Slurry Test vs. Runoff Readings... Discuss!

Hey there folks.

I am digging and searching for more information on this topic and cant find much using the search thread, and also have some interesting observations i would like to share with community.

I have been told doing a slurry test is a more accurate way to determine what is happening inside the coco medium. As opposed to just testing the runoff. What is surprising to me is how different these readings are coming out at. For instance:

I was getting runoff readings of 2.5 EC. in veg where i feed at about .5. This caused me concern so i did some slurry tests and these readings came out at .7

The way i do my slurry is to put about a cup of medium from the middle of one of my pots into a container and then add water at a 2/1 ratio. Then i let it sit for 30+ minutes. Then i either strain the coco out, or don't strain it (i am surprised to be getting the same reading, strained or unstrained), and put my probe in and get the EC reading. It is consistently A LOT lower than my runoff reading.

Anyone know why my slurry reading would be so much lower than a simple runoff reading? This has been happening with multiple meters, in many different pots.

It is recommended to use ditilled or RO water, which if i am looking for my coco PH i do, but when checking EC i just use my .2 EC tap water and subtract that out of the reading...

This stuff interests the heck out of me, and hopefully you as well, cause i wants to learn!!!

your doin right with the slurry according to canna.. thats why people feak out when they measure the regular runoff its misleading.

Tagged ..... I'm interested to see what the pro's say too .

Likewise. Don't have much experience with hydro.

cyat said:

your doin right with the slurry according to canna.. thats why people feak out when they measure the regular runoff its misleading.

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I`m confused as to how takin a "core sample" and diluting it compares to runoff ppm`s....

I mean....core readings are needed if yas run too high ppm`s /ec . but if yas run/maintain low ppm`s across the board , the plants never store residual salts and fuck with the medium as nutrient uptake is controlled by less is more...guaranteed...also....

Runoff readings are influenced by stored cal/mag and potassium that cause readings to elevate when feedin/ flushing a medium that doesn`t need flushed for fear of fuckin up that "cation exchange capacity"........so...

Things to ponder when variables are involved....

Peace...DHF.........

runoff testing has a pretty specific protocol, North Carolina State University maintains www.pourthruinfo.com which has pdfs that explain the procedure. Pourthru works best with small pot volumes.

dhf, not sure on the details but canna (in their coco papers)say measuring coco runoff is not accurate...

they recomend a ( i think ) equal parts coco and ro to test with accuracy

I measured my bcuzz coco on my first coco run and got a high reading the regular way...

ran water thru the some pots till the ppms went down, those flushed pots never grew as good as the unflushed ones

nf coco bricks are awesome no rinsing needed.. I prefer bagged and charged esp after bad experiences wit cheap coco, some I bought would NOT rehydrate

canna and bcuzz seem similar, gold label rocks, botanicare.. nice but dries out faster

sorry for rambling

Nice discussion folks, thanks for participating. Cyat, Canna is where i first heard about the slurry test. Years ago when i first started to dabble in coco. I dont ever remembering coming across any explanation of why it was better to slurry...

DHF said:

I`m confused as to how takin a "core sample" and diluting it compares to runoff ppm`s....

I mean....core readings are needed if yas run too high ppm`s /ec . but if yas run/maintain low ppm`s across the board , the plants never store residual salts and fuck with the medium as nutrient uptake is controlled by less is more...guaranteed...also....

Runoff readings are influenced by stored cal/mag and potassium that cause readings to elevate when feedin/ flushing a medium that doesn`t need flushed for fear of fuckin up that "cation exchange capacity"........so...

Things to ponder when variables are involved....

Peace...DHF.........

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Thanks for the info DHF. Interesting stuff indeed.

Can you explain a bit more about how/why readings are "influenced" by the stored cations in the coco? I always wondered if those cations were more available to be flushed out the bottom of the pots than taken up by the roots, at times. I would venture to say i have flushed plants that gave me a high runoff reading, and did more harm than good, similar to what Cyat mentioned. I am just wondering why this is happening.

Seems to be a pretty good consensus from folks on IC that flushing the coco when the runoff EC is >.5 above the input, is a suggested practice.

