#173 - Field Capacity from Orlando Florida
The Sprinkler Nerd ShowJune 21, 202435:3414.24 MB

#173 - Field Capacity from Orlando Florida

Andy revisits a past episode on field capacity, discussing soil moisture sensors, practical analogies, and their importance in irrigation systems.

Andy Humphrey kicks off the episode with "Little Silver Ring" by The Samples and shares insights from his trip to Orlando for volleyball Nationals, highlighting its economic impact. He then revisits a past episode on field capacity, discussing soil moisture sensors, practical analogies, and their importance in irrigation systems.


(0:00) Andy Humphrey introduces the morning song of the day

(1:31) Andy Humphrey reveals the song: "little silver ring" by The Samples

(1:44) Podcast introduction and target audience

(2:20) Andy Humphrey talks about being in Orlando for volleyball Nationals

(3:47) Discussion on the economic impact of the volleyball tournament

(7:36) Transition back to the Sprinkler Nerd Show

(8:15) Replaying a past episode titled "field capacity, my ass"

(9:40) Experiment with multiple soil moisture sensors

(12:21) Shout out to Juanita and replay of the April 2023 episode

(12:58) Defining field capacity

(14:23) Using a sponge as an analogy for field capacity

(17:09) Comparing field capacity to a percolation test

(19:16) Field capacity as the baseline for soil moisture

(22:10) Explanation of how to measure and apply field capacity

(27:20) The art of setting soil moisture sensors

(32:40) Analogies for understanding tank size and soil moisture

(33:14) Importance of field capacity in irrigation systems

(34:16) The future impact of soil sensors in the industry

(35:15) Closing remarks and wrap-up


And so I'm gonna play that for you today. Here because I don't have time to record a full episode with brand new content, but it did get me thinking about just soil moisture sensing and field capacity and understanding the numbers, and you're gonna hear all that on this episode, this replay episode. But I wanted to share a new concept that actually it's not completely new, but a concept that I have running, in ex in an experiment right now. And that is a concept of taking multiple soil moisture sensors, let's say, within one zone, and averaging them together to creating new soil moisture or that you might think of as a synthetic value. So there's always the angel question of, you know, how is this one spot where the soil moisture sensor is buried.

How how is that relative to the rest of either the site or the zone I'm not gonna get into all of those details. We'll save this for another episode. And and the and that is that is a valid question. And the only real way around causing a mistake is to actually just locate the sensor in in a really good area. But then what would happen?

What would it look like if instead of putting one soil moisture sensor in a zone? Or one soil moisture sensor in every zone, we put 3 soil moisture sensors in one zone and use that one zone as the baseline zone for the rest of the site. And it's not actually possible today to do this with any control system. Even a baseline control system. You cannot install you can install 3 soil moisture sensors in a zone, but they're all going to be independent, and only one of them can actually control the zone.

But what if you could take those 3 soil moisture sensors buried in the one zone, average them together to create a new synthetic, you know, synthetic value and use that average value to control the zone. What would that look like? That can't be done today, but I'm doing it. I'm actually I'm actually running this experiment. We have a a project that has I should know this number off the top of my head, but I don't.

Let's say if it has 8, it has 8 soil moisture sensors. And each soil moisture sensor is actually buried in in its own zone, 8 slow moisture sensors, 8 zones, but then I'm taking all of those 8 slow moisture sensors averaging them together and creating a new value that says here's the average soil moisture across the entire site. And what's interesting about that is, well, first, it it hasn't been done before. But second, it might make up for potential errors in location, formity, etcetera, etcetera, if multiple sensors are installed within one zone average together to create a new value So that's just one experiment that I'm running right now. It's going really well.

And, maybe maybe a manufacturer that might be listed into this podcast, we'll take that into consideration as they develop the platform. And, we'll just we'll just see how it goes. But Wanted to give a special shout out to Juanita. Thank you for being an active podcast listener. Appreciate hearing from you.

