Development of Custom Compost for Highbush Blueberry.

Blueberry plants are adapted to soils with high organic matter and acidic pH.  Composts can provide an alternative source of organic matter to the sawdust amendment used in Oregon, but are typically neutral in pH and can be high in soluble salts.  This study was to determine the best compost characteristics for blueberry production, and whether acidification of composts with elemental S is beneficial.   A three-day quick test to determine compost buffering capacity was developed using increasing amounts of 0.15M H₂SO₄   The decrease in compost pH was found to be linear with acid addition between pH 8.0 and 4.5.  The slope of the buffering capacity line was similar for  most composts (1.1 mol H+ kg^-1 C^-1 per pH unit) when normalized to compost C .  An 85-d greenhouse growth trial was conducted with Duke highbush blueberry in 3.8 L (1- gal) pots containing 30% compost by volume.  Prior to the growth trial, composts were either acidified with elemental S (2 unit pH drop) or were not acidified.   Plant growth was limited in composts with pH>7.5 and EC (10:1 water to compost)>4 mS cm^-1.  Plant-derived composts performed better than manure-derived. The acidification of composts with elemental S increased plant growth.  In this experiment, pH was a greater limiting factor to plant growth than soluble salts. Elemental S addition increased EC, but it also increased plant growth.