Chemical Characteristics of Custom Compost for Highbush Blueberry

Abstract

Recent development of markets for blueberry (Vaccinium corymbosum L.) produced under Organic certification has stimulated interest in production of composts specifically tailored to its edaphic requirements. Blueberry is a calcifuge (acid-loving) plant that responds favorably to mulching and incorporation of organic matter into soil. Many composts are high in pH and soluble nutrients, and may not be suited to blueberry. This chapter describes recent improvements in developing chemical criteria for composts that indicate potential suitability for blueberry. The experimental work, which was conducted in western Oregon, USA, confirmed that acidic pH (<5.5) is the most important characteristic needed in a custom compost for blueberry. Composts with pH < 6 are rare, so a testing protocol to quantify the pH buffering capacity of compost (CBC) and the quantity of acidity needed to reduce compost pH to 5.0 was developed. Median compost buffering capacity (n = 36) was 0.20 mol H+/kg/pH unit, and median elemental S (S^o) addition required for acidification to pH 5 was 8 g S^o/kg, assuming full reaction of S^o to H⁺. In compost acidified with S^o to a final pH of 5–6, EC was increased 1.6 fold, accompanied by increased solubility for K and Na (1.3–1.4×); P, Ca, and Mg (3.2–3.6×); and SO₄-S (5.2×). Blueberry plants accumulated K supplied by compost, accompanied by reduced plant uptake of Mg, reduced growth, and possible Mg deficiency. Compost acidification to below pH 6 improved blueberry plant growth and Mg uptake. We conclude that compost can be used to increase soil organic matter for blueberry, but that compost N must be limited to low analysis values (total N < 20 g/kg) in order to avoid problems with high pH, EC, and excess K. Because all compost feedstocks that met USDA-Organic certification requirements needed acidification to reach the desired pH level for blueberry (<5.5), future research should focus on economical, safe, and reliable methods for compost acidification that are acceptable under Organic certification rules.