And, with your recommendation, I'm gonna replay the episode from April, I think April 28th 2023 called field capacity, my ass. It is great content. I highly encourage you to listen to the entire episode. And if you don't have time today, bookmark it, listen to it again, and I'm, excited to talk about field capacity more in the future because it's more relevant than it ever has been. So thank you so much for listening.

Appreciate all of you. And, we'll just roll the intro here and get right into the episode on field capacity.

Okay. So in order to understand field capacity and talk about field I first think we should define field capacity. Field capacity refers to the maximum amount of water that a soil can hold against the force of gravity. Alright? It's kind of the point at which the soil is fully absorbed I won't say saturated, fully absorbed with water, and then the excess water has drained away.

K? After that excess water has drained away. That's field capacity. At field capacity, the soil is sort of holding as much water as it possibly can and any additional that we add to the soil will actually drain away due to that force of gravity. One great way to illustrate this, maybe this would create a mental picture in your mind, is to think taking a sponge in your kitchen sink.

K? If the sink is full of water, you put the sponge into the water, you let the water soak into the sponge, fully, then you lift the sponge out of the sink, it starts to drip, drip, drip, drip, That's saturation as soon as the water stops dripping out of the sponge. That's field capacity. K? That's the point at which the water can hold the the the moisture after gravitational drainage.

K. So the sponge is a great way to describe this. Great way to understand it. Great way for you to describe this to your customers, your clients. And seeing this on a graph is awesome as well.

So typically, I like to see this on a graph. So historically, working with baseline controllers, I would pull up a soil moisture graph report. I would look at this, on the graph to understand the concept and teach people about it. Okay. So the way that it looks on the graph.

And again, I think probably the best way for you to visualize this is that sponge because we can all picture a sponge dripping. Okay. And what that looks like on the graph is a spike. When when the soil is full of water and it is dripping, it it's not holding water. The water is actually draining out of that soil.

So you see this spike wet. Boom. It spikes up. And it's draining quickly. Right?

The water is dripping out of that sponge. So you get this needle tipped tall spike. And then as the drop start to slow down out of the sponge, that's that's kind of like a curve. It represents a curve, a dry down curve on the graph. And and that curve, which is sometimes called, like, the shoulder, the shoulder of that curve, that illustrates where field capacity is.

Okay. And you can kinda measure it. So if we're looking this looking at this on a graph, you could take your cursor, you could hover over the line, and you could get a a numeric value for field capacity. Okay? So more on that in just a moment.

The other way that we can describe field capacity instead of using that term, which is very scientific, but it's important for us to understand it, but the average person may not. So One way to describe another way to describe that to the average person is just using the term wet. Sounds easy, silly, but it but it's true. It's wet. Not too wet, not too dry, just right.

It's the goldilocks of wet. It's perfectly wet, not too wet, not too dry, just right. It's the goldilocks of wet. K? And again, you can measure this.

It's a scientific value. Feel capacity is scientific. How much you allow that to deplete is the art. You can you can deplete it as much or as little as your plants can sustain. But every soil has a field capacity value that can be measured.

Alright? Let's go with another way to think about this. I'm gonna give you a few examples because maybe one will stick more than another, and these are just analogies that I've used over the years to help describe the concept. A third way to describe this is a lot like a percolation test. Okay.

And a lot of landscapers know this and home builders understand this because if you have a well for your water supply. Actually, not a well. What am I saying? If you have a septic, system, A lot of wells also have septic systems versus city water. But if you have a septic system, typically, you have to do a percolation test to see how the water drains through that soil to see whether that soil can even house or hold a septic tank.

Okay. So typically a percolation test, you dig a hole You fill it up with water. You watch it drain over time. And then that would determine if you would be allowed to get a permit from your town to put in the drain field. Okay.

