Conversion between nitrate (NO₃) and nitrate-nitrogen (NO₃-N):

The reason for this conversion is that nitrate molecule weighs 62 grams per mole; the nitrogen content of nitrate is 22.5% of the total weight of the molecule.

Nitrogen content of irrigation water*

* water analyses from most labs report NO₃  in units of ppm, but it is very important to pay attention to which units the results are reported.

How much of the nitrogen in water should be credited to your crop is debatable.  Consider that lettuce transpires 5 to 8 inches of water between germination and maturity in the Salinas Valley during the summer.  Extra water applied beyond crop ET would be lost as drainage and therefore would not contribute N to the crop.   The extra water also would likely leach plant available soil nitrate below the root zone.   In addition, some ground water that is high in nitrate is also high in salts and may require a leaching fraction (extra water applied to leach salts below root zone) to attain maximum production.    The good news is that you can account for the N contribution from the nitrate in the irrigation water using the quick nitrate soil test for previous irrigations.  However, this test will not estimate the contribution of N from the irrigation water for future waterings.

Our best estimate of how much N the irrigation water would contribute to future waterings is to divide the crop evapotranspiration by the irrigation efficiency.  For example, for 7 inches of crop ET and an 80% irrigation efficiency, the following values would approximate the N contribution of irrigation water for the indicated range of nitrate concentrations:

* multiplied by 0.8 to account for the irrigation system efficiency

As can be seen, waters containing less than 45 ppm NO₃ generally do not contribute a significant amount of nitrogen for crop growth. However, if well waters contain more than that amount they begin to contribute greater amounts of water for crop growth.