Pump And Fertilize: How To Factor Irrigation Water Into Nitrogen Budgets
September 5, 2016 
UC-Davis and UC Cooperative Extension researchers have conducted field trials simulating water of different nitrate concentrations. Photo credit: Michael Cahn
Water quality regulations have been tightening for growers in California over the last decade. During recent years, attention has focused on nitrate contamination of groundwater.
Groundwater often has nitrate concentrations above the drinking water standard of 10 parts per million (ppm) nitrogen in coastal valleys where vegetables and berries are produced, and in regions with intensive dairy production, such as in the southern San Joaquin Valley.
Because much of the drinking water supply in rural and low-income areas is from groundwater, there is an abundance of pressure for state officials and regulators to address this drinking water impairment from the public, as well as from social justice and environmental groups.
Water quality regulations in many regions of California now require farmers to report the amount of nitrogen (N) fertilizer they apply to their fields and the nitrate concentration of their irrigation water. Baseline numbers from the first two years of reporting on the Central Coast clearly showed that a majority of vegetable producers apply much more nitrogen than their crops take up.
A Proactive Approach
Water quality regulators want growers to take proactive steps to reduce nitrogen inputs to their crops. One way many growers could make significant reductions in their use of fertilizer is by accounting for the N applied through their irrigation water when developing a nutrient budget.
The term “pump and fertilize” has been used to describe this accounting because conceptually a grower is pumping water and using the water as fertilizer for his crop. The benefit of this practice for the grower is lower fertilizer costs, and the benefit for the environment is reducing nitrate loading to groundwater.
In many vegetable production regions, such as in California’s Salinas Valley, the nitrate concentration of water pumped from agricultural wells averages more than 20 ppm N. This N concentration would translate to 37 pounds of N per acre for a lettuce crop receiving 8 inches of irrigation water.
Many wells in this region have even higher nitrate concentrations, and the amount of N that potentially could be applied to crops by irrigating with this water could be as high as 70 to 90 pounds of N per acre. Since most lettuce crops take up 130 to 150 pounds of N per acre, high nitrate water could substitute for about of half the fertilizer N normally needed to produce a crop.
Despite the potential benefits of implementing “pump and fertilize,” adoption by growers has been slow. One reason is due to doubts by growers that the nitrate in irrigation water is completely available to crops. Another reason growers are reluctant to account for the N in irrigation water is because they are uncertain about how much to credit the N in water applied to leach salts from the soil profile. Some growers also have expressed concern that the nitrates significantly increase the salinity of water, making it less beneficial to their crops. Finally, growers who use multiple wells to irrigate their fields have difficulty estimating the average N concentration of irrigation water.
Chemically speaking, nitrates in fertilizer and groundwater are exactly the same. Even when urea and ammonium forms of N are applied, they quickly convert to the nitrate form of N because of warm soil temperatures. However, growers are concerned that N concentrations in high nitrate water may still be too low to be absorbed by crop roots. Fertilizer N applications usually boost the N concentration of the soil water to levels much higher than are found in irrigation water.
Field Trial Results
During the past three years, we conducted replicated field trials to evaluate how much of the nitrate in irrigation water is taken up by lettuce and broccoli. Crops were drip irrigated after establishment using water of different nitrate concentrations.
These trials demonstrated that nitrate in water was equally beneficial to the crop as fertilizer forms of N, and concentrations of 40 ppm could supply a substantial amount of the N required to reach maximum yield.
Even nitrate concentrations as low as 12 ppm N were utilized by the crop. When extra water was applied to create leaching fractions of 40% to 50%, crops were able to utilize the N in irrigation water with the same efficiency as from fertilizer. Nitrate did boost the salinity of water, but the amount was small: approximately 0.07 deciSiemens per meter for each 10 ppm increase in Nitrate-N concentration.
Although the results of these experimental trials confirmed that growers can confidently take credit for background level of nitrate in the irrigation water, growers 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.
Use Drip To Minimize Leaching
Drip provides better control of irrigation volumes than sprinklers and furrow systems, which may minimize excessive leaching, and also offer more opportunities for fertigating N to correct any observed deficiencies. Because the plant uptake of water during establishment is low, it also is reasonable to wait until after establishment to take credit for the nitrogen in the applied irrigation water.
Soil nitrate levels should be monitored after crop establishment to understand if the soil has a sufficient supply of N. If using multiple water sources for a crop, the nitrate concentration of the blended water needs to be determined in samples collected at the field. Finally, applied water volumes need to be accurately monitored to estimate the amount of N that was applied through the irrigation water.
With water quality regulations continuing to become stricter for agriculture, it makes sense for growers to start implementing practices that can both lower farming costs and are beneficial for the environment. By accounting for the nitrate in irrigation water and using the soil nitrate quick test to monitor soil N levels, growers might be able to make significant progress in reducing the amount of fertilizer nitrogen needed to produce their crops, and demonstrate that they are addressing water quality concerns.
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