So the faster a soil drains the more sand there's going to be in the soil. And the slower it drains, the more clay we're gonna have in the soil. Or, like, based on that drainage rate, it can also describe, you know, kind of the compaction and the structure because the soil type is only one variable. Right? So if we say I have Sandy Lom, well, that does just because you have Sandy Lom or you think you have Sandy Lom or you sent your soil test to a lab, and they said you have sandy loam, it doesn't mean the soil actually behaves like you have sandy loam because there is compaction and there's structure in the soil.

They can also affect how water moves through the soil. So What we're really chasing is that field capacity and we want to measure it and we measure it with the soil moisture sensor. Okay. Got it. Am I tracking with you, right?

Picture the sponge, picture the, the perk test digging the hole, watching the water drain through the soil, we can measure those, measure the rate, and determine the soil type or actually more. We can determine that field capacity, which tells us about the soil type with the soil moisture sensor. Alright? So back to field capacity, feel capacity being perfectly wet, so Goldilocks wet. Alright?

Now that's where things start to get really fun because if you can understand that concept, you can now understand how everything else about soil moisture works. It's all relative to field capacity. And I think this is where most current manufacturers, if not all current manufacturers have missed the mark. I'm gonna tell you why here in just a minute. Again, this is my opinion that most of them have missed the mark because they focus on volumetric soil moisture.

They focus on the the data coming out, the actual reading of the soil at any moment, but one reading alone is not enough. Alright? So I'm gonna tell you what that means here in a minute. But I wanna also mention that typically, you know, engineers of soil moisture sensors, either just probes, control systems, engineers, know, like, what we're talking about. They know what this means.

They know exactly what the readings are, how to set the sensors And from my personal experience, teaching and training on the subject matter for, god, it's crazy, almost 20 years. I know how to set the sensors, but I also know that the average knuckle dragger has no idea. And the first question that they always ask me, like, every time, how do I set the sensor? Andy, I put the sensor in. How do I set it?

And so that's kinda what I wanted to talk to you about and provide you with some understanding of field capacity allowable depletion, permanent wilting point, all that shit can be so confusing that what we really wanna focus on is field capacity. Alright? And here's my tip If field capacity equals wet and wet equals full, k? Think of this like a tank, a tank of soil, full is 100%. So I like to take the volumetric moisture percentage and fine field capacity and set that to be full, because this makes the most sense to the average person.

Is my soil full? Is my soil empty or where in between those values is it, but the actual volumetric soil moisture percentage doesn't matter so much after we understand field capacity. Because we're gonna set field capacity equal to 100% and then the tank size is going to be determined from that. Alright? And that's where I think most manufacturers miss the mark because they should set their calibration relative to field capacity and set a new tank size.

Okay? So we're gonna get I mean, I'll give you a few examples here. In in a moment, but I want you to think about that concept. K? If field capacity is wet and wet is full in full must equal 100%.

So field capacity equals 100%. K? Then we wanna know what is 50% full and what is empty. And that's really the skill that we should be thinking about, k, because if we take one reading, and which is actually something that people typically ask me. So let me give you let me give you that example.

Oftentimes after someone installs a slow moisture sensor system, I would get a call. Right? And the the client says, Andy, you know, something like this. My moisture sensor is reading 28.5 percent What does that mean? How do I set the sensor?

K. The question is, literally, most of the time, just like that. My moisture sensor is reading 28.5, or it's reading, it's reading some number, and they wanna know what does it mean? How do I set the sensor? And the funny thing is that my response is usually like, I have no freaking idea.

K? How could how could I after with 20 years of slow moisture sensory experience, not know what 28.5% means. It's actually that I do know what it means. It means 28.5%. But I don't know what what that means in terms of wet or dry.

K? Because I need more information. 28.5 percent or the the value that you're taking at any moment does not mean anything all by itself. It has to be made in relationship to field capacity.


Is that making sense? If we took a reading of 20 percent or 25 percent or 30 percent, one number all by itself doesn't mean anything. It only means something when compared to field capacity. And I made a couple notes here. Wanna give you an example.

So this is one piece actually. I'm actually cheating here. Not reading word for word, but I I made some notes. So let's say for instance, we measure field capacity at 26%. Okay?

