Any such thing as GOOD perlite?

[fanaticactus]

Steve, considering all the fine cactus afficianados who contributed valuable information, I'm actually very pleased about the way this thread has gone. I truly don't think I got "more that I bargained for", since I didn't have any preconceived notion (other than an answer about perlite) what I'd learn about the new thinking in potting up cacti. I'm most grateful for all the input I've received and I hope to put it all to good use. I've only got one Tephro, by the way. Like 90% of my collection after this past winter, it looked terrible. I left it alone and put it in what I thought was a suitable place in the GH, and it suddenly exploded with growth. I got it so long ago, I don't remember what mixture I used, but I'm positive there was some kind of soil in it. I was tempted to repot it but it looks so good now I don't dare. Here it is:

DSC03701.JPG (61.53 KiB) Viewed 695 times

Thanks to all the good advice you've all given me, I've just ordered 3.5 gal. of pumice. Now, as for the amount to combine with DG... I imagine it depends on the climate where one lives, but are there general guidelines or doesn't it much matter? Are we talking 50/50 or more like 70 DG/30 pumice, or somewhere between? And do I dare ask about the advisability of mixing in a dash of earthworm castings for a bit of fertilizer or sphagnum for texture and more moisture retention?

You could try 1:0 ratio of pumice to DG and your plants would do great!

DG is fine I guess and I used it to create my cactus garden in my yard, but for potted plants I use straight pumice with no problems. I'm not really sure what adding DG to a mix is supposed to do over just having pumice. I think a lot of people think they have to make a mix for mixing's sake without having a clear reason why. I'm not directing that at you Steve! I know everything you do is well thought out. That is just a general statement about people making mixes without understanding what each component is meant to do.

Saxicola, the only reason I've decided to use DG is to retain a bit more moisture than I think pumice would by itself, and it would perhaps not necessitate watering as often. I could be wrong, but it seems denser than pumice and able to hold a bit more water for a longer period. Plus, I still have one and a half large bagsful of it and it would help stretch my mixture. Pumice is not available here and I'm paying more for shipping than I am for the 3.5 gal. of pumice itself. So the less often I have to buy it, the better--as long as the new mix works well. It will be my first experience with pumice, so experimentation is the key.

[Steve Johnson]

Saxicola, the only reason I've decided to use DG is to retain a bit more moisture than I think pumice would by itself, and it would perhaps not necessitate watering as often. I could be wrong, but it seems denser than pumice and able to hold a bit more water for a longer period. Plus, I still have one and a half large bagsful of it and it would help stretch my mixture. Pumice is not available here and I'm paying more for shipping than I am for the 3.5 gal. of pumice itself. So the less often I have to buy it, the better--as long as the new mix works well. It will be my first experience with pumice, so experimentation is the key.

Sorry to catch you in a factual error, but -- the density of DG makes it less porous than pumice. In fact much less, and its contribution to water retention is negligible. Are you absolutely positive that you do have decomposed granite with all the "play sand" fines sifted out?

By the way, your Tephro looks great! Whatever you're doing with it mix-wise, I wouldn't change a thing if I were you! I'd love to show you what my pine-cone (or cat-turd, if you will ) looks like right now, but I don't want to steal your thunder. Keep your eyes peeled for a new thread from old Steve Johnson in the General forum real soon.

[CactusFanDan]

Tephro's crave abuse, you need to treat them really mean to keep them in line. I mean really mean.

[fanaticactus]

Sorry to catch you in a factual error, but -- the density of DG makes it less porous than pumice. In fact much less, and its contribution to water retention is negligible. Are you absolutely positive that you do have decomposed granite with all the "play sand" fines sifted out?

See? That's exactly why I need you guys and your advice! Not ever having had any pumice at hand, I've not been able to compare its properties and the DG's side-by-side. It seemed to me that the Oil-Dri I've been using stays moist a few days if left in a flowerpot (with drainage) in the rain. Yes, I've used Oil-Dri exclusively with the understanding it's exactly the same stuff as Napa 8822. Since I refuse to shop Walmart anymore, I went to another auto supply place and got a bag of their equivalent. I believe all of the bags have listed the contents as 100% diatomaceous earth. Perhaps it's not the same as what I've been calling "decomposed granite", but if that's just another way of saying "pulverized rock", it's yet another thing that's not available here even at our gravel companies. Now I'm getting confused again.

[Steve Johnson]

Now I'm getting confused again.

Yep -- sorry about that, but you're confusing diatomaceous earth (DE) with decomposed granite (DG). Two completely different materials. Landscapers in the southwest use DG a lot, but it's unlikely you'll be able to find it in Vermont. If you want to mix in DE with pumice, make sure the DE's structure doesn't break down after repeated waterings. It's a good argument in favor of using a test pot to do this before you put any actual cacti in it. Either that, or just skip the DE and go with straight pumice.

[fanaticactus]

I did do some pretesting with Oil-Dri and found it to be OK; and I read others' experience with it vs. Napa 8822 here on the forum and they gave it a thumbs-up too. I also have some crushed black, cinder-like material (but NOT actual cinders) that goes by various names and is used for growing live plants in aquariums. It supplies small amounts of many trace elements and is very porous (but obviously won't float!) to bring oxygen to the root system. I've also had good luck with that as an additive. It helps aerate the mix quite well.