpoint). Sure, I can look up definitions, and I have, for bulk density, penetration tests, infiltration, etc. So I figured I would start a thread to document my misguided exploration of the topic.

I figure I need to be able to get infiltration readings, penetrometer readings, bulk density, and some "screwdriver tests". Some readings on actually porosity would be nice too.

First off is the infiltration test. @g-man and @Ridgerunner will be cringing. 

I looked at a few guidelines for infiltration tests, clearly my equipment is sub-standard, but the idea is see how long it takes for one inch of water to infiltrate a given area of soil. I used a tomato can with both ends cut off. I should probably remove the band that holds the lid on for the end going in the soil. This was a plausibility test, but I drove the can into the soil. and added some water. For a "real" test I will need to get the diameter of the can and calculate the amount of water needed to make up one inch and then measure that out a graduated cylinder.


One video I saw suggested the measured water should be poured out over a piece of plastic wrap which is then pulled out when the timer is started. 


The timer stops when water is only "glistening on the surface". I didn't measure the actual amount of water, but it took 1 minute 47 seconds to infiltrate. I think there is a fair amount of error in calling when to stop the timer.


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## dwaugh (Aug 25, 2020)

So there are some "legitimate" soil penetrometer that use a slide hammer, I think. I found a long drill bit that I can grind a cone on the "chuck" end. I think it would be better to drop a weight in a tube on the probe, but I had a "slide hammer" part of nail puller that I could drop on the probe. I will work on a better and more repeatable hammer, but even with this setup I think I could get repeatable results, although not tied to any standard.

Below is the not yet sharpened probe

Below is the slide hammer

After 10 blows (I will have to find a way to calibrate the drop height) the probe went in 11.1 cm. 


Goofball, yes, but I think I can this test to be at least repeatable.


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## dwaugh (Aug 25, 2020)

Bulk density will be harder. It seems I need to get the dry weight of a given volume of soil.

So I have a hand aerator that will pull a core. Clearly it will be compacted somewhat. 


I pulled some cores and collected them from each of the two sides of the corer.


The core with was measured, there was certainly some variability, but the mean was around 13mm. More work will be needed to get a better average, and then for sake of repeatability I will probably just use that average. 


Very imperfectly I cut the cores in areas that didn't break apart at right angles to the length and took measurements. Clearly a lot of error here.


Next step is get a wet weight. 


Next step is baking off the moisture. I probably have the temperate too high...


With some refinement, I think I can get some reasonable values... Or not.


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## BobLovesGrass (Jun 13, 2020)

Thank you for these efforts.
I think the water test might be easier if you just premeasure an inch worth in a can, volume doesn't even matter an inch in a same diameter can is repeatable.

Next time I open some cans I will save a couple and try and repeat your test


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## dwaugh (Aug 25, 2020)

BobLovesGrass said:


> Thank you for these efforts.
> I think the water test might be easier if you just premeasure an inch worth in a can, volume doesn't even matter an inch in a same diameter can is repeatable.
> 
> Next time I open some cans I will save a couple and try and repeat your test


I think what you are suggesting is that before I cut the cut to bottom of a can fill it to one inch and the measure that volume as my standard? That sounds like a good idea. Certainly I need to use a fixed, and measured, volume that as is as close to one inch as I can get.

My hypothesis is that although bulk density has been somewhat discounted as useful metric in golf courses (which generally have sand caps), it might be more useful for home lawns with non-sand soils. I'm fairly certain that I have compacted soils, often water logged during rain, that I could push a 3 foot screwdriver through.

I suppose proof is in the pudding, so I will also set up some test plots in which I aerate.


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## dwaugh (Aug 25, 2020)

Somewhere in the uncanny valley between science and "performance art", we have some numbers.

I got a soil bulk density of 1.2 which is a reasonable number..... I do think the cores are too disturbed and there is probably too much error in the volume calculation. So I will need to take some larger cores (which will have a lower percentage of disturbed soil). There is also some data on moisture content, the cores lost 10 grams of water after oven drying.


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## dwaugh (Aug 25, 2020)

More boring results. 
So I found a larger ring to use for infiltration testing. I ball parked the numbers to fill it with one inch of water (there is room for fine tuning).





I think some of the results were interesting. In the middle of the backyard, 3 replicates, non-overlapping, but in the same area resulted in 1 minute 12 seconds, 1 minute 52 seconds, and 1 minute 51 seconds.

Then I tested an area that is constantly damp, but gets no more water. I got 10 minutes 28 seconds, 3 minutes 45 seconds, and a 3rd test I stopped after 8 minutes because the mosquitoes were out and I was using a flashlight.

I think the low number in the damp area may have been because it was over an aeration hole that I made with my hand aerator. Clearly, despite not ideal testing situations, the two spots have different infiltration results.

In the damp area with lower infiltration rates, is was way easier to push a screwdriver into the ground. I can still push a screwdriver in the areas with higher infiltration rates, but there is more resistance. So this gives some clear areas that are worth investigating.


