how much perlite to mix with potting soil

How much perlite to mix with potting soil


From an address by
George (Doc) and Kay Abraham
to The Perlite Institute
April 27, 1979

Before discussing various tests that have been conducted, we have to explain why perlite is such an important soil component in today’s container grown and potted plants.

Water is a nutrient. A plant is 90% or more water but it also needs oxygen. Roots supply water and oxygen to plants (perhaps 98% of the oxygen a plant uses comes through its roots. Since both water and air (oxygen) move in, through, and out of soil mixes, both water retention and drainage, and aeration are important considerations in any potting mix.

If the soil is poorly drained, or packed due to fine organic particles, oxygen is cut off from the plant. Also, whenever a plant is watered, it forces air out through the bottom of the pot or it bubbles up to the surface. In a loose, porous soil this is fine because it creates a suction effect which draws in fresh air from the top into the air spaces in the soil.

However, if soil is too moist, or lacks proper drainage, oxygen is excluded and roots gasp for air. Also, a lack of oxygen favors high concentrations of carbon dioxide (a waste product), cutting down on root growth, and certain soil organisms, which like high CO2 concentrations, grow. In the stagnant soil these organisms produce toxic chemicals, which have a disastrous effect on the health of plants. Many gardeners discover this first-hand when they set out container-grown plants and often note that the bottom one-inch is often stagnant and smelly.

This coupled with a low pH (that is, high acidity) and a toxic form of ammonia (NH3) helps to kill plants. The role of perlite particles in a soil mix allows air spaces to form where oxygen and water have free movement.

While most growers seem to prefer coarse grades of perlite, an educational effort is needed to share with them how medium and finer grades of perlite are just as effective in preventing oxygen starvation.

General Observations about Perlite

In general, after years of testing and experimentation, several observations can be made about perlite and its use in potted plants:

1-Plots and pots containing fine, medium, and coarse perlite have had exactly the same weight and size as those using traditional peat mixes. Many tests have proved there should be no hesitation in using finer grades, and that 100% perlite could be used and be just as successful as traditional peat mixes.

2-Tests on the effects of various grades of perlite on the rooting of cuttings and in the germination of seeds found that in all cases, regardless of size, rootings were the same in time, size, and quality. In fact, finer grades required less water to maintain healthy roots. In seed germination, however, coarse grades of perlite did not do as well as finer grades.

3-Perlite also was tested for use in drying out flowers. Flowers were placed in pans of fine perlite and covered lightly. After standing for 3-6 days (or long enough for drying to occur), the flowers were removed and dusted off. Flowers also can be dried using fine perlite and a microwave (approximately 3-5 minutes).

4-100% perlite has been used growing orchids. The fine and medium grades have been tried, but coarse perlite did the best. (For more information on growing orchids in perlite see Growing Orchids in Perlite)

    Perlite is one of nature’s best media for growing plants. It does not appear to make any difference which grade is used except with certain plants like orchids.

It is possible to grow most plants in perlite alone, although usually the finer grades and medium grades will work better and require less water.

Seeds can be started in any grade of perlite, but with smaller seeds, finer grades of perlite would be recommended.

Perlite is good for greenhouse benches. And as an added benefit, insects and snails do not like perlite!

Perlite (especially the fine grade) is excellent for drying flowers.

Perlite is ideal for outdoor containers. They can be moved around easily because perlite in the mix lightens it, besides improving drainage.

Hopefully the summation of this presentation has been helpful. If you have any questions, or need more information, please look at our other plant guides, or contact us:

The Schundler Company
150 Whitman Avenue
Edison, NJ 08817

Using Perlite in potted plants and container plants with a discussion of how fine, medium, and coarse grades can be used in most situations.

Should You Add Perlite to Your Soil?

A common soil amendment for cannabis plant is a substance called perlite. This can be added to soil or coco coir to improve its air-holding capabilities and increase overall drainage ability. It’s not a requirement for cannabis growth, but it’s so useful that nearly all recommended potting mixes contain at least a little perlite.

Horticultural Perlite – A great amendment for soil or coco when growing cannabis. Perlite looks like little white rocks, but the pieces feel oddly light and airy, almost like popcorn.

Nearly All High-Quality Cannabis Soil Mixes Contain At Least a Little Perlite

A 50/50 Potting Mix of Coco & Perlite – Perlite provides more oxygen to the roots, resulting in faster growth. It also prevents nutrient buildup. This perfectly complements the ability of coco coir to hold onto a ton of water. Learn how to mix up your own coco/perlite mix!

Perlite for Growing Marijuana

Perlite is one of the most common soil amendments. It is highly recommended for any cannabis soil or coco mix that doesn’t contain some already.

Perlite appears as very light, airy white “rocks” that feel almost like popcorn.

Adding perlite increases the overall drainage ability in a potting mix, helping prevent overwatering.

Perlite helps prevent nutrient buildup which makes it a good choice when growers are giving nutrients and supplements in the water

More oxygen in your soil or coco results in faster growth. Roots love oxygen! Perlite increases the amount of oxygen available to the plant roots because it does not retain water. As a result, air pockets form around the perlite even when the growing medium is wet.

How Much Perlite to Add?

It’s recommended to add perlite so it makes up around 10-50% of the total volume of potting mix.

Add 10-20% perlite if you want better water retention and don’t plan on using a lot of extra nutrients. This is because a lot of extra perlite can cause the nutrients leach out faster from the soil as water drains through easily.

Add 30-50% perlite if you plan to use a lot of added nutrients or supplements and are looking to get the fastest growth from your plants.

I have used many types of perlite including Epsoma, Black Gold, Shultz and even Miracle-Gro perlite. Any 8-quart bag of perlite will work. Perlite can often be found in garden stores or the garden section of home improvement stores like Home Depot and Lowes. I normally advise against all things Miracle-Gro, but their 8-quart bag of perlite is okay if you can’t find anything else.

What is Perlite?

The light, almost fluffy perlite we use for gardening does not occur naturally. It is actually manufactured from expanded volcanic rock.

It all starts with lava (like the kind from a volcano!) which cools and sometimes turns into obsidian, a shiny black glass that can be mined from the ground.

Example of obsidian, which is formed out of lava

As centuries pass, obsidian absorbs water from the air. This hydrated obsidian is mined and crushed into small pieces. The pieces are then expanded by adding huge amounts of heat. Due to the high amount of water contained inside, the heat causes the perlite pieces to pop like popcorn. That is why perlite feels so light – it is made up mostly of air!

Production of Horticultural Perlite

Should You Add Perlite to Your Soil? A common soil amendment for cannabis plant is a substance called perlite . This can be added to soil or coco coir to improve its air-holding capabilities and ]]>