Early Season N Fertilization Management Strategies in Strawberry Production

Annual strawberry (Fragaria x ananassa Duch.) production is a major industry in the Salinas Valley of central California, an area with widespread NO₃-N impairment of both surface water and groundwater. Pending environmental water quality regulation will require more efficient utilization of N fertilizer. Seasonal N application averages approximately 230 kg·ha^-1; about half is applied in a controlled release N (CRN) form prior to crown planting in October–November. The remainder of the seasonal N is fertigated through drip irrigation during the subsequent spring and summer. Because strawberry is usually grown in rotation with lettuce and other leafy greens, residual soil NO₃-N at the time of crown planting often exceeds 15 mg·kg^-1. Winter rainfall provides N leaching potential while cool temperatures limit crop growth until March. These circumstances call into question the value of a large preplant N application. To investigate the efficiency of pre-plant CRN application, trials were initiated in Fall 2010 in three commercial fields comparing the growers’ standard CRN application with a half rate (all fields) and no CRN application (field 3 only). Full rate CRN application was 121 (fields 1 and 2) and 86 kg·ha^-1 (field 3). All treatments received all in-season N fertigation applied by the growers. Above-ground plant biomass, biomass N, and soil NO₃-N were determined in monthly sampling. By early April, plant biomass N averaged < 20 kg·ha^-1, with no significant effect of preplant CRN rate in any field. Biomass N in fields 1 and 3 averaged 177 kg·ha^-1 by mid-August, again with no significant effect of preplant CRN rate. Seasonal marketable fruit yield in fields 1 and 3, which averaged 1.55 and 2.00 kg plant^-1, respectively, were also unaffected by preplant CRN rate. In field 2, a transient but statistically significant reduction in biomass N was observed in the half rate CRN treatment in the June sampling, at which time soil NO₃-N was < 1 mg·kg^-1. Marketable fruit yield in the half rate CRN treatment, which until that time had been equivalent to the full CRN rate, fell progressively behind and finished the season 10% lower than the full rate treatment. We conclude that a large preplant CRN application is an inefficient practice in this production system. This study suggests that reducing preplant CRN rates while monitoring soil NO₃-N to guide in-season N fertigation, could improve N fertilizer efficiency.