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How to calculate a nitrogen credit from irrigation water

calculate nitrogen credit irrigation water fertigation minnesota

By: Vasudha Sharma, Extension irrigation specialist & Fabian Fernandez, Extension nitrogen management specialist

Nitrate (nitrate-N) is commonly found at some level in irrigation water. For corn in Minnesota in a normal irrigation year, when nitrate concentrations in irrigation water are below 10 parts per million (ppm), or 10 milligrams per liter (mg/L), we don’t recommend making adjustments to your fertilization plan. This is because the University of Minnesota’s corn fertilizer guidelines already account for this. If nitrate concentrations are greater than 10 ppm or the amount of irrigation during the season is substantially greater than normal, the nitrogen added through irrigation should be accounted for because:
  1. Excess nitrogen in some crops can result in vigorous and excessive vegetative growth, leading to uneven or delayed maturity and reduced quality.

  2. The nitrogen credit from irrigation water means you can save money on nitrogen fertilizer costs, especially in dry years like 2021 (more on this below).

  3. Exceeding a crop’s nitrogen needs can result in nitrogen loss and groundwater contamination.
Moreover, particularly in years like 2021 when we are experiencing excessive heat and very little rain, there is less potential for nitrogen loss from the fertilizer that has been applied and from nitrogen mineralization from the soil. So, it’s important to account for the nitrogen in the irrigation water in the total nitrogen application plan. Also, don’t forget that applying nitrogen past-tasseling will not help improve corn yields

How to calculate a nitrogen credit from irrigation water

Use this formula to calculate the nitrogen credit per inch of irrigation water applied:

nitrogen credit per inch of irrigation water applied

Using the above equation, if irrigation water has 1 ppm of nitrate-N, 0.23 pounds of nitrogen per acre are applied for each inch of irrigation water. Note that mg/L is equivalent to parts per million (ppm).

Cost savings

If a farmer applies 8 inches of irrigation water during a growing season that has 20 ppm nitrate-N, there will be a nitrogen credit of 36 pounds of nitrogen fertilizer per acre (lbs N/ac) (see table below).

If we use a price per pound of nitrogen fertilizer of $0.50, the cash value per pound of nitrate-N per acre is = $0.50/lbs N x 36 lbs of N/acre = $18/acre

If a farmer has a 120-acre center pivot field, the farmer will save a total of = $18 x 120 = $2,160

Table 1. Nitrogen credit from irrigation water

Nitrate-N in irrigation water 1 in. (of irrigation water) 2 in. 3 in. 4 in. 5 in. 6 in. 7 in. 8 in.
10 ppm 2 lbs.
5 lbs. N/ac 7 lbs. N/ac 9 lbs. N/ac 11 lbs. N/ac 14 lbs. N/ac 16 lbs. N/ac 18 lbs. N/ac
15 3 7 10 14 17 20 24 27
20 5 9 14 18 23 27 32 36
25 6 11 17 23 28 34 40 45
30 7 14 20 27 34 41 48 54
35 8 16 24 32 40 48 55 63
40 9 18 27 36 45 54 63 72
45 10 20 31 41 51 61 71 81
50 11 23 34 45 57 68 79 91

Water sampling and analysis

We recommend that growers collect an irrigation water sample at the beginning of the irrigation season and test it for nitrate concentrations. In most cases, the nitrate level in irrigation water is consistent throughout the year. However, in dry years like 2021, when growers irrigate their fields more often than usual, nitrate concentrations in groundwater can increase. For this reason, it is a good idea to test irrigation water again before your last fertigation event to help you make the most informed decision.


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  1. This is useful. We more often are taking sulfur credit than N credit. Would the calculations be done the same way?

    1. Dan Kaiser says,"You can measure sulfate sulfur in the irrigation water. However, the total amount coming over the season may or may not reflect where you will get a response. I don't have a specific number but what I know is that most irrigated soils, if irrigation starts early, there likely won't be a need for sulfur and also there is very little to no need for side-dress sulfur under irrigation."

      And Fabian Fernandez adds, "One more thing to add is that just like nitrate-N, sulfate-S is readily available for plant uptake."

    2. Irrigator AssociationJune 7, 2023 at 8:11 AM

      Dakota County Soil and Water is starting a test program


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