Hands sift through compost ready for use

COMPOST MATURITY TESTS

Many veteran composters do not test their compost, instead drawing on their experience to know when compost is ready based on its look, feel, and smell. Inexperienced composters should remember, though, that immature (unfinished) compost may stunt or kill plants, and should determine compost is mature before using it as a growing medium or incorporating it into soils.

The simplest test is to put your compost in a couple of pots and plant some radish seeds in the compost. If 3/4 or more of the seed sprout and grow into radishes, then your compost is ready to use in any application. Radishes are used because they germinate (sprout) and mature quickly.

If you want to conduct more scientific tests of your compost, follow the three simple procedures outlined below. Note that procedures 2 and 3 require less-specialized equipment and are "easier" to complete. Any of the procedures will determine compost maturity. Click any item below to learn more (show all items).

1. Plant germination in compost extracts

This test will help determine if your compost is "finished." Germination is when a seed sprouts. Immature compost may contain phytotoxins that will often kill seed embryos. Seeds grown in immature compost won't sprout or may die immediately after sprouting. To complete this test, you will need:

  • sample of your compost
  • 1 paper cup (approx. 8 oz)
  • 1/2 cup clean water
  • cheese cloth
  • 1 eye-dropper
  • 6 clear glasses
  • aluminum foil
  • 6 pieces of tissue or paper towel
  • 15 seeds of a fast germinating plant, such as radishes
  • tape, labels, and a marker or pencil
Performing the Test
  1. Put 1/4 cup of your compost in the paper cup;
  2. Add the 1/2-cup of clean water;
  3. Allow mixture to soak for two hours, to make a compost soup;
  4. After two hours, filter the compost soup through cheese cloth and collect the liquid (called filtrate or "compost tea");
  5. Prepare six glasses by putting a piece of tissue or paper towel at the bottom of the glass and adding five seeds to each glass;
  6. Place about 20 drops of clean water on the paper in each of three glasses (this will be your control, to see if there is a problem with your seeds or your compost), seal the glasses with aluminum foil and tape, and then label the dishes "control";
  7. Place about 20 drops of compost tea on the paper in each of the other three glasses, seal the glasses with aluminum foil and tape, and then label the dishes "compost tea" and
  8. Keep the glasses in a dark place at room temperature (around 80 degrees F, or 27 degrees C), and then observe and record germination after 24, 48, and 72 hours using. You can use our prepared worksheet or create your own.

To calculate "Average %" germination, add the total number of seeds germinated per liquid type per time period and divide by 15 (total number of seeds in all three glasses), then multiply that result by 100 to yield a percent value. As example, if your sample showed one seed germinating in Glass 1 after 24 hours, three seeds in Glass 2 after 24 hours, and 1 seed in Glass 3 after 24 hours, the "Average %" germination for 24 hours would be: (1 + 3 + 1) / 15, or 0.2. Multiplying by 100 yields a 20 percent germination rate. You can see a full example at the bottom of our prepared worksheet.

Interpreting results

Phytotoxins in immature compost will often kill seed embryos. Compare germination rate in compost extract liquid to germination rate in the control group; 80 percent germination is considered satisfactory.

2. Plant germination in compost

This test gauges compost maturiety based on seed germination rates. To complete this test, you will need:

  • bucket of potting soil that has worked for you in the past
  • sample of your compost
  • 12 planting containers (all same size): six each for the control and compost
  • 60 seeds of a fast germinating plant, such as radishes
  • tape, labels, and a marker or pencil
Performing the Test
  1. Label each of six containers with the date and then number each container in sequence (e.g., Control 1, Control 2, Control 3, …);
  2. Fill each of the containers with the potting soil;
  3. Label each of six remaining containers with the date and then number each container in sequence (e.g., Compost 1, Compost 2, Compost 3, …);
  4. Fill each of these containers with your compost;
  5. Place all containers in your nursery area, with no pattern (e.g., random);
  6. Plant six seeds in each container and water them;
  7. Irrigate regularly to keep containers moist;
  8. After seven days, count and record the number of seeds that have germinated in each container; and
  9. Calculate germination rate, as follows:
    1. Count the number of seeds germinated in each pot
    2. Add up the number of seeds that germinated in the six pots for each media type
    3. Divide the total number of seeds germinated per media by 36 (for six seeds in each container), and then multiply by 100 to get a "Germination rate (%)."
Interpreting results

A germination rate in the compost extract liquid markedly lower than the rate in the control group indicated the compost is immature and needs further curing.

3. Plant growth in compost

This test helps to measure the quality of the compost and to see if it is providing nutrients to plants or, potentially, robbing nutrients from plants. To complete this test, you will need the containers and seedlings started in the "Plant germination in compost" test. Note: these seedlings will be y days old.

Performing the Test
  • Trim samples to leave just three seedlings per container;
  • Irrigate regularly to keep containers moist;
  • You may fertilize once, if you want, but fertilize no more than half each of the compost and control containers, taking care to add the same amount of fertilizer to each container;
  • After 21 days, harvest the crop from each container, wash the roots to remove soil, weigh the sample, and record the data from each container separately;
  • Calculate the average weight per plant for the fertilized versus the unfertilized plants in each media type; and
  • Arrange/rearrange your containers to set up a new random pattern of control and compost containers, and repeat this step every seven days.
Interpreting results

Poor plant growth indicates that the compost is unfinished. Microbes are using nutrients in the compost to decompose the organic matter, and, therefore, nutrients are not yet available to the plants. To compensate for nutrient deficiency, fertilize the plants with a high nitrogen inorganic fertilizer. If plants seem healthy and weights are equal or better than the control, then the compost is probably ready for use as a potting media.