Arthur Villordon and Tara Smith

Soil moisture management during the first five days after planting is critical for establishment and determining the potential yield of sweet potato storage roots, which are the edible belowground portions of the plant. After the establishment stage, competent adventitious roots, which are roots that arise from nodes, undergo storage root formation. The number of potential storage roots is determined as early as 15 days after planting. During these critical stages, excessive soil moisture can reduce the development and potential yield of storage roots. On the other hand, reduced water availability can suppress storage root formation and lead to nonuniform storage root sizing and unpredictable yields. Almost 90% of the harvestable storage root number is determined during the first 30 days after planting, and soil moisture availability needs to be adjusted based on environmental conditions and crop growth. This underscores the importance of irrigation scheduling, which is the application of the right amount of water at the right time. Irrigation scheduling maximizes irrigation efficiency and reduces waste.

Conventional irrigation scheduling methods include calendar-based and subjective observations of plant foliage status. Several advanced technologies are available for scheduling irrigation, including weather stations, airborne and space-borne remote sensing platforms, computer models and soil moisture sensors. Soil moisture sensors, in particular, can be used effectively to improve irrigation management. As a tool for irrigation scheduling, soil moisture sensors have been shown to increase crop yields while conserving water. Soil moisture sensors provide site-specific, timely data that can be used to guide in-season irrigation management decisions. According to the U.S. Department of Agriculture National Agricultural Statistics Service Irrigation and Water Management Survey from 2018, only 11.9% of producers who irrigate used soil moisture sensors to schedule irrigation. However, with the growing necessity for sustainable use of water resources, there is an increasing need by producers to make more accurate and timely decisions to reduce costs and maximize profits. The availability of more affordable and reliable soil moisture sensors technologies is expected to increase the adoption of these tools by producers in the near future. 

In 2021 a newly developed next-generation soil moisture sensors system was evaluated at the Sweet Potato Research Station (SPRS). This sensor system offers innovative features including the capacity to measure soil temperature and electrical conductivity at three depths. It also includes an aboveground sensor system that measures several environmental parameters, including photosynthetic radiation, air temperature, relative humidity, rainfall and normalized difference vegetation index, or NDVI. When properly calibrated, NDVI measurements can be used to measure crop development. It can also be used to assess in-season nutrient status. In contrast to other systems that provide only sensor readings, this particular next-generation soil moisture sensor system incorporates sophisticated analytics that enable the user to perform analyses of crop performance across production fields throughout the season.

The sensor system is relatively easy to install and features onboard wireless telemetry that uploads data to a cloud-based server. Near real-time updates of soil moisture data can be accessed via a website or on smartphones or tablets. The sensor system was used to manage irrigation scheduling in research station plots. A critical step prior to using these sensors for decision making is calibrating soil moisture availability with sensor readings and determining field capacity. Once this is established, basic knowledge of crop rooting depth and soil textural properties and estimates of soil volumetric water content are needed to establish the management allowable depletion, which is the soil water content below which the crop will show signs of stress. These parameters are then used to estimate the plant-available water content as the difference between field capacity and wilting point. The allowable depletion is established as a threshold based on a fraction of plant-available water content at which a water deficit will occur without irrigation. Allowable depletion can differ by soil texture and crop type, as well as growth stage. In sweet potatoes, prior work has shown that allowable depletion can range from 50% to 70% depending on crop stage. Allowable depletion, whether measured or modeled, lends itself to decision support regarding how much to irrigate as well as when to irrigate. The next-generation soil moisture sensor system was deployed in well-characterized research plots that have been previously calibrated with earlier generation nonwireless soil moisture sensor systems. In particular, the next-generation soil moisture sensor system was calibrated, and readings were compared to previously established field capacity and allowable depletion values. Irrigation scheduling was conducted mainly through data accessed either via the web interface or smart devices.

In 2022, the next-generation soil moisture sensor system was evaluated on a producer's field. The main goals of the on-farm trials were to validate research station findings as well as use sensor-based soil moisture data to schedule irrigation using a center pivot system. This farm has been testing SOIL moisture sensor systems for possible use as management tools. Sweet Potato Research Station researchers worked with the farm’s agronomists in setting up and calibrating the sensors. The production field was divided into two management zones to facilitate comparison of the next-generation soil moisture sensor system versus irrigation, and they were compared using plant canopy status for signs of stress. As with the on-station trials, irrigation scheduling was based on data accessible via web interface or smart devices. After each irrigation event, researchers and the agronomists assessed the accuracy of the readings by visiting the field. At the conclusion of the study period, storage root yield sampling and grading were performed by farm staff. The agronomists collected storage root yield data as part of their routine sampling procedures. The use of the next-generation soil moisture sensor system to schedule irrigation events was associated with a 33% increase on total storage root yield compared to conventional irrigation scheduling using visual cues. More importantly, the yield of the economically important U.S. No. 1 storage root yield grade was increased by 63% when irrigation was scheduled using next-generation soil moisture sensor system data. The on-farm results are consistent with prior work that shows the benefits of the use of next-generation soil moisture sensor systems. This work demonstrates the importance of proper calibration and testing of next generation sensors prior to commercial deployment on-farm. The ease of deployment and near real-time data availability for time-critical in-season management decisions are features that will facilitate the adoption of soil moisture sensor systems by growers.    

Arthur Villordon is a professor at the AgCenter Sweet Potato Research Station in Chase. Tara Smith is an interim executive associate vice president for the AgCenter and director of the Louisiana Cooperative Extension Service. She is based at the Dean Lee Research and Extension Center in Alexandria.

This article appears in the winter 2023 edition of Louisiana Agriculture.

The Arable sensor system was a next-generation wireless sensor system used to detect soil moisture in sweet potato fields. Photo by Arthur Villordon

The use of the next-generation soil moisture sensor system to schedule irrigation events was associated with a 33% increase on total storage root yield compared to conventional irrigation scheduling using visual cues. Figure by Arthur Villordon