STRAWBERRY NITROGEN FERTILIZATION FROM ORGANIC NUTRIENT SOURCES

Organic strawberry production is increasing in California in response to growing demand and more stable or higher prices for organic fruits in the US and international markets. In 2007, 4.0% of total strawberry acreage in California was farmed organically, but there has not been a solid base of information for growers to develop cultural practices for organic production. In this paper, we review current nitrogen (N) management practices and discuss challenges associated with N management in organic strawberry production in California. The N availability is critical for early plant development on which fruit yield and quality depend. Studies indicate that the synchrony of N supply from organic nutrient sources with crop N demand is inconsistent and unpredictable with organic nutrient sources compared to soluble conventional N materials. Organic growers apply compost, manures, or green manures three to eight weeks before planting for early N nutrition. In addition, growers supplement the pre-plant incorporated fertilizer materials with pre-plant granular or pelleted materials band-applied below the transplant. Studies have shown that most incorporated N materials are completely mineralized by six to eight weeks and thus, it is difficult to plan pre-plant incorporated N programs to match transplanted strawberry plant N needs. One study reported high N loss from an organic strawberry farm during the winter rains. Growers also supplement pre-plant application with different liquid organic materials injected through the drip irrigation system, but dosage and timing are uncertain. Liquid organic materials are continually and unpredictably transformed by bacteria, they lack uniformity, and a portion of the N is in suspended solids that may be filtered out or lost behind drip orifices. Data from recent strawberry N fertilization trials - including N absorption patterns and N availability from cover crops - are now beginning to answer some of the key questions for organic strawberry producers.