Nitrogen in Irrigation Water

Nitrate in groundwater has the potential to provide a substantial amount of plant-available nitrogen to crops. Accounting for the nitrate in irrigation water allows for substantial decreases in fertilizer costs. To measure the concentration of your irrigation water, you can have the nitrate content analyzed at a commercial test lab. It is important to note that results from these tests can be expressed in units of ppm NO₃^- or ppm NO₃^-—N. The more useful measurement for calculating irrigation water contribution to plant-available N is NO₃^-—N. To convert from NO₃^- to NO₃^-—N, multiply your ppm NO₃^- by 0.225. For more conversions and in-depth discussion of the implications visit this blog post.

EXAMPLE:
A recent study¹ showed that nitrate concentrations in ground water can consistently reach concentrations greater than 45 ppm NO₃^-. To convert this nitrate content to lbs N supplied per acre-foot of water, we must first convert from 45 NO₃^- to NO₃^—N, (45 ppm NO₃^-*0.225 = 10ppm NO₃^—N). Using Table 3, we can see that this reading of 10ppm NO₃^—N would translate to 27 lbs of plant-available nitrogen per acre-foot of irrigation applied.

Adapted from East San Joaquin Water Quality Coalition website (esjcoalition.org).