Composting, a controlled process for stabilization of organic matter, can turn organic waste into a valuable soil amendment. Compost can return nutrients and organic matter to the soil, a proven practice for soil health enhancement. It can improve crop growth and provide environmental benefits by improving soil tilth and the soil’s capacity to absorb and hold water and plant nutrients. A properly managed composting process can destroy weed seeds, plant pathogens, and human pathogens.
Compost analysis assists buyers of bulk compost to be confident that they are receiving good value for their money. This publication is designed for wholesale buyers of compost for resale, farmers, nursery managers, and public/private landscape managers.
This publication focuses on the following:
Selecting a commercial analytical laboratory to analyze your compost is an important first step. Because compost testing requires skill and experience to produce reliable results, it’s best to select a lab that specializes in this type of analysis. Unfortunately, only a few laboratories have this expertise.
Look for labs on the Compost Analysis Proficiency (CAP) Program website (https://compostingcouncil .org/compost-analysis-proficiency-program/). CAP is sponsored by the U.S. Composting Council (USCC), and labs participate voluntarily as a way to improve testing accuracy and precision based on standard methods published in a peer-reviewed manual, Test Methods for the Examination of Composting and Compost (USCC, 2001a). Lab representatives should be able to produce a CAP proficiency testing report that reviews the lab’s analytical performance at your request. You can read more about how the CAP Program ensures testing quality in the sidebar “Laboratory proficiency: How reproducible are compost analyses?”.
Compost testing labs typically offer several analysis “packages” designed to evaluate compost for use as a soil amendment in the field or as a component of a soilless mix (potting soil) for container-grown crops. Before submitting a sample, we recommend contacting the lab to discuss analysis options and to determine sample submission details (e.g., sample size needed, shipping instructions). We also recommend requesting an example of a compost analytical report to determine what test interpretations are provided and whether they are relevant to your needs.
To get the most value from a compost analysis, the sample must represent the compost in the field. Sampling instructions are provided in Extension publications PNW 533, Fertilizing with Manure and Other Organic Amendments (Bary, 2016), and PNW 673, Sampling Dairy Manure and Compost for Nutrient Analysis (Moore, 2015).
Compost moisture, or water content, is expressed as a percentage of compost wet weight. A compost with 60 percent moisture contains 40 percent dry matter. Composts with high moisture content (above 60 percent) are usually clumpy and difficult to spread. Composts with low moisture content (below 40 percent) are dusty. The higher the moisture content, the lower the amount of organic matter per ton of compost. Compost moisture and dry matter are determined by drying the sample in an oven at 70°C (158°F).
Bulk density is expressed in pounds per cubic yard. Bulk density allows you to convert between weight units (tons) and volume units (cubic yards). This conversion is often necessary because labs report nutrient concentration on a weight basis, while field application is often on a volume basis.
As a rule of thumb, screened composts that contain 50 percent moisture will have a bulk density of about 1,000 lb/cu yd. Very wet composts can have a bulk density of over 1,500 lb/cu yd.
Laboratories can perform a bulk density test, or you can perform one in the field. Field determination of bulk density often is more informative than laboratory measurement because it uses a larger volume of compost. Also, multiple samples can be evaluated to obtain an average bulk density. For a simple method to calculate compost bulk density, see “Additional tests conducted by the compost vendor or user”.
In many situations, organic matter is the most valuable component of compost for soil health improvement.
Total organic carbon and organic matter are expressed as a percentage of compost dry weight. Organic carbon (C) represents about half of the organic matter weight. Thus, if you know the organic C content, you can estimate total organic matter content. For example, a compost with 25 percent total organic C contains about 50 percent organic matter.
Two methods can be used to estimate C or organic matter:
Often, low organic matter values in compost (below 25 percent) result from soil or sand being mixed into the compost during turning. This is common when compost is prepared on bare ground.
