Irrigation Water Nitrogen Contribution Calculator

Version 0.3, August 2017

A tool for converting nitrate and ammonium concentration in applied irrigation water into pounds of N applied per acre.

This tool helps you convert lab results and irrigation information into the amount of N that comes from your irrigation water source. Accounting for this when planning your N program is good practice.

Mineral nitrogen (nitrate and ammonium N) in your irrigation water is, from the perspective of growing plants, indistinguishable from the N forms in fertilizers that are injected into irrigation water or otherwise applied to the soil using conventional application methods. In other words, plants cannot tell where N came from. Unless you are recycling wastewater or animal waste effluent, most of the dissolved N in applied irrigation water will probably be nitrate. If you test your water, then you can easily use those results to calculate how much N is applied with irrigation. If you find the concentration to be relatively consistent, then sampling need not be frequent. For example, if semi-annual samples give similar results for two years, then checking once every two years thereafter would alert you to shifts from this baseline level.

Like any applied N, even when best practices are used to maximize efficiency, there is usually some N lost on the way to the plant. The proportion of applied N that is lost can be minimized (N used more efficiently) when N applications coincide in time and amount with the rate of plant uptake, and when they are applied where roots are active and able to take up water and other essential nutrientss (i.e., not too deep, where moisture is sufficient, and salinity is not elevated; see Reference 1). Nitrogen from irrigaiton water that is synchronized with crop N demand will have far less N lost, and much greater N uptake efficiency, than N in irrigation water applied during times when crop N demand is low or absent. For example, the efficiency of N in irrigation water applied during an off-season leaching event will be quite low.

Growers should therefore consider N in irrigation water as meeting the crops' N needs much as they would consider fertilizer: by calculating the amount of N applied, and then multiplying by the anticpated efficiency. The result of this calculation should be SUBTRACTED from N that needs to be supplied from applied fertizer, and fertilizer application rates REDUCED accordingly.

The calculator tools below help you to do this. The background of the calculations is provided on the "References" worksheet of the offline calculator (click the download link below), if you wish to incorporate correct calculations into your own tools. An additional version of the calculator is provided for growers or CCAs who work with many fields, so that a database or spreadsheet accommodating many fields in a consistent manner is preferable. The calculations are identical, regardless of the calculator format.

Be sure to look at the resources on the "References" tab from the offline calculator to better understand how to interpret the results of these calculations.

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.

If you wish to perform the calculations offline, download the offline calculator. You must have Microsoft Excel in order to use this file.
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 % of Efficiency:
Calculation Result: