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OK, maybe not how to mix it, but how to tell if its any good after you've mixed it Ignoring for now all the chemistry like nutrients and pH.

Two of the most important things about a soil are how much water it holds and how much air it holds, especially how much air it holds when it is wet. You can easily measure these two things in the comfort of your home with just an old coathanger and a .... wait, that's a different program

You'll need four identical pots (or one pot where you can open and close the holes in the bottom), weighing scales, your soil ( ), and some water.

Fill one pot with water (no holes!) and weigh it. This is just to measure the volume of the pot. Fill one pot with dry soil and weigh it. Fill one pot with holes with soil and water it until water comes out of the holes (or use whatever method you would normally use to water your plants), then weigh it. The difference from the previous pot is the amount of water retained. Fill one pot without holes with soil and water it until its full, then weigh it. The difference from the previous pot is the amount of air which was left after a normal watering.

You now know how much water is retained by your mix and how much air remains in the mix after it is watered. I will suggest that good values are about 25%-30% water retention and 15%-20% air remaining in the wet mix. More water retention is usually good, although 25%-30% is enough for most succulents. You'll find that things like peat and cat litter absorb a lot of water. Air retention is critical to avoid succulent roots drowning and rotting. Peat and large irregular or honeycombed granules hold the most air.

Sounds easy and it is easy but there are some complications if you want to go further. You may find you can add more water if you wait a few minutes from the first watering. Some substances like peat and cat litter slowly absorb water into interior spaces and can then hold more. Some substances have very small pores and water may not easily penetrate these, so your measurement of the air spaces may be too low. You can use a detergent to reduce this effect. Also stirring the soil may help the water to fill all the air gaps. It may also turn your crumbly mix into mud, that's certainly filling all the air gaps

One last little check is to open up some holes in the bottom of your fourth pot of saturated soil. Ideally water will drain out leaving you with something like the third pot. Some soil mixes won't let go of their water for various reasons and this can be a problem. Such soils can get over-saturated if they stand in water for a while or are left in the rain to very slowly fill up.

Good info Ian. I "sticky'd" it and moved it to cultivation!

iann

I have all the info but I'm not totaly sure how to get the results... I think?

3" plastic pot = 28 grams (I used the same pot but taped up the holes to hold water weight, and released them to drain the water)
3" pot with water = 450 grams
3" pot with diatomite = 190 grams
3" pot with diatomite and water = 486 grams (holes taped so pot was full)
3" pot with diatomite and water = 346 grams (opened holes so water drained out)

Something doesn't look right to me. Anyway after 3 days it is now 320 grams. Not sure what all this means.

Ivan

Volume of pot = 422cc Sounds about right for a 3" pot.

I saw the diatomite picture in the other thread. Looks good, maybe with a little grit for weight depending on how heavy your soil is. 190 / 422 means a specific gravity of 0.45 which is somewhat less than pumice. Theoretically it would float but it probably has an open pore structure and would rapidly take up enough water to sink.

The next reading confirms the open pore structure. The saturated diatomite actually weighs more than water alone. "Solid" diatomite has about double the density of water and you could work out the percentage of the granules that is air (assuming they all filled with water) but it isn't a very useful property.

486 - 346 means 140g of "air", or more correctly 140g of missing water which is 140/422 (=33%) air in a pot that you just soaked. That's plenty, maybe even too much. Of course, unless you're planning to grow in pure diatomite it will be lower in the final mix.

346 - 190 = 156g of water in the soaked pot. This is 156/422 (=37%) water. That's a lot! A typical clay loam would hold that much water, although less air. Again, adding other ingredients in the mix will lead to a lower value.

So, very high water retention and very high porosity, sounds like a good mix for rooting but I'd want something a little more "meaty" in there for growing actual plants.

Thank-you iann

Those numbers I gave are "quick" numbers. Another words I jotted them done right away as opposed to letting the water and diatomite mix sit and soak and settle before reading. In fact I'm sure I could have added a bit more water that way once some of the air escaped. But my tape was getting wet and I would lose the water all over the scale so I just looked at it and tore the tape off to let the water run out. But I did notice once the water was out than it was out. In a lot of cases even if the water was out once you tipped the pot a bit more would come out. Let it sit and tip again and a few more drips/drops would come out again. With the diatomite this did not happen. A good thing I would think. It would allow the cactus to absorb what it needed/wanted. The rest would just sit and create humidity which again I believe is good.

Thanks again for your help and I will continue reading through posts and see what others have had success with. A grit as you noted. I would like to add one if not a couple more ingredients.

Diatomaceous earth = diatomite ?