Composts with high levels of organic matter (more than 65 percent) may not have been thoroughly composted. These materials may contain considerable unstable organic matter that will be lost (as carbon dioxide gas) via rapid decomposition after field application.
If you know the percentage of organic matter, you can calculate organic matter supplied per cubic yard of compost as follows:
organic matter/cu yd = bulk density x % dry matter x % organic matter
Example: A fresh “as-is” compost has a bulk density of 1,000 lb/cu yd and contains 50 percent moisture. Organic matter in dry matter is also 50 percent. This compost will supply 250 lb organic matter/cu yd, calculated as follows:
1,000 lb compost/cu yd x 0.50 dry matter x 0.50 organic matter = 250 lb organic matter/cu yd
Compost pH is a measure of acidity/alkalinity. Most plant-based composts are moderately acidic (pH 6) to moderately alkaline (pH 7.5). Manure-based composts usually have pH of 7 to 8. The high pH of most manure-based composts makes them unsuitable for acid-loving plants such as rhododendron and blueberry.
Electrical conductivity (EC) is an indicator of soluble salt content. Electrical conductivity is usually reported in units of mmhos/cm, mS/cm, or dS/m. These units are equivalent and have the same interpretation.
High salt levels may injure plants, with seedlings and transplants being most susceptible to injury. After compost application, salts are usually moved downward via leaching. However, when irrigation water moves soluble salts across a planting bed, as it does with drip irrigation, salts can become concentrated, increasing the risk of plant injury (Figure 2). See PNW 601-E, Managing Salt-affected Soils for Crop Production (Horneck, 2007), for more information on crop sensitivity to soluble salts.
On the other hand, most soluble salts are soluble nutrients, so compost with a high salt concentration may be a good source of nutrients when applied at a low rate.
Acceptable EC for compost used in field situations depends on the compost application rate, soil EC prior to application, depth of compost incorporation (tillage), soil texture, and irrigation water management.
An online calculator, available from the University of California (Crohn, 2016), can be used to predict soil EC after compost application. To use the calculator, you’ll need to know the following about your compost:
The calculator assumes uniform mixing of soil and compost after application and no dilution of EC by leaching. Calculator predictions have been verified for selected soils and composts (Reddy and Crohn, 2013).
Two protocols are used for water addition to compost in preparation for pH and EC determination: the 1:5 compost:water method and the saturated extract method. The saturated extract method is preferred for compost to be used in potting media. For all other applications, the 1:5 compost:water method is preferred.
The amount of water added for the 1:5 method is greater than for the saturated extract method. Because of greater dilution, the EC determined in a 1:5 extract will be two to five times lower than in a saturated extract. Conversely, the pH determined via the 1:5 method is usually 0.1 to 0.3 pH unit higher than the pH determined via the saturated extract method.
Total nitrogen (N) is the sum of two types of N:
Compost organic N is estimated as total N minus inorganic N. Usually, more than 95 percent of total N in compost is organic N.
The carbon to nitrogen (C:N) ratio is the ratio of total C to total N. Well-composted materials reach a stable C:N ratio of 10 to 15, similar to the C:N ratio found in soil organic matter. Woody composts typically have higher C:N ratios (above 20). These composts may increase the N fertilizer requirement. Composts with C:N ratios below 10 supply a significant amount of plant-available N in the short term.
Ammonium (NH4-N) and nitrate-N (NO3-N)—sometimes called plant-available N, inorganic N, or mineral N—are soluble inorganic ions that are released as organic N is decomposed. In most composts, inorganic N is usually less than 5 percent of total N, with the remainder in organic form. Mature composts usually contain more NO3-N than NH4-N.
See “Interpreting compost nitrogen analyses” for additional information on using N analyses to assess compost maturity and N fertilizer replacement value.
In addition to N, composts supply other macro- and micronutrients that are important for plant nutrition, including phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), zinc (Zn), manganese (Mn), copper (Cu), iron (Fe), and boron (B).
