MINNEAPOLIS — 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:
- Excess nitrogen in some crops can result in vigorous and excessive vegetative growth, leading to uneven or delayed maturity and reduced quality.
- 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).
- 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:
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).
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).
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|
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.
Support for Minnesota Crop News nutrient management blog posts is provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).
— Vasudha Sharma, University of Minnesota Extension irrigation specialist & Fabian Fernandez, Extension nitrogen management specialist
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