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## dwaugh (Aug 25, 2020)

More data of possibly questionable value....
So this was an attempt to try out my soil penetration test before trying on some test plots.



So a added a ~45 degree bevel to the end of a 7mm steel rod (A); I realized that the grass/thatch were causing issues, so I used a pipe flange and short length of pipe to allowed me to use a wood space (B), which when the point was on the ground surface, I could space the pliers at a fixed interval above the top of the pipe, after which I could push the rod, at fixed distance, into the soil, about half an inch (I have the wood block and can get a real thickness) (C); Then the rod was marked so that the "drop weight " could be set to fall a fixed distance (D). Clearly not in such an ersatz manner, but there is a legitimate test like this using a drop weight.... So I used 9 drops of the weight (probably not relevant, but F=M*A, so I can weight the mass of the weight, the height of drop and actually get the amount of force applied. The final recording was the distance of penetration after 9 blows.

So yes, the above was inelegant and is full of sources of error (not to mention soil moisture).

I'm not sure if this test will be useful, as it would be too hard to try and calibrate it with a standard test, at the least it's repeatable and can be used in a qualitative manner. Anyway, the results (below) show multiple tests in two areas. There were an unequal amounts of replicates in each location, and obviously I had to move around some, but for each location all the runs were within 1 square foot. It would seem site 1 was more resistant to penetration than site 2, more so if you removed to two outlier points (which I did not). I can't compare these results to anything, and I have no idea what variance is using a calibrated instrument. Of interest, although these are not the exact spots, but site 1 is in the area where I got fast infiltration, and site 2 is were I got slow infiltration. More refinement is needed, but this would suggest that at least in my lawn, the higher the penetrometer reading, the slower the water infiltration. Which is maybe not what one would expect?


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## dwaugh (Aug 25, 2020)

More data that no one will find interesting. So I redid the soil bulk density more or less following this guide with the materials I had on hand: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_050956.pdfg


I cut out some of the existing grass which was not deeply rooted (A) and scraped the surface to be flat, but not disturb it with a small masons trowel; so the soup can (B) didn't work so well as a core puller, but despite some deformation issues which were isolated toward the top, I think I got a decent core. I dug down around the core/can to get it out without disturbing the sample (C); then, using a knife I cut the bottom of flat with the soup can.

So, not perfect, but not so bad (maybe), after weighing and drying the soil, I ended up with a bulk density of 1.29 (g/cm^3). That's actually fairly close to my first test using the smaller cores which was 1.24 (g/cm^3).


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## Glen_Cove_5511 (Jun 13, 2020)

Enjoying this "exploration" @dwaugh. Very interesting and I appreciate you taking the time to document and post everything.


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## dwaugh (Aug 25, 2020)

@Glen_Cove_5511 I'm glad you are enjoying, you must be easy to entertain (like I am) 

Here are the water infiltration test results


NIST calibrated peanut can (A); Fill with water based on diameter of can so that 1 inch is added (463ml in this case) on top of plastic wrap (B), after that the plastic is pulled out and timer started; the can was pushed down about 2 inches, after the test, while trying to remove the can, all two inches of soil pulled up, it doesn't seem the roots go much deeper than 2 inches here (C); (D) would suggest some subtle layering, possibly alignment of clay particles?; the bottom of the hole shows two large worm holes, and some smaller ones (E) this may have been where the water was going...


Above are the results, for 3 sequential tests at each of the two location I tried out. Orange line shows results for the test showing in the images, blue line is a different location. Subjectively, the orange line with slower infiltration is in an area that is easy to penetrate with a screwdriver. Although it's possible, maybe likely, there is no significant difference between the two lines. These results seem reasonable.


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## dwaugh (Aug 25, 2020)

Who knew, but the local auto parts store sells tools for taking soil cores to test bulk density, as a bonus they have stainless steel models. I've pulled a few cores and am currently drying the soil.

I did have to grind an edge on one end


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## dwaugh (Aug 25, 2020)

Results of more catiwampus testing using the exhaust pipe sampler in the post above.


Above, A, clear a flat area by scraping the surface; B, drive in core tool; C, undercut core in pipe; D, scrape off bottom of core.


The bulk densities of both soil would indicate the soil is not compacted, but the "screwdriver test" suggests the soil with a higher penetration resistance is actually less dense and has a higher water infiltration rate. The nominally more compacted soil is easy to penetrate.

So far, I'm beginning to think that the concept of soil "compaction" in the context of lawns, is nebulous. At the very least, penetration tests and bulk density are only proxies for what's going on.


