Nutrient Management Research Database

This project evaluated appropriate P management practices for lettuce production in the Salinas Valley of central California in light of elevated soil P levels from prior fertilization, and concerns over adverse water quality impacts. A field survey revealed that soil test P level (bicarbonate procedure) in soils in long-term vegetable rotations commonly exceed 70 mg kg-1, both under conventional and organic management. Trials were conducted in 6 commercial lettuce fields in which the effect of preplant P fertilization (the typical grower practice) was compared to no P application; soil P levels ranged from 54-171 mg kg-1. In only one of 6 fields was lettuce productivity improved with P application; the responsive field was the earliest spring planting, and had the lowest soil test P. Tissue analysis showed that P application in fields with high soil P levels barely influenced plant P uptake. Furthermore, currently suggested midrib PO4-P sufficiency levels appear to be higher than necessary.

• Soil P levels (bicarbonate, or Olsen-P) in the Salinas and Pajaro valleys commonly exceed 80 ppm.
• Even crops with high P requirements such as lettuce are unlikely to respond to P fertilization at levels above 40-60 mg/kg, and most crops won’t respond to P fertilization when P exceeds 15-30 ppm.
• This project aimed to develop P fertilizer guidelines for coastal lettuce production and establish the relationship between soil characteristics, soil P test levels, and the potential for P runoff losses.

Results

• Most fields did not show responses to preplant P fertilization ranging from 42 to 130 lbs P2O5 per acre, which was expected due to the generally high bicarbonate P levels. The one field that did show a yield response was likely due to colder temperatures, which limit P availability.
• The results of this project show that soil P levels are high enough for most coastal vegetable production systems to avoid P fertilization.
• Limiting P fertilization will lower the agricultural contribution to water pollution issues.