Irrigation Water Nitrogen Contribution Calculator

Version 0.3, August 2017

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. 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 agoutreach@plantierra.com.

Offline Calculator Options
  1. If you wish to perform the calculations offline, download the offline calculator. This version allows you to enter inputs and view calculated results. You must have Microsoft Excel in order to use this file.
    Download Offline Calculator
  2. As an alternative to the calculator, you may also download a lookup table that does not require inputs. Select from the Microsoft Excel or PDF version to download.
1 Specify Single or Multiple Water Source:
Single (Irrigation)
Multiple (e.g., Surface Water and Groundwater)
2 Enter Irrigation Water Concentrations (mg/L, same as ppm):
(NO3)-N   (NO3)
Other N in Water
3 Enter Applied Water:
4 Enter Estimated % Efficiency:
Calculation Result: