FlytrapCare Carnivorous Plant Forums

Although Quikrete Commercial grade (and many of their other sands) are a 99% silica sand, right out of the bag they it is NOT an acceptable potting media!

Quikrete states its commercial grade sand is a "Washed silica sand." When they say "washed" what they actually meant to say is "washed with some VERY hard water."

Suspicious of mineral burn on my plants, I recently purchased a TDS meter and individually tested each of my potting components (Sunshine brand Sphagnum Peat, Black Gold Perilite and Quikrete Commercial grade silica sand). I took a small volume of each of my components and poured distilled water over it and then stirred the mixture and then tested the water that ran out of the filter with a TDS meter.
My results were less than pleasing:
Sunshine Sphagnum Peat: ~80-85 ppm (acceptable, but not great. From now on I'm going to run distilled water through it until it runs clear)
Black Gold Perilite: ~ 3ppm (great!)
Quikrete Commercial Grade sand: ~750 ppm ( )

At this point in time I quickly stopped testing my materials and ran to the rescue of my poor plants. After several gallons of distilled water ran through the pots, I was able to get the TDS readings of the run-off down to acceptable levels (60-80 ppm).

As for Quikrete sand: You MUST wash this sand before use. I was able to get the run-off out of a coffee filter down to acceptable TDS levels with the following method:
Place the desired volume of sand into a coffee filter (depeinding on the size of your pot you should consider picking up some large coffee filters at the grocery store). I nestled the filter into the drain of my sink, but you could easily put it into a shallow cup, pretty much anything to hold it that can be easily drained. I then ran tap water over the sand, lightly stirring, until the water ran clear (initially it will be VERY murky). Once the water was clear, I discarded any residual water, and rinsed for another short while with tap water. This allowed me to get the run-off down to the ppm of my tap (~170ppm for reference). I then rinsed the sand yet again with distilled water approximately 2-3 times (filling my container, stirring the sand, dumping water off the top, then filtering some through) and the run-off came out at ~40ppm. Safe!

Cliff's notes version: Quikrete Sand: Easy to find, death out of the bag, but usable with some preparation.

Thanks for the heads up!!

I found the same problem with my pool filter silica sand that I use. I threw the empty bag away quite awhile ago and don't remember the brand name, but it wasn't quikrete brand. I thought it was fine because when I rinsed it the TDS of the filtrate was 17 mg/L, but a pink venus I planted in the mix that had this sand died in a matter of 30 days, so I decided to try soaking it for 24 hours. I soaked 35 pounds of it in 3 gallons of distilled water and mixed it once an hour for the first 4 hours and then let it soak until the next day (the filtrate off this sand never looked murky though). The results for the first 4 hours after mixing were below 29 mg/L but the result after soaking for 24 hours was 115 mg/L, so you may want to soak it for awhile and see if your reading goes up at all.

Lowes sells 100lb bags of silica sand that is rated at 17 ppm. The brand is Carmeuse. I got the 20 grit kind. Hope this helps.

Update* - After soaking the sand in distilled water for an hour the ppm went up to 25. Will do some more testing to make sure its safe.

Anything under 50ppm is easily acceptable, so I wouldn't be at all concerned about the 25. I'll look for that sand next time I'm in lowes. I still have about 90lbs of Quikrete though, but sand is a lot cheaper than the amount of distilled water it would take to clean all my sand. Unfortunately stores in my area aren't showing that brand of sand online.

Anyway, I may wash an soak some sand for a bit longer and see if the TDS reading goes up at all.

Another thing to remember is that just because the run off from the bottom of your pots is below a certain mg/L (PPM) that doesn't mean that your soil mix is the same. The run off is most likely much lower than the actual TDS of the mix.

markus13 wrote:Another thing to remember is that just because the run off from the bottom of your pots is below a certain mg/L (PPM) that doesn't mean that your soil mix is the same. The run off is most likely much lower than the actual TDS of the mix.

I'm actually going to disagree with you here. If you simply put water in the tray and sat the pot into the water then yes, but if you top water and allow water to run through the potting mix, I would argue that the water that drains would actually be a higher concentration of dissolved solids than any one part of the mix. As the water drains through the mixture, it's going to pick up salts/minerals the entire way through, from all the parts of the potting mix from your pot. We're going to play "ideal physics world" here for a second to simplify our example. Assume the potting mix has a constant amount of dissolved salt per unit volume. As the water travels through each unit volume, it picks up a certain amount of salt. The final result would be a simple integral calculus problem where the water would have a higher TDS than any single unit volume of soil. Now, outside of ideal physics land, where the water isn't going to pick up ALL of the salt contained in any volume of soil and we have concentration gradients to deal with complicates the matter, but I don't think it completely negates the concept.

