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Trials suggest less nitrogen, water for head, romaine lettuce in Salinas Valley

University of California, Davis, agricultural research is yielding two cost-cutting options for head and romaine lettuce growers in California’s Salinas Valley.

Field trials conducted by UC Cooperative Extension (UCCE) farm advisors Richard Smith and Michael Cahn, Monterey County, suggest that less nitrogen (N) fertilizer and water may be required to grow head and romaine lettuce in the Valley.

“The nitrogen uptake for head and romaine lettuce in the trials was about 120 pounds per acre,” said Smith, vegetable crop production advisor. “The N amount produced an almost equivalent-sized plant at harvest.”

Nitrogen amounts applied to lettuce vary in the Valley. Smith knows one grower who last year applied 250 pounds of N per acre while another applied 75 pounds.

The actual nitrogen amount required for crops, Smith says, depends on the grower’s overall fertilizer program, existing nitrogen in the soil from the previous crop, soil temperature, and the amount of irrigation water and rainfall that leaches through the soil.

“If you can save 30 to 40 pounds of nitrogen per acre, it’s a cost savings to the grower,” Smith said.

Nitrogen on average is applied five times to lettuce, Smith says, when using a standard sprinkler system: before planting, as an anti-crustant during planting, two side-dresses around plant thinning, plus an application near harvest to green out the crop.

Nitrogen develops proteins in plants and contributes to building enzymes and chlorophyll. “Without nitrogen, the lettuce plant doesn’t develop,” Smith said.

Cahn, an irrigation and water resources advisor, says growers apply on average about 18 inches of water to grow head and romaine lettuce in the Valley. Yet that’s substantially higher than the amount Cahn applied in the trials to yield high quality lettuce.

“About 12 inches of water per acre were sufficient in the trials,” Cahn said. “That’s about one-third less than the average amount. For those farmers growing two head or romaine crops per season, the reduction could save a foot of water annually.”

Smith and Cahn conducted the trials in sandy soil in South Salinas, a clay loam soil in Kings City, and a clay soil in San Ardo. Cahn held irrigation trials in romaine lettuce during the 2007-2008 crop seasons. Cahn and Smith added nitrogen to the irrigation studies in 2008. The trials will continue this year in other Valley areas with different soils.

The trials were conducted within commercial grower’s fields in cooperation with Fresh Express. Three large trials were located within 15 to 28 acre fields. Each trial consisted of six large plots 2.5 to 3.5 acres in size. Three plots at each trial were farmed by the grower’s normal method and three plots were farmed following the irrigation and nitrogen fertilizer recommendations of the farm advisors.

The farm advisors used the quick nitrate test to guide nitrogen applications. Evapotranspiration data from the local California Irrigation Management Information System (CIMIS) weather station directed the irrigation schedule.

“Some growers in Kings City said we couldn’t grow a successful lettuce crop with a foot of water because of the higher temperatures,” Cahn said. “We got pretty close.”

The preliminary findings are welcome news for growers financially pinched by skyrocketing input costs and a worsening California drought that’s creating tighter water regulations. Energy prices last year blasted N prices above $1,000 a ton. Energy costs have retreated with N prices in January in the 90 cents per pound range.

The long-term benefits of reduced N in vegetables fields can decrease leaching through the soil into groundwater.

“I think growers realize they will probably have to make some changes in how they fertilize,” Smith said. “Nitrogen in the soil in a lettuce field is great; it’s a benefit. The minute it leaves or runs off it’s a pollutant.”

The economic value of crops in Monterey County in 2007 topped $3.8 billion, including $1.1 billion from lettuce, according to the county agricultural commissioner’s office.

Valley growers use groundwater to irrigate crops — water drawn from wells tied to the nearby Salinas River. Growers pay for electricity to power pumps to lift water from the well for field use. Cahn says pumping a third-less water could significantly reduce electricity costs.

“Growers pay about 15 cents per kilowatt hour for electricity,” Cahn said. “Depending on well depth, the electrical costs to lift the water from the well to the surface and pressurize it with a booster pump to 50 to 70 pounds-per-square-inch are about $70 to $80 per acre-foot.” Pumping water for drip irrigation is cheaper due to reduced pressurization requirements.

Smith and Cahn concur that reducing nitrogen and water use could improve agriculture’s position with regulators including the Central Coast Regional Water Quality Control. The group says water quality problems in agricultural areas across the region include levels of nitrates, a key form of nitrogen, which exceed drinking water standards.

A five-year agriculture conditional waiver adopted by the board in 2004 limits waste water discharges. Specifically, farmers must implement irrigation, nutrient, and pesticide management plans, plus erosion control efforts.

As of June 2008, more than 1,700 farms were enrolled in the program; representing almost 400,000 irrigated acres and about 85,000 acres of tailwater.

The ag waiver is now in the re-evaluation stage. Smith hopes the water board will recognize the positive efforts by farmers to reduce water and nitrogen use and not implement a ‘one-size-fits-all’ policy based on just one grower’s N use.

Growers keep close tabs on soil nitrate levels using the pre-side-dress nitrate quick test. Originally developed to measure nitrate levels in Midwestern corn, the test was adapted for vegetables by Tim Hartz, UC Davis Extension vegetable specialist. The quick test provides instant information on nitrogen levels in the soil.

“If a grower is considering a nitrogen application but the nitrate quick test shows enough nitrogen is already present, perhaps a side-dress can be skipped,” Smith said.

Vegetables in the Valley are irrigated by sprinklers initially. The sprinklers are either kept in the field or replaced by drip or furrow irrigation. About one-third of Valley vegetables are drip irrigated, Cahn says.

Salinas Valley vegetable growers can learn the details of Smith and Cahn’s field trials during the 2009 Irrigation and Nutrient Management Meeting and Cover Crop and Water Quality Field Day on Feb. 24, at the Monterey County Agricultural Center, 1432 Abbott Street, Salinas, Calif.

The trial results do not apply to winter-grown head and romaine lettuce production grown in California’s Imperial County and Arizona’s Yuma County due to different soil types in the low desert, Smith says.

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