Performance Evaluation of Nitrate-Nitrogen Sensing Technologies in Organic and Conventional Iceberg Lettuce Systems under Subsurface Drip Irrigation

In arid agricultural regions such as Yuma, Arizona, efficient nitrogen management depends on the close coordination of irrigation and fertilizer applications because water movement within the soil profile directly affects nitrogen dynamics and plant uptake efficiency. Traditional laboratory analyses of soil nitrate are time-consuming, costly, and provide only periodic snapshots that may not represent real-time field variability. Recently, near-real-time nitrate-nitrogen sensors have emerged as a promising solution for monitoring soil nitrogen availability continuously within the active root zone. These sensors allow growers to make timely, data-driven adjustments to irrigation and fertilization practices, improving nitrogen use efficiency and minimizing environmental losses.

Advancements in electrochemical and ion-selective electrode (ISE) sensor technologies have enabled in-situ monitoring of nitrate concentrations with improved accuracy and temporal resolution (Eldeeb et al., 2023; Bristow et al., 2022). Several studies have demonstrated strong correlations between sensor readings and laboratory analyses across a range of soil textures and nitrate concentrations, validating their potential for precision nutrient management. However, the accuracy and reliability of these sensors are influenced by soil moisture fluctuations, salinity, and temperature, factors that are particularly challenging under the arid, high-evapotranspiration conditions of Yuma’s desert soils. Therefore, evaluating the performance of nitrate sensors under these conditions, especially in subsurface drip–irrigated organic and conventional lettuce systems, is critical for determining their field applicability and potential integration into nitrogen management strategies

In lettuce (Lactuca sativa), efficient management of organic fertilizers in organic systems or synthetic nitrogen (N) sources in conventional systems is crucial to improving yields, reducing environmental impacts, and optimizing resource use (Hartz et al., 2000; Hochmuth et al., 2022). One of the significant challenges in N management for growers is quantifying and monitoring N availability during the growing season within the active root zone, especially at the peak of lettuce growth stages (i.e., heading stage).

Near real-time sensors are transforming modern agronomy by providing continuous, timely data on key variables such as soil moisture, temperature, salinity, and nutrient levels. These sensors enable growers to make informed, datadriven decisions on irrigation and fertilization, significantly improving resource use efficiency and crop performance. Reducing reliance on delayed laboratory analyses and manual field checks, near real-time monitoring helps minimize input losses, enhance environmental sustainability, and support precision agriculture practices. As a result, growers can respond quickly to changing field conditions, thus increasing productivity and profitability (Bandgar & Biradar, 2024; Chatziparaschis et al., 2023).

Near real-time sensors assist growers in making informed daily agronomic decisions throughout the growing season. Particularly, they can provide continuous monitoring of N levels in the active root zone from planting to harvest. This evaluation aims to utilize an innovative nitrate-N sensor to generate data that can be analyzed and applied in developing N management strategies for both organic and conventional lettuce production systems in Yuma, AZ.

Site description, fertilization, and irrigation management

This study was conducted during the Fall 2024 through Spring 2025 growing seasons at the Valley Research Center, University of Arizona Yuma Agricultural Center, located in Yuma, Arizona. The experimental location lies within an arid desert climate zone, characterized by extremely low annual rainfall (less than 3 inches) (AZMET). The soil at the study site is classified as a Gila silt loam (fine-silty, mixed, superactive, calcareous, hyperthermic Typic Torrifluvent), typical of irrigated alluvial soils in the Yuma Valley (USDA–NRCS, 2022). It has a clay loam texture, with a field capacity of 31.9% volumetric water content and a permanent wilting point of 15.5%. The particle size distribution consists of 21% sand, 48% silt, and 31% clay, values consistent with previous characterizations of Yuma Valley soils (Mohammed, 2025). These soil properties are important for local stakeholders because they influence irrigation management, nitrogen mobility, and sensor response. In soils with moderate water-holding capacity like this, maintaining optimal soil moisture is essential for accurate nitrate-N sensor readings and efficient fertilizer use under subsurface drip irrigation. Topsoil (0–12 inches) organic matter is relatively low (1.5%), consistent with regional soil characteristics in arid environments (Mohammed, 2025).

Nitrogen fertility strategies differed by production system. The conventional treatment included a pre-plant application of 200 lbs/acre of synthetic nitrogen fertilizer on October 28, 2024. For organic treatment, 2,000 lbs/acre of chicken manure pellets (4-4-2) were applied before planting. An additional 1,800 lbs/acre of organic fertilizer (9-6-1) was sidedressed on January 8, 2025 (Nature Safe Fertilizers, Darling Ingredients Inc.). Irrigation scheduling followed locally recommended practices commonly adopted at the Yuma Agricultural Center, ensuring optimal water application timing and efficiency across both treatments.

