Fertilizer Rates for Organic & Conventional Potted Blueberry
Blueberries are not easily grown in Florida soils because of high organic matter and low pH needs. The media for growing blueberries requires good drainage (total porosity of 50-85%), a pH of 4.2-5.5, and a relatively low nutrient concentration in the soil solution. People everywhere are growing blueberries in pots to overcome these problems, but fertilization requirements in pots are not well known. Canadian researchers wanted to determine the optimal fertilization rates for highbush blueberry plants growing in three different potted systems: organic substrate with organic granular fertilizer, organic substrate with organic liquid fertilizer, and conventional substrate with controlled-release conventional fertilizer.
Researchers potted one-year-old ‘Duke’ plants into five-gallon pots in the spring with either organic or conventional potting mix. The conventional media was composted pine bark, coir, and peat while the organic media was 100% coconut coir. Three fertilization methods were tested: conventional controlled release, organic granular and organic liquid at the same rates of nitrogen (N) per pot: 0.16, 0.26, 0.36, 0.46, or 0.56 ounces/pot and the plants were grown for two years. All plants were watered when the average substrate moisture was at or less than 20% volumetric water content and all were grown outside. During the winter in Canada they were put in a covered outdoor structure. The irrigation water for the organically grown plants was pH adjusted by adding apple cider vinegar.
Conventional – The conventional controlled release fertilizer was Osmocote Plus with a 5-6 month duration, applied right after transplanting to the top of the pot. Applications were made again the next year in spring.
Granular Organic – The granular organic fertilizer was divided into three applications – one quarter of the total for each of the five rates was incorporated into the potting mix before planting and the remainder was applied in two equal top-dressed applications one month and two months after planting. The fertilizer was a mix of Bio-Fert General-Purpose fertilizer plus bloodmeal. The total amount was applied as one top-dressed application the following year.
Liquid Organic – The organic liquid fertilizer was diluted with tap water and applied at each watering event after a one-week establishment period. The fertilizer was Bio-Fert General-Purpose Liquid + calcium oxide.
Plants were evaluated for plant growth (plant height and width), leaf chlorophyll content, nutrient deficiency symptoms, yields (in the second year) and overall appearance.
Healthy, productive plants were produced with both organic and conventional fertilizers with the appropriate amounts of nitrogen. Plants grown with the conventional and organic liquid both grew best, with lower nutrient leaching, with the lowest two levels of N fertilizer, while plants with the organic granular did best with the medium range of N. The high fertilizer rates, regardless of method, showed the lowest growth and poorest overall appearance with interveinal chlorosis. Blueberry roots are very fine and susceptible to root damage from too much fertilizer.
Fertilization Methods for Organic and Conventional Potted Blueberry Plants. M.J. Clark and Y. Zheng. 2020. HortScience 55(33):304-309.