7.5 Tiny Bytes: Penetrometers and the Real Cost of Compacted Soil

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And now it's time for a tiny bite of knowledge.
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Welcome back to Tiny Bytes.
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Some podcasts explore the mysteries of the human heart.
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That is not this podcast.
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Today, we are going to explore how hard it is to shove a metal rod into soil.
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Welcome.
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We're so happy you're here.
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On today's episode, we're talking penetrometers and soil compaction.
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Penetrometers are that rod that we stick into the soil to measure and diagnose soil compaction.
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Let's talk about soil compaction first.
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Specifically, what really is soil compaction?
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And I think this is something that's interesting for us to think about.
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You know, soil is made up of mineral and pore space.
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Compacted soil has had a reduction in pore space.
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And so when we're trying to measure compaction, we are trying to measure exactly what force a root needs to push to go through that compacted layer because it be gets to be very dense and very hard for a plant root to push its way through.
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And it can actually have a pretty big impact on your crop.
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Actually, it can easily reduce crop yields up to 10% or more.
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And by the way, I just want to give a big shout out and credit to Jody Deon Hughes from University of Minnesota, who has actually previously defined soil compaction as a reduction of pore space and some of these facts that are contained.
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All right.
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So just thinking about soil compaction and what it looks like out in the field.
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Like I think all of us that are in agronomy probably know exactly what part of the field we're thinking about, right?
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The headlands, those areas that get a lot of traffic, um, fields when they're moist or they're wet and they're driven on what happens, like depending on what level of saturation they're at, some levels can be, some levels of moisture can be a lot more damaging and more harmful towards contributing towards compaction than others.
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But you know, when you're getting those big ruts that are pushing mud out the outside, that can leave a history and a scar of compaction in soils.
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And like what you'll probably see, right, is that like the headland areas, the crop that comes on the headlands does just doesn't look as good as the area that's farther into the field.
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Usually because it's not.
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Yeah.
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Usually because it's not.
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And compaction in soil, compacted soils restrict oxygen movement to roots.
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It limits the amount of water that roots can take up.
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It causes the root systems to be stunted because it takes so much force for those roots to grow through.
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And it reduces access to that deeper water, especially in a drought situation like some places are actually experiencing right now.
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And so compaction is very interesting.
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Other sideline impacts to farming can also be increased fuel costs.
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It costs a lot, it takes a lot more fuel to pull a tillage tool through a compacted soil versus a well-aierated soil or soil with good porosity.
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So we've talked a little bit about why soil compaction is important, but let's also talk about how we can measure it.
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Because soil compaction can actually occur in a field at different severities.
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Yeah.
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Okay, why is that important?
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A soil can be compacted, but the level of compaction can vary greatly.
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And there is actually a way that we can measure how compacted something is and whether it needs to be like say subsurfaced with a different piece of tillage equipment, or if it doesn't have to be a subsoiler.
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So, yes, there are different levels of compaction that exists.
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And it's important to know the difference in where these places are because in some places you may have to get a subsoiler to help reduce that compaction farther down in their profile.
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In some areas, you may only need a more shallow tillage tool to um address and manage that compaction.
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And so that's why it's important to go out with a tool like a penetrometer, that nice steel rod, and use that to measure what the compaction level is.
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Right.
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And so what is a penetrometer?
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A penetrometer is like we said at the beginning, it's basically a metal rod.
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And at the top of it, it has um a pressure gauge, and it actually measures PSI or pounds per square inch, the amount of force as you push that rod through the soil.
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That's the basic overview.
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So, um, how you operate this is you take your penetrometer, you take it out into the field, you push it down into the ground.
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Um, and and you as you push down, you need to make sure that you've got a very steady pressure on that.
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Do you like my animation here?
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Very great for our podcast listeners.
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And in case you wanted to know, listeners, I'm wearing my fiery trash can of death uh t-shirt, which is the same as a button in our software.
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It's very fun.
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But anyway, so you push down on the penetrometer with very even pressure pushing that down.
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And as you push down, the pressure gauge is going to indicate that resistance in pounds per square inch.
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And the driving shaft should also show your depth.
