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.
- Plant germination in
extracts of compost.
- Plant germination in compost.
- 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 |
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6 clear glasses |
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Aluminum foil |
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6 pieces of tissue or paper towel |
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15 seeds of a fast germinating plant, such as
radishes. |
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Tape, labels, and a marker or pencil |
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 |
|
|
|
|
|
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48
hours |
|
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72
hours |
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|
|
|
|
Filtrate
(compost tea) |
24
hours |
|
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|
|
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48
hours |
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72
hours |
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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.
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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
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 |
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Compost |
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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.
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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 7
days old.
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.
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