Nitrate-N and soil moisture sensors (AquaSpy, Inc.) were installed after crop emergence, once uniform plant establishment was confirmed. Each probe was positioned midway between two healthy lettuce plants within a representative section of the field that reflected average soil texture, moisture, and crop vigor. The probes were installed vertically into pre-augered holes to ensure good soil contact at multiple depths (3, 6, 9, 12, 15, and 18 inches), allowing continuous monitoring of nitrate-N and soil moisture within the active root zone. Installation was performed carefully to avoid disturbing adjacent roots or creating air gaps around the sensor shaft, which can affect readings. After installation, the soil surface was gently compacted and leveled to maintain consistent irrigation flow and prevent preferential water movement near the sensors. Sensor calibration and activation were completed on-site using the manufacturer’s field app to ensure stable baseline readings before data collection.

This installation approach was designed so that a grower or farm manager could replicate the setup with minimal technical expertise. By following a few key steps, selecting a representative site, ensuring proper soil contact, and verifying baseline readings, the sensors can provide reliable real-time data on soil nitrate-N and moisture dynamics under subsurface drip irrigation. Additional photographs illustrating the step-by-step installation process are recommended to guide practical adoption (see Figure 1).

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Sensor data collection and soil sampling protocol

Sensor data were collected using the AquaSpy data management platform, which records nitrate-N and soil moisture readings at 3-inch intervals down to 24 inches. The sensors transmitted data hourly to the cloud-based platform, allowing continuous monitoring throughout the growing season. Data were reviewed weekly to verify consistency and identify changes following irrigation or fertilizer applications.

To validate sensor accuracy, soil samples were collected manually from the same sensor locations on the same day as selected sensor readings. Samples were taken at two depth intervals 0–12 inches and 12–24 inches, to correspond with the primary lettuce root zone and the depths most sensitive to nitrogen movement under subsurface drip irrigation. Each sample was composited from three cores around the sensor probe within a 12-inch radius to account for smallscale variability. Samples were immediately placed in labeled plastic bags, stored in a cooler, and transported to Ward Laboratory (Kearney, NE) for nitrate-N analysis.

Sensor and laboratory data were compared by matching sampling dates and depths. Descriptive comparisons were made to evaluate the magnitude and direction of differences between the two measurement methods. Special attention was given to changes in nitrate-N levels following irrigation and fertilizer events to assess the sensors’ ability to detect temporal shifts in N availability under field conditions.

Nitrate sensor comparison vs. soil lab analysis

The comparison between nitrate-N sensor readings and laboratory-analyzed soil samples demonstrated consistent and comparable trends across both production systems. Although no formal statistical analysis was conducted, the observed differences between the two methods were minor (Table 1). The sensors effectively captured relative changes in soil nitrate-N concentrations over time, particularly following fertilizer applications and irrigation events, demonstrating their practical reliability for tracking N dynamics under field conditions. Additionally, the sensors effectively captured the increase in nitrate-N levels following the side-dress application of organic fertilizer on January 8, 2025, reflecting their responsiveness to in-season fertility events. This is particularly valuable for growers and crop advisors because it demonstrates that near-real-time sensors can detect changes in soil nitrate-N soon after fertilizer application. Such responsiveness allows for more precise evaluation of nutrient availability, helps confirm whether applied nitrogen is reaching the active root zone, and supports timely adjustments to irrigation or additional nutrient inputs, improving both N use efficiency and environmental stewardship.

Soil nitrate concentration

Irrigation events had a substantial influence on soil nitrate-N concentrations throughout the growing season. In both organic and conventional lettuce systems, noticeable changes in nitrate-N levels followed irrigation, indicating that water movement within the soil profile affected nitrate distribution and sensor detectability. These irrigation-related shifts were more pronounced in the conventional system, where higher fertilizer inputs and more frequent irrigation contributed to greater fluctuations in nitrate-N concentrations.

Across the growing season, soil nitrate-N concentrations differed between the two production systems and soil depths (Figures 2 and 3). In the conventional lettuce system, nitrate-N concentrations in the first foot of soil ranged from 5.8 to 14 ppm, while the second foot ranged from 2.3 to 15 ppm. In contrast, the organic system exhibited more stable patterns, with the first foot ranging from 3 to 17 ppm and gradually declining as the season progressed. In the second foot, nitrate-N levels ranged from 7 to 19 ppm. Descriptive comparisons of these patterns indicate consistent trends between sensor-measured data and field-observed responses, confirming that the sensors effectively captured dynamic changes in soil nitrate-N associated with fertilizer applications and irrigation events.

