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.

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 the table below, 10ppm NO₃^—N would translate to 27 lbs of plant-available nitrogen per acre-foot of irrigation applied.

	Concentration of nitrate-N in ppm (mg/L)
Acre-Ft water applied	1	5	10	15	20	25	30	35	40	45	50
0.1	0	1	3	4	5	7	8	10	11	12	14
0.2	1	3	5	8	11	14	16	19	22	25	27
0.3	1	4	8	12	16	21	25	29	33	37	41
0.4	1	5	11	16	22	27	33	38	44	49	55
0.5	1	7	14	21	27	34	41	48	55	62	69
0.6	2	8	16	25	33	41	49	58	66	74	82
0.7	2	10	19	29	38	48	58	67	77	86	96
0.8	2	11	22	33	44	55	66	77	88	99	110
0.9	2	12	25	37	49	62	74	86	99	111	123
1	3	14	27	41	55	69	82	96	110	123	137
1.25	3	17	34	51	69	86	103	120	137	154	171
1.5	4	21	41	62	82	103	123	144	164	185	206
1.75	5	24	48	72	96	120	144	168	192	216	240
2	5	27	55	82	110	137	164	192	219	247	274
2.5	7	34	69	103	137	171	206	240	274	308	343
3	8	41	82	123	164	206	247	288	329	370	411

Adapted from Determining the Amount of Nitrogen in Irrigation Water from East San Joaquin Water Quality Coalition. These materials are not maintained by UC ANR and may not meet current accessibility standards. If you need assistance accessing this content or require an accessible format, please contact East San Joaquin Water Quality Coalition (esjcoalition.org).

References

1. Harter, T., J. R. Lund, J. Darby, G. E. Fogg, R. Howitt, K. K. Jessoe, G. S. Pettygrove, J. F. Quinn, J. H. Viers, D. B. Boyle, H. E. Canada, N. DeLaMora, K. N. Dzurella, A. Fryjoff-Hung, A. D. Hollander, K. L. Honeycutt, M. W. Jenkins, V. B. Jensen, A. M. King, G. Kourakos, D. Liptzin, E. M. Lopez, M. M. Mayzelle, A. McNally, J. Medellin-Azuara, and T. S. Rosenstock. (2012). Addressing Nitrate in California's Drinking Water with a Focus on Tulare Lake Basin and Salinas Valley Groundwater. Report for the State Water Resources Control Board Report to the Legislature. Center for Watershed Sciences, University of California, Davis. 78 p.