Posted By: Marlowe DeVille /
Reprinted from Ag Alert, January 18, 2017. Author: Bob Johnson
The nitrogen in irrigation water can be counted the same as in fertilizer when calculating a nutrient budget for either lettuce or broccoli, the latest University of California studies in Salinas confirm.
Even when nitrate nitrogen concentrations in the water are far below average for Salinas Valley agricultural wells, the nutrients can still be safely included as helping to meet crop need.
“During the past three years, we conducted replicated field trials to evaluate how much of the nitrate in irrigation water could be taken up by head lettuce and broccoli,” said Michael Cahn, UC Cooperative Extension farm advisor.
Cahn worked on the trials with UCCE vegetable crops specialist Tim Hartz, farm advisor Richard Smith and staff research associate Laura Murphy.
The point of their studies was to demonstrate that vegetable growers could deduct the nutrients in their irrigation water from their fertilizer applications, which would both help satisfy water quality regulators and save a few dollars.
“Water quality regulators would like growers to take proactive steps to reduce nitrogen inputs to their crops, “Cahn said. “One way many growers could make significant reductions in their use of fertilizer nitrogen is by offsetting their nitrogen fertilizer rates by a portion of the nitrogen applied through their irrigation water. The term ‘pm and fertilize’ has been used to describe this practice, because conceptually a grower is pumping water and using the water as fertilizer for the crop. The benefit for the grower is lower fertilizer costs, and the benefit for the environment is reducing nitrate loading to groundwater.”
There are only a few situations in which the studies indicate it would be unwise to fully deduct the irrigation water nitrates from fertilizer applications.
Nitrate-nitrogen in the water applied soon after planting, before the young seedlings are established, is best not included in a nutrient budget for the season.
“Because the crop water use is low during the first weeks after planting, it is also reasonable to wait until after establishment to take credit for the nitrogen applied in irrigation water,” Cahn said.
The trials also showed that when broccoli is heavily irrigated to leach out salts, the crop is not as efficient at taking up all the nutrients in either irrigation water or applied fertilizer.
But the nitrate-nitrogen in well water is the same as in fertilizer, and in the Salinas Valley the contribution to crop nutrient needs can be quite substantial.
“A significant number of wells have nitrate-nitrogen concentrations in the range of 30 to 50 parts per million, and the amount of nitrogen that potentially could be applied to crops by irrigating with this water could be as high as 55 to 90 pounds per acre,” Cahn said.
Well water with very high levels of nitrate-nitrogen could contribute as much as half of the nutrients needed to grow lettuce crops, which take up between 130 pounds and 150 pounds of nitrogen.
The studies also showed, however, that lettuce can take up nitrate-nitrogen even from water with concentrations as low as 12 parts per million, which is well below the average for Salinas Valley agricultural well water.
“In the Salinas Valley, the nitrate concentration of water pumped from agricultural wells averages more the 20 parts per million, “Cahn said. “This nitrogen concentration would translate to 37 pounds per acre for a lettuce crop receiving eight inches of irrigation water.”
Recycled water from Monterey Reginal Water Pollution Control Agency, the sole water source for approximately 12,000 acres of coastal fields near Castroville, is relatively high in ammonium nitrogen.
One study confirmed that this ammonium nitrogen can also be included in a crop nutrient budget.
“The source of nitrogen in the irrigation water, ammonium or nitrate, had no significant effect on nitrogen recovery by the crop, “Cahn said. “Presumably, under normal summer temperatures, ammonium would quickly transform to nitrate when added to the soil.”
Although the results of these experimental trials confirmed that growers can confidently take credit for the background levels of nitrate in the irrigations water, one should still be cautious when implementing this practice.
“Experimenting on fields where the water source is known to have a consistently high concentration of nitrate but is not excessively high in salts is recommended, “Cahn said.
The recommended place to experiment is on fields with highly efficient irrigation systems.
“Drip provides better control of irrigation volumes then sprinklers and furrow systems, which may minimize excessive leaching and offer more opportunities for fertigating nitrogen to correct observed deficiencies, “ Cahn said. “Because the crop water use is low during the first weeks after planting, it is also reasonable to wait until after establishment to take credit for the nitrogen applied in irrigation water.”
Although caution is advisable, the studies show that counting the fertilizer in groundwater is nutrient budgets has potential as a winner both environmentally and economically.
“Despite the potential benefits of implementing ‘pump and fertilize’ adoption by growers has been slow,” Cahn said. “One reason is due to doubts that the nitrate in irrigation water is completely available to crops. Chemically speaking, nitrate in fertilizer and ground water are the same.”