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It’s a question we get asked all the time, and the answer isn’t as straight forward as it may seem. In this blog, we cover aspects of what factors should be considered in determining the number of sensors needed to get a good understanding for your application. 

Soil moisture is a parameter which can show big differences over short distances. Hydrologists agree that soil moisture values are far from homogenous across fields and landscaping projects. There are many factors influencing (hyper)local soil moisture levels, which is the key reason why our sensors have a low Total Cost of Ownership (TCO). It is also the reason why we have ‘soil moisture only’ philosophy for our sensors – it’s a factor which simply needs to be measured more granularly than for example soil temperature. We believe it is better install multiple low-cost soil moisture sensors over a field or landscaping project and average the soil moisture values, instead of placing one, more expensive sensor at a central location.

The key question is: how many sensors do we need on our farm or project? Unfortunately, there is no ‘golden rule for the number of sensors per hectare’, but from practical experience we know that a good total picture can be derived from some cleverly placed sensors.

In this blog post we will talk about the factors influencing soil moisture levels and provide rules of thumb for deciding how many sensors are needed.

Key factors influencing soil moisture are:

  • Terrain accents, like higher areas, lower areas, sloping, shade etc.
  • Crop type (root depth)
  • Soil type (and differences in soil type across fields or projects)
  • The underground (hard pans, underground water flows, etc.)
  • If irrigated: the number of irrigation zones, valves, lines

Based on the above factors and years of experience in field, we have developed a couple or rules-of-thumb:

  • Terrain accents: if there are altitude difference in fields and/or landscaping projects, it can be assumed that the higher parts are drier, and the lower parts are more wet. This simple physics (gravity), and higher parts are often more exposed to wind and solar radiation. If a field has a higher part and a lower part, we advise at least 2 sensors – one for each scenario. Additional insights can be provided by one or more sensors in the ‘middle’.
  • Crop type: if different crops are grown, with different characteristics like for example root depth, leaves, canopy density, and moisture preference, we advise to place at least one sensor per crop type.
  • Soil type: different soil types show different moisture behavior. Sandy soils behave completely different than clay soils. If both soils are present in a field or landscaping project, both should be measured.
  • The underground: this can be a tough one, because we can’t ‘look into the soil’. For example a hard pan can prevent water to infiltrate into lower levels, leading to wet patches. Often, soil moisture sensors are an ‘early warning’ system, giving an indication that a certain area needs to be examined further.
  • Irrigation system: for irrigated land/projects, one could choose to put one sensor in every irrigation zone. Of course, this can also be aggregated to 1 sensor per X irrigation zones. Furthermore, with drip irrigation systems or in an orchard, it can be insightful to put multiple sensors along an irrigation line.

Of course, in very large areas It might not be economical to place one sensor per hectare or even one per 2 hectare. In these cases, we advise to split up the terrain or project in ‘zones’ with similar characteristics. A good way to identify these areas is by using moisture data from satellites.

By the way, it is often thought that satellite data competes with sensors. We think satellite data is extremely useful and should be used, but it does not provide the insights of real time measurements in the root zone of the plants. Hence the data should be combined, to get the best insights to support decisions.

Soil Calibration Guide 

To help support you decisions for placement of sensors based on soil type and soil moisture behavior, we’ve put together a free Soil Calibration Guide. 

Download the free soil calibration guide