How Much Perlite to Add to Potting Soil?

Perlite can be found as an aggregate in many potting soils and growing media. Above all, they are used to store water efficiently, to improve soil aeration and to improve soil drainage. When used properly, perlites can help optimize the soil structure in the long term and stimulate plant growth. We explain what perlite is and how much perlite to add to potting soil in this article.

Formation of Perlites

So-called volcanic glasses are called perlite and have changed into loose rock structures through numerous weathering and transformation processes. So originally, hard obsidian became loose rock. Due to the numerous replenishment of volcanic material, perlites will continue to be formed in the future.

In the hobby and professional gardening area, only inflated perlite is used. The rock is heated up to 1000°C in industrial plants, which can increase the volume up to twenty times. Bloated perlite has a low density, is usually white in color and looks like popcorn.

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Use of Perlites in the Garden and for Pot Cultures

According to the Fertilizer Ordinance, perlites are classified as soil additives. It is not a fertilizer, but a substance that improves the soil properties or the structure of the soil. Perlites for plant cultivation are pH-neutral, low in salt and have a high pore volume.

PH value	Neutral
Origin	Volcanic
Salinity	Enormously low
Use	Cultivation, water storage, improvement of soil quality, special substrates

In horticulture, perlite is used as an additive for soil improvement or as an individual substrate for mineral potting soil. As soil improvers, perlites ensure the storage of water due to their high permeability and the relatively high pore volume (around 95%) and can help to avoid soil compaction. In addition, the aggregate helps to positively influence the aeration of the soil and prevent salt damage.

How Much Perlite to Add to Potting Soil?

Many potting soil manufacturers use perlite to ensure better water retention in their soil and to make potting soil looser in the long term. The porous surface of the expanded perlite creates space for both air and water, which among other things is beneficial for root growth. Depending on the purpose of the soil, between 5 and 35 percent are added to the potting soil. In succulent soil, a higher percentage of perlite is mixed into the substrate.

In the private garden area, perlite can be used primarily on soils that are difficult to retain moisture. This includes mainly sandy soils or those that are in areas that are not sufficiently covered by precipitation. In the case of very watering-intensive plants (deep-rooted plants), adding perlites can help to avoid damage caused by lack of water (especially on hot days). In most cases it is sufficient to mix the garden soil with 10 to 20 percent perlite.

Available Grain Sizes of Perlite

Perlites are usually available in three grain sizes:

• 0-3mm
• 0-6mm
• 2-6mm

Grain size 0-6mm can be used for growing media or for use as a soil improver. If you want to use perlite for your own mineral substrates or potting soil, the grain size 2-6mm should be selected, otherwise the substrate tends to silt up. All three grain sizes can be used to loosen potting soil.

Perlite in Stores

Despite their popularity, perlites are relatively difficult to obtain commercially. Many plant centers or hardware stores only rarely offer the aggregate. You can occasionally find what you are looking for at building material yards or composting plants. If there is no local dealer, perlite can be bought online at a reasonable price. Different container sizes are available as required, although 100 liter packaging is probably the most common and also the cheapest.

It is important to be careful when purchasing perlites. There are quite a few containers that can be used for construction projects as well as for horticulture. However, there are also coated perlites (mostly with a silicone layer), which no longer have any useful properties for soil and earth.