Moisture sensors help manage water fronts and signal when plants are thirsty
There are farmers who water, and there are farmers who irrigate, says Farm Journal Field Agronomist Ken Ferrie. New technology can help irrigators manage water more precisely and effectively.
In-field moisture sensors help keep water fronts from separating in the soil—the key to efficient water use and stress-free crops. When you think about how water moves through soil and into your crop, you quickly realize moisture sensors are a big step forward.
“Every soil holds a certain amount of water,” Ferrie explains. “How much is a function of soil texture—sand versus clay loam, for example. Some of that soil water is available for crops and some isn’t.
Soil moisture sensors, placed at various depths, can tell you if moisture is at field capacity and if water fronts have separated. Using sensors helped
Farm Journal Field Agronomist
Ken Ferrie discover some areas were being over-watered and others
“Unavailable water is held so tightly to clay particles and organic matter that microbial organisms and plants can’t strip it away. While fine-textured soils, such as clays and clay loams, have more waterholding capacity, they also contain more unavailable water than sandier soil.”
Having enough available, or usable, water is a concern. Available water is stored between soil particles and hooked to the particles themselves and to other water molecules. “When the macropores between soil particles are filled with water, the soil is at field capacity,” Ferrie says. “If more water is applied by rainfall or irrigation, the additional water ponds, runs off the surface or is lost through tile lines.”
When all the available water is pulled out of the soil, by evaporation from the surface or transpiration through plants, plants begin to wilt. “A grower’s goal is to manage the available water, which might be 2" per foot in clay or 0.4" to 0.5" per foot in sand,” Ferrie says.
Water management is like nitrogen management, Ferrie continues. “Whether it’s water or nitrogen, you consider what nature provides on a daily basis, along with the soil’s holding capacity. Managing nitrogen or water in sandy soil is different from managing them in silt loam. Light soil provides less of each.”
As the crop matures, it requires more water. “Efficient irrigation fills in the gap between the water provided by rainfall and the soil itself and the requirements of the crop, and it never lets the crop run out of water,” Ferrie says. “Farmers who weigh all these aspects and react accordingly are irrigators. Those who simply apply a certain amount of water per week are only watering.”
If you apply water without knowing the soil’s waterholding capacity, the infiltration rate and the crop’s requirements, you might over-water. “Our studies show you can lose yield by over-watering, just as you do by under-watering,” Ferrie says.
“Under-watering has the greatest risk. If you apply too little water, the water fronts—the water you’re applying and the water in the soil profile—might separate too far. If that happens, water no longer ‘pulls’ itself up through the soil (because water molecules are attracted to each other). An hour or two after you irrigate, you might notice the crop is stressed again. The applied water has evaporated out of the soil, and the soil water remains deep in the profile.”
To make sure the water fronts are meeting, good managers watch the field, check to make sure water is infiltrating and dig pits to check the water fronts. “If the fronts are not meeting, you may have to slow the irrigator,” Ferrie says.
Moisture-sensing technology, which has proven itself in Ferrie’s studies, makes monitoring water fronts easier and more precise. “Sensors are accurate and timely and, now, they are affordable,” he says. “Ten or 15 years ago, they would have been too expensive to be practical.”
Sensors measure available water capacity at various depths and send the information to computers and mobile devices, letting farmers check on their water fronts when they wish. “Whether it’s water or nitrogen, we no longer have to guess how much to apply,” Ferrie says.
There are a variety of sensors on the market and more in development. Moisture sensors fall into two categories. Some provide a volumetric measure—the percentage of water in a given amount of soil. Others provide tensiometric measurements by using electrical resistance to measure the physical force holding water in the soil (in kilopascals—kPa). The tension of the water correlates to the water content of the soil.
“Both types are effective,” Ferrie says. “But volumetric sensors must be calibrated to the soil texture. Two soils might have the same water percentage but a different kPa reading.”
If you’re using variable-rate irrigation (VRI), moisture sensors are almost a necessity. “Their information can provide a basis on where in the field you need to change the rate and timing,” Ferrie says.
“When we began studying VRI, we learned from our sensors that not only the rate of application had to change but also the speed of application,” Ferrie adds. “In some areas, sensors revealed some of the irrigation water was not infiltrating fast enough. To use our water, we had to lower the rate and apply more frequently. The sensors also showed we were over-watering some parts of the field and under-watering others.”
Irrigation consultants can monitor sensors for you. If you install and monitor the sensors in-house, here a few tips to help you get started:
- Not all sensors are alike, so read the directions. “For example, tensiometric sensors need to be saturated with water before you place them in the soil,” Ferrie says.
- Think about the best areas in the field to put sensors. “We like to put them in both high-yielding and low-yielding areas of the field,” Ferrie says. “Place sensors in different soil textures because they will have different irrigation requirements.”
- Think about where to locate sensors so they do the most good. “We like to place sensors directly under corn rows, so we get readings as close as possible to the roots,” Ferrie says.
- Place sensors at various depths, so you obtain readings from the entire soil profile. “We like to place them at 4", 8", 12" and 24",” Ferrie says.
- There are many ways to receive sensor data. You can enter the field and download data manually or have it sent to your office computer. With VRI, you might need to upload data to the Web for decision-making and then download it to your irrigation system for application.
“If you are remotely loading the data, think about how you will send it from the sensor to the base station,” Ferrie says. “When we started using sensors, we set one of the site antennas too high, and the center pivot rig knocked it over. If your fields include hills and valleys, you may need to set up a relay station.”
- Investigate technical support services before you buy. “There will be a learning curve, and you might need some assistance,” Ferrie says.
- The first year, you might want to irrigate normally and use the sensors to see if your water fronts are staying together. “You may find you’re over-watering some areas and under-watering others,” Ferrie says.
- To benefit from sensor technology, your irrigation system must be properly calibrated. “Because sensors cover only a small area, they can’t tell you if your nozzles are applying the same rate,” Ferrie says.
If you haven’t looked into software and sensors to help make irrigation decisions recently, now is a good time to do so. “Water conservation and water rights have everyone thinking about how to be more efficient with water,” Ferrie says.
As a limited yet vital input, water demands a high level of diligence. The Water Management series details how farmers can manage earth’s most valuable resource to boost yields and profit.
Thermal Imaging Or Moisture Sensors?
“Thermal imaging is a valuable tool and can provide information somewhat similar to data from moisture sensors,” says Farm Journal Field Agronomist Ken Ferrie. “But you only receive information about every two weeks, whenever the aircraft flies over your field. In-field moisture sensors can give you hourly updates.”