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So it should be marked like every three inches, so you can see at every three inches what that PSI reading is.
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The readings that you record as you go down through the soil profile are going to be the cone index, and they quantify the strength at each depth interval.
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Basically, what is happening here is that the penetrometer is simulating the mechanical effort that a plant root has to put forth to grow through that soil profile.
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So, what do the values mean?
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Basically, at 200 psi, you've got a healthy soil structure.
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Um, the roots can penetrate through very freely without restriction.
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So that's that's a pretty good soil.
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And below 200 is good too.
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So yeah.
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Below 200.
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Sorry.
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Thank you, Tody.
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The soils must be at 200 psi.
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Could you imagine what the PSI of a peat soil is?
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It's probably not 200, it's probably way less.
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Um, 200 to 300 psi.
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It's kind of a caution area in there where there's some resistance to some root growth.
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You know, monitor it closely.
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And above 300, that is a severe limiting um compaction.
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Plant roots are gonna practically stop penetrating.
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This is gonna kind of be in that significant threshold um to restrict plant growth.
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So sometimes roots can still penetrate through soils greater than 300 psi, but that's if they are finding cracks and pores that continue to be present.
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So it's not in the area where all of the pore space has been completely compacted out.
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Maybe all of a sudden that plant root finds that crack to like go through, or one little pore that didn't quite um get compacted all the way out.
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But keep in mind the reason why that 300 or greater psi exists is because there's just not that much opportunity for that.
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So keep that all in mind.
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Another thing to keep in mind when you go through the uh soil profile, if you go from a smaller particle soil texture type into a larger one, you can feel a sudden change in the PSI.
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And in that instance, it is not necessarily compaction that you're running up against, but it is an actual change in the soil texture as you go through the soil profile.
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So if you're going through something that is a uh a clay and all of a sudden you hit a sand layer and you and it feels like you're hitting a brick wall, it's because, well, essentially, if you compare the particle size, you kind of are.
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So it can feel very different.
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So if you do run into a particle size change, that is one thing to consider as well.
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And when you're out there working with the penetrometer, there are a couple of things to keep in mind in order to get consistent measurements.
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And so one thing to think about in terms of timing is making sure you're going out when soil is at field capacity.
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So when the soil is moist but not muddy.
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So this is typically 24 hours to a few days after a soaking rain.
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What this means for most of us here, at least in the upper Midwest, that best time of year is probably gonna be spring because the whole profile has usually been thoroughly moistened during the winter and the early spring.
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And it's not too shortly after that you're gonna be at field capacity.
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Um, if it is too muddy out there, do know that your compaction levels that you're measuring with the penetrometer can be underestimated because the soil kind of acts like a liquid instead of that force.
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So it's gonna be easier to drive that rod through the soil than if it was drier.
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And then on that same coin, that same flip or the other flip of the coin, other side of the coin, if the soil is too dry, compaction could be overestimated because the roots um will penetrate it when that soil dampens.
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So just keep that in mind.
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And then, of course, like some there's some very probably obvious advice too, but make sure you're not trying to measure the resistance of a rock or tree roots or plant roots or quiz if you're if you're not trying to measure your equipment tracks.
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Um, don't take measurements in equipment tracks or high traffic areas unless you're trying to diagnose what the compaction levels are in those areas.
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And so do keep that in mind.
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You know, it's it's gonna be similar to taking a soil core.
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Um, remove the litter off like scrape the litter off the surface of the soil so that you're reading the penetrometer um resistance of the soil and not other residue that's on top of it.
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I think that's really good advice, Jody.
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Really good advice.
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And for any of you out there who have pulled soil samples, the cool thing about uh a penetrometer reading is as long as you follow those rules that Jodi just said and you use steady pressure as you push the penetrometer through the uh through the soil, you don't have to worry about the core quality because there is no core, which is really nice.
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So that was a great conversation talking about shoving a metal rod into soil.
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Yeah, I'm really happy that we did not cover the intricacies of the human heart.
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This was far more interesting.
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Thank you all for joining us.
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Tune in next time for a tiny bite of knowledge from GK Technology, where we have a map and an app for that.