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CropTalk: Managing Irrigation with VRI and Moisture Probes

July 2019

Published on Thursday, July 4, 2019

There's often talk about variable rate (VR) fertilizer applications or VR planting, but have you ever heard about variable rate irrigation (VRI)? In the same manner that a prescription can be created to apply different rates of fertilizer or seed to defined areas of a field, farmers with pivot irrigation can also vary the rate of water within a field. Much like creating a VR planting prescription, a VRI prescription requires a strong data set that would include elevation, soil texture, and water holding capacity.

Many irrigation pivots can do VRI by simply adding a telemetry system that can control and vary the speed of the pivot. This would potentially divide the circular area under the pivot into pie-shaped slices that can each be assigned a different speed. Varying speed would, in turn, vary the rate of water applied. Program the pivot for the specific management zone that covers most of the area in the pie slice and there you have it, variable rate irrigation! Speed control is the easiest method of VRI, and while there are systems capable of section or nozzle control; these systems add significant cost to the system.

After the very wet start to the 2019 growing year, it may seem odd to read an article about irrigation. However, the decision of when and how much to irrigate can be more difficult in a wet year than in a dry year. Tools such as soil moisture probes can provide invaluable information to all farmers, whether their fields are irrigated or not.

Soil moisture probes provide farmers and agronomists with insight into what is happening below ground. These probes provide a set of “eyes” in the soil, and the sensors report moisture content, soil temperature, and salinity at various depths in the soil profile. For example, take a probe from CropMetrics™, an irrigation optimization company. The probe is 36 in. long and has 9 sensors that begin at 2 in. deep and continue every 4 in. down to a 34 in. depth. Having sensors at multiple depths adds much higher resolution to the data and a better picture of what is happening with both soil moisture and rooting depth.

If I walk out into a field, I can easily dig down to 4 or 5 in. with my hands and find that maybe the soil is dry to that depth. However, to have a better understanding of what soil moisture values are, I need to know what is happening at the active roots. By having multiple sensors at different depths, we can easily identify where the active root zone is located, as that is the depth at which the crop's roots are removing moisture from the soil.

If we think of the soil as a cylinder around a plant, the depth of that cylinder is our storage tank for water. If our storage tank is 3 ft. deep, then we would have more water than if the storage tank was 1 ft. deep. In a wet year, it can be more difficult to water because it's possible that the roots are shallow and therefore cannot access a large portion of available soil water. Knowing where the roots are is key!

In many areas, water is the limiting factor for yield; however, irrigation comes at a cost. For those who have the ability to irrigate, VRI can add value to that system. Soil moisture probes can be a valuable tool in understanding and managing water profitably with or without irrigation. If you have any questions about VRI or soil moisture probes, please feel free to reach out to your local Beck’s representative. 

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Sean Nettleton

Sean Nettleton

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