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Overhead irrigation in a field

Agent and Grower Perspectives on Soil Moisture Sensors for Irrigation Decision Making

Dr. Charles Barrett, Regional Specialized Agent in Water Resources, discussed agent and grower perspectives on soil moisture sensors during the 2020 Virtual Ag BMP Summit.

Dr. Charles Barrett holding a soil moisture sensorSoil moisture sensors are long probes that are lined with pairs of brass rings. To sense the soil moisture content, one ring sends an electrical signal into the soil and the other ring in the pair receives this signal. The probes then measure the frequency change and use an algorithm to translate this information into the soil moisture content.

The producer sees the results in the form of a graph similar to the one pictured above. As the diagram shows, each pair of rings on the sensor aligns with a line on the graph. The graph allows the farmer to easily identify irrigation events, how deep in the soil the water reached, how deep the crop’s roots are reaching, and how quickly the water is taken up. The steady decrease depicted on the graph above shows the plants absorbing the moisture in the soil. This crop clearly only has roots in the top few layers of soil because the bottom two layers do not show a decline in moisture content. If the plant’s roots grow deeper into the soil, the graph will show decline in the deeper layers as well.

Florida’s soils are sandy and allow water to quickly travel deeper. An indication that plants are having to reach deep into the soil to find water is the soil moisture graph showing activity in progressively deeper layers. Irrigation may need to be increased if this is the case. Providing the appropriate level of irrigation to the crop is important because putting immense levels of energy into root development can stress the plant.

Extension agents frequently recommend the use of soil moisture sensors because they can prevent grower stress. Farmers are interested in this information because it shows them where their irrigation is going and how quickly the plants are taking up the water. The farmer can then determine whether a new irrigation management plan is needed. Knowing exactly how their plants are reacting to irrigation events provides peace of mind for growers who are concerned with obtaining optimal plant health.

Frequently, farmers who try soil moisture sensors realize that they need to alter their irrigation strategy. Instead of having large irrigation events that are spread out, it is typically better to use shorter irrigation events that are closer to each other. The amount of time between irrigation events should be determined based on how quickly the plants can take up the water.

Using a custom irrigation plan reduces crop stress because the plant receives the exact amount of water it needs without expending energy on growing deeper roots. The farmer also saves money by reducing diesel and electricity use due to less time spent irrigating. Additionally, the cost of using soil moisture sensors can be offset by purchasing inexpensive models and utilizing cost-share programs. Another advantage is the ability to maintain a high yield.

Growers appreciate using soil moistures sensors because they offer tangible benefits. By utilizing a grant provided by the Florida Department of Agriculture and Consumer Services (FDACS) Office of Agricultural Water Policy, farmers can also purchase sensor data subscriptions. They can test out the product before purchasing through the lending program provided by extension agents all over the state. If the grower decides to continue using the soil moisture sensors, they can do so affordably by utilizing these grants and cost share programs.

Dr. Charles Barrett installs a soil moisture sensor into a grower's field.

Dr. Charles Barrett installs a soil moisture sensor into a grower’s field.

Another result of this program is a better relationship between extension agents and farmers. The program allows extension agent to go out into the field with farmers to give them the sensor and help them make better business decisions based on the results. Agents must be patient with farmers because they will view the sensor results from a different perspective (one of maximizing profit potential using their current irrigation plan). Agents can help growers understand what the root zone looks like and how they can alter their irrigation decision making according to crop needs.

Agents can also help farmers visualize how irrigation affects the root zone by performing the blue dye test. Blue dye is applied to fertilizer and then the field is irrigated to see how the blue dye travels. By digging into the soil after the event, the agent and grower can see how deep the dye traveled and compare it to the soil moisture sensor results. This test not only proves that the soil moisture sensors are accurate, but it also visually displays how quantity of water irrigated can drastically affect the depth reached.

Even growers who use seepage irrigation can use soil moisture sensors to know when to raise or lower the water table. They are especially useful for winter crops that are affected by cold fronts. The grower can increase the water storage capacity to utilize anticipated rainfall by lowering the water table. Essentially, the soil moisture sensors can be used by all growers, regardless of their chosen irrigation method.

