Not all Composts are Equal: Recommendations on

Composts and Organic Soil Amendments for Blueberry

By Dan M. Sullivan (Oregon State University) with assistance from Lisa DeVetter (Washington State University)

A summary of the proceedings and presentation provided for the Oregon State University Blueberry School.


Ryan Costello (M.S. graduate, Crop & Soil Science, OSU)

David R. Bryla (USDA-ARS Horticultural Crops Unit, Corvallis, OR)

Bernadine C. Strik (Dept. Horticulture, OSU)

     With the arrival of spring comes the beginning of fertilizer applications in blueberry. Sources of fertilizers and soil amendments vary based upon their origins and compositions. Some of these organic soil amendments have beneficial attributes making them suitable for blueberry production, whereas others may actually cause damage to plants or require additional inputs to make them more suitable for the specific conditions required for blueberry.  The purpose of this article is to summarize some of the recent work led by Dr. Sullivan and his team addressing organic soil amendments in blueberry with an emphasis on compost.


     Blueberry prefers acid soil (pH 4.5 to 5.5) that is low in soluble salts.  Blueberry usually responds favorably to soil amendment with organic matter.  The industry standard soil amendment (Douglas-fir sawdust) is low in salt, has acidic pH, and provides organic matter. But sawdust increases nitrogen (N) fertilizer needs, as plant-available N is consumed as sawdust decomposes in soil.



     Compost is the product of controlled biological decomposition of organic materials. Composts provide nutrients, organic matter, and they have been heat-treated to eliminate weed seeds and plant pathogens. Unlike sawdust, composts do not increase N fertilizer need and they usually provide plant-available N (reducing N fertilizer need). However, composts usually have higher pH and contain more salt than needed for blueberry.


     Soil amendments not suitable for blueberry because of high salts [or electrical conductivity (EC)] and pH (> pH 7.0) include whole dairy, poultry, and feedlot compost manures. Mushroom compost and other composts that have added lime and gypsum are also not recommended for blueberry. Animal manures applied in a liquid or slurry form also contain high salts that may damage blueberry, despite having a benefit in other berry crops. Table 1 lists various sources of soil amendments from Oregon and their measured pH and EC values, which demonstrates why certain sources of compost are not recommended for blueberry due to high salts and pH.


Table 1. Analysis of soil amendments from Oregon, including composts, sawdust, and peat. Material pH and EC were determined by the saturated media extract method.





















     Yard debris compost has been shown to be the most suitable compost for blueberry based upon on-going research conducted at the OSU North Willamette Research and Extension Center in Aurora, OR. In the Organic Systems Trial, a yard debris compost was applied at 1 to 2 inch depth at establishment (2006) and additional compost was applied after three growing seasons. The yard compost treatment resulted in the following: 1) soil organic matter increased from 3% (no compost control) to 4%; 2) maintenance of optimum soil pH; 3) increase in berry yield by 0 to 10% versus no compost control, depending upon year; and 4) soil exchangeable potassium (K) increased from 250 ppm (no compost control) to 400 ppm, but leaf K did not increase with compost application. Despite these results, the yard debris compost did not improve economic return because it increased weed growth and resultantly increased cost for weed control.


Guidelines on Compost Selection for Blueberry

     Blueberry is sensitive to high salts and pH. Composts used for blueberry should not injure plants with excess salt and maintain pH in an acceptable range. Guidance provided here is conservative and is designed to avoid plant injury when compost is applied at soil amendment rates (i.e., 1-2 inches of compost applied as mulch or soil amendment to a blueberry bed). A compost analysis is essential to determine suitability for blueberry.


First, consider the C:N ratio of the compost.


  • Composts derived from manures usually have low C:N (<12), high total N (>2%), and will supply too much N for blueberry, even if they do not injure plants by supplying too much salt or raising soil pH. Low C:N ratios in compost are almost always associated with other negative characteristics for blueberry, including high salt, high EC, and high potassium (K). Therefore, composts containing more than 2% N (dry weight basis) produced from poultry manure, dairy manure, or feedlot manure are not recommended for blueberry. Other high N composts (e.g., peppermint and mushroom compost) are also unsuitable for blueberry.
  • Composts with C:N of 12 to 25 (1 to 2% N, dry weight basis) may have value for blueberry, but caution is advised. Some of these composts may be acceptable for blueberry, while others will have excessive pH, EC, and/or potassium (K).
  • Woody composts with C:N above 40 (<1% N, dry weight basis) supply organic matter, but usually have close to zero value in supplying plant-available nitrogen. Application of these composts will often increase the need for supplemental N fertilizer application during the first growing season following application.


     Target values for other key compost analyses are provided below. Keep in mind that research is ongoing, and target values may be adjusted as more research is completed.


  • Compost pH.  Goal: Maintain optimum soil pH. Target < pH 6; Acceptable pH < 7.5.  Note that composts tend to be weak liming materials and acidification through addition of sulfur may be needed to maintain optimum soil pH.
  • Compost EC.  Goal: Avoid plant injury.  Target depends on EC analysis method.   Target EC < 4 dS/m with the saturated media extract (SME) method, < 2 dS/m with the 1:5 method, and < 1 dS/m with the 1:10 method.  Units: 1 dS/m = 1 mmhos/cm.
  • Compost K.  Goal: Avoid plant accumulation of excess K, and possible K/Mg nutrient imbalance. Target total K in compost < 0.7% (dry weight basis); acceptable < 1.3%.


Take Home Messages

     There are many other aspects to consider regarding organic soil amendments and composts for blueberry. A few of the take-home messages specific to utilization of compost in blueberry include:


  • Compost analyses can provide important information to guide compost selection.
  • Composts are weak liming materials and can raise soil pH.
  • Yard debris compost is usually an acceptable compost for blueberry.  But excess K may be a long-term problem.
  • Most, if not all manure composts are not suitable for blueberry. They contain too much salt (including K) and they often raise soil pH above the target range for blueberry (pH 5.0 to 5.5).
  • Most composts will make a weedy mulch, because they are high in nutrients and have small particle size. Some growers and researchers have suggested that it may be possible to overcome this problem by placing compost under geotextiles, like “weedmat” and landscape fabrics.


For further questions, email the main author at: