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Can you put Perlite in Compost? Here’s When and Why

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Perlite is a naturally occurring volcanic glass that has become a popular additive for soil and garden mixes. 

Perlite is a great addition to matured compost because it helps retain moisture and nutrients. It also adds bulk and aeration and should only be used to enhance the completed product which is when the compost is fully matured. Adding perlite too early during the composting process will cause the compost to retain unwanted moisture.

Using perlite in compost will make better soil but at the right time. We will explain the benefit of adding perlite to compost as well as the best time to add perlite to ensure your plants are being provided with the best soil possible.

What is perlite?


Perlite is a volcanic glass that is mined and then heated up in a kiln which causes it to expand and create tiny pores within its structure.

The pores trap water vapor from the atmosphere, allowing the moisture to evaporate slowly out of the soil rather than rushing through the soil at high speeds like water would.

This process reduces evaporation rates and increases the amount of time plants have access to the water they need to thrive.

Reusing Perlite on Compost

Perlite is a naturally occurring mineral that has many uses including insulation, air purification, and soil amendment.

It’s often used as a filler material in potting mixes because it provides a lightweight porous medium for plant roots to grow into.

When placed in compost, perlite can potentially be able to help aeration if the compost is not being turned as often as it should. However, this may lead to some problems if used too early within the composting process. 

Remember perlite helps retain moisture which is bad for a thriving compost.

When is The Best Time to Use Perlite on Compost?

Perlite in mature compost (1)

The best use of perlite is when it is placed in compost that has been fully matured or decomposed and ready for use. 

Once your finished compost is mature, you can test its readiness by using a trowel or shovel to dig down into the center of the pile. If there’s no smell, then the compost is ready to use.

The next step is sifting.

While it’s not always necessary to sift your compost before spreading it in the garden, it makes a better planting medium without all those lumps and clumps, and also makes sure that only finished compost goes into the soil.

Most time this is when the compost has been sifted to remove the heavier stuff. The lighter composted material remains which, over time can become compact or drawn closer together. 

Adding perlite after the point of sifting will help ensure the compost remains aerated and remains beneficial for the plants on which it is intended for use.

With the addition of perlite during composting, the final product is a better potting soil and soil amendment!

When Not to Use Perlite on Compost

Perlite Retains Moisture

Soil Retains Moisture

Another reason why perlite is not recommended for composting is if the compost is still too wet. Wet compost tends to stick to the sides of containers and can clog drainage holes.

Adding perlite to wet compost can make it harder for the compost to dry out properly. If the compost is too wet, adding perlite will only cause the problem to get worse.

Too much moisture will prevent heat from escaping and will slow down the decomposition process. Moisture should be kept at less than 20 percent. 

Using perlite too son in compost will make it difficult to maintain the ideal moisture content for composting.

Perlite Can Harbor Bad Bacteria


If the perlite was not stored properly or used on plants that are infected with some sort of disease then it will affect other plants when used in compost.

Perlite is not recommended for use if the plants that were growing in it may have been infected with a disease or may have had root rot.

Root rot is caused by fungi which are easily spread through contaminated soil.

If your compost was infested with these types of pathogens, perlite could allow them to enter the plants more quickly.

How to Clean Used or Old Perlite

Perlite is made from volcanic ash, which means it has lots of tiny holes in it. This makes it absorbent and great at absorbing odors and impurities as well as nutrients within the soil.

Apart from the nutrients, the impurities like bacteria and fungal spores may also reside within the pores or even the surface of the perlite, which is why there is a need to have it cleaned before reuse.

You can soak the perlite in a solution of 1 part bleach to 9 parts water for 20 – 25 minutes and then rinse it before use.

Bleach works by releasing oxygen molecules in a process called oxidation. The oxygen molecules released by bleach break up the chemical bonds of chromophores. The changed chromophore molecules either reflect no color or a color outside the visible spectrum.

an atom or group whose presence is responsible for the color of a compound.

After the cleaning process, it can be dried off and then mixed into the matured compost pile for later use.

The Benefits Of Using Perlite in Composted Soil

Perlite is known for its ability to improve air circulation throughout the soil. It is porous so it allows gases to escape while preventing any harmful organisms from entering.

It also helps retain moisture and reduces evaporation.

This means that it keeps the roots cool and prevents them from drying out when exposed to hot weather.

In essence, perlite improves the overall health of the plant’s environment.

In addition, it increases the rate of nutrient absorption in the soil and improves plant growth.

When using perlite in soil, it is important to keep an eye on how much fertilizer you apply. Too much nitrogen can lead to excessive vegetative growth and decreased fruit production when too much nitrogen fertilizer is absorbed and retained.

The Takeaway

– Perlite is good for matured compost because it retains moisture.

– But you don’t want to add perlite to compost that is too moist.

– The final product is a better potting soil and soil amendment.

– Don’t reuse perlite if it had a plant with any type of disease.

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