The FDACS program is steadily increasing involvement of both agents and growers to spread throughout the state. Farmers have the opportunity to see whether this technology will benefit them, and then many purchase soil moisture sensors or participate in the cost share program. As they utilize the soil moisture sensors, farmers can schedule irrigation for when the plants need it and have the peace of mind that comes with knowing exactly what is happening in their fields.

If you are interested in learning about technical and cost share assistance for soil moisture sensors, contact your District Conservationist here: https://offices.sc.egov.usda.gov/locator/app?service=page/CountyMap&state=FL&stateName=Florida&stateCode=12. To learn more about utilizing soil moisture sensors, visit the USDA’s Field Office Technical Guide here: https://efotg.sc.egov.usda.gov/#/.

Watch the recording of his webinar.

Written by Natasha Roberts, CLUE Communications Intern

Q&A

Q: Have you investigated water availability for microbes and other fauna, in addition to water availability for crops? Also, do you have any insights regarding cover crops in the soil moisture sensors?

A:  As far as looking at things specifically for soil microbes, we have not, but this will give you a good indication of what the soil moisture levels are. If there’s water there for crops, there’s water there for every other fauna in the soil; I think that it’ll be a matter of competition at that point. As far as the cover crops, yes. Joe Love has done a bunch of work with soil moisture sensors out in cover crops and it’s amazing to see what these cover crops can do with their deep root systems – with no irrigation, with irrigation, with dry periods, with rainy periods – its all over the board. You will be impressed! Looking at soil moisture sensors with cover crops has been an eye opener. Cover crop roots are aggressive and for the most part are very responsive. They’re very interesting to watch on a soil moisture sensor because there’s a lot to see. Some crops, like perennial crops such as fruit trees, it’s not as interesting because there’s periods of time where they’re really taking up water and then there’s periods of time where they’re not. From the work that I’ve seen in citrus, when there’s a big flush going on you’ll see some root activity and then when you see things quiet down the lines don’t really move that much. It’s the opposite for cover crops – those things are gangbusters all the time it seems like.

Q: Are there any soil moisture sensors of this type installed at FAWN stations? Could be an opportunity to train growers in working with them. Rainfall is the only moisture inputs at the stations and would be interesting to watch the dynamics on the FAWN website.

A: Soil moisture sensors are not live through FAWN yet, but Rick Lusher has installed up to 4 or 5 probes on farm stations where we are starting to look at this data and I’ve provided feedback on how that data should be displayed to make it easier for everyone to understand, so yes those things are in the works already. What gets complicated about putting soil moisture sensors on FAWN stations is that some stations have bahia grass growing underneath the FAWN station and some just have bare ground. So something has to be done there to where everybody knows what they’re looking at – if you’re looking at bare ground it’s not going to behave the same as a bahia grass would, and a bahia grass may not behave the same as a crop in that vicinity would – but it could give us some insights into what’s going on with the soil moisture, especially if you’re growing sod or some other crop, bahia grass would be a really good indicator of what you’re going to be seeing on your farm if your in close proximity and you’re getting similar rainfall. We’re working on it.

Q: At what point during crop development, say for field corn, does increased regrowth become a cost to the plant in terms of yield decline?

A: I think early on when we’re vegetative we’re okay growing big roots. When we start to switch over to reproductive is when we want to stop thinking about growing roots and we want to switch into growing fruit. So, all the way up until tassel, probably a little bit before tassel, I’d say let’s stop focusing on building roots and let’s switch gears into reproductive. Corn is so much more drought resistant early on where it’s not going to hurt yield later on. When the crop goes reproductive it’ll still be drought resistant, it won’t die, but you’ll have a lost yield.

Q: How expensive are the probes that can measure 4 different depths?

