Development and Testing of a Variable Rate Nitrogen Application System through an Overhead Irrigation System

Fertigation is the injection of fertilizers, soil amendments, water amendments and other water-soluble products into an irrigation system. The common method of fertigation is using a pumping apparatus that injects the nutrients from a tank filled with nutrients at the base of the irrigation system into the main irrigation water line. This method applies fertilizer uniformly across the entire field. However, this method does not account for variation in soil texture and water holding capacity, all which could subsequently have a major impact on crop fertilizer management strategies.

Yield response to N fertilizer also varies significantly among different sections of a production field due to the variation in soil even in small fields (less than 4 hectares in size) in the Southeast region of the USA. The spatial soil variability adds to the difficulty in N application, use, and proper timing for a cropping season. However, there is no variable-rate fertigation system available to apply the correct amount of N within a field through an overhead irrigation system. Therefore, the objective of this study was to develop and test a variable-rate N application system that works independently of irrigation water flow for site-specific N application.

The variable-rate fertigation system (VRFS) was designed to apply different rates N using a pulse width modulation technique. The VRFS utilized the Clemson Lateral Irrigation Control software which controlled the solenoids in each zone by turning the N supply on and off (pulsing) for each zone.

In this study, four tests were conducted to determine the uniformity of the VRFS. In test # 1, the pump output showed a linear slope relationship and was the same for water and N. In test # 2, nozzle flow and uniformity were determined using four different irrigation system travel speeds at N application rates of 31, 59, 88, and 113 kg/ha. There was a strong correlation (R² = 0.9998) between irrigation system speed and N rate. In test # 3, the uniformity across the length of the irrigation system was determined. The nozzles produced an average flow of 31.1, 58.7, 87.6, and 112.7 kg N/ha with an overall average error of 0.1% across all N rates. Results also showed the system was capable of accurately applying N based on prescription maps with an error of less than 1.8%. Test # 4 was conducted to determine the accuracy of the map-based controller system for applying variable rate N. There was a strong correlation between target N and actual N rates (R² = 0.9999).

In summary, the overall performance of the system was promising. The VRFS applied the correct amounts of N within each zone by either manually controlling the pulsing mechanism or utilizing a prescription map that could apply different rates throughout the field within each individual zone.

Article by Phillip B. Williams, et al, from USA.

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