For compost intended for field application, nutrient analysis is routinely used to estimate effects of compost on soil in the field. For example, if compost contains 2 percent of a nutrient (dry weight basis), a dry ton of compost will supply 40 lb of that nutrient.
Acid digestion of compost is used to prepare samples for analysis. This method measures all of a nutrient present in compost, whether it is in plant-available form or not, so the result is known as total nutrient analysis. See the sidebar “Evaluating total macronutrient concentrations in compost” for additional information.
If compost is intended for use in potting media, the saturated extract method for nutrient extraction is preferred. Compost is saturated with water, and water is extracted from the compost under a vacuum. The nutrients present in a saturated extract are a “snapshot in time” and are subject to change, so they are not useful for long-term planning.
The saturated extract method is useful for determining chloride (Cl) and boron (B) content of compost. This is important because too much water-soluble B and Cl can injure plants. When Cl is above 500 ppm (mg/L) or B exceeds 1 ppm (mg/L), as determined via the saturated extract method, sensitive plants may be injured.
Routine soil tests, such as the Bray or Olsen method for P or the DTPA method for micronutrients, are not appropriate for high-organic-matter substrates such as compost. However, routine soil tests are recommended to document long-term effects of compost on soil nutrient status. Wait at least 6 to 12 months after compost application before sampling soil for nutrient status. This will allow time for compost nutrients to equilibrate with the soil and will provide more reliable information for planning crop nutrient additions.
Compost maturity and stability are not the same thing.
Testing laboratories do not measure maturity directly. Instead, “maturity ratings” are assigned, based on a battery of quantitative tests. Measurement of compost stability is one of the major criteria used in assessing compost maturity. See the sidebar “Industry standards for compost ‘maturity’” for an example of compost maturity ratings sanctioned by the U.S. Composting Council.
A few labs that specialize in compost testing offer stability tests. Stability is usually determined by measuring carbon dioxide loss during incubation of a compost sample. The most reproducible stability test is a 3-day measurement of compost respiration rate at 37⁰C (99°F). The greater the respiration rate, the less stable the compost. “Very stable” composts have decomposition rates below 2 mg CO2-C/g organic matter/day, and “stable” composts have decomposition rates below 8 (USCC, 2001a).
High concentrations of ammonia in a compost can inhibit microbial activity, thereby reducing the measured carbon dioxide evolution rate and rendering stability test data invalid. Improved compost stability testing protocols are the subject of ongoing research (Wichuk and McCartney, 2010).
The following compost tests are sometimes useful, depending on how you plan to use the compost.
Particle size is of limited importance when compost is incorporated in the field for soil improvement. When compost is used as a surface mulch, larger particle sizes are desirable.
Recommended compost particle size distribution (by weight) for field application of compost in agriculture in California (Crohn, 2016) is as follows:
— Soil amendment: 95 percent passing 0.6-inch (16-mm) screen; 70 percent passing 0.4-inch (9.5-mm) screen
— Mulch: 99 percent passing 3-inch (76-mm) screen; less than 25 percent passing 0.4-inch (9.5-mm) screen
You can perform your own plant growth test to verify the absence of herbicides in compost (see “A pea bioassay to test for contamination by persistent herbicides”). This protocol is suitable for detecting the risk of plant injury from a variety of broadleaf herbicides, including clopyralid and aminopyralid.
Bioassays may be especially important for certified organic farms, where herbicides in compost can impact certification status. Contact your organic certifier for guidance.
Contamination of compost with weed seed is most likely when cured compost is left uncovered outdoors for long periods, particularly in areas with abundant weeds nearby. Some weed seeds may also survive an improperly managed composting process.
As-is bulk density can be measured on-site, using a scale and a 5-gallon bucket with vertical sides, as follows:
1. Weigh the empty bucket and record its weight.
2. Fill the bucket with 5 gallons of water and mark the water level on the inside of the bucket. Empty the bucket. Use a ruler to measure and mark lines at one-third and two-thirds of the 5-gallon line.
