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Monitoring the Root Zone of a Crop

Updated: Apr 5

Soil Moisture Probes - Travis Rokey


Introduction

While soil moisture probes are a great piece of monitoring equipment to have, they often present problems in the way of equipment malfunction, rodents chewing wires, and an unusual attraction to combines and sprayers. We have been exploring different options for monitoring crops, specifically the root zone; today we will cover different options we have found to be successful, along with the importance of monitoring the root zone and how you can use that data to promote success and profit.

Why Monitor the Root Zone?

You may be asking yourself, if I can see my plants, why should I monitor my root zone and care about what is going on below the surface? Three good reasons:

1.      Plants drink their nutrition, so nutrients need to come in with water through the roots. Well timed irrigation water application is key to ensuring nutrients are available for plant use.

2.      Optimal soil moisture will help drive your soil biology and help that biology be as efficient as possible.

3.      The right amount of water, at the right time, makes money.

There's a lot of research that goes into your nutrients (NPK), but really that's only 4 percent of what a plant uses. The other 96 percent are hydrogen, oxygen, and carbon. We really need to pay attention to water and food source for microbes if 96% of our yield comes from that.

What Options do I have for Soil Moisture Monitoring?

Alright, it sounds like a good idea to monitor the root zone. Now what? Over the years we have tried many different soil moisture probes and different programs to find what works. One option discussed by Kurt is Irriwatch. (Learn more HERE – go to time stamp labeled Irriwatch). This is a virtual whole field monitoring system by satellites. Today we are going to discuss in field soil moisture probes, which are more like a microscope on one specific area in your field. A few options we are currently using: 

·       GroGuru – wireless permanent probe

·       Sentec/Zenseio – wired permanent probe

·       AquaSpy – annual

·       SoilTech Wireless – annual

Some of the issues with soil moisture probes include the following: ensuring the hardware is working correctly, getting it in and out of the field at the right time, and ensuring it is kept safe from rodents.

Permanent probes solve the problem of getting probes into the field at the correct time. They tend to have less hardware to cause problems, and wires for rodents to chew. GoGuru and Sentec/Zensio probes are two that provide excellent in field data.

The GoGuru Probe

The GoGuru probe is a capacitance probe that goes 3 feet in the ground. It has a battery that lasts 5-7 years and a transmission unit. With SDI, we often bury it at the same depth as the tape, and at the proper distance from an emitter to ensure it is reading correctly. You can farm directly over the top of it.

The telemetry unit needs to be close to the transmission unit; it is easy to move for planting and harvest. If you don’t move that during planting, it will not give accurate data as there are no roots around the probe.

Another part of GoGuru we really like is the ease of access to data; it is available on an app. The app has different algorithms for different crops. On the website, you have a map so you can drop pins and you can see where those probes are at.

Sentek/Zenseio



This is another permanent probe but with no infield hardware but does require hard wiring going from the probe to edge of field RTU. It ties into Galcon controllers via API. There is hardware at the edge of the field, but we try to place it by a valve of the drip system or a fence. The max wire length is 200’, so there is a constraint on where the probe can be placed. There is a telemetry unit that reports back to the base station. You can have unlimited probes on that base, as long as they are within a couple mile range.

Aquaspy



Aquaspy has been around awhile, and NutraDrip has used their soil moisture probe unit for quite a few years. It’s a good probe with good information but has a lot of parts and pieces, and opportunities for malfunction. There are wires, a solar panel, and you have to stay above the crop canopy. This does have a very good crop specific interface for a mobile app.

Soiltech Wireless



This is a new probe that is a one sensor unit. It’s basically a robust yellow plastic brick that you bury. It’s very easy to install but only measures the soil moisture at one depth.

Permanent Probe Versus Annual Probe Data

While annual probes work well for some, they don’t give you nearly as much data to learn with.

For example, this picture shows 4 years worth of data from a permanent probe. This gives a more complete picture of soil moisture through the irrigation and off season. This is from 2019-2022, with a corn and soybean rotation. From this data, we can see that soybeans tend to dry out the soil more than corn. This might indicate a need for pre-season irrigation.

This is just a snapshot of what it would look like if you had seasonally installed probes. If you just were looking at the growing season, there's a lot of data you're missing  in the off season period.

 

 

Interpreting the Data from Your Moisture Probe

You chose your soil moisture probe, and decided to invest in your farm and learn more about the soil.  Now, let’s discuss how to USE and interpret the data from the moisture probe, and profit from it: both in money and knowledge.

Let’s look at a few different graphs with varying information. This probe goes down several layers, so each line will tell us how much moisture made it to that related soil depth. At the beginning, you can see the moisture doesn’t make it to the bottom layer, but by the end of the graph, you can see the wetting events. Different soil types respond in varying ways to irrigation. That is why we can’t tell you to put on a certain amount each day and you will be successful. Not only can that waste water, but it can decrease your yield; not what we are after! Soil moisture probes give us a better picture of when and how much to water.


This graph shows us the classic stair step action during the day when plants are using water. It slows down during the night while the plant resting, then back down again for another day. Use this as a heart rate monitor for the plant, measuring its pulse. If it's really drinking hard, you can tell it's healthy. It's doing what it's supposed to and there's moisture that it can use.

When we start seeing a drop down, that crop has stress. It's not drinking what it should. If you're working hard and building up a sweat, you're going to be thirsty. Your body will want water. Same with a corn or soybean plant. It's going to be pulling that water.

Occasionally, you will see that level come back up at night. That is water moving up from deeper levels in the soil.

The spikes are wetting events: irrigation or rainfall. You can then count the days from the wetting event until the plant is stressed again.

Another important aspect is watching roots. Each of these lines shows a different depth: shallow to deep. You can see when those stair steps started happening; this shows root activity by that level of the sensor. Those roots gradually grow deeper throughout the season.

