Field Guide to Urban Gardening

Field Guide to Urban Gardening

In Field Guide to Urban Gardening, author Kevin Espiritu of Epic Gardening shares the basics of growing plants, offers tips on how to choose the right urban gardening method, and troubleshoots the most common problems you’ll encounter.

MORE INFO: Amazon

Perlite

Introduction

When you open up a bag of commercial potting mix, you expect to see little white specks in it without really questioning why they’re there. But what is perlite, really? What is perlite made of? What does it do for the soil, and is there a reason to add more?

Perlite is a form of amorphous volcanic glass, although it’s often confused by new gardeners as being some lightweight material like styrofoam. It’s occasionally called expanded pyrite and has the nickname “volcanic popcorn”, and I’ll get into why in the next segment. If you looked at a piece of horticultural perlite under a microscope, you would see that it’s quite porous. The cavities in perlite help store nutrients and some moisture that the plant might need, but drain excess water away. It is non-toxic, clean, disease-free, and extremely lightweight and easy to work with.

Perlite is often used in industrial settings as well as in the garden. It’s commonly mixed into such products as lightweight plasters, ceiling tiles, or masonry for stability or as an insulator. It’s also popular as a filtration agent, often used for filtering spent grain or other solids out of beer or in the biochemical industry.

There’s many other uses, but to gardeners, it’s an essential ingredient in their garden.

START | NEXT


Espoma PR8 8-Quart Organic Perlite

Perlite helps loosen heavy soils and prevent compaction. It aerates the soil and promotes root growth. This is 100% all natural perlite for organic gardening.

MORE INFO: Amazon

How is perlite made?

FLICKR: Maja Dumat

Volcanic glass

Perlite begins as a naturally-forming volcanic glass, a special variety which is created when obsidian makes contact with water. This type of volcanic glass has a much higher H2O content than other varieties. Like most other materials from volcanic origin, it’s in the grey to black range with some color variation, and is very dense and heavy. So why does the stuff we use in gardening appear to be white and lightweight?

Expanded perlite is formed when normal pyrite is heated. Heating perlite to a range of 1,560-1,650 °F (850-900 °C) causes the mineral to soften. As it does, the water that’s trapped in the volcanic glass vaporizes and tries to escape. This causes the glass to expand to 7-16 times its original volume, and remaining trapped air changes the color from dark to a brilliant white due to the reflectivity of the remaining water inside the glass.

This newly-created material is much lighter in weight than its previous form and has numerous crevices and cavities. It can easily be crushed with moderate pressure, but does not crumble under the light pressure exerted on it by other soils, and it doesn’t decay or shrink. It is clean and sterile.

The typical chemical composition of perlite varies slightly, as most volcanic glass does. However, perlite which is optimal for the expanding process typically consists of 70-75% silicon dioxide. Other chemicals include:

  • aluminum oxide (12-15%)
  • sodium oxide (3-4%)
  • potassium oxide (3-5%)
  • iron oxide (0.5-2%)
  • magnesium oxide (0.2-0.7%)
  • and calcium oxide (0.5-1.5%)

All of these are natural minerals, and are often part of other soil blends. It has a pH of 6.6 to 7.5.

START | NEXT


Hoffman 16504 Horticultural Perlite

Hoffman Perlite is made in the USA. It is a lightweight soil conditioner that loosens clay soil, reduces caking, and improves drainage and aeration. Blend peat moss and perlite for a custom soilless mix. Natural materials won’t upset soil pH.

MORE INFO: Amazon