A: Most of the probes we’re using fall between $1800-$2200; that will get you between 4 and 5 depths. I usually look at EC (electrical conductivity) or some measure like that. You can get them a little bit less expensive than that; you can drop that range down to $1000-$2000 for 4-5 depths without EC. There will also be costs just for the data plan that will be somewhere around $150-$200. Depending on what kind of service you’re getting from your service provider it can go up from there. If the grower takes it upon himself to install and remove the sensor then it’s less work for the vendor and the vendor would charge less. Think of it as a menu and you can order what you want off that menu and your price will be affected by what you order, almost like an à la carte.

Q: Is there a way to come up with an average water savings, such as savings per acre for different crops, maybe as a percent compared to the typical water use, without soil moisture sensors?

A: Yes, I think it’s very difficult to just look at soil moisture sensors every year on somebody’s farm. I think it’s easier where I can say this specific grower changed their behavior and now this new behavior resulted in a reduced application of irrigation or an increase in irrigation uptake efficiency, etc. But the way to do it is to do what Dr. Dukes did over at our research farm and measure over the course of a few years what a grower standard would look like and what a soil moisture sensor based irrigation schedule would look like and compare the differences. We ranged somewhere between 30-50% irrigation depending on how wet a year was. You’re not going to save a lot of irrigation in a dry year. You’ll save a lot more irrigation water application in a wet year because you’re watching that rainfall and you’re making better use out of that rainfall – if you don’t have any rainfall you have to irrigate.

Q: Would you comment on your view of using EC data from the soil moisture sensors?

A: I love having EC data to look at. However, it’s a very complicated piece of the puzzle, especially for the untrained eye or novice soil moisture probe user. If the squiggly lines when I showed you soil moisture looked messy, EC data is even messier. The thing about EC is I never look at it in less than a month timeframe and I look for general trends. Because the way EC is measured you’ll see a dilution effect from an irrigation application or a rainfall event. It doesn’t mean that those nutrients are gone, it just means that the sensor can’t see it. So when your soil has a high water content your EC looks like its gone down. As that soil dries out you see where that EC is and you can see if that EC has moved to a deeper depth in the soil profile, if it stayed right where it was, or what has happened with it, so you have to be patient. So, I caution everybody looking at that to be patient and not make quick judgments or panic. “I lost all my nutrients with that rainfall event, I gotta hurry to go out there and buy more.” No, that’s not how these things work. Wait and let soil dry down a little bit and then we can see. Maybe your nutrients are still in your root zone and they didn’t leach as much as you thought. Or it’s just below where your roots are right now, but you’re early in the season and your roots are at 8 inches and you know they’re going to get to 12 and 16 inches – hold out on applying more nutrients. Those roots will dive down and tap into those nutrients so you don’t have to reapply; we see a lot of that early in the season. And that’s when we get those cold fronts. Up here in North Florida we’ll get a cold front come through and it might push some nutrients a little deeper, like the blue dye I showed, it might push it a little bit deeper down into the soil profile, but if you wait a week those plants suck it right back up and green right back up no problem. And the plants are already going to slow down because of the temperatures of the cold front, so it’s not an issue. I think people just panic and are a little bit too quick to hit the panic button. Just relax.

Q: What’s the right approach to scheduling irrigation in flatwood soils where all the roots are in just the top eight inches?

A: Yeah, the right approach is always more frequent irrigation events and shorter durations. You hear this as spoon-feeding. I don’t really love that term, but yeah, rather than doing one long event you do many short events. That’s my advice.

Q: Have you ever been asked how many acres per sensor? Have you ever been asked a question like that?

