Having no formal education in gardening I am always questioning what I do. Recently there was a discussion on one of the forums about vermiculite. I do not remember the specifics about the discussion but it led me to this inquiry.
I had sent this email to the Schundler Co. the largest vermiculite/pearlite producer on the east coast.
Hi my name is John and I have used vermiculite extensively in the garden and a hobby greenhouse. I have read that vermiculite does not breakdown do they mean chemically or structurally? After about three years I cannot find any of it. Should I be replacing it after so many years?
This is their response.
It is a “soft” rock which is to say it may have just been crushed to the point that you don’t recognize it. For aeration and water retention you should replenish it.
Now I get in trouble it got me to thinking. I have put literally tons upon tons of compost on my garden every year for 36 years. After all that time and compost the height of my garden has not changed. This leads me to believe that when compost is totally decomposed it must be about as thick as a layer of dust. Also I have learned that it takes a very long time for peat moss to decompose. Some of my friends from the forums are SFG purest and justly so they use the recommended mix of 1/3 peat 1/3 vermiculite and 1/3 compost with very excellent results. Can I assume if I had a 6” raised bed and filled it with the recommended mix and did not top dress it for 3 to 4 years I would end up with about 2” of peat with just a residue of compost and vermiculite? If this is true then shouldn’t you top-dress with a mixture of 50% compost and 50% vermiculite and blend/till it with the remaining mix from the previous year to maintain the integrity of the SFG?
Any comments to set my thinking straight would be appreciated