Study Proposes Improved Nitrogen Management Practices for California Strawberry Growers

Changes in nitrogen management would allow strawberry growers to simultaneously maintain high yields and decrease the amount of pollution linked to the spread of the chemical.

More than 80 percent of the United States' strawberries are grown in the coastal valleys, which have seen a 140 percent increase in yields in the last 50 years due to the introduction of new plant varieties produced via selective breeding.

The strides in production are not without their costs to the local community, however. As crop yields have increased, water quality has suffered, with regional monitoring often revealing groundwater outside the limits of federal drinking water standards.

This has translated to pressure placed on strawberry growers as locals demand a change in management practices to protect vital local resources.

Amidst the conflict, researchers from the University of California, Davis, Department of Plant Services and the University of California Cooperative Extension in Salinas, teamed up to examine nitrogen fertilization and irrigation management practices in fall-planted annual strawberry fields.

"Our primary objective was to document plant and soil nitrogen dynamics (in annual strawberry production) under the environmental conditions and current grower management practices of the central coast region of California," co-author Timothy Hartz of the Department of Plant services said in a statement. "Additionally, we evaluated strawberry response to preplant controlled-release fertilizer (CRF) application rates in three commercial field trials."

According to Hartz, the findings revealed "that current nitrogen fertilization practices did not efficiently match the crop [nitrogen] uptake pattern observed."

For many of these fields, the strawberries are planted after vegetable crops, Hartz explained.

"These fields typically have significant residual soil mineral nitrogen," he said. "Therefore, justification for preplant controlled-release fertilizer (CRF) in this production system appeared to be to ensure [nitrogen] availability throughout the winter, when NO3-N leaching by rainfall is possible."

Based on their findings, however, the researchers determined that preplant CRF rates had little effect on strawberry nitrogen accumulation all the way through to the June sampling, "by which time the vast majority of controlled-release fertilizer nitrogen had been released."

Efficient drip irrigation management was carried out in many fields, the researchers found.

"In only one of the nine highest-yielding fields was seasonal irrigation more than 120 [percent] of evapotranspiration," Hartz said. "The consistency of crop [nitrogen] uptake over the spring and summer provided a guideline for [nitrogen] fertigation. Adjusting for higher fruit yield potential under California conditions, this supports prior research that found [nitrogen] fertigation averaging 0.5 to 0.9 kg/ha per day to be adequate for peak production."

The authors of the study concluded: "Our results suggest several ways in which [nitrogen] management could be improved in this production system. The replicated controlled-release fertilizer (CRF) rate trials indicated that routine use of high CRF rates was not an efficient practice. Reducing CRF rates, particularly in heavier textured soils that are less easily leached, could substantially improve [nitrogen] use efficiency."