A: Yes, all the time. So I get three main questions: how many of them do I need, where do I put them, and which one is the best. So you asked one of the main ones – how many do I need? It’s a tricky question. I’ve actually made a YouTube video on this that we’re fixing to post, but I’ll summarize and just say that if you’ve got a field that is 80% or 90% of one soil type. And then you get another soil type filling up the rest of that hundred percent; so, you’ve got a dry spot or a wet spot out in that field. Don’t put it in the dry spot and don’t put it in the wet spot. If you put it in the dry spot you’re going to over irrigate and if you put it in the wet spot you’re going to under irrigate. You always put it in that more average or more representative soil type or zone. And it depends on your zones too. If you’re zoning, if your doing drip irrigation and you’ve got a lot of zones, what’s the age of your crops in that zone? If you have a zone of older more established plants next to a zone of younger plants you may need two sensors in that type of situation, unless you can ballpark it. It really just depends on each situation how many you’re gong to need, and also your pocketbook; how fat is your pocketbook and how much do you want to spend on soil moisture sensors. How much do you value the information that you’re getting from them? Up here in North Florida we can put one out and irrigate one or two pivots off of it. One or two pivots might seem like a lot; 140 acre pivot, but we’ve got a lot of deep sands up here and most of that corn is planted within a few weeks of each other, and so pivot side by side might be that same day or the next day, so the plants are the same age, the soil’s the same, so it lends itself to covering more acreage with fewer sensors.

Q: So what I hear you saying in a manner of speaking is, “know your field.”

A: And growers do, right? A lot of times they’re just interested in putting it in this one spot because they think it would be cool. Hey, if that’s going to be what’s driving that decision, awesome. I’ll put it in that spot first because it shows me that the person wants to look at what’s going on right there. Ninety percent of learning how to use a soil moisture sensor is just getting in there and looking at it. It’s like riding a bike; you’re not going to get good at it if you don’t practice.

Q: What’s the name of your YouTube channel?

A: North Florida Research and Education Center Suwannee Valley, NFRECSV. If you go there it’s a playlist for soil moisture sensors. That playlist is a list of 1 right now and we’ll have more populating there shortly. https://www.youtube.com/playlist?list=PLn-umAc9HXgDyMWN-dstplSR9QWUiU9H8

Q: Would you talk about cost-share available?

A: I will because that’s a huge success story that the state has been able to accomplish. All over the state FDACS will share up to 75% on soil moisture sensors. And then the water management districts have gotten in on the game as well. So up here in Suwannee, if you’re in a BMAP area you can get 90% cost-share, which is out of this world. Down in St. John’s there’s cost-share available as well, I think it’s up to 75% if I’m not mistaken. And then SWFWMD (Southwest Florida Water Management District) also has a cost-share program and they’ve actually brought me in to give some talks about soil moisture sensors to help them advertise the use of soil moisture sensors and also advertise that they have a cost-share program for it. So anywhere you are in the state, pretty much, you can get at least 75% for soil moisture sensors and start saving water today.

Q: What about NRCS, do they cost-share soil moisture sensors?

A: I don’t know is the short answer. I know that there has been interest there and that I’ve talked to folks at NRCS that want to, but I don’t know anybody that has been successful in getting cost-share for soil moisture sensors just yet. I just don’t know.

An attendee wrote in that NRCS does not cost-share equipment, so thanks for clarifying that.

Q: Do you want to talk a little bit about any upcoming EDIS docs about irrigation scheduling? I think you’re a co-author on one that’s in the mill?

A: Sure. So, there’s a lot going on with that. I think Vivek is starting to lead the charge on that. I started working on revamping an old EDIS doc that was getting ready to sunset that Tom Obreza had originally done for crops in North Florida, and I worked on it and worked on it. Then Vivek came in and was looking for stuff to do and said, “hey I’ll take it.” And he has done some amazing work on it in a short turnaround and got it submitted to EDIS. So for North Florida we’ll have most, if not all of the crops that we’re growing up here have an up to date irrigation schedule that is in the format that I showed that looks like the bell-shaped curved. And then the goal there is to put that on FAWN and make that available for the whole state. So if you’re growing tomatoes you can get on FAWN and say “I’m in this region of the state and I’m going to grow tomatoes”, and then print an irrigation schedule for the season. That’s where we’re headed with that. I guess I can touch on those other two main questions about soil moisture sensors, which one’s the best is a hot topic. I work for the University of Florida, and with that I’ll just say that I am science-based and unbiased when it comes to soil moisture sensors, but that’s actually the truth. I don’t really care what soil moisture sensor anyone uses, I can pretty much read them all. And I’ll help anybody with anything that they’ve got or find them help if I can’t do the helping myself. But I’ve used them all. I’ve used individual probes, the one’s you walk around with and make a map with it with GPS; I’ve used probes and all kinds of sensors. And so to me it doesn’t matter which one you’re using, it matters if you understand the information that you’re getting from it. They’re all good. That technology is 20-30 years old, which is pretty old for technology. There’s nothing new here with this. I’m not reading something that someone else could not figure out; everybody can figure this stuff out. It’s just different vendors offer different levels of service. So wherever you are in Florida find a vendor you can trust. Because with every technology something can and might go wrong with them, and you might need to rely on that vendor to come out and get it fixed because you might be in the middle of a season making day-to-day irrigation decisions. My biggest thing is go with a vendor you can trust. Their product is going to be good. If you can’t understand it, you don’t get good service from them, or you just want to double check what they’re telling you, come to me or your county agent and we’ll get you squared away on what the sensor is trying to tell you.