We can also see the depth of infiltration of a wetting event by watching to see which sensors spiked. Later in the season, you can see the water went deeper; the soil was dryer and had more room for the water to move down.

 

Quick story about the importance of this from Travis Rokey: “I had a customer down in Southwest Missouri. I was talking to him about drip irrigation. He really liked the idea, liked the technology of drip irrigation, but said ‘I have 2 pivots. I've been farming them for 25, 30 years. There's a lot of years that the corners are out yielding the pivot.’ He said ‘I have all the water I need. I can just pump that water on and nothing's changing.’ He said ‘I need help managing this. I don't want to spend all this money on drip irrigation and not pay just like my pivots.’ So this past year was the first year we installed these permanent moisture probes. We started watching it through the season. He irrigated like he normally did. We were hot and dry, unlike here last year, and I was watching his graphs and the water kept being used up and used up. So I called him up. I'm like, hey, are you irrigating? Are you running your pivot? Is it working? Yeah, every 3 to 5 days that pivot is making it over that probe and he's putting on 6/10ths.

 This was his graph from last year, (the black line, it was installed in April) all the way through October, but it just kept dropping. And we've got a problem. If you're putting on 6/10ths, you'd think it would be doing something. If you look really close, there's a few instances in here where it's leveled out, that uptake wasn't as steep, but since this probe is buried down, I think we buried it 10 inches, that 6 tenths of an inch was only getting soaked up in that top a little bit.

Here's a rain event and it should have seen a spike from this inch rain. It slowed down the uptake, obviously, just a little bit. But the water was not making it very far into the soil. So I went out there after the pivot made a pass, and that water was only soaking in 2 to 3 inches. The rest was running off. We learned really quick we had a water infiltration issue. He had pulled soil samples. We had looked at a water test. Everything came back pretty good. Bicarbonates weren't that high. Sodium really wasn't an issue. When we sent him to the lab and the lab combined the water and the soil together (for a saturated paste test), he came up with a whole bunch of issues. Basically, that soil was sealing up, and it wasn't letting water soak down. This year we're going to try some different amendments to see if we can't get that (water) to soak in. Due to the soil moisture probes, we have a better idea of what is happening, and where to go from here.”

This is a graph from the field where the soil was having a hard time pulling in the moisture. As you can see, there has been a lot of soaking rain, and the moisture profile is finally rising slightly but is still dry in the lower two sensors. A lot of the rain events or moisture over the winter only made it down 30 inches.

 

 

To contrast that, consider this graph from a probe in northeast Kansas, just down the road from the NutraDrip shop. It was a wet year last year; you can see a lot of rain events during the crop growing season. Notice the root activity that goes all the way down to the 40 inch sensor. At the end of the growing season you can see a rain event where the moisture soaked down to the 40 inch mark. That means the roots have 40 inches of soil that has moisture, plus access to all the nutrients available in that soil.

Soil moisture probes can also tell you if your soil profile is full; on this probe, the moisture throughout December and January “filled it up”.

This is that other sensor a mile away that is still showing dry.  The little blip on the shallow 12 inch sensor (top blue line) was from a really cold snap in the first part of February. The frost got down to 12 inches; the probe is perfectly fine through frost but won’t read the soil moisture correctly. It will give goofy readings where the frost level is.

This past winter as I was digging through data, I noticed something interesting. I was always under the assumption that if it is hot and dry, it will force the plants to grow roots deeper to find moisture. That is true in some cases, but I have found on multiple probes by me that roots will be pulling moisture from the shallow sensors, 12-16 inches. After a rain event, they will suddenly pull moisture from the next probe. This tells me that having optimal soil moisture – not too much, not too little- encourages root growth. The whole plant Is healthy, so it is able to send its energy into growing roots deeper; focusing on growing roots at the beginning of the season is so important.

 

 

This is an example of a situation that was over watered on tight clay soils.  At the beginning of the season, the plant started using moisture, and the graph started dropping. The plants showed stress, so the grower turned his irrigation system on. The system ran and never really turned off. It was hot and dry, and the plants looked stressed. Turns out, we were adding stress because the roots never got down to the 24 inch sensor depth. How much of a crop can you grow off roots that deep? It wasn’t until the end of the season when he shut his irrigation system off that the roots grew down deeper into the 24 inch range. That's really when that plant should have been filling the grain versus growing roots. This is a situation where moisture probes can help save you money; by knowing the soil, and how little it will accept water, the grower can make better decisions during the growing season to increase commodity yield and decrease water consumption.

 

How Does Soil Moisture Probe Data Make Me Money?

So how does this data make me money? The obvious would be to help prevent over or under watering while irrigating and losing yield from that. You can optimize soil moisture to optimize your biology. Another benefit is evaluating new practices and treatments and having solid data to show you whether to continue or try something else. Like the example above, we know we need to work on water infiltration. There are many different products that we haven’t tried; with soil moisture probes, we can try a little bit and see if that helps the moisture levels on our probes. If that’s not working, we can try something else.  Soil moisture probes help you know if your treatment was successful before you run your combine.

An example of evaluating treatments via a moisture probe: in Texas a grower was using a water structuring device. By using two soil moisture probes, they were able to see the difference in how water soaked into the ground. The data above shows two pivots side by side: one had the water structuring unit, and the other one did not.

Conclusion

The majority of the data we went through in the last section is from GroGuru probes, but there are other options that will still give us good solid data to help make better decisions on when and how much to irrigate and how to be efficient with the resources we have available. We are still learning about the data provided from these probes, especially the permanent probes. If you are wanting to learn more about your soil and how it responds to moisture, a soil moisture probe would certainly be a benefit for you and your farm.

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