University of Florida
Error processing SSI file

Compost Maturity Tests

Most composters don't do any testing of their compost.  After a while, you'll get a "sense" of the look, feel, and smell of finished compost.  For uses other than top-dressing/mulch, immature (unfinished) compost may stunt or kill plants. Therefore, the grower should determine compost maturity before using compost as a growing media or incorporating compost into soils.

The simplest of testing method 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.

  1. Plant germination in extracts of compost.
  2. Plant germination in compost.
  3. Plant growth in compost.

Procedures 2 & 3 require less specialized equipment and are "easier" to complete. You may try one or all of these to determine compost maturity. 

Plant germination in extracts of compost

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:

  • A sample of your compost.
  • 1 paper cup (~ 8 oz.)
  • ½ 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

  • Step 1 - Put ¼ cup of your compost in the paper cup.
  • Step 2 - Add the ½ cup of clean water.
  • Step 3 - Allow mixture to soak for 2 hours, to make a compost soup.
  • Step 4 - After 2 hours, filter the compost soup through cheese cloth and collect the liquid (called filtrate or "compost tea").
  • Step 5 - Prepare 6 glasses by putting a piece of tissue or paper towel at the bottom of the glass and adding 5 seeds to each glass.
  • Step 6 - Place about 20 drops of clean water on the paper in each of 3 glasses (this will be your control to see if there is a problem with your seeds or your compost). Seal these 3 glasses with aluminum foil and tape. Then label the dishes "control."
  • Step 7 - Place about 20 drops of compost tea on the paper in each of the other3 glasses. Seal these 3 glasses with aluminum foil and tape. Then label the dishes "compost tea."
  • Step 8 - Keep glasses in a dark place at room temperature (around 80°F (27°C)). Observe and record germination after 24, 48, and 72 hours using the worksheet below.
Number of seeds germinated
Liquid Type Time Glass 1 Glass 2 Glass 3 Average Average %
Control (water) 24 hours          
  48 hours          
  72 hours          
Filtrate (compost tea) 24 hours          
  48 hours          
  72 hours          

To calculate Average % germination, add the total of seeds germinated per liquid type per time period and divide by 15 (total number of seeds in all 3 glasses). See the Example below:

Number of seeds germinated
Liquid Type Time Glass 1 Glass 2 Glass 3 Average Average %
Control (water) 24 hours 1 1 1 3 of 15 20%
  48 hours 3 2 4 9 of 15 60%
  72 hours 4 4 5 13 of 15 86%
 Filtrate (compost tea) 24 hours 0 0 0 0 of 15 0%
  48 hours 1 0 2 3 of 15 20%
  72 hours 2 1 4 7 of 15 47%

In this example, the filtrate gave these results:

After 24 hours, there was no germination in any of the glasses for 0 out of 15 average germination, or 0 divided by 15 times 100= 0% germination.

After 48 hours, there was a total of 3 germinated seeds in the glasses for 3 out of 15 average germination, or 3 divided by 15 times 100 = 20% germination.

After 72 hours, there was a total of 7 germinated in the glasses for a total of 7 out of 15 average germination, or __ divided by __ times 100 = __ % germination.

Interpretation of 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% germination rate is considered satisfactory.

Try your own experiment. »

Top

Plant Germination in Compost

This test will test your compost for maturity, based on seed germination rates. To complete this test, you will need:

A bucket of potting soil that has worked for you in the past. This will serve as your control group, to test that the seeds and environmental conditions did not affect your results.

  • A sample of your compost.
  • 12 planting containers (all same size), 6 for the control and 6 for the compost trial.
  • 60 seeds of a fast germinating plant, such as radishes.
  • Tape, labels, and a marker or pencil

Performing the Test

  • Step 1 - Label each of six containers with the date and then number each container in sequence: Control 1, Control 2, Control 3, etc. Then fill these 6 containers with the potting soil.
  • Step 2 - Label each of the six other containers with the date and then number each container in sequence: Compost 1, Compost 2, Compost 3, etc. Then fill these 6 containers with your compost.
  • Step 3 - Place the containers in a randomized pattern in your nursery area.
  • Step 4 - Plant six (6) seeds in each container and water them in.
  • Step 5 - Irrigate regularly to keep containers moist.
  • Step 6 - After seven days, count and record the number of seeds that have germinated in each container.
  • Step 7 - Calculate germination rate.

The following table will help you record your findings.

Media Type Pot 1 Pot 2 Pot 3 Pot 4 Pot 5 Pot 6 Total germinated Germination rate (%)
Control
Compost

Notes on how to use this table.

  • Record the number of seeds germinated in each pot, under the columns labeled "Pot 1", "Pot 2" etc.
  • Add up the number of seeds that germinated in the six pots for each media type in the "Total Germinated" column.
  • To calculate the "Germination rate (%)" divide the "Total Germinated" by 36 and multiply this number by 100. (Note, 36 is the total # of seeds planted in each media type, which in this procedure is 6 pots times 6 seeds per pot).

Interpretation of results - If your germination rate in the compost extract liquid is significantly less than germination in your control group (pure water) then the compost must be immature and need further curing.

Top

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:

Performing the Test

  • Step 1 - Your seedlings are seven days old. Rearrange the containers to set up a new random pattern of control and compost containers, and repeat this step every 7 days.
  • Step 2 - Thin seedlings to 3 seedlings in each container.
  • Step 3 - Irrigate regularly to keep containers moist.
  • Step 4 - You may fertilize once, if you want. Fertilize only half of the compost containers and only half of the control, and mark on the labels which containers you fertilized. Be sure to add the same amount of fertilizer to each container.
  • Step 5 - After 21 days, harvest the crop from each container. Record the data from each container separately. Wash roots before weighing to remove soil.
  • Step 6 - Calculate the average weight per plant for the fertilized versus the unfertilized plants in each media type.

Interpretation of 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.

« Back  to managing a compost system