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Ryan C. Costello, Dan M. Sullivan, David R. Bryla, Bernadine C. Strik, and James S. Owen

New markets for organic northern highbush blueberry (Vaccinium corymbosum L.) have stimulated interest in using composts specifically tailored to the plant’s edaphic requirements. Because composts are typically neutral to alkaline in pH (pH 7 to 8), and blueberry requires acidic soil (pH 4.2 to 5.5), we investigated elemental sulfur (S0) addition as a methodology for reducing compost pH. The objectives were to 1) characterize initial compost chemistry, including the pH buffering capacity of compost (acidity required to reduce pH to 5.0), 2) measure changes in compost chemistry accompanying acidification, and 3) evaluate plant growth and mineral nutrition of blueberry in soil amended with an untreated or acidified compost. Ten composts prepared from diverse feedstocks were obtained from municipalities and farms. Addition of finely ground S0 reduced compost pH from 7.2 to 5.3, on average, after 70 d at 22 °C, and increased the solubility of nutrients, including K (from 22 to 36 mmol(+)/L), Ca (from 5 to 19 mmol(+)/L), Mg (from 5 to 20 mmol(+)/L), and Na (from 6 to 9 mmol(+)/L). Sulfate-S, a product of S0 oxidation, also increased from 5 to 45 mmol(−)/L. The composts were incorporated into soil at a high rate (30% v/v) in a greenhouse trial to evaluate their suitability for use in blueberry production. Shoot and root growth were strongly affected by compost chemical characteristics, including pH and electrical conductivity (EC). Potassium in compost was highly variable (2–32 g·kg−1). Concentration of K in the leaves increased positively in response to compost K, whereas shoot dry weight and root growth declined. Leaf Mg also declined in response to compost K, suggesting that elevated K concentrations in compost may cause Mg deficiency. Composts with the highest K were also high in total N, pH, and EC. Compost acidification to pH ≤ 6 improved growth and increased leaf Mg concentration. On the basis of these results, composts derived from animal manures or young plant tissues (e.g., green leaves) appear to be unsuitable for high-rate applications to blueberry because they usually require high amounts of S0 for acidification and are often high in EC and K, whereas those derived from woody materials, such as local yard debris, appear promising based on their C:N ratio, compost acidification requirement, and EC.

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Bryan K. Sales, David R. Bryla, Kristin M. Trippe, Jerry E. Weiland, Carolyn F. Scagel, Bernadine C. Strik, and Dan M. Sullivan

Biochar, a carbon-rich, fine-grained residue obtained from pyrolysis of biomass, is known to improve soil conditions and to suppress infection by soilborne pathogens. However, its use as a soil amendment has received relatively little attention by the horticulture industry. Two 12-week experiments were conducted in a greenhouse to determine the potential of using biochar, produced from mixed conifers during conversion of wood to energy, as a soil amendment for highbush blueberry (Vaccinium hybrid ‘Legacy’). Plants in the first experiment were fertilized once a week with a complete fertilizer solution, whereas those the in the second experiment were fertilized once a month with a solution of ammonium sulfate. In both cases, the plants received the same amount of N in total and were grown in pots filled with unamended soil (sandy loam) or soil amended at rates of 10% or 20%, by volume, with biochar or a 4:1 mix of biochar and bokashi (biochar-bokashi). The bokashi was produced from fermented rice (Oryza sativa L.) bran and was added to increase nutrients in the amendment. Half of the plants in each soil treatment were inoculated with Phytophthora cinnamomi Rands, which causes root rot in blueberry. Although pH of the raw biochar was high (8.5), soil pH averaged 4.5 to 5.5 in each treatment. In the absence of P. cinnamomi, plants grown with 20% biochar or 10% or 20% biochar-bokashi had greater leaf area and 30% to 70% more total dry weight than those grown with 10% biochar or in unamended soil. Biochar also improved soil aggregation and increased root colonization by ericoid mycorrhizal fungi. The percentage of roots colonized by mycorrhizal fungi was 54% to 94% in plants grown with the amendments, but was ≤10% in those grown in unamended soil. Plants inoculated with P. cinnamomi were stunted and showed typical symptoms of root rot. Root infection by the pathogen was unaffected by biochar or biochar-bokashi and negated any growth benefits of the amendments. Overall, amending soil with biochar appears to be a promising means of promoting plant growth and mycorrhizal colonization in blueberry, but it may not suppress phytophthora root rot.