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“Flat” Bottom Graph

18’’ sensor does not show much movement, should I be worried?

No extreme changes on your deeper probes could reflect reality!


Fig. 1: (Corn, NE. V9 stage on July 24) “Flat” 18’’ graph. The response to irrigation is mostly shown on the 8’’ graph. The 18’’ graph is settled around field capacity

Relatively flat graphs, especially for the bottom sensor, do not necessarily indicate dormant soil or sensor malfunction. It most likely indicates some kind of equilibrium, a state of stability, especially at the early stages of the crops. Such graph can still inform you about over or under watering, and it can even teach something about where the active roots are taking up water.

What could possibly result in relatively flat graph?

  1. Young crop - When the crop is young and active roots do not populate the 18’’ sensor depth, sensor reaction will remain around field capacity if irrigation is accurate and does not reach deep enough. The roots of a well watered corn crop at the V9 stage (Fig. 1) have just begun to reach to 18” and as expected little movement is shown on the sensor at that depth. As the crop grows, and active roots penetrate deeper both the 8” and 18 sensors begin to respond to irrigations and extraction by the crop (Fig. 2).


    Fig. 2: (melons planted in March, AZ) Gradual increasing reaction of the 18’’ graph to growing roots.

  2. Shallow roots - Roots will be found where the soil provides sufficient amounts of water and oxygen. Shallow roots can result from frequent accurate irrigations where water does not penetrate deep. We suggest taking a soil moisture sample before beginning irrigation. A flat bottom graph later in the growing season is a good reason to check on the soil and adjust your irrigation schedule accordingly.

  3. Sensor measuring capacity - The sensitivity of the Cropx sensor is reduced at water contents above 40% vwc. Thus, the response of the sensor in dampened at these high moisture levels and the graph shows stable measurements around 40% vwc. At these moisture levels the concern is over-watering and irrigation should be adjusted. (or else hope it stops raining). Once the moisture drops below 40% vwc, the graph will act accordingly.   

  4. Expectation - Water moves slowly in through the soil. For most agricultural soils speed of 0.5 to 1.5 inches per hour are common. This means it can take a watering event 12 to 24 hours to show up at the 18” sensor. The Cropx sensor is designed to minimally disturb the soil while the mushroom & spiral design prevents water from moving along the tube soil interface. Some other sensors are inserted into disturbed soil and allow water to flow along the sensing tube. These sensors often show water penetration to 36” in a few hours even though that does not reflect actual movement in the soil. The expectation of seeing rapid responses to irrigation are not necessarily realistic.

Bottom Line

The most reliable way to validate your sensor is to go out to the field and witness it yourself. Even though you wish to save that by using remote devices, the confidence you’ll gain by confirming the reliability of your devices, worth the trip. With that said, if you suspect your “flat” graph as malfunction, go ahead and feel the soil. Most likely, you would find that the sensor is “right”. Once you experience that, the “flat” line would be more realistic and therefore would provide you more confidence with managing your field.


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