Soil and Plant Nitrogen and Organic Strawberry Fruit Yield Associated with Varying Rates of N Applied as Different Certified Organic N Sources

Fertilization is one of the most challenging cultural practices for organic strawberry growers in California. Nitrogen (N) is the most important and costly nutrient to manage and cost-effective N management information is needed for efficient organic strawberry production. Nitrogen management can be especially challenging on low organic matter soils that typify some California strawberry growing areas. Compost and green manure cover crops are widely used as economical sources of N for organic vegetable production but long turn around period between organic amendment pre-plant application and N need by strawberry transplants often limits the usefulness of these amendments for organic strawberry growers. Strawberries establish and grow slowly during the early season and growers are forced to depend upon in-season fertigation injection of liquid N sources for much of N nutrient needs. N availability from liquid N sources is variable and N availability may not adequately match crop need for N. Efficient utilization of these materials requires additional data on field performance. We conducted this trial between October 2008 and August 2009 to determine the plant and soil N effects, plant growth, plant N uptake, and fruit yield of plots receiving varying rates of three organic liquid N sources. We compared True Organic (4N-1.8P-1.2K), Neptune's Harvest (4N-0.9P-0.6K), and Phytamin 434 (4N-1.35P-2.4K) certified organic fertilizers at rates of 6.8, 13.6, or 20.4 kg N hectare-1 week-1 injected into the fertigation system. Total seasonal N applied varied from 177 kg N hectare-1 to 533 kg N hectare-1. Weekly residual soil nitrate N (SNN) varied between 0 and 15 ppm from early January to early April 2009 and was unrelated to material or amount of N injected. In early April, weekly SNN increased markedly and weekly SNN from early April to the end of the season ranged from 20-35 ppm. Differences in weekly SNN between types of material were not significant but the Phytamin materials were associated with highest SNN most weeks. Total season plant N uptake ranged from 53 to 64 kg N hectare-1 and was not consistently related to N rate or fertilizer material. Total yield ranged from 33,750 to 44,332 kg hectare-1. Variability in soil N availability from organic N sources appears to severely limit N uptake, plant growth, and yield. Leaching of N from rainfall and irrigation may play a role in limiting organic strawberry production with marginal N availability from uncertain N sources.