Accounting for Nitrate in Irrigation Water

With the increasing scrutiny and regulatory pressures to reduce nitrate leaching and nitrogen (N) fertilizer use, determining how much N is contributed from the irrigation water becomes important, especially in organic systems where the costs of N fertilizers are high.

The first step is to identify the nitrate concentration in your well water, which can be done by sending a sample to a commercial lab. The nitrate test strips could also be used, but with limited precision. 

Once the nitrate concentration is determined in ppm of NO₃-N (NO₃ ÷ 4.43 = NO₃-N), an estimation of the amount of water to be used in the entire crop season (in ac-in) is also necessary for the calculation.

Conversion factor: ppm of NO₃-N × 0.227 = lb of N/ac-in of water

Example: a strawberry crop that uses 24 ac-in of water (2 ac-ft), with well water of 20 ppm of NO₃-N will have a contribution of approximately 109 lb of N/ac (20 x 0.227 x 24 = 109).

Keep in mind that not many wells in Ventura County present such high nitrate concentrations. Also, if irrigation amounts exceed crop water use, the excess water should not be accounted for the N contribution calculation. 

The study in the link below demonstrates that the nitrate from irrigation water was at least as efficiently used as N fertilizer:

Field trials show the fertilizer value of nitrogen in irrigation water