If field capacity is 26%, And the client asks me, what does 28.5 percent mean? I I know right away, shit. That's higher than field capacity. That means your soil's freaking wet. Saturated and saturation because I can compare 28.5 percent to the known field capacity of 26.

K? But maybe what if field capacity was 32? And they asked me, what does 28.5% mean? Then I know Well, it's not yet up to fuel capacity. Your tank is about, you know, 50% full or 60% full.

You have to take the slow moisture sensor reading that you get, and you have to compare it to you have to compare it to field capacity. Alright? And every moisture sensor reading that is compared to field capacity has to be compared to field capacity on the very same sensor. You can't compare different sensors to each other because they're all installed in different environments. And how the sensor is installed in each environment could be different.

Even if the soil type is the same, how the sensor is installed can affect the reading a bit, which means field capacity on one sensor might be 30. And in the same soil condition, you know, 100 yards away, it could be 32% based on how that sensor is installed. So you wanna compare the number that you're getting and feel capacity only on one sensor. Never compare 2 sensors to each other. Alright?

So for this reason, what I usually recommend is that during installation, you pour a bucket of water over the sensor or any amount of water that's going to take to saturate the sensor, which means get it more wet then feel capacity. Get it kind of as wet as you can, you know, to a point. And then wait 24 hours. Kinda like doing that perk test. You wanna get it wet.

Then you wanna let gravity take over and you wanna see what it does after 24 hours. You can come back to the site, take a reading, Or if your system is connected remotely, you can look at the graph. You wanna give it some time. You wanna, like, gravity do its thing. I don't like that sponge.

You wanna pick that sponge up out of the sink. Let it stop dripping. So you wanna wet your soil, get it nice and wet, let gravity take over, and then you wanna take another reading. And that reading is field capacity. That's the right or downer.

That's your baseline. That's field capacity. And everything else becomes relative to that number. If you take a reading in the future and it's higher, you know, your soils and saturation. If you take a reading in the future, and it's 50% lower, you know you're freaking dry.

Okay. So the next question I usually get after we find field capacity, is how do I set the sensor? And this becomes the art. So I mentioned before that measuring field capacity is a science. Because you can actually measure it.

The amount that you allow for depletion or dry down is the art, and there's only best practice And there's observation and there's knowledge of your plant material and the health of your plant material because all of those things can affect how dry and for how long can you maintain dry. But I have one general rule that I usually give to all clients as a starting point And that is 20 percent depletion, not 20 percentage points, right, not going from 30 down to 10, not 20 point 20 percentage points, but 20% of a number. And that is how you determine your tank size. You take field capacity, you subtract 20% of it, or you multiply it by 0.8 And that will give you your dry number. Alright?

So let's look at a specific value. Let's pretend here today, that we measure fuel capacity and it's 30%. What would 20% depletion be? What would 20% dry be? What is that number?

Well, I I typically do the math. 10% of 30 is 3. We'll double that to get to 20. That would be 66 percentage points. Field capacity was 30.

6 percentage points is our tank size, so 24 becomes the depletion. That's dry. We will let the soil go from 30 when it's wet down to 24 when it's dry, and then we'll fill it back up again. And I'm today, I'm not gonna be talking about how to automate we're just talking about the numbers itself. Right?

So if you take your reading and it's 25%, you know, you're not dry yet because we just said 24 was dry. If you take a reading at 17, you know, you're way below your depletion point. Alright? And I use 20% as a starting point because you never wanna start too dry. You never want to you always wanna start a little bit conservative, see how it goes, then you could lower it a little bit, then you could lower it a little bit more.

But the analogy of using 20 percent helps to describe how the tank size can be measured. So if we think about the field capacity number. And we think about taking 20% of it. So we started with 30%. We took 20% away.

That was 6 that was 6 percentage points. Okay. And then turn on was 24. The type of soil that we're working with actually determines the size of the tank. So generally speaking, and this is kind of a, I guess we could call it a rule, clay soil holds more water.

