Save Water by Measuring Temp of Plants and Intensive Chemigation

By
Khardi J Mukuyu
For farmers in places like Colorado, where population growth and drought have made water a critical issue, managing irrigation water more effectively has emerged as a top priority.

“With water becoming more precious, we need to exploit the potential of every tool available,” says Kendall DeJonge, an Agricultural Research Service engineer in Fort Collins, Colorado. “Using an infrared radiometric thermometer (IRT) to determine a crop’s water needs just makes sense.”

Currently in research applications, IRTs are placed on field posts or center pivot irrigation systems. Temperature readings are gathered hourly or daily on crops. Scientists then interpret the data by using one of several indices, including the crop water stress index (CWSI), which is considered the gold standard for quanti fying water stress. Developed in the early 1980s, the CWSI also requires that users know air temperatures and humidity levels.


Because the CWSI method is fairly technical and requires additional measurements, many farmers will guesstimate when to irrigate, which could mean water is wasted by overirrigating. As a result, yields will suffer because crops aren’t getting enough to drink. What farmers need is a simple, yet effective, method to monitor the water demands of a crop.


Simplified process

That’s where DeJonge comes in. The scientist has found a way to simplify the process by using canopy temperatures to determine if crops are water stressed.

His research centers around IRTs, affordable sensors that can determine the crop canopy temperature and whether a crop is water stressed.


He and his colleagues compared the CWSI with five other formulas for interpreting IRT data to see how well they could detect crop water stress over two years in a corn-sunflower rotation. All of the indices used crop canopy temperatures to determine water stress levels.

The team developed two indices for the study, which are simpler than CWSI. The Degrees Above Non-Stressed (DANS) index is calculated by comparing a stressed plant’s temperature with the temperature of a nonstressed plant in the same environment. The second study, Degrees Above Critical Temperature (DACT), is based on an established crop temperature threshold, with plant water stress determined by how many degrees above that threshold the plant temperature reaches.


Crop canopy temperatures for DANS and DACT were taken each day at 2 p.m., when water stress levels were usually highest. Researchers monitored soil water levels and crop water use. They fully irrigated part of the field, while intentionally stressing other areas.

Even though they require much simpler measurements, the findings show the DANS and DACT indices are just as effective as CWSI.

DeJonge plans to develop crop water coefficients that establish water needs of specific crops under different scenarios. With that data, IRTs could soon be widely used by farmers. He also foresees farmers using handheld IRTs in the not-so-distant future and eventually using the thermometers with drones to calculate water needs over extensive areas.

Intensive Chemigation
 By
khardi J Mukuyu

It doesn’t take an agronomist to realize that Jeff Raybould wouldn’t be growing many crops on his 2,000-acre farm west of St. Anthony, Idaho, without irrigation. After all, much of the main farm borders sagebrush-covered BLM (Bureau of Land Management) land, and the St.

 Anthony Sand Dunes are only a couple miles away, which means that most of Raybould’s fields already contain more sand than soil.

Consequently, 100% of Raybould’s operation, which produces potatoes, malt barley, and alfalfa, is under sprinkler irrigation. However, those pivots are utilized for much more than water.

They’re also a crucial tool for applying chemicals and fertilizer while managing crop inputs to avoid leaching or crop damage.

37 years of chemigation experience
“I’ve been using fertigation and chemigation since 1978,” Raybould says, noting that because of the sandy soil, he applies nitrogen a little at a time, as the crop requires it. “However, I have a variety of liquid products I’m putting on barley and potatoes via injection pumps. My pivots are making a circle approximately every two days, and there’s something being injected nearly every time they go around from about late June until early August. That includes several different fertilizer solutions, as well as fungicides and pesticides that I put on using the same Agri-Inject pumps.”

Raybould, who is a member of the Idaho Water Resource Board and president of the local irrigation district, says he typically starts a potato crop by applying a portion of the fertilizer as a preplant application. A little more nitrogen is banded at planting, followed by a topdress application a short time later.

“The balance of the fertilizer on potatoes then goes on through the pivots starting when the crop is about 4 to 6 inches high and continuing through maturity,” he explains.

 “In general, I’ll put on about 15 units of nitrogen every other time the pivot goes around, or about every four days.”

Tissue analysisCombined, his potatoes receive 15 to 18 applications of fertilizer and four to six applications of chemicals through pivots.

“I have a program built for each crop, which includes a certain amount of nitrogen, phosphate, potassium, sulfur, and trace minerals. However, I take plant tissue samples twice a week, so I’ll often tweak the blend to meet the plants’ needs.”

Raybould’s program for barley generally includes 120 to 140 units of nitrogen when the crop follows potatoes and 200 units when barley follows barley. That’s usually applied through four to five fertigation applications that end when the crop is in the boot stage. As a general rule, the rotation involves one year of potatoes and two consecutive years of barley, with alfalfa fitted in where it makes sense.

“The advantage of putting on fertilizer and chemicals with the pivots is twofold,” he says.

 “With fertilizer, the timing of the application can be matched to the growth stage of the crop and the time when it needs it the most. The other advantage is I’m not having to drive on the crop or spend extra money on fuel.”

The one exception, he says, is when he has to spray for wild oats.

 That herbicide can’t be put on through the pivot and has to be applied with a sprayer or aerial application. Otherwise, fertilizer, most herbicides, insecticides, and fungicides are applied via chemigation.

“In these sandy soils, I simply can’t put on more fertilizer than the plants will use in a few days or weeks,” Raybould explains. “If I were to put on half of the nitrogen at one time and I got an inch of rain, it would quickly leach to below the root zone.

 So for me, chemigation is the only way to go” to effectively and economically apply crop inputs.”

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