Developing Nitrogen Management Strategies for Organic Strawberries in the Central Coast of California

Nitrogen management in organic strawberries in central coastal California is challenging due to winter rains typically occurring soon after transplanting, unpredictability of N mineralization, the long growth period of strawberries, and the high cost of organic fertilizers. To develop a nitrogen management strategy that is simultaneously productive, reduces environmental impacts, and is cost effective, replicated on-farm experiments using cv. Seascape were conducted in Watsonville, CA for four seasons. Split plot randomized design trials, with supplemental N rates (suppl.) as biweekly applications of liquid organic fertilizer from Mar. to Oct. as main plots and varied pre-plant applications (pre.) as sub plots, were conducted in fields with various organic management histories and soil types. Every year levels of treatments were decided through discussions among growers and researchers. Treatments were: 1^st season (2005-06); suppl.: 56, 112, and 168 kg-N/ha, pre.: none, feather meal 56 kg-N/ha, and compost 11 Mg/ha, 2^nd season (2006-07); suppl.: 84, 168, 252 kg-N/ha, pre.: none, Sudan grass, and compost 22 Mg/ha, and 3^rd and 4^th seasons (2007-08, 2008-09); suppl.: 0, 168, and 337 kg-N/ha, pre.:  0, 84, and 168 kg-N/ha using blood meal. Marketable and cull fruit yield was monitored weekly from Apr. to Oct. Soil inorganic N in 0-30 cm depth (monthly) and N uptake by strawberry plants and fruits (bimonthly) were determined from Nov. to Oct., from which apparent N-loss during the winter (N-loss) was estimated. To evaluate tissue tests for organic strawberries, NO₃-N in petioles and TN in leaf blade were monitored monthly from Feb. to Oct. Cost effectiveness of N application schemes was evaluated based on fruit yield and price of fertilizers used. Overall, for pre-plant application, Sudan grass cover crop, compost 11 to 22 Mg/ha, or organic fertilizers up to 84 kg-N/ha worked well. 168 kg-N/ha of blood meal was ineffective especially under rainy winter causing greater N loss. For supplemental N, 112-168 kg-N/ha was found to be appropriate. 337 kg-N suppl. was effective only when pre-plant N was lost by winter rain. In contrast, supplemental N was ineffective at any rate during a warm-dry winter in a silty clay field with >20 yrs. of organic management. Amount of N-loss varied in a range of -42 to 121 kg-N/ha depending on residual inorganic N level in soil, amount of winter rain, pre-plant N rate, and soil type. Soil inorganic N and petiole nitrate look promising as tools for N management in organic strawberries.