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Effect of Methyl Bromide Alternative Fumigants on Fertility in Strawberries

Ajwa, H.A., S. Klose, M. Bolda and D.V. Shaw. 2006. California Strawberry Commission


Observations from growers fields at the central coast of California showed that strawberry (Fragaria X ananassa Duchesne) plants from fields drip-fumigated with chloropicrin (CP) or InLine revealed reduced yields at later growth stages compared to methyl bromide and CP (MeBr CP) fumigated fields. A field experiment was conducted from 2004 to 2006 to evaluate the effect of different fumigants (CP and MeBr CP), planting times (9 or 4 weeks past fumigation), and N fertilization rates (1x standard rate, 2x standard rate) on strawberry growth, fruit yield and quality, and soil NO3- N concentration and supply rate over two strawberry growing seasons. Strawberry growth was monitored by measuring plant foliage diameter and foliage nutrient content. Fruit yield and quality (e.g., fruit appearance and size) was determined from March to October 005. Soil NO3- N concentrations and supply rates were determined on a monthly basis from August 2004 to September 2005 (first growing season) and from August 2005 to ongoing (second growing season) by a combined soil extraction method (1 M KCl extract) and ion exchange resin technique (PRS™ anion probes, Western Ag Innovations Inc., Saskatoon, Saskatchewan, Canada ), respectively. PRS&trade anion probes were buried at 0-15 and 15-0 cm depth at the center and edge (first growing season) and at the edge (second growing season) of raised beds to monitor soil NO3-N supply to the plant over the entire burial time. Soil samples were collected at the same depths and bed locations and extracted with KCL in the laboratory to determine soil NO3-N concentration at sampling time. Results from the 2004/2005 growing season indicated that plants from fields fumigated with chloropicrin showed similar yields, fruit size and quality compared to methyl bromide/chloropicrin fumigated fields. NO3-supply rates (PRS™-probe analyses) can be used as a reliable tool to measure available soil nitrate during the growing season, and schedule fertilizer applications. Although not reflected by strawberry yield and fruit quality, NO3-N supply rates measured by resin exchange technique indicated differences in soil fertility between the two fumigants and fertilization rates tested. An impact of planting time was evident in plots fumigated with MBCP only, with higher soil fertility in plots planted at 9 weeks relative to those planted at 4 weeks past fumigation. These findings demonstrate the need for a continuation of this field study over a third growing season for data validation, including a laboratory experiment to study shifts in microbial community structure and N mineralization and nitrification rates. The results of this ongoing study are expected to answer the question whether higher strawberry yields in MB/CP fumigated soils relative to CP treated soils are related to the efficacy of the fumigants, to planting time, or to N availability.