Great topic. I buy pumice by the half yard. My soil mixes usually have anywhere from 35-50% porosity. In Northwest climates, drainage and porosity are most important. Pumice aids in the exchange of nutrients in soil solution, as well as the free exchange of oxygen and carbon dioxide to root systems, essential for optimal root growth. The pumice I use contains minute levels of micronutrients. For bulbs, this is a BIG plus. Sand, on the other hand, is heavy, and can very well close the porous space you just created by adding a larger conditioner such as pumice. In my climate, during the rainy season, sand , when it is wetted, is nearly impossible to dry out, considering their is little sun or heat to evaporate the moisture. As for soil sterilizing, if I'm growing a crop I truly prize, I take this precaution. Sterilized soil can( and will) get reinfected, but hopefully said plants will be off to a good start before they have to use their resources fighting off disease. Rick K On 7/12/12, Jane McGary <janemcgary@earthlink.net> wrote: > Gastil wrote >>Having heard good things about pumice on this list, >>I went to our OSH and purchased a small bag of Black Gold brand pumice. >>It was about 10 times heavier than I expected pumice to be, but they >>assured me that was just because it was wet. >>At home, I noticed it had a lot of dust, which I did not think I >>wanted in my growing medium >>(not having read the note from Jane yet about the dust containing >>useful minerals). >>When I washed the pumice, I noticed it sank. I expected pumice to float. > > If the Oregon white pumice is bagged during damp weather, it will be > wet. When I buy bags of it (4 cubic feet per bag, which is a BIG bag) > in winter, the bag weighs 80 to 90 pounds; in summer it weighs about > 60 pounds. (Must remember to stock up in August!) And yes, it will > sink, even when it's dry. > > You can buy both washed and unwashed pumice here. I prefer the > unwashed product because of the available nutrients in the fines. > This pumice also raises the pH of a mostly acidic soil mix slightly. > > A geologist friend of mine once told me that mixing peat with pumice > will eventually cause the pumice to break down because of a chemical > reaction. I have not noticed this, but I don't usually keep the seed > mix (the only one with peat) more than three years. > > I would never put vermiculite in planting soil, but I have used it to > root cuttings and it's also excellent for packing bulbs for shipping, > especially if they have to be kept very slightly moist. > > It perhaps should be mentioned that many of the ingredients being > discussed in this thread need to be handled with care not to breathe > in much of the dust. I worry sometimes about developing silicosis > from using so much sand, but a recent chest x-ray (for bronchitis) > gave me the "all clear." Vermiculite is said to be hazardous in this > respect. Milled sphagnum moss (not the same as sphagnum peat) has > been implicated in transmitting a pathogen, but I don't recall the > details; I use the former occasionally when growing plants from seed > that are very prone to damping off. > > Like Peter Taggart, I used to use forest humus as the organic > component of my bulb soil -- when I had a forest. (Still have it, > please buy it!) I never bothered to sterilize it, though I did sieve > it and pick out the visible bugs. My feeling is that if you're > growing your container plants outdoors, especially in the country, > there isn't much point in sterilizing the soil. Soils for seeds > should be more sterile, and for cuttings, even more. > > Jane McGary > Portland, Oregon, USA > > _______________________________________________ > pbs mailing list > pbs@lists.ibiblio.org > http://pacificbulbsociety.org/list.php > http://pacificbulbsociety.org/pbswiki/ >