Technology and Advantages of Soil Moisture Sensors

Soil moisture sensors (SMS) can provide invaluable information to support irrigation scheduling decisions, along with field observations and crop knowledge. Growers and field managers can optimize crop transpiration rates and consequently maximize yield and quality by making sure that soil moisture at the root zone stays within ideal levels throughout the crop cycle. Soil moisture sensors can be an important ally for that. Research shows significant yield gains for soil moisture sensors guided irrigation, including 9% for celery and 10 % for strawberries, relative to grower’s practices. Recent results conducted on strawberry and almonds show that SMS can greatly help with managing deficit irrigation, saving 10 to 16% of water use while obtaining maximum yield (Gendron et al., 2018, and Collin et al., in press)

Although soil moisture sensors of different types have been around for at least 50 years and their benefits quantified in research projects, their use in irrigated agriculture is still not widespread. Crop type, familiarity with technology, agronomic understanding of the information, willingness to invest time and resources for training staff, and capacity to integrate the information into the decision-making processes are usually the main factors influencing the adoption rate of SMS. Adoption can also be influenced by sensors choice at first-time acquisitions, where unsuitable choices may discourage continued use. Yet choosing the right product can be overwhelming for the end-user who is unfamiliar with the technology and faces innumerous options of brands, models, service platforms and prices. New companies and products have appeared in the market as never before.

            There are approximately four major types of technology for quantifying in situ soil moisture status: neutron probes, TDR and FDR (both measure the dielectric constant of soil and water), tensiometers and gypsum blocks. Among those, the most common are TDRs/FDRs and tensiometers. Tensiometers are usually more suitable to crops that are sensitive to water stress such as berries and vegetables; these sensors tend to be more accurate and responsive to moisture changes in the near field capacity range. Tensiometers are also favored by many users for their suitability across different soil textures as they don’t require soil calibration. This isn’t the case for sensors that quantify volumetric water content, as they require site calibration for increased accuracy in addition to being sensitive to soil-water salinity. A common practice of SMS users is to use research-established moisture thresholds for determining when to start irrigation. This specific threshold varies according to crop types and irrigation systems, mostly due to large differences in wetted perimeter and rooting depth. This results in drier thresholds for deep rooted and overhead irrigated crops, and wetter thresholds for shallow rooted, drip irrigated crops such as strawberries and baby spinach. Volumetric water content sensors are usually simpler to install and may require less maintenance, depending on the quality of the tensiometer.

Recent improvements in hardware of certain tensiometer brands have significantly increased their robustness, resulting in lower maintenance requirement and increased accuracy and reliability. Other important advancements in the soil moisture sensing industry are significant software upgrades, which allows for improved data access, visualization, and friendlier user-interface. In addition, alert messages can be texted or emailed to users once the soil moisture reaches a pre-determined threshold.

Given the broad range of products and services available on the market, it is important that end-users understand their specific needs before committing to a sensor. In addition to choosing a type of sensor, one can choose how to access data: readings can be taken where the sensors are installed, or data transmitted to smart phones or computers through the internet. Increased data accessibility is usually more attractive to high-value crops and crops with low tolerance to water stress. Besides data transmission, data storage is also a factor to consider. Most sensors have the option of including a datalogger so the information can be graphed for better understanding of how soil moisture fluctuates throughout the season. Some sensors will require costumers to set up, install and maintain them, while others offer it all as part of the service, including a website or app for data access. Irrigation agronomic support service is also included in some cases. Needless to say, prices vary accordingly. Most important is to define which sensor or sensor package works best based on crop type and frequency of data use. Consulting local Farm Advisors and consultants can also help with making the right choice.