So when you measure field capacity in a clay soil, it's going to always be higher. So let's just say it's 33%. K? And the range is probably anywhere from 29 to, you know, 38% generally speaking. Clay soil holds more water, so field capacity is a bigger number.

Sandy soil holds less water, so field capacity of sand will be lower. Right? Let's say field capacity of sand is 22%. And we're just assuming field capacity of clay is 33%. But if we use the general rule of 20 percent depletion, 20 percent of a bigger number is a bigger tank.

Right? The more clay in the soil, the larger the tank size will be. The more sand in the soil, the smaller the tank size will be. Because 20% of a smaller number is a smaller number, 20% of a bigger number is a bigger number. So the smaller the tank, the more frequent it will need to be filled up.

The larger the tank, the less frequent it will need to be filled up. Okay? It also means it'll take more take more water volume to fill it up, but the depletion will occur faster in soil and in clay I'm sorry. It's faster in sand than it will in clay. And I noticed this from my own sort of, growing up in Vermont, there is the soils very heavy clay.

Lake Champlain used to cover a big portion of the state of Vermont and this the soil that was left at the bottom of the lake, you know, Eons ago has tons of clay in it. It holds a shit load of water. Mud season in Vermont, sucks. However, it's freaking green all the time because the soil doesn't dry out. Now, where I live, Now in Michigan, it's a sand dune.

It's a big ass ant hill. The sand soil holds very little water dries out quickly, and you have to irrigate the hell out of it. K? The climate is actually very similar here in Michigan to where I grew up in Middlebury, Vermont, but the irrigation market in Michigan is huge. The irrigation market in Vermont is small.

Vermont is green, Michigan is brown because of their native you know, soils. And again, I'm making some analogies here and some similarities. I'm not a scientist that knows any evidence of this other than from my own experience, but just sharing that with you because the soil type makes a difference. Another way to look at this is that if you had a 12 gallon gas tank, you need to fill up more often than if you had a 20 gallon tank. K?

Now to fill up the 20 gallon tank takes more volume, takes more gallons, but you can go longer in between. K. So that's kind of some analogies of of, how this works. And that's really it. Like, that's all there is to it.

K? Essentially, that's all there is to it. So it can be often it can be easy to often overthink this, overcomplicate this, and, like, totally miss the point. And the point is that you've got to find field capacity. Field capacity becomes like your baseline.

Field capacity is everything. Field capacity is not my ass. Fuel capacity actually is everything, and everything becomes relevant to it. Right? It's that it's the known variable.

You gotta find out for every single, sensor that you're that you're working with. And this is why in my opinion, it's actually impossible to input the soil type into the ET formula. We all know controllers, one of the inputs is soil type. Now it can get relatively close, but if 30 of us went out and We're in a class. We went outside and were asked to determine the soil type.

We'd probably come up with 10 different answers. Not all thirty of us would pick the same soil type. So I think it's really fascinating that controllers ask the user to input the soil type, but the user cannot determine the soil type. It's it's like almost impossible to pick the soil type. Alright?

Without sending it to a lab, of course, but even if you send it to a lab, the lab can't tell you if it's compacted and what the soil structure is. It can only tell you where it falls on the soil in the soil table, you know, which is better than nothing, or you could just put a damsel of moisture sensor in the ground and measure field capacity. Okay? So I guess just wrapping this up, I I believe soil sensors is the next revolution that we're gonna see in this industry. I mean, we are gonna see soil sensors hit hard, guys.

And I think as professional irrigators, professional distributors, consultants, designers, landscapers, landscape architects, it's so important to understand this concept and how it works because it's going to affect our business. Alright? So remember, everything is relative to field capacity. Learn it, understand it, Have a beer with it, think about it, have another beer with it, and think about it. Matter of fact, have as many beers as you can and think about it.

It will be way more fun that way. So field capacity, my ass, not so much field capacity is everything. That's it, guys. Have an awesome weekend, and we'll catch you on the next episode.