Many people are interested in growing blueberries in containers to provide optimum soil qualities of high organic matter, low pH, and controlled irrigation that are not easily provided in fields. Organic matter requirements are expensive and can be met in various ways, although the need for low pH in these organic materials can be difficult to obtain. These Oregon researchers wanted to test various lower cost media than peat and compare plant growth.
They tested 10 different soilless medias with 10% perlite and varying proportions of sphagnum moss, coconut coir, and douglass fir bark as well as a commercially available “blueberry” mix of peatmoss, perlite, and other proprietary ingredients. The commercial mix was Sunshine Professional Growing Mix #4 from Sun Gro Horticulture Distribution Inc. Young ‘Snowchaser’ highbush blueberry plants were planted in one-gallon pots and grown for 128 days. Plants were destructively harvested at 128 days after planting to measure dry weight of stems, leaves, and roots and nutrient content. Media properties were also measured including bulk density, porosity, particle-size distribution, cation exchange capacity (the ability to hold on to and release nutrients), percent C and N, concentration of ammonium N, electrical conductivity (EC), pH, and nutrient content.
By 128 days after transplanting the total dry weight was nearly twice as much in the commercial mix and in media with at least 60% peat or coir than in media with at least 60% fir bark. The researchers felt this was mostly because the fir bark media did not receive enough irrigation because the bark had lower porosity and water holding capacity than peat, coir, or the commercial mix and dried out more quickly between irrigations. Bark also reduced plant uptake of a number of nutrients. Uptake efficiency of P, K, and Mg differed between plants grown in peat and coir, mostly because of the initial nutrient concentration differences between the medias. Before planting, peat had the highest concentration of Mg and Fe of all the media, while coir had the highest concentration of P and K. Leachate pH was lowest with peat and highest with coir at the beginning, but by the end of the experiment they all had similar pH.
Peat and coir both appear to be good substrates for container production of highbush blueberries whereas bark was less suitable, especially when it exceeded 30% of the total media composition. Their findings illustrated the importance of pH management of substrate, particularly in the long-term to maintain N in the desired ammonium form. Changes in media properties such as nutrient retention or porosity over long term production must be monitored for changes and managed accordingly. Future research should focus on changes in media properties over the long term and on maintaining adequate moisture levels in the media. Blueberries generally start producing fruit earlier in containers than in the field, but economics dictate that they must be grown in containers for at least several years to cover the costs of container production.
Source: Suitability of Sphagnum Moss, Coir, and Douglas Fir Bark as Soilless Substrates for Container Production of Highbush Blueberry by P.J. Kingston, C.F. Scagel, and D.R. Bryla. HortScience 52(12):1692-1699. 2017.