By Bob Johnson, Ag Alert
Three years of studies on commercial fields point to the conclusion that many desert region baby spinach and lettuce growers could reduce nitrogen applications, some by as much as 70 pounds an acre, without damaging their crop.
As growers in Imperial and Yuma valleys have switched to mixed baby leaf salad greens in recent years, nutrient management plans to target nitrogen applications to crop need have failed to keep pace.
“Over the past decade, the production of high-density mixed leafy green vegetables on large beds, 80 and 84 inch, has increased dramatically,” said Charles Sanchez, research scientist at the University of Arizona Maricopa Agricultural Center. “Work on the fertilizer requirements for these crops are lacking, and many growers have simply utilized the fertilizer practices they currently use on full-season iceberg, romaine and leaf lettuce.”
Sanchez spent the last three years comparing nitrogen practices in the desert with recommendations generated by CropManage, a University of California software that helps growers save money and be more efficient by targeting irrigation and nitrogen applications more precisely.
In his most recent trial at six baby-spinach fields, Sanchez found that three of the growers were close to the software’s nitrogen fertilizer recommendations, while the other three applied between 23 and 70 pounds an acre more with no apparent benefit.
Sanchez presented his studies at the California Department of Food and Agriculture Fertilizer Research and Education Program conference in San Luis Obispo in October.
Efforts are underway to make irrigation and fertilizer management tools developed and widely used on vegetable crops in the Salinas Valley also available to growers in the southern desert region, which supplies the nation with mixed leaf baby greens from December to March.
Ongoing concern about nitrate in groundwater due to decades of Salinas Valley vegetable production prompted efforts to help growers target their nitrogen applications as precisely as possible.
UC Cooperative Extension irrigation farm advisor Michael Cahn, who works out of the Salinas office, led development of the program that Central Coast growers use for strawberries, head, romaine, leaf and baby lettuce, broccoli, Brussels sprouts, cauliflower, cabbage, celery, cilantro, mizuna, bell peppers, raspberry, spinach and tomatoes.
The software calculates California Irrigation Management Information System and NASA weather data, soil quick tests of available nitrogen, U.S. Geological Survey soil type maps, previous fertilizer applications and university trials showing how much nitrogen and water the crops need over the course of the season.
Sanchez discussed progress for adding desert-grown baby leaf vegetables to the list of crops covered by CropManage during the conference.
“We completed field work on six production field sites,” he said. “Water balance is complete for these six sites. Salinity and nitrogen laboratory analysis and computational work related to the calibration of CropManage is ongoing.”
Sanchez’s work also calculates irrigation requirements, including the amount of water needed to leach salts out of the root zone of these salad greens.
While the Salinas Valley may be renowned as America’s “Salad Bowl,” Arizona these days produces 30% of the nation’s lettuce. After the Salinas Valley harvest is complete, Arizona growers harvest more than $300 million in head, leaf and romaine lettuce in the winter, making lettuce the state’s leading cash crop.
Imperial County growers harvest more than $200 million in lettuce annually during the winter months.
Desert producers are playing catch-up when it comes to calculating the nitrogen these short-season salad products need and when.
“These crops are grown at a higher density than full-season lettuce. They are harvested young and are shorter season—20 to 40 days—compared to the 80- to 180-day full-season crops,” Sanchez said. “We have no information how these factors affect fertilizer needs, no information on how irrigation interacts with nitrogen and no information to modify nitrogen fertilizer recommendations for these crops.”