Q: Tell us a little bit about what’s on the horizon in the future with your soil moisture sensor statewide demo program.

A: Sure. So, we’ve received some funding from FDACS to make this program more available statewide. Right now we’re refurbishing some soil moisture sensors that Dr. Dukes had out in the field for a big trial that we’re going to be able to deploy across the state for our agents or our FDACS technicians; put them out on farms where farmers are interested in looking at using soil moisture sensors on their farm. I find that that’s the best way to get folks to adopt this kind of technology is to try it, get it out there, see what it looks like on their farm, and then decide to adopt it or not. Most of the time after they’ve tried it and figure out what this data means to them they’ll go and adopt the technology and start saving water.

Q: Economic analysis on the soil moisture sensors?

A: It’s basically a no brainer, especially with cost-share, that they’re cost effective. The mathematics are super simple on a diesel system, or even on an electric one. If it costs you $7 to pump an inch of water per acre and you’ve got a 140 acre pivot, you’re going to add up those $7 really quickly and you’re going to very easily pay for one soil moisture sensor. I figure if I can save one irrigation event I’ve paid for my sensors for the year, usually on a center pivot system. With a drip system where you’re applying lower volumes of water more frequently, it might take you a few more events to add up to it, but soil moisture sensors are so cheap for how many acres you can cover with one, the economics kind of speak for themselves. That said, I have not done an economics analysis on it formally to put something in writing.

Q: But you would invite any of our colleagues who might be interested?

A: Love it! Love that idea!

Q: If there were some sort of technological advancement that could happen in some period of time in the future, what would it be that would make this just business as usual?

A: I don’t know that we need a technological advancement to do it. Maybe on that train of thought, making the data interpretation a little bit more user friendly. I’ve seen some attempts of that. Personally, I love the amount of information that we get with all the lines; I love to dive into it and look at it. I know that folks that have 50 sensors might not have time to spend on every single sensor like that. I think the biggest thing to get folks to adopt it more widely and make it more user friendly is more hands on experience with it. I have a statewide program that I’m working on and that’s why I’m doing it. If somebody else has a better idea I’d love to see it and I’d love to collaborate with them, but that’s what I think is going to be the best way to do it – get it out there into people’s hands. When you’ve got a grower – Lincoln use to always call them a “lighthouse grower” – somebody that other growers look up to and respect, if you have that lighthouse grower, can get them to adopt it, and everybody else around sees that and thinks, “shoot that could work for me, alright, let me try it,” that type of stuff and word of mouth really helps. That’s what I think is really going to push this over the finish line.

Q: Center pivot and drip economics sounds good, what about seepage?

A: Yeah, what about seepage. I would say on a seep into situation it’s still pumping costs. You’re still looking at electricity – you’re pumping a lot of water. So, it’s probably even more effective on a seep system for saving water because of how much water it takes to raise that water table, we’re talking days of pumping that we can potentially reduce by the end of the season. So, it’s even easier on a seepage system than it is on a drip or center pivot system because those systems use less water