I've used Diatomaceous earth the baked type for pool filters can get in your lungs. the raw type is probably safer. I think the PH is high around 8 but I never measured it.

Wolf Creek ranch on Ebay has a bunch of info on it. I think they eat it over there ?

I thought it would kill root mealy bugs nope good for growing Epithelanthas though

I used as much as 1/3 by volume some cactus don't like it too much so I expect it really does have a high PH ?

I use regular sandy soil and add pumice or lava rock to it, and a little Bonemeal. No peat or any store bought Mix no reason to IMO. Use sandy soil not clay the test is get it wet and see if it sticks together in a ball or just falls apart, the fall apart is the sandy soil good for potting cactus. Stick together and stain is the clay type good for making bricks.

Diatomaceous earth = diatomite ? answer no

"Diatomite is a porous rock that is made from fossilized remains of diatoms. It is high in silica, very porous and lightweight. Diatomite is very absorbant and has uneven shape and size which resists overcompaction. As reported in the September 2006 Issue of the AOS Orchids Magazine,"

Same stuff, the orchid growers are making up their own definitions again. However, you don't want the raw stuff. As a rock it is too crumbly, and pre-crumbled into a powder it is a pesticide but not a soil ingredient. The "diatomaceous earth" in soil needs to be a particular kind that is found mixed with clay and then baked at a high enough temperature to form hard pellets that don't dissolve in water. The high temperature calcining turns the clay a pinky-beige colour. Low-fired clays will remain grey or dull brown and are probably not suitable because they will crumble to dust or mud.

You can also use other forms of fired clay pellet but the diatomaceous earth kind is highly porous and makes a very nice mix.

OK

I didn't say you couldn't add powers as a major soil ingredient, just that you shouldn't Soils usually need coarse material added for extra drainage, not fine material for less drainage. Usually this applies to the rookie mistake of adding loads of fine sand.

Just checked the MSDS, which describes them as pink granules. They state a pH of 5.5, which I suspect is more marketing than anything else since they also describe them as insoluble. I've never detected any pH change from cat litter in water.

Thanks Iann, it helped go further in the question of the potting mix.

I'm testing some mixes here, using different proportions of the material I have here (which is not pumice, perlite or coir).

I've tested 4 different potting mixes (M1, M2, M3 and M4)

Here are the results:

Water storage %:
M1 = 26%
M2 = 20%
M3 = 42%
M4 = 45%

Air "storage" %:
M1 = 18%
M2 = 10%
M3 = 14%
M4 = 5%

(I think I must repeat the experiment because results for M4 does not make sense, if you know the components of it).

Anyway, given these number, based solely on this information, how would you rank the mixes from the best to the worse? Or, could you please make any comments on the % obtained?

Thank you in advance,

Marcelo

oK. Do not answer to my post above. As I found the numbers bizarre, I repeated the experiment with 3 repeats for each substrate and extracted the numbers from the mean.

Numbers changed drastically (showing the importance of repeats and a more controled experiment):

Water storage %:
M1 = 15%
M2 = 20%
M3 = 28%
M4 = 21%

Air "storage" %:
M1 = 23%
M2 = 13%
M3 = 16%
M4 = 21%

Ok, now, how would you guys rank the mixes from the best to the worse? It is possible to make any comments on the % obtained?

Thanks!

Marcelo

More air is almost always good, certainly in the range you have. How much water you want depends on your climate and what sort of plants you have. For vigorous leafy succulents, or in an arid climate, more water tends to be a good thing. For highly xeric plants with big roots, or in a very humid climate, less water would be good.

M1 would be good if you have OCD about watering. Your climate should give you a lot of room for error. I would always go for the most open mix, the one that holds most air, but I have a cool humid climate and I grow a lot of Lithops Mixes that hold less air aren't necessarily "bad", but they are less forgiving if you overwater, or water at the wrong time.

Thanks again, Iann

Now I read your reply I think I get to the Answer to the Ultimate Question of Life, The Universe, and Everything: it depends!

There is no universal potting mix, or perfect potting mix for all situations...

You mentioned different kinds of plants and climates. Well, there are also the different pot materials (plastic, clay), etc.

So I could not have asked "which one is the best", because it depends on several factors.

Well, but you also helped me understand the importance of good aeration.

And yes, I think I have OCD about watering. But I dont like M1, it is very sandy and when it dries out it shows none structure at all. I dont feel it as a good potting mix.

I have to control my OCD and try something more like M3 or M4.

Far from have ended my search for an adequate potting mix, but now I know I can relax a little, no need to panic, and just adjust what I have here, because I'm not that far from a acceptable result.

Thanks