Managing nitrogen efficiently is one of the most important and complex challenges for lettuce growers in the desert Southwest. In Yuma Valley, where over 90% of the nation’s winter lettuce is produced, irrigation and fertilization must be closely coordinated to ensure nitrogen (N) remains available in the crop root zone. Because nitrate-N is highly mobile in soil, both under- and over-application can impact crop yield, input costs, and environmental quality. Traditionally, growers and crop advisors have relied on periodic soil sampling and laboratory testing to assess nitrate levels. While accurate, these methods are time-consuming and represent only a snapshot of field conditions at the time of sampling. This limitation often makes it difficult to track rapid changes in nitrogen availability following irrigation or fertilizer events.
Introducing Near-Real-Time Nitrate Sensing Technology
Recent technological advances are helping close that information gap. Near-real-time nitrate sensors are designed to monitor soil nitrate-N directly in the field, transmitting readings continuously to an online dashboard that growers can access on their computer or smartphone. These sensors, along with soil moisture probes, offer the potential to better understand nitrogen movement and crop uptake dynamics across the growing season.
During the 2024–2025 lettuce season, the University of Arizona Yuma Agricultural Center conducted an evaluation of this emerging technology using the AquaSpy (Inc.) nitrate-N sensor system under both organic and conventional iceberg lettuce production.
Fertilization and Sensor Setup
The conventional treatment received 200 lbs N acre⁻¹ of synthetic fertilizer applied pre-plant. The organic system received 2,000 lbs acre⁻¹ of chicken manure (4-4-2) before planting and 1,800 lbs acre⁻¹ of organic fertilizer (9-6-1) side-dressed midseason. Nitrate and soil moisture sensors were installed after crop emergence between two healthy lettuce plants, with probes positioned vertically at depths of 3, 6, 9, 12, 15, and 18 inches (Figure 1). The sensors collected hourly data on nitrate-N and soil moisture and transmitted it to a cloud-based platform for remote access.
Figure 1. Nitrate-N sensor-soil moisture sensor from AquaSpy and soil moisture sensor
from Sentek were installed between two healthy plants in the organic lettuce production
field at the Valley Research Center at the University of Arizona, Yuma Agricultural Center,
Yuma, Arizona.
Results and Observations
The sensors performed well under both systems, capturing seasonal changes in nitrate-N levels that corresponded with fertilizer applications and irrigation events (Figures 2 and 3). The data showed a clear rise in soil nitrate-N following the January 8, 2025, side-dress application of organic fertilizer and subsequent decreases following irrigation cycles, reflecting nitrogen redistribution in the soil profile. Comparisons with laboratory-analyzed soil samples confirmed that sensor readings followed the same general trends, with differences typically within 3–4 ppm of lab values. Although no formal statistical analysis was conducted in this initial evaluation, the consistency between methods demonstrated that near-real-time sensors can effectively capture nitrogen dynamics in the active root zone. Soil moisture levels strongly influenced sensor performance. Under persistently dry conditions, nitrate readings tended to stabilize or decline, likely due to reduced nitrate mobility and limited diffusion in the soil solution. These results reaffirm that maintaining uniform and adequate soil moisture is essential for both crop uptake and accurate sensor measurements.
Figure 2. Soil nitrate concentrations in the first and second foot of the soil profile under
the conventional lettuce system during the 2024–2025 growing season.
Figure 3. Soil nitrate concentrations in the first and second foot of the soil profile under
the organic lettuce system during the 2024–2025 growing season.
Ongoing Evaluation
This study represents an initial evaluation of nitrate-N sensor technology under Yuma Valley conditions. Additional research is underway to expand testing across different fields, soil types, and seasonal moisture conditions. Future work will focus on calibration, long-term accuracy, and integration with precision irrigation systems to better support adaptive nitrogen management strategies. The early results are encouraging: near-real-time nitrate-N sensing has the potential to become a valuable component of precision agriculture in the desert Southwest, helping growers maintain productivity while conserving water and nutrients.
To read the full Extension article, visit: https://extension.arizona.edu/publication/performance-evaluation-nitrate-nitrogen-sensingtechnologies-organic-and-conventional