Despite these responses, persistently dry soil conditions at the study site may have limited the sensor’s ability to accurately reflect nitrate availability, as low soil moisture can restrict nitrate mobility and plant uptake. It is well established that insufficient soil water reduces nutrient transport to roots and can lead to nitrate immobilization, thereby lowering nitrogen uptake efficiency (Irmak & Mohammed, 2023).

Irrigation events influence nitrate sensor performance

Irrigation played a key role in shaping nitrate sensor readings, as noticeable shifts in nitrate concentrations often followed irrigation events. This pattern suggests that water movement through the soil helped mobilize nitrate, increasing its detectability by the sensors. A significant correlation was observed between irrigation events and nitrate sensor performance, especially in the conventional lettuce system, where nitrate concentrations exhibited pronounced fluctuations throughout the growing season (Figure 4). However, the pattern was distinctly different in the organic lettuce system. Overall, the soil tended to be drier than in the conventional system, which likely contributed to more stable and less variable nitrate concentrations in both the first and second feet of the soil profile (Figure 5).

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The ability of nitrate-N sensors to provide near realtime information on soil nitrogen dynamics has practical implications for improving nitrogen management in desert vegetable systems. In the Yuma Valley, lettuce growers typically rely on pre-season fertilizer applications based on expected crop demand and short production cycles, often with limited in-season monitoring. According to the University of Arizona Vegetable Fertilizer Guidelines (Desert Ag Solutions, 2024), N recommendations for iceberg lettuce generally range from 180 to 220 lbs N acre⁻¹ under conventional systems, with adjustments based on soil type, irrigation method, and seasonal conditions. These guidelines emphasize maintaining sufficient available nitrogen during the rapid growth and heading stages, when crop uptake is highest.

Near real-time nitrate-N sensors can complement these established guidelines by providing field-specific, timesensitive data that helps growers evaluate the effectiveness of fertilizer applications and make informed in-season decisions. For example, increases in sensor-measured nitrate-N following fertilizer or irrigation events can confirm that N has reached the active root zone, while declining readings may indicate crop uptake or potential leaching losses. Interpreting these data trends, growers can fine-tune nitrogen scheduling, adjust irrigation timing to minimize leaching, and avoid unnecessary additional applications.

Integrating sensor-based monitoring with existing nitrogen recommendation frameworks provides a practical pathway toward adaptive nitrogen management, helping growers maintain crop yield and quality while reducing fertilizer costs and environmental losses. Over time, incorporating sensor data into routine management could allow for more site-specific and sustainable nitrogen recommendations for both organic and conventional lettuce systems in Yuma and other arid regions.

This study provides an initial field evaluation of a nearreal- time nitrate-N sensing technology in organic and conventional lettuce production systems. The findings are preliminary and based on limited comparisons between sensor data and laboratory analyses; however, they indicate the potential of this technology to monitor soil nitrate-N dynamics and detect in-season changes following irrigation and fertilizer events. This ongoing research aims to expand data collection, incorporate multiple site replications, and conduct statistical analyses to validate sensor performance under varying soil and moisture conditions. Continued evaluation will support the refinement of calibration methods and the development of practical guidelines for integrating nitrate-N sensors into nitrogen management strategies in arid vegetable production systems.

The accuracy and reliability of sensor readings were closely tied to soil moisture conditions. Maintaining adequate and consistent soil moisture throughout the growing season proved essential for ensuring dependable nitrate measurements. Under dry or limited irrigation conditions, sensor performance was reduced. Therefore, the use of these sensors is not recommended under deficit or limited irrigation strategies. Utilizing nitrate sensors may assist growers in monitoring soil nitrate concentrations in real time, enabling them to make more informed decisions that enhance sustainability and improve profitability, while also saving time and reducing the need for frequent laboratory testing, which often requires considerable time and resources.

The author gratefully acknowledges the support of the School of Plant Sciences and the Yuma Agricultural Center at the University of Arizona. Special thanks are extended to the University of Arizona Cooperative Extension for their continued guidance and resources. Appreciation is also given to the Western Alliance to Expand Student Opportunities (WAESO) for funding the internship students who contributed to this project. The author sincerely thanks AquaSpy for their generous support and advanced sensor technology, which played a critical role in enabling real-time data collection and improving nitrogen and irrigation management in the field. The author also extends appreciation to Nature Safe Fertilizers for generously providing the organic fertilizer used in this study and to Soiltech Wireless for providing the precision weather station at the Yuma Agricultural Center.

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Desert Ag Solutions. (2024). Fertilizer Guidelines for Vegetable Crops in Arizona. University of Arizona, College of Agriculture and Life Sciences. >https://desertagsolutions.org/research-areas-and-initiatives/improve-plantnutrition/fertilizer-guidelines-vegetable-crops-arizona  

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Mohammed, A. T. (2025). Soil characterization for lettuce production trials in arid regions: Field and laboratory data summary. University of Arizona, unpublished data.