pH and

The Schundler Company
10 Central Street, Nahant, MA 01908


By Bruce Schundler

There often has been confusion about the pH of vermiculite---is it neutral or alkaline, is it constant or does it change, and do some vermiculite deposits have higher pH's than others do.

Essentially the pH of vermiculite can be close to neutral but usually because of the presence of associated carbonate compounds, the reaction often is alkaline. As such, the pH of vermiculite does change, and can range from 6 to above 9.5, and the vermiculites from some mines tend to be more neutral while other mines will tend to have higher pH's. Also, depending on how vermiculite is processed (exfoliated or expanded), the pH of the expanded particles can change. After being processed, vermiculite can become more alkaline as it slowly absorbs more atmospheric moisture.

Unfortunately most of the literature and reference books available to growers and professionals is based on vermiculite that was mined in Libby, Montana by the Zonolite Company and later by W.R. Grace and Company. This mine was one of the two largest deposits in the world, and was the largest in North America until it closed in the late 1980's.

Contrary to what many thought and to some of the sales literature which had been distributed over the years, even vermiculite from the Libby deposit could and did have broad pH ranges. In one former product guide of the W.R. Grace Company, vermiculite was given a range of from 3-11 (based on intentional changes in processing characteristics.) And in third party testing by the Department of Agriculture in Maryland, there were years when this vermiculite was tested to be in the 7.6 to 8.3 range.

The problem, of course, is that many horticultural articles and reference books describe vermiculite as being "neutral" or as having a neutral pH; and growers can be confused or can make mistakes if they are unaware of the pH changes possible with vermiculite. The literature now distributed by organizations like The Vermiculite Association ( and current manufacturers state that vermiculite has a pH range of from 6 to 9.5, but none of this information seems to be getting into the hands of the writers, teachers, and growers who need to know.

The vermiculite being processed in the United States today comes from mines in South Africa, China, Brazil, Zimbabwe, and the states of South Carolina and Virginia. The pH of vermiculite from each of these mines can be slightly different, and the amount of water retained and strength of the particles can also vary. Nevertheless, once these differences are noted, they can be used successfully and easily, and have been for years and years in both this country and abroad.

To be sure, vermiculite should be checked whenever pH and changes in pH are critical. And growers should not assume the pH of vermiculite coming from the same supplier or the same source necessarily will remain constant. Just as the pH and other characteristics of peat moss can change dramatically from deposit to deposit, and from year to year, and just as water can affect pH significantly and needs to be tested regularly for both pH and alkalinity, vermiculite too should be among those ingredients for which some testing needs to be done.

Its many beneficial properties in helping to buffer mixes and in helping cation exchange capacities make vermiculite an excellent soil amendment in almost any mix. But its variations in pH need to be taken into consideration, and adjustments have to be made---especially if changes in pH and especially higher pH values would be detrimental to the plants being grown in a media!

For more information, contact your vermiculite manufacturer or supplier, or contact:

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

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