Varying Rates of Pre-Plant Controlled Release N and in-Season Fertigated N Affect Soil and Plant N and Strawberry Fruit Yield

Conventional strawberry (Fragaria x ananassa Duch.) N fertilization practices in California have traditionally been based on a combination of pre-plant controlled release (CR) fertilization banded below the plant row and in-season fertigation. Nitrogen has been detected in surface and ground water in the Santa Maria Valley of California and a conditional pollution waiver requires farmers to carefully manage N. The CR fertilizers are considerably more expensive as a group than other potential N sources and the N application rate is important to lower production costs minimize excess nitrate and where possible. We conducted this trial between October 2008 and August 2009 with “Albion” strawberry transplants to determine the plant and soil effects of different pre-plant CR N rates and in season N rates and effects on weekly fruit yields. Pre-plant CR rates were 0 (0CR), 74 (74CR), and 154 kg N ha-1 (154CR) as 18N-3.5P-7.8K. Nested within each of these treatments were 2.85, 5.7, and 11 kg N ha-1 week-1 as calcium ammonium nitrate (17N-0P-0K) and treatments were replicated 5 times. Weekly injections began January 10, 2009 following transplanting on November 14, 2008. Total seasonal N applied were 88 kg N ha-1 in the 0CR-2.85 kg N ha-1 week-1 and up to 422 kg N ha -1 in the 154CR -11 kg N ha -1 treatments. Weekly soil nitrate N (SNN) in 154CR plots generally varied from 25 -100 ppm in contrast to 0CR plots that generally varied 10 to 20 ppm. All plots fell below 10 ppm during a rainy period in mid February when three weeks of rain prevented injection treatments and may have leached N below the sampling zone. SNN generally varied directly with the amount of pre-plant CR N applied, but weekly-injected N amounts were not consistently related to SNN. Total plant dry weight and total N uptake was lower for the 0CR than higher CR N rates but was not different between 74CR and 154CR and were not related to weekly N treatment rates. Highest total seasonal fruit yield exceeded 85,500 kg ha-1 and was higher in 74CR and 154CR treated plots than 0CR treatments but yields in 74CR and 154CR treatments did not differ significantly. Yields of weekly N application rate plots only differed in the 0CR plots and were directly related to N rates. These results can guide growers on efficient N fertilization for profitable strawberry production that minimizes off-site N movement.