Testing the Quality of Compost

Composting is the process of converting organic waste into a valuable soil amendment through the actions of microorganisms. High-quality compost improves soil health, provides nutrients for plant growth, and supports beneficial soil life. However, immature or poor-quality compost can potentially harm plants if applied inappropriately. Therefore, testing is an essential step to ensure your compost is stable, nutrient-rich, and safe to use in your garden or farm.

This article will provide a comprehensive overview of the most important tests used to evaluate compost quality. We will cover the following key topics:

  • Why Test Compost Quality?
  • Signs of Mature, High-Quality Compost
  • Compost Maturity Tests
  • Nutrient Content Tests
  • Contaminant and Toxin Tests
  • Other Important Tests
  • DIY Home Compost Testing
  • Sending Samples to a Lab
  • Interpreting and Using Test Results
  • Improving Compost Quality

Let’s explore the world of compost testing and unlock the secrets to creating rich, beneficial compost for your plants!

Why Test Compost Quality?

Compost testing gives vital insights into the properties and performance of the finished compost. Here are some of the main reasons why compost testing is so important:

  • Evaluate maturity – Immature compost can harm plants. Testing determines if compost is adequately stabilized and ready to use.
  • Optimize as a soil amendment – Tests reveal the compost’s potential benefits for improving soil health, fertility, and plant growth.
  • Avoid plant damage – Testing prevents issues like phytotoxicity, nutrient deficiencies, or overabundance.
  • Screen for contaminants – Ensure compost is free of toxic metals, chemicals, trash, pathogens, etc.
  • Troubleshoot composting issues – Pinpoint any problems with your composting process that need adjustment.
  • Verify quality claims – Validate compost quality for commercial marketing purposes.

Bottom line – compost tests take the guesswork out of using this valuable material. Investing some time and money into testing will pay dividends through healthier soil and plants.

Signs of Mature, High-Quality Compost

Before diving into testing methods, let’s look at some telltale signs that your finished compost is likely to be mature and of high quality:

  • Dark brown or black color – Indicates a high level of organic matter decomposition.
  • Soil-like odor – Mature compost has an earthy smell, not unpleasant odors like ammonia.
  • Fine texture – Compost particles should be relatively uniform in size, not easily identified.
  • Low moisture content – Compost shouldn’t be soggy wet or bone dry.
  • No extreme heat – Piles should not feel hot, which indicates active decomposition.
  • Contains few recognizable materials – Little unfinished waste or debris remains.
  • Supports plant growth – Applying compost results in faster, healthier plant growth.

While these are good general indications, laboratory testing provides definitive proof of compost maturity and quality. Now let’s explore the specific testing methods.

Compost Maturity Tests

Determining whether compost is adequately mature is typically the first priority. Immature compost can contain high levels of organic acids and ammonia that are phytotoxic to plants. Here are some common ways to test maturity:

Radish Seed Germination Test

One simple method is the radish seed germination test. The steps are:

  • Plant radish seeds in pots filled with compost samples. Use 10 seeds per pot and at least 3 pots per sample.
  • Grow the radishes for 4-7 days and count the total number that germinate across all pots.
  • Compare to radish seeds grown in standard potting soil.
  • If 75% or more of the seeds in the compost sprout, it indicates acceptable maturity.

This bioassay tests phytotoxicity using radish seeds as an indicator species. Mature compost will have minimal impact on the germination rate compared to the control soil. It’s inexpensive and straightforward to conduct at home.

Solvita Compost Maturity Test

Solvita is a commercial test kit that measures CO2 and NH3 levels given off by a compost sample. The gases react with patented pads that change color based on the concentrations. Feed the data into an index to determine if compost is mature, curing, active, or unstable.

Solvita provides a fast, standardized maturity rating. However, the kits have an upfront cost and limited re-use compared to a bioassay. They are convenient for frequent monitoring during the composting process.

Dewar Self-Heating Test

This lab test measures the rise in temperature of a compost sample placed in an insulated container for 72 hours at room temperature. The temperature increase indicates the level of remaining microbial activity.

