Statewide
In my previous tip I noted the value of existing subsoil (below 6”) nitrate that is readily available to grain sorghum and other crops, which should be fully credited to grain sorghum N requirements nor soil nitrate between 6” and 24” deep. Now for irrigated crop producers, let’s consider the amount of nitrate-N (also expressed as nitrate-nitrogen or NO3-N) that potentially exists in irrigation water. This N—like soil N—should be fully credited to crop production with only a minor exception or two.
For reference read the three-page Extension publication “Nitrates in Irrigation Water: An Asset for Crop Production” (Texas A&M AgriLife Extension Service, E-619, type ‘nitrate’ in the search box at http://agrilifebookstore.org). This recent 2012 document explains the justification for valuing nitrate in irrigation water, which by reducing fertilizer N requirements, can save producers input costs. Most Texas irrigation waters contain 3 to 10 parts per million (ppm) nitrate-N, but some waters are over 20 ppm.
In general, for each 1 ppm of nitrate-N in your irrigation water you are adding 0.23 lbs. of N per acre. A producer who irrigates 12” and has 6 ppm nitrate-N in the water is applying about 14 lbs. of N per acre. For a 6,000 lbs./A yield goal, this level of long-term N concentration is about 12% of your total crop requirement. The cost of N fertilizer, were you to buy it, would currently be about $9/acre for this example, and that does not include application costs.
