Managing Nitrates in Groundwater
How California Vegetable Growers Can Adapt to New Nitrogen Rules
Written by: Dr. Michael Cahn, UC Cooperative Extension Farm Advisor
Growers in the Central Coast region of California produce most of the cool-season vegetables for the U.S. from March to mid-November.
After several years of multi-cropping vegetables, the groundwater in many of the coastal valleys has become contaminated with nitrate. Some wells have concentrations of nitrate several times the federal drinking water standard of 10 ppm (parts per million) nitrate-N. Rural communities that rely on groundwater as their sole source of drinking water must purchase bottled water or install small reverse osmosis systems.
The regional and state water boards have imposed several iterations of water quality regulations on growers on the Central Coast to address nitrate contamination of ground and surface water.
The latest version, known as the Agriculture Discharge Order, sets limits on how much nitrate can be discharged by farming operations. Growers will need to estimate their annual discharge of nitrate by calculating how much nitrogen (N) they apply to all their crops and how much nitrogen is removed from their fields through the harvest of product or other means.
The applied nitrogen calculation includes nitrogen derived from fertilizer and organic amendments as well as the nitrate in irrigation water. Although nitrogen removed is mainly in the harvested portion of the crop, the estimate could also include nitrogen sequestered in soil organic matter or taken up by a winter cover crop.
How Much N to Discharge
The first nitrogen discharge target proposed for December 2023 is 500 pounds N per land-acre per year. By the end of 2027 the limit will be ratcheted down to 300 pounds N per land-acre per year and then decreased to 200 pounds N per acre in 2031.
Because vegetable growers produce two to three crops per season on each farmed acre of land, these limits will become increasingly difficult to meet. The average rate of nitrogen applied to one crop of lettuce produced on the Central Coast is currently 150 pounds of N per acre, and the average amount of N removed at harvest ranges between 45 and 65 pounds of N per acre.
Additionally, most growers irrigate their crops with water containing elevated concentrations of nitrate that may contribute more than 50 pounds of N per acre per crop.
Besides adjusting to the extra paperwork that will be required for tracking and reporting on nutrient management, growers will need to step up implementation of practices that reduce the applied amounts of nitrogen and find ways to increase the removed amounts.
The fact that much of the nitrogen taken up by a vegetable crop remains in the field after harvest as plant residues suggests that fertilizer rates can be decreased for the following vegetable crop.
Tools such as the soil nitrate quick test can identify when the top foot of soil is rich in mineral nitrogen so that fertilizer is not unnecessarily applied. Equally important for optimizing nitrogen is careful water management to minimize leaching of nitrate below the root zone of the crop.
Additionally, you will need to credit nitrate in your irrigation water when figuring the fertilizer needs of your crops. This can be challenging when different wells on a ranch may have different concentrations of nitrate or because the nitrate concentration of the groundwater may vary during the season.
It’s Time to Adopt New Methods
To enhance removal of nitrogen, you will need to find strategies to incorporate nitrogen scavenging winter cover crops into your farming operations. A rye or mustard cover crop can often take up 100 to 150 pounds of N per acre during the winter, thereby preventing the leaching of nitrate during storm events while also helping to infiltrate clean water back into the aquifer. However, the challenge with winter cover crops is that they conflict with preparing the ground for planting in the spring.
For most of my career I have collaborated with growers and my Extension colleagues to develop and demonstrate practices that will improve nutrient and water management in vegetable cropping systems on the Central Coast, but progress in implementing these practices on farms has been relatively slow.
Now it appears that some rather tough regulations are just around the corner, and the vegetable industry will likely need to make some large adjustments to how they farm to meet the challenge of these upcoming nitrogen discharge limits.
A 2014 report published by University of California, Davis, has publicized the on-going issue of nitrates in our groundwater. This report specifically focused on the Salinas Valley and Tulare Lake basins, areas where high amounts of fertilizer applications have occurred in past decades, or on-going cattle/dairy operations produced large amounts of manure.
Nitrogen is required by all crops to produce a healthy, safe result: fresh fruits and vegetables that are marketed in our local grocery stores and farmers markets, produced either conventionally or organically. Improvements in nitrogen management have reduced the amount of fertilizer applied to crops in the past decades; in fact, studies show that, while yields have increased exponentially in the past two decades, the actual use of nitrogen has leveled off and has remained constant for many years now.
But we can all do better. New science and research is developing improved ways to manage and apply nitrogen to crops. Salinas Valley farmers and ranchers have been early adopters of these new methods and remain vigilant in their search for better ways to manage nitrogen when growing fruits and vegetables. Nitrogen is an input cost, and in recent years that cost has increased considerably and farmers won’t apply fertilizer they don’t need.
Studies show it has taken decades for the nitrogen applied generations ago to reach the groundwater as nitrates. And it will take decades to make improvements; cleansing groundwater is a long, expensive process. Today’s farming practices are much improved over those of even one generation ago, and the contributions to the problem in groundwater should be viewed as a legacy issue, not as a punitive action to those currently farming responsibly. With all things in our society, science and research have improved our knowledge base.
The UC Davis report will be peer-reviewed to validate the study findings; much data previously collected was not included in the report and needs to be validated to show that groundwater quality in the Salinas Valley is not impaired to the degree that the report determines. Until then, we maintain that the groundwater in Monterey County is being improved through the pro-active solutions growers have adopted in recent years.