Perlite in the Garden: To Use or Not to Use?

Those small white particles we have all seen in potting mixes really do have an important role to play, though we may often take them for granted. They are called perlite and work to prevent the soil from becoming compacted.

By doing this, perlite functions as an aid to maintain adequate air or oxygen in the soil, which in turn improves plant respiration. To better understand whether perlite is right for your growing operation, it is best to look a bit deeper into just what it is and what it does.

Characteristics of Perlite

Non-combustible, perlite is essentially expanded glass that is retrieved from areas where a volcano has been active. Perlite is initially quarried, crushed, and then fractionated or divided up into its various parts: those that will be capable of being expanded into useful perlite, and those that are not.

The useful form of the product will have a typical grain size between 0.14 and three millimeters. These particles will later be expanded at high heat to give the final product its airy texture.

Perlite is non-fibrous, which makes it safe for use. However, it can contain perlite dust particles, so it is best to work with it moist to prevent the inhalation of these particles.

Nutrient-rich water is attracted to the small crevices in the perlite particles, and this capillary feature is why it can also provide moisture to your plants as well as aide in providing oxygen.

When is it Best to Use Perlite in the Garden?

Whether to use perlite or not greatly depends on what type of grow system is being used. Regular soil is an obvious candidate as it is quite heavy and can become compacted. Some organics like peat moss or coco mulch can and will also compact.

This happens over time as the soil or media is watered; each particle gets closer to the others and air pores are shrunk or eliminated. When this happens, water will not drain through the grow media very well and air will not easily penetrate. The grower will likely notice a decrease in plant vigor and production, though they may not be aware of the cause.

Roots also may fail to expand or grow, and they may contract various diseases due to a lack of oxygen and overabundance of moisture. Moss can grow on the medium’s surface if it remains wet most of the time.

Perlite makes a positive contribution towards resolving these issues when used in the proper ratio. If your grow pots are sheltered from wind, perlite placed on the surface of the grow media will wick away moisture from the surface and reduce or eliminate mold and moss.

Perlite, however, is lightweight, which makes it problematic for grow systems where it might float. Deep water culture is an obvious one. With ebb and flow, the perlite particles may slowly travel towards the surface of grow pots, causing uneven distribution within the pot.

Any grow system where flooding is a part of normal operations may not be appropriate for using perlite.

Additional Tips for Using Perlite in Your Grow Medium

Though it is not a synthetic material, perlite can have a detrimental effect on the local ecological environment because it is strip mined. So, when buying perlite, it is worth researching the various firms that mine it and learn how they implement the maintenance of soil composition and ecology, and what steps they take to restore the environment. When done properly, there is no need to leave the landscape damaged after the mining process is finished.

As perlite does not hold much more than one per cent moisture, it is usually best used in conjunction with other materials. Vermiculite is a popular companion to perlite as it does hold considerably more water. In fact, vermiculite will hold from between 30 and 50 per cent of its volume in water, and 200 to 300 per cent of its weight.

When mixed 50/50 with perlite, it will do an excellent job of providing much-needed media porosity and air, as well as help to sustain plants from drought. If mixing perlite or vermiculite with peat moss, a combined ratio of between 25 and 35 per cent by volume is best.

The ability of perlite to improve aeration while it aids in providing water makes it an excellent product for many hydroponic applications. Perlite is a naturally occurring inorganic, chemically inert, and sterile product, so it does not represent a threat by adding unwanted biologics or pests into your system.

It is also an insulator so it can effectively reduce relatively high ambient air temperatures from transferring into the soil or media when used as a surface or lining product.

The question of whether to use perlite or not will first be decided by the type of system you are using. Some systems will not work well with it—mostly because it floats. Otherwise, this beneficial natural material can help to maintain a healthy root system by assuring that the soil or grow media stays porous and provides oxygen for root respiration.

Healthy roots mean greater yield and better-tasting crops. Any time you notice your grow media is becoming hard or compacted and seems to drain more slowly, you may want to incorporate some perlite into the mix.

Check out What Makes a Good Gardening Potting Mix? for more tips and tricks on mixing.