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.

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.

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.

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.