3. Fill the bucket with compost to the one-third line. Drop the bucket 10 times from a height of 1 foot. Add compost up to the two-thirds line and drop the bucket 10 times. Add compost to the 5-gallon line and drop 10 times. After the final drop, top off the bucket with compost to the 5-gallon line.
4. Weigh the full bucket and subtract the weight of the empty bucket. This is the weight of 5 gallons of compost.
5. Multiply this weight by 40 to calculate the bulk density in Ib/cu yd.
An illustration of the bucket method (with a sample calculation) is given on page 16 of PNW 533 (Bary, 2016).
Bary, A.I., C.G. Cogger, and D.M. Sullivan. 2016. Fertilizing with Manure and Other Organic Amendments. PNW 533, Washington State University. http://cru.cahe.wsu.edu /CEPublications/PNW533/PNW533.pdf
California Compost Quality Council. 2001. Compost Maturity Index. http://compostingcouncil.org /wp/wp-content/uploads/2014/02/2-CCQC -Maturity-Index.pdf (verified 17 Aug 2018).
Crohn, D.M. 2016. Assessing Compost Quality for Agriculture. ANR 8514, University of California. https://anrcatalog.ucanr.edu/Details .aspx?itemNo=8514
Horneck, D.A., J.W. Ellsworth, B.G. Hopkins, D.M. Sullivan, and R.G. Stevens. 2007. Managing Salt-affected Soils for Crop Production. PNW 601-E, Oregon State University. https://catalog.extension.oregonstate.edu /pnw601
Moore, A., M. de Haro-Marti, and L. Chen. 2015. Sampling Dairy Manure and Compost for Nutrient Analysis. PNW 673, University of Idaho. https://www.cals.uidaho.edu/edcomm/pdf /PNW/PNW673.pdf
Reddy N. and D.M. Crohn. 2013. Compost induced soil salinity: A new prediction method and its effect on plant growth. Compost Science and Utilization 20(3):133–140.
U.S. Composting Council. 2001a. Test Methods for the Examination of Composting and Composts (TMECC). https://compostingcouncil.org /tmecc/ (verified 17 Aug 2018).
U.S. Composting Council. 2001b. Composting testing programs. https://compostingcouncil.org /programs/ (verified 17 Aug 2018).
Washington State University and the Washington State Department of Ecology. 2002. Bioassay Test for Herbicide Residues in Compost: Protocol for Gardeners and Researchers in Washington State. https://s3.wp.wsu.edu/uploads /sites/411/2014/12/PDF_Clopyralid_Bioassay .pdf (verified 17 Aug 2018).
Wichuk, K.M. and D. McCartney. 2010. Compost stability and maturity evaluation—a literature review. Canadian Journal of Civil Engineering 37(11):1505–1523.
As defined by the U.S. Composting Council, compost maturity describes the suitability of compost for a particular use. It is a subjective overall rating derived from summarizing and evaluating laboratory results for compost stability (Group A tests) and for other tests that indicate the potential for compost toxicity to plants (Group B tests). Toxicity tests include the ratio of ammonium-N to nitrate-N, ammonia/ammonium-N concentration, volatile organic acid concentration, and plant growth/seed germination. A current list of sanctioned Group A (stability) and Group B (toxicity) tests is available from the U.S. Composting Council.
The Composting Council rating system defines three categories of compost maturity: very mature, mature, and immature (USCC, 2001a; California Compost Quality Council, 2001).
Very mature composts are considered suitable for any application. Potential uses for mature and immature compost are more restricted.
Keep in mind that these test interpretations are based on short-term effects of compost on plant growth.
Nitrogen in inorganic forms (nitrate or ammonium) or organic forms (as reflected in the C:N ratio) is the only nutrient specifically considered in Composting Council maturity ratings. Plant damage observed in seed germination or seedling growth tests (Group B tests) may be related to excess soluble nutrients (salts), but soluble salt levels are not explicitly considered in industry ratings of compost maturity.