Nutrient Management Research Database

http://ucanr.org/sites/nm/files/76744.pdf

Decades of heavy phosphorus fertilizer application to vegetable fields in the Salinas and Pajaro Valleys have resulted in substantially increased soil P concentration. Soil test P levels frequently exceed the threshold for expected crop response to continued P fertilization; however, many growers continue to apply P to such fields. While this generally does not cause agronomic problems, it may be a significant contributor to the undesirably high P concentration found in the Salinas and Pajaro River systems. Parts of both watersheds have been listed by the California EPA as 'impaired' for soluble nutrients, based on the prevailing Federal water quality standards. This project was undertaken to reevaluate the current P management recommendations for lettuce production in light of this potentially serious environmental problem.

• Soil was collected from 30 conventional and organic fields in long-term vegetable rotation in the Spring of 2002
• Soils will be used to correlate soluble and total P concentration of runoff water with soil test P values and soil hydraulic properties
• 6 trials were conducted in commercial lettuce fields in the Salinas Valley in 2002 to evaluate the effect on crop productivity of P fertilization in fields with moderate or high soil test P
• The fields chosen had between 54-171 PPM bicarbonate P
• In some fields the growers did not apply P fertilizer, and researchers established plots that received a preplant fertilization of 130 lb P2O₅/acre.  In other fields the growers applied P and 4 plots were established in which the P application was skipped.  
• Plant P status was assessed biweekly throughout the crop season, including harvest.
• Plant biomass and data on marketable yield and head size was collected

• In the first trial, response to preplant P was observed, although soil bicarbonate P level was 54 ppm, which is above the commonly suggested response threshold.  The first trial also had a trend toward smaller plants in the 0 P plots
• The other trials did not show response to P fertilization
• P fertilization had low impact on tissue P concentration for all trials