Compost Maturity – Solvita CO2/NH3 Test
1 Purpose
This procedure describes the determination of compost maturity and stability using the Solvita Compost Maturity Test (Woods End Laboratories). The test simultaneously measures carbon dioxide (CO2) and ammonia (NH3) emissions from a compost sample during a 4-hour incubation, providing a dual indicator of biological activity and nitrogen stabilization. The intersection of the two readings yields a Compost Maturity Index (CMI) that indicates whether compost is immature, active, curing, or finished (Vargas et al., 2005).
CO2 evolution reflects ongoing microbial decomposition of organic carbon. NH3 volatilization indicates incomplete nitrogen stabilization and is associated with odor, phytotoxicity, and high pH. True compost maturity is reached when both gases have declined to low levels. Because the CO2 and NH3 cycles do not always track together, the dual-measurement approach provides a more reliable maturity assessment than either gas alone.
This SOP is distinct from SOP-L408 Solvita CO2 Burst, which measures rewetted soil respiration over 24 hours using only a CO2 paddle.
2 Scope
- Applies to: Assessment of maturity and stability of composted organic materials, including green waste, food waste, biosolids compost, manure compost, and vermicompost
- Does not apply to: Soil respiration measurements (see SOP-L408); raw feedstock characterization; long-term (7–28 day) laboratory respirometry; finished potting mixes where compost is a minor component
- Location: EA Program laboratory, SGM 133
3 Responsibilities
| Role | Responsibility |
|---|---|
| Lab PI | Approve SOP, ensure training compliance |
| Lab Manager | Maintain Solvita kit inventory, stock gel paddles |
| Trained Personnel | Perform assay, record and report results |
| New Students | Complete training before independent use |
4 Definitions
- Compost maturity: A measure of how fully decomposed and stabilized a compost product is; mature compost no longer undergoes rapid decomposition, does not cause phytotoxicity, and can be safely applied to crops or soils.
- CO2 index (Solvita): The Solvita scale value (1–8) corresponding to the CO2 concentration in the headspace above moist compost; lower values indicate more active decomposition (less mature); higher values indicate stabilization.
- Maturity index: A combined score calculated from the CO2 and NH3 Solvita indices; the primary output of the Solvita compost maturity test; used by USCC, US Composting Council, and others to certify compost quality.
- NH3 index (Solvita): The Solvita scale value (1–5) corresponding to the NH3 concentration in the headspace above moist compost; high NH3 (low index) indicates immature compost with excess ammoniacal nitrogen that can harm plants.
- Phytotoxicity: Inhibition of plant germination or growth caused by immature compost; typically results from high NH3 concentrations, volatile fatty acids, or other metabolites produced during active decomposition.
- Solvita paddle: A plastic stick holding two gel-pad sensors (CO2 and NH3) inserted into the compost jar headspace during the test incubation; color changes are read with the Solvita DCR reader.
5 Safety and Hazards
- Biological: Compost may contain elevated microbial loads, fungal spores, and trace pathogens. Handle as a potential biohazard. Avoid inhaling dust when opening bags or scooping dry compost.
- Chemical: NH3 released from immature compost is an irritant at high concentrations. Work in a well-ventilated area when handling fresh or actively decomposing compost.
- Physical: Minor risk of glass breakage from incubation jars.
5.1 Required PPE
5.2 Emergency Procedures
If compost dust is inhaled and respiratory irritation occurs, move to fresh air. For skin contact with compost, wash thoroughly with soap and water. Refer to SOP-L002 Laboratory Safety.
6 Equipment and Materials
6.1 Equipment
| Item | Notes |
|---|---|
| Solvita Compost Maturity Test kit | Includes CO2 and NH3 gel paddles, incubation jars, scoop, and color charts |
| Solvita Digital Color Reader (DCR) | Recommended for quantitative readings; provides numerical color values |
| Incubation jars (approx. 475 mL) | Glass or polycarbonate; provided with kit |
| Timer | 4-hour countdown |
| Thermometer | For recording ambient and sample temperature |
| Lab notebook or data sheet | For recording results |
6.2 Consumables
| Item | Notes |
|---|---|
| Solvita CO2 gel paddles | One per test; store sealed at room temperature in the dark |
| Solvita NH3 gel paddles | One per test; store sealed at room temperature in the dark |
| Solvita CMI Calculator spreadsheet | Available from Woods End / solvita.com for precise index calculation |
7 Procedure
7.1 Sample Collection and Preparation
- Collect a representative compost sample of at least 500 g from the pile or windrow. Sample from the interior of the pile (at least 30 cm below the surface) to avoid surface drying artifacts.
