Target Compost Analytical Values for Application to Highbush Blueberry

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 presentation will review the chemical criteria for “custom” blueberry compost, based on plant growth in recent mulch and soil amendment trials. Acidic pH (<5.5) is the most important compost characteristic for blueberry. Chemical analyses of compost can be used to identify potential compost suitability 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. Composts that require > 8 g·kg^-1 S^o for acidification (to pH 5) and contain total K > 10 g·kg^-1 (by acid digestion and ICP determination), total cations (sum of Ca, Mg, K, Na) > 1.5 mol (+)/kg, and have electrical conductivity (EC) > 4 dS·m^-1 (by SME) may damage transplants when applied at high rates (> 5 cm depth) to blueberry beds. Even when applied as mulch, composts with high EC can kill transplants. Composts that meet all of the target chemical analysis criteria are rare. Therefore, composts that meet “most” of the analytical criteria should be considered for trial use. To meet target compost analysis values, compost N usually should be limited to low analysis values (< 20 g·kg^-1 N) because high pH and EC and excess K are often present in composts with higher N analyses. Typical municipal yard debris composts in western Oregon usually meet the suggested target compost analytical criteria for pH and EC. However, they often contain K concentrations near the suggested limit, which could be a potential long-term soil problem (K⁺ leaches slowly). An ongoing field trial (2006 to present) in western Oregon has demonstrated that yard debris has a positive effect on yield but a negative effect in terms of weed control costs relative to alternative mulches such as sawdust or weed mat. In the future, blueberry production systems that combine geotextile weed mat and compost show promise and may help growers realize the benefits of compost use without incurring additional weed control costs.