Leafy green growers face new fertilizer standards

The next generation of leafy green vegetable growers will contend with regulatory restrictions on the use of nitrogen fertilizer that are unimaginable today.

Water quality regulators set strict standards on fertilizer use in the Central Coast region to reverse the effects of nitrate leaching from agricultural fields to groundwater.

"With Ag Order 4, they will be using a nitrogen fertilizer target based on the amount applied minus the amount removed with the crop," said Richard Smith, University of California Cooperative Extension farm advisor.

Researchers are studying how to reduce nitrate contamination by targeting fertilizer applications more precisely, recovering nitrates before they run off or leaching below the fields. They are breeding crops that use nutrients more efficiently. And they're working to convince regulators to amend some proposed regulations to achieve realistic standards for commercial agriculture.

The recently adopted standards for the Central Coast region begin in 2023 by allowing application of 500 pounds more nitrogen a year than is removed by the crop and decreasing that steadily until 2051 — when only 50 pounds above removal would be allowed.

"Once we start ratcheting down to 300 or 200 pounds, it might be difficult for growers in double or triple crop situations," Smith said.

The 300-pound standard Smith referred to as potentially difficult would go into effect as early as 2027.

The devil could be in the details as growers and researchers are still talking with water quality regulators about how to calculate the amount of nitrogen removed by scores of vegetable crops.

Changes made the regulations more realistic in how much nitrogen to account for in composts and organic fertilizers that release nutrients slowly, how to score nitrates in irrigation water that is not taken up by seedlings and how much credit to allow for cover crops and high-carbon composts that hold on to nitrogen and prevent leaching.

Researchers are also working on methods of managing nitrates before they leave the field, and on vegetable varieties that could give growers a greater margin of error because they perform well under nitrogen stress conditions.

"We have identified lettuce genes that are candidates to improve nitrogen use efficiency," said Pawan Kumar, U.S. Department of Agriculture research geneticist based in Salinas. "We are looking for genes that would improve lettuce nitrogen use efficiency and compared the Salinas variety with a UC genotype." California Polytechnic State University, San Luis Obispo, researchers have also identified some lettuce types that are better under nitrogen stress conditions, others that do well under drought stress and are working to combine the two traits.

"At low nitrogen levels, nondomesticated species are good nitrogen accumulators, but we don't know why," said David Still, director of the agricultural research institute at Cal Poly Pomona. "Can we combine good nitrogen accumulation with large root biomass under water stress conditions? Now these traits are separate."

The difficulty is that nitrogen efficient plants use the available nutrients to support photosynthesis in the leaves, but drought tolerant plants need the ability to expand their roots when needed.

"If you want to select for nitrogen use efficiency, you have to do your experiments using limited amounts of nitrogen," Still said. "We have identified lettuce types with higher leaf nitrogen concentrations than commercial varieties and are doing crosses. We know all these plants do great if you give them enough nitrogen. But there is no correlation between nitrogen use efficiency at full rates and at lower rates."

Meanwhile, researchers are looking for ways growers can contain nitrates before they leave the farm, either by running off or by leaching below the root zone.

"Overhead sprinklers frequently have runoff that includes nutrients, sediment and pesticides," said Michael Cahn, University of California Cooperative Extension farm advisor. "Growers are going to be responsible for not causing toxicity in their runoff."

Growers have tried using sediment ponds to contain and reuse their tailwater, but sediment ponds can have food safety issues regarding E. coli levels in the water to be reused.

Cahn recently revisited his 2003 and 2004 trials showing that injection of polyacrylamide (PAM) can cause significant amounts of sediment to drop out of tailwater before it reaches the sediment pond, but this system was too expensive in equipment, materials and labor.

"We designed a system for mixing granular PAM with irrigation water in a cheaper system," Cahn said. "We were able to reduce sediment and runoff water from 5 or 6 inches of sprinkler germination. We can reduce sediment in the runoff by 95% before it enters the retention pond. We are also looking at this system for making chlorine treatment of tailwater more effective."

Meanwhile, efforts continue to convince Central Coast water quality regulators to make their standards more realistic before they go into effect in two years.

"They have adjusted the amount of nitrogen in the water to just the water the crop takes up, of nitrogen in compost that is counted and the amount from organic fertilizer to just what is available," Smith said.

The regulators have been reluctant, however, to adjust their figures for the contribution of high carbon composts that hold on to nitrogen and prevent leaching, or of cover crops that scavenge for nitrates and pull them up into the root zone for the next crop.

Most perplexing could be the attempt to come up with an accurate figure for nitrogen removed at harvest for each crop.

"The Regional Water Quality Control Board wants a number, but the reality is there is quite a bit of variation," Smith said. "There is a substantial difference in the amount of nitrogen removed depending on the variety, the irrigation strategy and other management practices."

Smith did studies showing that with a romaine lettuce crop, for example, an average of 54 pounds an acre of nitrogen is removed from the field.

But the average number disguises the variability, as the lowest number in Smith's study was 44 pounds an acre, but the highest was 73 pounds.

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)