Compost that warms up significantly still contains a high level of easily decomposable organic matter and is immature. Mature compost shows little self-heating. The Dewar test is thorough but more time and equipment intensive.

Other Maturity Indicators

Other parameters can also indicate maturity:

  • Oxygen uptake rate – High rates mean immature compost with abundant food for aerobic bacteria.
  • CO2 evolution – High CO2 production indicates vigorous microbial breakdown of organic matter.
  • Colorimetric pH – Dark black to neutral brown colors signify stability and maturity.
  • Cation exchange capacity – Higher CEC values correspond with mature compost.

Nutrient Content Tests

Once compost maturity is verified, the next step is determining its value as a soil amendment. Testing key nutrient levels reveals the fertilizer potential of the compost.

pH Test

pH profoundly impacts nutrient availability. Most plants grow best within a slightly acidic pH range of 6.0-7.0.

  • Use a pH probe, meter, or indicator solution to determine compost pH.
  • Adjust pH if needed before application by blending with acidic or alkaline materials.

NPK Testing

NPK refers to the primary macronutrients – nitrogen, phosphorus, and potassium. These are reported as percent dry weight concentrations.

  • Nitrogen (N) promotes lush vegetative growth but can leach out quickly.
  • Phosphorus (P) aids root, flower, and fruit production.
  • Potassium (K) is key for water regulation,disease resistance, and carotenoid pigments.

Testing NPK levels allows matching compost to your plants’ nutritional needs. For example, nitrogen-rich compost benefits heavy feeding vegetables, while low-nitrogen mixes are ideal for flowering ornamentals.

Carbon:Nitrogen Ratio

The C:N ratio represents the balance between carbon-rich organic matter and nitrogen in compost.

  • Mature compost generally has a C:N ratio around 15-20:1. Higher values indicate excess woody waste and unfinished compost.
  • Very low C:N ratios below 10:1 release nitrogen too quickly, which can burn plants.

An optimal C:N ratio for plant growth is typically 15-20:1. This provides a steady nitrogen supply without excess losses.

Additional Nutrients

Testing for secondary macronutrients and micronutrients can refine your understanding of the compost:

  • Calcium, magnesium, sulfur – Major secondary nutrients.
  • Iron, manganese, boron, copper, zinc, molybdenum – Key micronutrients.
  • Sodium, chlorine – Monitor for excessive salt levels if symptoms appear.

While NPK gets the most attention, plants need the full spectrum of nutrients. Compost testing can detect deficiencies to create a balanced blend.

Contaminant and Toxin Testing

In addition to beneficial nutrients, compost can also contain harmful substances that require testing. These include:

Heavy Metals

Toxic metals like arsenic, cadmium, lead, and mercury can accumulate in composted sewage sludge or urban waste.

Check municipal, industrial, or biosolid composts for heavy metal concentrations. Excessive amounts may warrant avoidance depending on intended uses.


Composting destroys most disease-causing organisms, but a small risk remains in some cases.

  • Test for indicator organisms like E. coli and salmonella in compost destined for food crops.
  • Ensure proper high-temperature composting to eliminate pathogens.


Lawn clippings, leaves, and plant debris may carry chemical residues that persist in finished compost.

Testing is advised if compost inputs potentially contained pesticides or herbicides. Even at low levels, these can damage sensitive vegetables and ornamentals.

Physical Contaminants

While not toxic per se, physical contaminants like plastic, glass, and metal can be dangerous to machinery or plants.

  • Screen compost through 1/4 inch mesh to remove plastic, glass, and debris.
  • Visually inspect compost and discard any foreign objects.
  • Educate homeowners/collectors about keeping feedstocks contaminant-free.

With proper diligence during the composting process, physical contaminants can be minimized or avoided.

Other Important Tests

In addition to the major categories above, some other compost tests provide useful supplemental data:

Bulk Density Test

Bulk density indicates how much volume a given weight of compost occupies. Typical values range from 400-1100 lbs/cubic yard.