I agree that running low ppm is the way, but certainly buildup can happen even running low ppm, no? Like say a pest gets down into the root zone and starts eating roots, or rot or something. Of course these are hopefully uncommon occurrences, and what your saying rings true when all is right in the room.

Thanks for sharing your experience with us.

inreplyavalon said:

Hey there folks.

I am digging and searching for more information on this topic and cant find much using the search thread, and also have some interesting observations i would like to share with community.

I have been told doing a slurry test is a more accurate way to determine what is happening inside the coco medium. As opposed to just testing the runoff.

Click to expand...

I prefer the slurry test for all media that is so testable. I've tried using run-off testing in order to make certain determinations when I'm experiencing a problem and I have yet to make success using that method. This is why I prefer the slurry method.

inreplyavalon said:

What is surprising to me is how different these readings are coming out at. For instance:

I was getting runoff readings of 2.5 EC. in veg where i feed at about .5. This caused me concern so i did some slurry tests and these readings came out at .7

Click to expand...

Yup, hell of a difference. What about pH? Wouldn't be surprised if you're able to (more accurately?) note actual conditions in the rhizosphere using this method, too.

inreplyavalon said:

The way i do my slurry is to put about a cup of medium from the middle of one of my pots into a container and then add water at a 2/1 ratio. Then i let it sit for 30+ minutes. Then i either strain the coco out, or don't strain it (i am surprised to be getting the same reading, strained or unstrained), and put my probe in and get the EC reading. It is consistently A LOT lower than my runoff reading.

Click to expand...

You do it differently than I do, and you let it set a lot longer than I have found is really necessary. I begin with the water and test that, because I want to know its basic parameters of EC and pH. Then I add samples taken from all pots (or from all problem pots), mix them, then add them to the water sample--enough to make a slurry. I let that sit for 7-10 minutes, and I've hit upon that time because I played around with letting it sit for less and more time, up to 30 minutes, and I found no significant changes occurred in readings after the 10 minute mark, 7 is usually sufficient.

Straining is not necessary because the meter can't read particulate matter if it's not electrically conducive.

inreplyavalon said:

Anyone know why my slurry reading would be so much lower than a simple runoff reading? This has been happening with multiple meters, in many different pots.

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Yes. Because you're taking a measure of what's actually IN the pot, not what can be washed out/through.

inreplyavalon said:

It is recommended to use ditilled or RO water, which if i am looking for my coco PH i do, but when checking EC i just use my .2 EC tap water and subtract that out of the reading...

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Cleanest water you can possibly use. I use RO/DI, but if I had distilled on hand I would probably use that, at least to make a comparison of results to RO/DI water.

The problem with using the tap water is that you don't have only the EC to be concerned with, if it's alkaline (as in resistant to pH shift via hardness) then your pH readings will not be anything resembling accurate. We go to this trouble in order to gain accuracy, do we not? So why would you then proceed to use a more inaccurate method? If you're after accuracy, I think that using the RO, RO/DI or distilled water.

It's just like people who drop the $$$ on a really good pH tester, to then go on to only calibrate using the 1-point method instead of the much more accurate 2-point method, or don't bother calibrating at all. Makes no sense.

Later today I will do slurry tests AND runoff tests of at least Canna and B'cuzz.

I'm interested to see what differences there might be between runoff and slurry readings.

SeaMaiden said:

Yes. Because you're taking a measure of what's actually IN the pot, not what can be washed out/through.

Cleanest water you can possibly use. I use RO/DI, but if I had distilled on hand I would probably use that, at least to make a comparison of results to RO/DI water.

The problem with using the tap water is that you don't have only the EC to be concerned with, if it's alkaline (as in resistant to pH shift via hardness) then your pH readings will not be anything resembling accurate. We go to this trouble in order to gain accuracy, do we not? So why would you then proceed to use a more inaccurate method? If you're after accuracy, I think that using the RO, RO/DI or distilled water.

It's just like people who drop the $$$ on a really good pH tester, to then go on to only calibrate using the 1-point method instead of the much more accurate 2-point method, or don't bother calibrating at all. Makes no sense.

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Ahh great reading about your technique Sea maiden! I am with you on RO being prefered, i guess i just typically concern myself more with EC readings than PH, because my PH reading with a slurry test always seem to be in range, when i have tested with RO. But you're right, why not do it the correct way if i am going to do it at all...

