Savannah Valley District

Soil Moisture Sensor Placement

Becky Davis, Water Resources Agent

Properly irrigating crops with the optimal amount and timing can improve crop yields without causing runoff and loss of nutrients to ground and surface waters. However, before irrigating any plant or crop, information should be gathered to determine when it is necessary. This information a.k.a. data, can come in various forms. The weather forecast can provide useful information on probable rainfall, humidity, and wind speed. Another way of assessing soil moisture is by simply touching the soil to feel how wet it is. Knowing the soil moisture at the root of the plants gives the best indication of additional water needed. Therefore, that is where the soil moisture sensors should be to get to the root of the matter! Placement of the sensors depends on the type of sensor used, soil type, crop to be watered, and the form of irrigation used.

The sensor depths to be used depend upon the crop, soil texture in the profile, and the extent that the roots grow. It is also crucial to have the sensors within the row of the crop to be amongst the roots and away from tractor travel in between the rows.

Photo series 1. WaterMark 200SS soil moisture sensors are attached to PVC pipes of varying lengths to correspond to the desired depths. The placement of the sensors is in the crop row.

Sensor placement in relation to the rest of the field is important because one or two sets of sensors will be representing the water needs of the whole field. Therefore, obtaining data such as soil type will greatly improve the estimated irrigation needs. A general rule of thumb is to place the sensors in the dominant soil type for the field (on left in Photo series 2). In fields with multiple soil types covering similar acreage, other decision-making factors come into play such as the potential yield of a crop in certain soil types. For example, in a field in which the crop performs best in loamy soils, the sensors should be placed in the loamy zones. In other words, monitor the moisture in the area of the field where the highest yield potential will be.

Another factor to consider for sensor placement in the field is the uniformity of the water distribution by the irrigation system. The map on the right in Photo series 2 was generated by the Center Pivot Irrigation Test (CPIT) program. The yellow areas indicate the locations of average water output by the irrigation system. Therefore, in this case, the best location for sensors would be along the widest yellow band especially since the area is an outer span of the pivot which covers more acreage than an area closer to the center. An overlap of these factors (note X in the Photo series 2) results in a better representation of the field for optimal irrigation rates as opposed to consideration of only one factor. For more information on this, please visit .

Photo series 2. A possible placement of a soil moisture sensor (X) using data provided by a soil type map and a water distribution map.

Other considerations for sensor placement include the slope of the land and be sure to choose areas that are not disturbed by the movement of the irrigation system and proximity to an access road.

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