- If the compost is very heterogeneous, collect from multiple locations and composite the subsamples
- The sample should be at “natural” moisture, not artificially wetted or dried. Compost that has been stored at 4 C should be brought to room temperature (20–25 C) before testing. Allow at least 30 minutes.
- Gently break up large clumps by hand but do not sieve or grind the compost. The test is designed for material in its as-produced condition.
Do not test compost that is above 40 C (i.e., still in the thermophilic phase). Elevated temperatures accelerate gas production and will give artificially high CO2 and NH3 readings that do not reflect the maturity of the material once cooled.
7.2 Setting Up the Test
- Fill a clean Solvita incubation jar loosely to the fill line with compost using the supplied scoop. Do not pack or compress the material. The volume should be approximately 150–200 mL.
- Remove one CO2 gel paddle and one NH3 gel paddle from their sealed foil packets
- Inspect each paddle: the gel should be uniform in color with no cracks, discoloration, or signs of prior exposure. Discard any compromised paddles.
- Insert both paddles into the jar so that the gel faces are exposed to the headspace above the compost surface. The paddles should stand upright and not be buried in the compost.
- Seal the jar immediately and securely
- Record the jar number, sample ID, compost source, ambient temperature, and start time
- Start the 4-hour timer
7.3 Incubation
- Place sealed jars at room temperature (20–25 C) in a location shielded from direct sunlight and drafts
- Do not open, move, or disturb jars during the 4-hour incubation
The standard incubation time is 4 hours. This is shorter than the 24-hour incubation used for the Solvita soil CO2 burst test (SOP-L408) because compost produces gas at much higher rates than soil.
7.4 Reading Results
- After exactly 4 hours, open the jar and immediately remove both paddles
- Read each paddle promptly. Prolonged exposure to ambient air will alter the gel color.
Using the Solvita Color Charts (visual method):
- Compare the CO2 paddle to the Solvita CO2 color chart. Record the color number (1–8 scale, where 1 = highest CO2 / least stable, 8 = lowest CO2 / most stable).
- Compare the NH3 paddle to the Solvita NH3 color chart. Record the color number (1–5 scale, where 1 = highest NH3 / least mature, 5 = lowest NH3 / most mature).
Using the Solvita Digital Color Reader (DCR, recommended):
- Insert the CO2 paddle into the DCR and record the numerical reading
- Insert the NH3 paddle into the DCR and record the numerical reading
- The DCR values can be entered into the Solvita CMI Calculator spreadsheet for a more precise Maturity Index
7.5 Determining the Compost Maturity Index (CMI)
Use the CO2 and NH3 color numbers to determine the CMI from the Solvita lookup table (printed in the kit manual) or the CMI Calculator spreadsheet:
| CMI | Interpretation | Recommended Action |
|---|---|---|
| 1 | Raw, very immature | Continue active composting |
| 2 | Immature, highly active | Continue active composting |
| 3 | Immature, active | Continue composting/curing |
| 4 | Moderately immature | Begin curing phase |
| 5 | Mid-range maturity | Continue curing |
| 6 | Moderately mature | Approaching readiness |
| 7 | Mature | Ready for most applications |
| 8 | Very mature, highly stable | Fully finished compost |
Most regulatory agencies and compost quality programs consider a CMI of 6 or above as “mature” and suitable for unrestricted use. A CMI of 7–8 indicates a fully finished product with minimal phytotoxicity risk. For sensitive applications (seed germination, potting mixes), target CMI 7 or 8.
7.6 Cleanup
- Dispose of used compost in the designated waste container or return it to the compost pile
- Wash incubation jars thoroughly with soap and water; rinse with deionized water and allow to air-dry
- Dispose of used gel paddles in standard trash
8 Quality Control and Acceptance Criteria
8.1 QC Requirements
- Run duplicate tests on at least 10% of samples (two jars from the same homogenized compost). Duplicate CO2 readings should agree within 1 color number; NH3 readings within 1 color number.
- Include a blank jar (empty, sealed with both paddles) in each batch to check for ambient CO2 and NH3 interference. Both paddles should read at or near their maximum (most stable) values.
- Verify that the compost sample temperature is between 20 and 40 C before testing. Record the temperature.
- Calibrate the DCR per the manufacturer’s instructions before each batch if using the digital reader.
- Store gel paddles sealed in their foil packets, at room temperature, away from light. Check expiration dates; do not use expired paddles.