This statement seems counter intuitive to me...
"Yes. Because you're taking a measure of what's actually IN the pot, not what can be washed out/through."

If it can be washed out of the pot, then its in the pot, no? So i would think that some of what is in the pot has a hard time washing out, hence the runoff readings would be lower than the slurry. Or at least not more than the slurry. Because the slurry is whats actually in the pot, which is what is being forced out of the pot(some of it)... Jeepers i dont know if i am making sense or not, but i am doing my best to explain what i got going round my head!

You're making sense, the problem lies in my inability to adequately explain what I mean.

Like you, I found vastly different readings using simply run-off measures vs slurry method measures, and found that the run-off measures are usually much higher or more out of range than the slurry method. I believe, but cannot state for certain, that it's because the run-off method reads what's washed through, which may end up being a more concentrated form than what's actually surrounding the roots.

Now, I read someone posting a good method for using run-off to make adjustments, but I can't remember who or where, only that it was in the last couple of months.

A caveat here is with organics--organic molecules have differing levels of electrical conductivity, from none at all to enough to make your meter go crazy. In researching quantification of organic molecules, I discovered that, while it can be done, it's prohibitively expensive and requires multiple pieces of equipment because you must first know what molecules you wish to test for, and not all equipment can be used to perform those tests.

Am I rambling?

SeaMaiden said:

You're making sense, the problem lies in my inability to adequately explain what I mean.

Like you, I found vastly different readings using simply run-off measures vs slurry method measures, and found that the run-off measures are usually much higher or more out of range than the slurry method. I believe, but cannot state for certain, that it's because the run-off method reads what's washed through, which may end up being a more concentrated form than what's actually surrounding the roots.

Now, I read someone posting a good method for using run-off to make adjustments, but I can't remember who or where, only that it was in the last couple of months.

A caveat here is with organics--organic molecules have differing levels of electrical conductivity, from none at all to enough to make your meter go crazy. In researching quantification of organic molecules, I discovered that, while it can be done, it's prohibitively expensive and requires multiple pieces of equipment because you must first know what molecules you wish to test for, and not all equipment can be used to perform those tests.

Am I rambling?

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DISCO! This is just the kind of insight i am looking to wrap my head around. Maiden thank you dearly for taking the time to clarify and i do believe this may be the case. At least it seems logical, and i am always trying to balance what i think logically, with the often mysterious ways of this universe.

As for the concentrated runoff, it seems combined with the fact that coco and plants roots have a symbiotic relationship, it would make sense that high runoff readings are not necessarilly an indication of a need to flush. A happy medium (pun intended) exists somewhere between the two!

Interesting confirmation that organics are not so cut and dry as far as runoff readings.

I have some acquaintances whose professional lives are in the sciences, some of them pretty high-minded, if you will. I wanted a way to reliably test my organic feeds for nutrient saturation levels by methods other than volumetrically, I wanted to remove variables. The long and the short of a two or three page thread was this--first, you have to know exactly what type of molecule you want to test. Then, you need the equipment that would be able to perform that function. Apparently this kind of equipment is rather specialized, so if I want to test for, say, seaweed saturation levels in the solution I've mixed up, I have to know what molecules in that mix are most important, get that equipment to then set up to test. Which may have a bit of a lag time in returning results. And that's assuming the equipment can be afforded, quite a lot of what was being suggested as examples to me were in the tens of thousands of dollars range.

I can guess why slurry readings are lower. The probes test the conductivity between two metal posts. Slurry not only has liquid, but also a bunch of coco between the probe posts, and I would expect that to seriously affect the conductivity between the posts. I would expect that courseness/grind would also have different effects on readings.
Slurry testing doesn't seem consistent enough, especially when testing different brands.

I just tested 4 brands of coco. I used a keg cup 1/2 full, added water to form a slurry, swished it around for 30 seconds, and then tested the runoff. Each test was perfectly consistent. I repeated each test twice with the exact same results.

Once the coco has been used, runoff readings ALWAYS test crazy high IME, it seems there's always some form of buildup, so I don't test or grow by runoff readings.

Testing new coco before it is used is the valid and important data for me.

