Nutrient Management Research Database

• Twelve field trials were conducted in commercial lettuce growing operations in 2002 and 2003.
• Growers decided how to manage P fertilization in their fields. Field soil test P levels varied from 53 to 171 ppm.
  
  • In ten field, growers applied preplant P, either broadcast or banded a part of bed formation.
  • Plots with no P additions were also maintained to make comparisons.
• Plant growth was measured by plant canopy development, meaning the percent of the soil bed that was covered by the crop biomass.
• Marketable yield and total plant residue was measured.
• Total P was also measured in both harvested lettuce heads and plant residues.

• P fertilization of fields with soil test P values greater than 50 ppm showed negligible responses to P fertilization.
  • This value is referred to as a "threshold value." If soil test P levels are above this point, response to P fertilization is unlikely.
  • The one field that did show a clear yield increase was planted in early spring, so low temperatures likely limited P availability.
• Almost all plants had P levels that were sufficient, and P fertilization did not influence plant P uptake.
• Lower temperatures were found to reduce plant available P. However, more research is required before making definitive adjustments to the soil test P threshold values.

Implementing soil test P results into P management budgets can help reduce costs.