Also, it seems silly to state "just because the run off from the bottom of your pots is below a certain mg/L (PPM) that doesn't mean that your soil mix is the same" because measuring the run off is really the only marginally consistent method for determining what the TDS of our soil is. I could take a small amount of my 750ppm sand and put it into 750 L of distilled water and it would appear to be a safe mix. There's no way to measure the TDS of the actual soil, because without water you have 0L of solvent, which means you have an indeterminate (divide by 0) amount of solute. I suppose if you use exactly 1L of water and you submerge whatever your "normal" potting mix consists of and then measure the TDS of that you could get a baseline for what the total amount of dissolved solute is in your entire mix, but again, this is going to be higher than any one part that actually matters in your pot. I feel that whenever we talk about acceptable TDS, everyone always refers to water that has run through and drained out of the bottom of the pot.

Just my $0.02, feel free to provide another view point.

Good to know. I'll make sure to wash it really well. Quikrete is the most readily available form of the pure silica sand for me.

I'm just going by my own observations with the mix that I use. I can run 0 mg/L TDS lab grade reagent water over my silica sand and it only has 17mg/L of TDS but if I soak it for 24 hours in just enough water to cover it and then test the filtrate the TDS is up to 115 mg/L. The same thing happened with the peat moss that I used in the past and the coir that I use now. It's because of these observations and a dead pink venus that I use Steves coir desalination steps on all parts of my medium until I get a TDS reading of <80 mg/L from the soak filtrate before any of them go into my pots. I could be wrong and making alot more work for myself by doing it, but with the cash that I've spent on plants, I would rather be safe than sorry (like my poor pink venus). The whole problem with your perfect physics world is that you leave out the most important variables like the absorption factor of your medium (it absorbs the solids left behind during evaporation while rehydrating) and the leaching factor at the end. Your assumptions are for a medium that would consist of particles that wouldn't absorb anything and just let the water flow through to pick up whatever is there and not at all like the medium I use. I'm a microbiologist/chemist by trade not a physicist. Believe me or not, that's up to you, maybe even try it yourself to see what your results are. That's just my $1.00 worth <---- inflation these days, everything costs more.

I just appreciate a healthy well informed discussion!

I acknowledged in my post that we don't live in ideal physics land (although I'd really like to get some of those frictionless pulleys somewhere) and I tried to make it clear that it wasn't a perfect model by mentioning concentration gradients. Nevertheless, I am also not a physicist by trade, but I have a pretty heavy math/physics background from my electrical engineering degree. It tends to make me want to break problems down into simplified components. Anyway, back on track... I like "safe than sorry" ways of dealing with things, and in the future I can assure you I'm going to be washing my potting media, I was merely questioning your statement because it didn't make sense to me. Your experience with your silica sand just makes me wonder if the discrepancy is just a difference in water volume and time. Obviously we can agree that soaking the sand does not actually create MORE salts/minerals in the silica sand. Maybe one way to test this would be to get equal amounts of water for two equal volumes of silica sand. Allow one set to simply sit and soak for a certain amount of time, and the other you can collect the filtrate and continuously run it through the silica sand for the same amount of time. It would be interesting to see the result. I was never trying to insinuate that one single pass of water through the pot would remove all of the dissolved solids. When I "washed" my potting media after I found out about my sand I literally had water running from the top, out the bottom of the pots constantly for about 4 or 5 hours (leaving little time for evaporation/leeching). That said, I never did that same thing with the sand alone.

"washed with some VERY hard water."

See people get mad at companies because something isn't right for their plants... but this stuff isn't even made for any plant. the stuff is used for building things. all the silica sellers ive spoken too say "don't buy the sand or i can't tell you it's ok"

I'm certainly not mad at Quikrete! The sand is more than acceptable for what it's real purpose is. I was just trying to make it clear that I believe the sand is actually washed as Quikrete claims, but whatever they use to wash it with isn't suitable for our plants. I'm not suggesting Quikrete should start washing a commercial grade product to fit our niche need!