Effects of Acidified and Non-Acidified Composts On Highbush Blueberry Growth and Nutrient Uptake.

Highbush blueberry (Vaccinium corymbosum L.) is adapted to soils with high organic matter and acidic pH, and it is often grown in the Pacific Northwest with coniferous sawdust as a soil amendment or mulch.  Composts could provide an alternative source of organic matter and nutrients, but acidification is needed to overcome high pH.  Ten composts created from organic wastes were acidified and evaluated with ‘Duke’ highbush blueberry.  Our objectives were: i) evaluate compost response to acid addition and acidify composts with elemental S, (ii) determine the effect of acidified and non-acidified composts on plant growth and nutrient uptake under low-N management, and (iii) identify appropriate screening criteria for blueberry composts based on compost analytical data.   Composts were titrated with 0-7 mL H₂SO₄. The average compost response was 0.85 mol H⁺•kg C^-1 per pH unit.  Ground elemental S was added to decrease compost pH by 2.5 units; pH decreased by 1.9 units after 70-d incubation, 76% of titration prediction.  One year old plants were grown in 2.4L pots containing 60% silt loam soil (pH 5.4), 10% pumice, and 30% compost (v/v) in a 78-d trial.  Experiment design was a 2 x 2 factorial with 5 reps; factors were compost amendment and acidification.  Controls were sawdust amendment and soil only.  The interaction of compost and acidification was significant for new shoot dry weight and root growth rating.  Acidification increased new shoot dry weight and root growth rating across all composts, and ameliorated compost pH-raising effect on soil.  Composts increased leaf K concentration, but decreased leaf Mg and Ca due to cation antagonism from high compost K.  Acidification increased aboveground uptake of most nutrients, and compost initial pH was negatively correlated with plant aboveground uptake of N, P, K and Mg.   High N composts also had high pH, preventing plant uptake of compost-supplied N.  Compost initial pH and soluble cation ratio of K:(Mg+Ca) were negatively correlated with new shoot dry weight and root growth rating.  Screening of composts for blueberry production should include pH, soluble salts, total nutrient content, and saturated media extract for soluble nutrients.  Composts best suited for blueberry would have a pH < 7.5, soluble salts < 2 mS•cm^-1, and a soluble K:(Mg + Ca) ratio < 2.