Gradations for

From the
Perlite Institute
The Schundler Company
The Schundler Company
10 Central Street
Nahant, MA 01908


By Bruce Schundler

For many years, horticultural perlite was understood to mean relatively large and often screened material. Usually it was used to provide aeration and drainage, and fine and medium grades were usually avoided.

In newer hydroponic applications, both coarse and screened perlite was initially being used and, again there was little confusion about what size or type of perlite was appropriate.

Beginning in the late 1980's and early 1990's, however, the grades of perlite being used and tested began to change. David Hall and others began working with 100% perlite media where the size and type of perlite being used was much smaller and was not screened. New horticultural studies by Hall and others began referring to fine and medium grades of perlite. Perlite Institute reports from The Netherlands in March and December of 1993, and work in hydroponics from Israel to England began referring to medium and fine grades of perlite being, and reports of turf and agricultural applications in India indicated finer grades were used. Eventually at meetings of the Perlite Institute and particularly during its Horticultural Committee meetings, there was confusion about what sizes and grades of perlite were being used and discussed in the many horticultural and agricultural uses of perlite, and in response a basic grading systems was developed.

Generally everyone in the perlite industry seemed to understand the basic parameters of "fine", "medium", and "coarse." Fines are those grades traditionally used in cryogenic insulation and fine plasters, medium grades have been used for plaster and concrete aggregates, and coarse grades are the two or three coarsest grades available from any perlite mining operation. To more carefully define these three grades, most studies have come up with sieve sizes or basic parameters, while others have just referred to "fine, medium, or coarse" grades in the hope that everyone will understand.

To avoid future problems, and misunderstandings, the Horticultural Committee of the Perlite Institute at the 1994 Mid-Year Meeting in Charleston, South Carolina approved a very basic specification for use within the perlite industry and others.

Standard Sieve or Micron SizePERLITE GRADATION
+16 mesh or 1 mm10% Max.60% Max.70% Min.
+100 mesh or 150 um60% Min.85%Min. 

When compared to the former (and now defunct) NBS Voluntary Product Standard for Horticultural Perlite (PS-23-70), these screen sizes are a little different (calling for a horticultural grade which is not as coarse as today's "coarse". Although this standard is not used anymore, we have included it here as a general reference:

Sieve Number
Cumulative Volume Retained
3/8 inch0.0% (max.)
+8 mesh 20.0 % (min.)
+20 mesh80.0 % (min.)
+100 mesh97.0% (min.)

For more information about these uses of perlite in turf grass applications, landscaping, and/or container growing, please call or contact us at:

The Schundler Company
10 Central Street
Nahant, MA 01908
(ph)732-287-2244 (fax) 732-287-4185

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