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## dwaugh (Aug 25, 2020)

What I thought was most surprising was the calculated soil porosity that I got (around 50%, see above post). I'm not convinced that porosity estimate is reasonable, or at least it warrants further investigation. The calculated porosity is based the density of solid rock. Today I took an undisturbed core from an adjacent location, got a starting weight, and then by ~tablespoon increments added the soil back into the metal ring and compacted each layer using a stainless steel ~1cm rod, with a occasional few taps from a hammer. The idea was to compact the soil as much as I reasonable could. This allowed my to calculate "porosity" as the difference between the compacted versus uncompacted over the the uncompacted volume, this gives a porosity of 25% which is certainly much less than the original 50% value. I suppose the reality is more complicated, and then we get into macro vs micro porosity.


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## dwaugh (Aug 25, 2020)

This is an interesting read on the topic https://www.yumpu.com/en/document/read/38430673/is-bulk-density-the-answer-pace-turf


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## ionicatoms (Mar 8, 2020)

:thumbup:


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## dwaugh (Aug 25, 2020)

ionicatoms said:


> :thumbup:


Thanks @ionicatoms

More boring data.



The new data is from sample 4 (but some other data was added to the table). Sample 4 is another bulk density sample in which I collected an undisturbed sample, and then compacted it in tablespoon increments (in the same core tube used to pull it), and then calculate the difference between the "as collected" volume, and "compacted volume", using the difference to calculate a porosity measurement. This value is very different than the porosity calculated in the traditional way in which the density is compared to the density of rock. Presumably the traditional measure of density is micro + macro porosity, the porosity based on the difference between the compacted and as collected sample is presumably capturing mostly the macroporosity.

Is there a take home point? I'm not sure.... I think in the loamy clay soil (not sure of the exact classification) I have, the "screwdriver test" might be not a good indicator of compaction. This is based on samples 1 and 2 taken in the same area which are "hard" to penetrate with a screwdriver, but seem to have higher infiltration rates and lower macroporosity. While samples 3 and 4 taken in the same area are "easy" to penetrate with a screwdriver, but have lower infiltration rates and lower macroporosity. The bulk density in both areas is seems about equal. Now sure, these are meatball numbers, there could be math errors, and there is little replication of the results.

So how does one know if their soil is "compacted"? :dunno: My guess it depends on the soil type and probably a holistic view is needed.


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## CarolinaCuttin (Sep 6, 2019)

I think the screwdriver test is only viable for a given level of moisture. The depths to which you can insert the screwdriver is going to increase as soil moisture increases, so unless you can get constant moisture it's not going to tell you much.

It also depends on what we mean by compaction, to me when a soil is being compacted I'm thinking about the absolute volume of space (air) between particles decreasing. Anything that decreases the volume of pore space is compacting the lawn.

I think what "liquid aeration" does is provide uniform wetting of the soil profile, mostly through the use of wetting agents. This makes it easier to insert a screwdriver, but I don't see how it is increasing the total volume of pore space. One can imagine a situation in which a reaction goes on in the soil to release gases which causes the soil to fracture and open up air pathways, and there are products on the market for professional turf managers that do just that, but I don't see that technology in any DIY products. The list of ingredients is pretty unremarkable.

To me there is no substitute for pulling a core, and if you can fill it with sand even better. For especially tough areas, a drill and fill can work wonders. Getting 12 inch vertical sand channels in the profile changes everything.


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## dwaugh (Aug 25, 2020)

CarolinaCuttin said:


> I think the screwdriver test is only viable for a given level of moisture. The depths to which you can insert the screwdriver is going to increase as soil moisture increases, so unless you can get constant moisture it's not going to tell you much.


Very true, certainly moisture will effect the screwdriver test, althogh as long as the soil is generally moist I think it has some value, but it's clearly no so simple.



CarolinaCuttin said:


> To me there is no substitute for pulling a core, and if you can fill it with sand even better. For especially tough areas, a drill and fill can work wonders. Getting 12 inch vertical sand channels in the profile changes everything.


This is what I think it interesting about the topic. In most soils, pulling cores and adding sand will actually increase the bulk density, but likely (I think) increase porosity (locally), and will certainly increase infiltration. But, if the holes are filled, the compacted soil in-between the holes can't "relax" and de-densify as the existing soil creeps into the holes from lack of lateral support. Now clearly there are reasons for sand filling holes, and clearly golf courses do that for any number of reasons. 
Oddly, I just got one of those soil drills you mentioned here and in another thread a few weeks ago, I've got some low areas that don't drain well and at this point in time regrading and adding drains in not practical for me, so I'm going to try that out to increase drainage in a few areas.


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## CarolinaCuttin (Sep 6, 2019)

@dwaugh I would say that the holes can still relax after the addition of sand since the sand channels aren't very rigid laterally in the profile. If we forcefully packed the sand in rather than just working dry sand in naturally I would agree with you.

I've used a regular 18V cordless drill with a 7/8" auger bit and treated small areas with drainage issues with success. I'm not kidding about going 12" deep it's easy to get down there and bust up layering. Please let us know how it works for you, I love the way you are trying to quantify these changes.


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