  • Higher bulk density composts offer more nutrients per volume. However, they may drain poorly and resist root penetration when used as a potting media.
  • Finding the ideal bulk density for your application optimizes compost performance.

Water Holding Capacity

This measures the percent water retained by compost after being saturated and drained.

  • Composts with good water holding capacity improve moisture retention in sandy soils.
  • However, too high a water content can suffocate plant roots. Test to find the ideal moisture retention properties.

Electrical Conductivity

EC determines the total salt concentration in compost based on electrical conductivity.

  • High salt levels harm germinating seeds and sensitive crops. Use EC to detect excessive salinity.

Particle Size Distribution

Screening compost through sieves of various mesh sizes reveals the range of particle sizes present.

  • A balanced mix of fine and coarse particles improves soil structure and aeration. Adjust grinding and screening accordingly.

DIY Home Compost Testing

Many basic compost tests can be performed at home using common garden tools and materials:

  • Maturity – Conduct radish or cress seed germination tests. Monitor odor and heating.
  • pH – Use a pH probe or meter, or wide-range indicator strips.
  • Moisture – Calculate moisture content percent by weighing compost before and after oven drying.
  • Salinity – Make a 1:1 compost:water extract. Measure EC with a cheap PPM meter.
  • C:N ratio – Send samples to a lab for carbon and nitrogen analysis. Estimate ratio.
  • Nutrients – Qualitatively assess NPK by plant growth. Consider inexpensive mail-in tests.

While lab methods are more precise, DIY tests allow frequent, low-cost monitoring to optimize your compost.

Sending Samples to a Lab

For standardized, quantitative data, having compost analyzed by a professional lab is advised. Here are some tips:

  • Research reputable labs in your region that offer compost testing services. State universities and extension offices are a good option.
  • Follow the lab’s guidelines for collecting representative samples, labeling, and shipping.
  • Expect to pay $30-100 per sample depending on number of tests. Costs add up with frequent analysis.
  • Define your objectives and select tests accordingly. Maturity, pH, NPK, and C:N ratio cover the basics.
  • Ask labs to interpret results and provide fertilization or amendment recommendations.

Laboratory compost analysis provides definitive quality metrics to fine-tune your process and products.

Interpreting and Using Test Results

Once you receive compost test results from a lab or DIY methods, how do you make sense of the data?

  • Compare results to ideal reference values for each parameter based on composting science literature.
  • Identify any problem areas – Is pH too high? Are nutrients deficient or excessive? Are contaminants present?
  • Relate results to intended uses – Does the compost meet the requirements of your specific application?
  • Consult with experts and adjust your composting process to achieve target properties.
  • Blend amendments like lime, fertilizer, sawdust, etc. to correct any deficiencies before applying compost.

Careful analysis of test results ensures you produce optimized compost for your soil’s needs.

Improving Compost Quality

Testing provides the feedback needed to troubleshoot issues and upgrade compost quality over time. Here are some tips:

  • Record and track test results from each batch to quantify improvements.
  • Refine carbon, nitrogen sources and ratios to achieve a balanced C:N ratio around 15-20:1.
  • Adapt aeration, moisture, and temperature to hit maturity benchmarks like the radish seed test.
  • Adjust pH through inputs and curing. Optimal is 6.5-7.0 to avoid extremes.
  • Reduce contaminants by controlling inputs and establishing sanitation procedures.
  • Screen thoroughly to remove physical contaminants and achieve desired particle sizes.

Use the testing process as a learning opportunity – your compost will become greater than the sum of its parts!

Concluding Thoughts

While compost testing may seem complicated at first, it provides invaluable insights that every composter should take advantage of. Assess maturity, nutrients, contaminants, and other key properties to ensure quality compost that radically enhances your soil. This guide equips you with all the fundamental testing knowledge and techniques you need to improve and master your compost. The future health of your garden starts under the laboratory microscope!