About Irrigation Water Nitrogen Contribution Calculator

Nitrogen (nitrate and ammonium N) in irrigation water is as readily available to crops as N in chemical fertilizers. So, when growers account for N in irrigation water, they can reduce rates of fertilizer N application. The Irrigation Water Nitrogen Calculator converts nitrate and ammonium found in irrigation water, as measured by a laboratory or other method, into pounds of N applied per acre. A fact sheet for this tool is available here. The calculator features the following options:

• A single source mode for consideration of a single water source.
• A multiple source mode for consideration of multiple water sources (e.g., surface water and groundwater).
• An offline version, which allows the tool to be downloaded and used offline. This version can also be joined with other calculations performed in a grower or CCA database or workbook.

Water Quality Sampling

To determine the amount of N supplied by irrigation water, samples must first be collected and analyzed. Most of the N in irrigation water will be in the nitrate form. Sampling recommendations include the following:

• Frequency. Sample irrigation water two times a year initially to understand potential variation. If results show the N concentration to be relatively consistent for two consecutive years, then consider reducing sampling to once every two years.
• Sampling. Samples can be collected and delivered to a lab for testing (preferred). Inexpensive nitrate test strips or other testing methods can also be used.
• Labs. Many agricultural labs can test water quality for N. County health departments can be contacted for a list of certified labs.

Your coalition may also provide irrigation water N testing services. Consult with your coalition for more information.

Use of Results

The Irrigation Water Nitrogen Calculator result (in pounds of N per acre) should be SUBTRACTED from the total crop N requirement. Other N credits, such as for N available in the soil and from organic amendments, should be similarly evaluated and SUBTRACTED from N requirement. The residual N requirement, after accounting for these sources, is the appropriate fertilizer N application rate. Leaching of irrigation water beyond the root zone is necessary to maintain appropriate levels of soil salinity. Similar to any applied N, some irrigation N will also likely be leached with the irrigation water because nitrate moves with water in soil. The amount of N lost can be minimized by maintaining irrigation water in the root zone through irrigation scheduling, making fertilizer N applications to match crop demands, and fertigating N during the latter hours of irrigation sets when possible. If leaching for salinity control requires off-season irrigations, then low-N water is preferable.

Authored by J. Dickey, reviewed by Ken Cassman, Eric Athorp/Kings River WQC, and Ryan Dodd/Provost & Prichard, Tim Hartz, and Daniel Geisseler (both UCCE). Please send comments and suggestions for improvements to outreach@formationenv.com.