Estimating N contribution from irrigation water containing nitrate

Understanding the nutrient value of nitrate in irrigation water often leads to confusion. Laboratories commonly report nitrate concentration of water samples in parts per million of nitrate (PPM NO₃).  However, as shown in Table 1, the reported nitrate value must be multiplied by 0.225 to express it in equivalents of nitrogen (PPM NO₃-N).   This conversion is required because the nitrate molecule has three oxygen atoms for every nitrogen atom, meaning that the nitrate molecule is 22.5% N by weight. 

Several more calculations must be made to determine the pounds of N contained in a volume of water.   Factors for converting PPM NO₃ or PPM NO₃-N to lbs of N per acre-foot (or acre-inch) are presented in Table 2.   For example, multiplying a water test of 45 PPM NO₃ by 0.62 would determine that an acre-foot of water contained about 28 lbs of N.  If the same water test was reported as PPM Nitrate-N it would be equivalent to about 10 PPM N and would be multiplied by 2.72 to convert to lbs of N per acre-foot of water. 

How much of the nitrogen in irrigation water should be credited as fertilizer value to a crop is debatable, especially for shallow rooted vegetables such as leafy greens.   It is unclear if leafy green crops can make use of nitrate in irrigation water when concentrations are less than 20 ppm NO₃-N.  The soil nitrate level that optimizes lettuce growth is considered to be in the range of 15 to 25 ppm NO₃-N.   But this concentration is expressed in terms of weight of nitrogen per weight of soil (mg of N/kg of soil).  Since soil is about 20% to 25% water by weight, the nitrate concentration in the soil water of most commercial lettuce fields would range between 60 to 120 ppm NO₃-N.   Field monitoring of nitrate concentration in soil water using suction lysimeter tubes installed in the root zone of lettuce has confirmed that most commercial fields are in the range of 50 to 150 ppm NO₃-N, which is substantially higher than the concentration of nitrate found in most irrigation water.   

Applying a leaching fraction to minimize the build-up of salt in the soil would also affect how to credit the fertilizer value of nitrate in irrigation water.  Much of the nitrogen contained in the extra water applied to leach salts would presumably percolate beyond the root zone and not be available to the crop.    In the Salinas Valley, growers report that ground water with a high nitrate concentration often has high levels of sodium and chloride salts.   These water sources are often from shallow wells, where both salt and nitrate have leached over time into the aquifer.    In these circumstances where elevated levels of salts are associated with high nitrate concentrations, a greater leaching fraction may be required and therefore less of the nitrate can be credited to the crop.

Irrigation efficiency can also factor into estimating the fertilizer value of nitrate in irrigation water.  Fields with irrigation systems that apply water with poor uniformity would have areas where water drains below the root zone and other areas where less than the desired volume of water is applied.     

Considering the above concerns, perhaps the fairest way to value nitrate in irrigation water is to take credit for the water that the crop uses for evapotranspiration (ET).  This would be equivalent to the volume of water that enters the plant roots or evaporates from the soil surface leaving salts including nitrate behind.   Lettuce transpires 5 to 8 inches of water between germination and maturity in the lower Salinas Valley during the summer.   Factoring in irrigation system uniformity also makes sense.   If the irrigation system uniformity is 80%, one may assume that 20% of the N in the applied water would likely percolate below the root zone and would not be available to the crop.  For a crop that has a seasonal ET of 7 inches and an irrigation uniformity of 80%, the following values (Table 3) would approximate the N contribution of irrigation water for the indicated range of nitrate concentrations.   Most wells in the Salinas Valley with elevated concentrations of nitrate are less 20 PPM NO₃-N; therefore, we would expect that for most wells with this range of nitrate concentration, the irrigation water would contribute 20 to 30 lbs of N/Acre to a lettuce crop.

Conclusions

Growers have long been advised to reduce their standard nitrogen fertilizer rate if their irrigation water is high in nitrates.    Extension publications commonly suggest that one can credit the nitrogen in irrigation water by multiplying the concentration of nitrate by the volume of water applied to a crop.   As can be seen from the above estimates, water containing less than 45 ppm NO₃ generally does not contribute a significant amount of nitrogen to crop growth (< 15 lbs N/acre). However, if well waters contain more than that amount, irrigation water may contribute greater amounts of nitrogen for crop production.  In the future, we hope to conduct field trials to verify these estimates of the fertilizer value of nitrate in irrigation water.

Irrigation water is only one of several possible sources of nitrogen for a crop.  The soil organic matter, added organic amendments, and previous crop residues continuingly mineralize releasing nitrogen.  Fertilizer N is also periodically applied through sidedress and fertigation applications or as a slow release product.   The good news is that you can account for the N contribution from the nitrate in the applied irrigation water as well as these other sources by using the quick nitrate soil test.  However, keep in mind that this test only shows what has happened, and will not estimate the contribution of N from these sources, including irrigation water, to predict future crop N needs.  

Table 1.  Conversion factors between NO₃ and NO₃-N

 Conversion between nitrate (NO₃) and nitrate-nitrogen (NO₃-N):

Table 2.  Factors for converting between nitrate concentration in irrigation water and pounds of N per volume of water.

Nitrogen content of irrigation water*

* water analyses from most labs report NO₃  in units of ppm, but it is very important to pay attention to which units the results are reported.

Table 3.  Estimated fertilizer N value of nitrate in irrigation water for lettuce production.

* multiplied by 0.8 to account for the irrigation system efficiency