8.2 Complementary Tests
For a comprehensive compost quality assessment, consider pairing the Solvita test with:
| Parameter | SOP / Method |
|---|---|
| pH and electrical conductivity | SOP-L402 (1:5 compost-water extract) |
| Moisture content | SOP-L400 (105 C, 24 h) |
| Organic matter (LOI) | SOP-L406 |
| Total C and N (C:N ratio) | SOP-L407 and elemental analysis |
| Seed germination bioassay | TBD |
9 Waste Disposal
| Waste Type | Disposal Method |
|---|---|
| Used compost | Return to compost pile or standard trash |
| Used gel paddles | Standard trash |
| Jar wash water | Drain |
10 References
ia, 9(2), 25–30.
11 Training Record
Before performing the Solvita compost maturity test independently, personnel must:
- Read this SOP in full
- Complete SOP-L002 Laboratory Safety training
- Review the Solvita Compost Maturity Test User’s Guide (included in the kit)
- Observe a trained operator perform the full procedure from sample collection through CMI determination
- Perform at least one test under supervision with acceptable duplicate precision (within 1 color number)
- Be signed off by the PI or lab manager
12 Appendix A: Solvita Compost Maturity Index Lookup Table
The following table reproduces the general structure of the Solvita CMI lookup. Consult the kit manual or CMI Calculator spreadsheet for the official version.
| NH3 = 1 | NH3 = 2 | NH3 = 3 | NH3 = 4 | NH3 = 5 | |
|---|---|---|---|---|---|
| CO2 = 1 | 1 | 1 | 1 | 2 | 2 |
| CO2 = 2 | 1 | 2 | 2 | 3 | 3 |
| CO2 = 3 | 2 | 2 | 3 | 4 | 4 |
| CO2 = 4 | 2 | 3 | 4 | 5 | 5 |
| CO2 = 5 | 3 | 4 | 5 | 6 | 6 |
| CO2 = 6 | 3 | 4 | 5 | 6 | 7 |
| CO2 = 7 | 4 | 5 | 6 | 7 | 7 |
| CO2 = 8 | 5 | 5 | 6 | 7 | 8 |
Note: This table is approximate. Use the official Solvita color charts and CMI Calculator for definitive results.
13 Appendix B: Interpreting Results in Context
13.1 CO2 Color Number Interpretation
| Color No. | CO2 Level | Stability Interpretation |
|---|---|---|
| 1–2 | Very high | Actively decomposing; unstable |
| 3–4 | Moderate to high | Active composting phase |
| 5–6 | Low to moderate | Curing; approaching stability |
| 7–8 | Very low | Stable; decomposition essentially complete |
13.2 NH3 Color Number Interpretation
| Color No. | NH3 Level | Maturity Interpretation |
|---|---|---|
| 1 | Very high | Strong odor; phytotoxic; immature |
| 2 | High | Significant ammonia; not ready |
| 3 | Moderate | Some ammonia; continue curing |
| 4 | Low | Approaching maturity |
| 5 | Very low / none | Nitrogen stabilized; mature |
13.3 Common Patterns
| CO2 | NH3 | Interpretation |
|---|---|---|
| High | High | Actively decomposing with unstable N; classic immature compost |
| High | Low | Carbon-rich material still decomposing but N is stabilized; may need more time |
| Low | High | Unusual; possible anaerobic conditions or recent N-rich amendment; investigate |
| Low | Low | Both carbon and nitrogen cycles complete; finished compost |
14 Appendix C: Troubleshooting
| Symptom | Probable Cause | Corrective Action |
|---|---|---|
| CO2 paddle reads 1 (max activity) after 4 h | Compost is very fresh or sample was too warm | Verify sample temperature was below 40 C; confirm composting has progressed past thermophilic phase |
| NH3 paddle reads 1 (max ammonia) | High free ammonia; fresh manure or urea input | Continue composting; ensure adequate C:N ratio (target 25:1–30:1) |
| Both paddles show no color change (max stability) | Compost is fully mature, or paddles expired | Verify paddle expiration date; confirm with a known immature sample |
| Duplicates disagree by more than 1 color number | Heterogeneous compost; inconsistent fill level | Homogenize sample more thoroughly; fill jars to consistent level |
| CO2 high but NH3 low | High-carbon, low-N material (e.g., wood chip compost) | Normal for C-rich substrates; continue composting until CO2 declines |
| CO2 low but NH3 high | Possible anaerobic pocket; recent N amendment | Turn the pile to aerate; reduce N inputs; retest in 1–2 weeks |