Guys I can`t touch the scientific end of things as SM and IRA are discussing , but it all makes sense but then doesn`t about this slurry thingy cuz as PA just testified after testing 4 different samples results varied and I haveta agree with the water being introduced without actually knowin what`s in the medium as far as organics and such like Sea brought up.....

Lotta good info and knowledge bein shared , but what I was gettin around to in my above post was from waaaay back when the old head method was to test the "core" of the coco directly in contact with the roots if things were wonky with the plants......IOW....

I think it`s probably on the same lines as the "slurry test" but without the slurry/addition of buncha water.....just guessing though as my scientific knowledge is limited at best....but....

Nowadays Inreply buildup`s a thing of the past with the addition of dripclean @ 1ml per gal instead of H&G`s suggestion of .4 ml...so that said.....

Never runnin over 750 ppm`s and allowing my ph to rise gradually over a weekly period from mid 5`s to upwards of 6.2 kept my rootzones and plants happy happy for several yrs before shutdown , and that`s all I can attest to that less is more and I was runnin DTW with skads of runoff each feed sequence to prevent residual salt buildup before the introduction of dripclean to the market....so....

Core samples are accurate in that they`re not diluted , but I never did any cuz I never had any problems , and I did test my runoff on many occasions "for ph " though finding that with the 15-20% runoff I used in my feed cycles that my runoff was always the same or lower than my input as it should be ftw....

Ya`ll carry on and thanks for the info although I haveta admit it`s somewhat over my pore `ol country ass head.......I just grow/grew and followed the rules and it worked repeatedly.....less is more with coco ....guaranteed....

Peace....DHF.........

you all touched on this briefly, before but its very important to use distilled or deionized water when doing runoff testing.

of course all of the various methods off soil testing produce slightly different readings but they shouldn't be varying to the extent that is being reported. Also most slurry methods use deionized water for testing purposes.

simply put the steps for proper runoff testing are as follows, or you can just download the pdfs from NCSU.

1. throughly irrigate your plant 1 hour before testing

2. place saucer under plant

3. Pour enough distilled water on the surface of the substrate to get 50 ml (1.5 oz) of leachate

4. Collect leachate for pH and EC. Make sureto get about 50 ml of leachate each time. Leachate volumes over 60 ml will begin to dilute the sample and give you lower EC readings.

5. Calibrate testing devices

6. Test sample

Arkady, are you the poster I was referring to previously, who posted on how to perform both types of tests?

puff adder said:

I can guess why slurry readings are lower. The probes test the conductivity between two metal posts. Slurry not only has liquid, but also a bunch of coco between the probe posts, and I would expect that to seriously affect the conductivity between the posts. I would expect that courseness/grind would also have different effects on readings.
Slurry testing doesn't seem consistent enough, especially when testing different brands.

I just tested 4 brands of coco. I used a keg cup 1/2 full, added water to form a slurry, swished it around for 30 seconds, and then tested the runoff. Each test was perfectly consistent. I repeated each test twice with the exact same results.

Once the coco has been used, runoff readings ALWAYS test crazy high IME, it seems there's always some form of buildup, so I don't test or grow by runoff readings.

Testing new coco before it is used is the valid and important data for me.

Click to expand...

I've tried straining the slurry mix before testing, and it hasn't made a difference, so I no longer bother with that step. Like I said before, though, I'm no longer using my EC meter because I'm running all organics. If I were using chemical salts, though, I would use the slurry test because I can't test run-off of my raised beds.

I don't think so, maybe, mostly I just like to link up agronomy website links for the appropriate testing methodology, you can still use ec and pH testing with organic soil and organic amendments, though honestly to get the most out of it you will likely have to take a soil sample down to the land grant school lab or county agricultural extension office for your home veggie beds... They will be more then able to give you a detailed analysis of the available nutrients in your soil and they should be able to provide a organic fertilizer plan for your 'tomato' crop.

Hey! I'm really growing tomatoes. And squash, and beans and corn and peppers and cauliflower and broccoli (I got some of that 'fractal' broccoli, called Veronica), and lettuce and onions and garlic and shallots and carrots and.. and... I think that's it.

I'm also already in the public eye, so "they" already know what else I grow.

In any event, I definitely prefer those types of sites to gain real information, they've proved most helpful in solving my problems.