EC for pot and watering management

[Rick]

With the revelation that the moss in some of my baskets is accumulating salts, and with the recent discussions about what makes difficult species difficult. Mike (Stone) inspired me to buy my own soil EC meter to see whats going on in the pots.

So I bought a Hanna HI 98331 direct soil EC meter (about $125 from Gemplers), and started poking around the GH.

The EC of my irrigation water is about 80 useimens/cm, while my well water is about 650us/cm for comparison

Most of the fresher baskets/pots, and baskets where roots are doing really good have EC around 200 or less. This also seemed to be the rough cutoff where live moss also seemed to be growing well too.

Some of the older baskets, with OK but sluggish plants had EC up to 600 or so us/cm. So I tried a flushing procedure to knock these down to less than 200. Amazingly it could take well over a half gallon of RO water to flush out a 4 inch basket. Time was also a factor. Just because you could fast flush out down to <200 with just a cup or 2 of water, the salts retained internally in the moss leached out salts over a few more minutes to bring the EC back up to >300. So I really needed to kill time with the pump sprayer to flush/equilibrate/flush/equilibrate for quite some time repeatedly to get stable low EC readings.

I also checked some pots still using CHC mix. For a start, the EC of air is 0, so a probe poked into a coarse airy mix of anything will not register EC. The probe tip needs to be fully "immersed" to get true readings. So you can get good readings with sopping dense moss, but not bark or CHC. So I set up the pots with CHC in small containers, and then filled them up with RO water to saturate the mix. Some of these actually cleared 1500 us/cm!! (A gratrixianum with bad roots). In comparison a good looking henryanum in spike (in old bark mix w some chc) had an EC of less than 350. Both of these flushed well (albeit slow) to get them both <200.

I'm not advocating any particular cut off EC value for any particular species, but I think this is a great way of gauging the age of a potting mix/repotting cycle or considering depth of watering. Obviously with an irrigation water of 80us, a pot EC has concentrated salts 7 to 8 times to get up to 600+. Since the EC of many fertilizer mixes are going to be in this 300 to 600 range, then its also apparent that you can mess up a pot fast with a weekly fertilizing if you don't flush A LOT and heavy between feedings.

Since much of my plants are mounted, they don't build up salts on the exposed roots, and I don't have to "flush". However, I've always tried to control my watering of potted plants to avoid "root rot", but apparently I've only been watering just enough to build up salts (during fertilizing) with the wetting/evaporation cycles, rather than flushing anything out. The other implication is that trace metals in the fertilizer are building up to toxic levels too.

So using the EC meter I might pick some cutoff levels that appear to support good growth of orchids and moss, and then regulate depth of watering or feeding rate to provide a stable EC level.

I'm already considering that the fertilizer watering should be more superficial (enough to wet the plants and conduct some water through the pot or basket, while between feed waterings are much deeper and done in conjunction with EC monitoring to ensure no conductivity buildup.

 

[Rick]

The practice of "pot flushing" is nothing new and frequently discussed, but until you get an EC meter you really don't have a clue as to how much it takes to make a difference.

 

[Rick]

Another surprise came out of a "semi hydro" pot of Phrag caricinum. It's in an 8" pot of nothing but hydroton balls, some live moss on top, and certainly a dense root network.

Earlier in the week I dumped out the sump water, cleaned the sump, and threw in a couple of cups of straight RO water (EC 0). So I was somewhat surprised to get an EC of 450. But the plant is within reach of the fogger, so maybe some well water got into it.

So I dumped out the sump, and put in a couple more cups of RO water, and within a few minutes the sump water EC was already back up to 250.

So even in a pot full of mostly inert media, there is still salt concentration to worry about.

 

[gonewild]

So even in a pot full of mostly inert media, there is still salt concentration to worry about.

I know Hydroton and leca is supposed to be inert but several years ago I did similar tests as you have just done and got similar results. Either the media is dissolving or it retains salts. In any case it is not inert as far as EC goes.

 

[Stone]

Apparently, With the course orchid type media, water doesn't remain in the pot long enough to aquire/disolve many salts before it has drained out. So for flushing, its recommended to flush intermitantly - that is go over with more water several over a period of several minutes. Or if you have the time, immersion is best. (ok for 10 plants but 100??)

 

[Rick]

(ok for 10 plants but 100??)

Exactly
Or repot and throw away the old media when you hit your limit.

Besides the time factor is the water consumption factor. I think I went through about 30 L of RO water to work over maybe 10-15 plants.

You're going to end up doing one or the other, but the beauty of the EC meter is that you can catch it before noticing a limp plant with burnt roots.

 

[Rick]

I know Hydroton and leca is supposed to be inert but several years ago I did similar tests as you have just done and got similar results. Either the media is dissolving or it retains salts. In any case it is not inert as far as EC goes.

I know that clay has a certain amount of ion exchange capacity, and the purpose of the hydroton is the porosity to help wick up water anyway, so its not totally a surprise to see some retained salts. I was just really surprised by the magnitude of it.

 

[Stone]

Another surprise came out of a "semi hydro" pot of Phrag caricinum. It's in an 8" pot of nothing but hydroton balls, some live moss on top, and certainly a dense root network.

Earlier in the week I dumped out the sump water, cleaned the sump, and threw in a couple of cups of straight RO water (EC 0). So I was somewhat surprised to get an EC of 450. But the plant is within reach of the fogger, so maybe some well water got into it.

So I dumped out the sump, and put in a couple more cups of RO water, and within a few minutes the sump water EC was already back up to 250.

So even in a pot full of mostly inert media, there is still salt concentration to worry about.

I just worked out the conversion. 1 dS/m = 1000 uS/cm so now I can better understand your numbers!
So 250 would be acceptable? but once we get over 350 to 400 we should probably take action.
With the Phrag in the hydroton, How long after fertilizing did you take the mesurement? did you irrigate with plain water in between and how many times? This will help us work out how many times we should use plain water before feeding or how dilute we should make our fertilizer or both.

 

[Rick]

So 250 would be acceptable? but once we get over 350 to 400 we should probably take action.
With the Phrag in the hydroton, How long after fertilizing did you take the mesurement? did you irrigate with plain water in between and how many times? This will help us work out how many times we should use plain water before feeding or how dilute we should make our fertilizer or both.

I can't say if 250 is better or worse than 400 without knowing the composition of the salts. That's why I put in the EC values of my well water and orchid irrigation water for comparison. Also I thought the EC field values for PK were up to 450. For another comparison, when I made up Klite in EC 80us/cm water, at 1/4 tsp/gal the EC was 700 us/cm (basically the same as my well water which has no significant nitrogen, potassium, and a teeny smidge of phosphate). My well water is mostly calcium sulfate, with a modest amount of magnesium, chloride, and bicarbonates.

My "irrigation water" is well water diluted 8-10 fold with RO. The Phrag in hydroton was fertilized a week ago. It sits in a shallow bowl, but earlier in the week the bowl was dry so I added a cup of "irrigation water". A day or so after that the bowl was dry again so I washed out all the algae, and added another cup of irrigation water. The plant is in an 8" pot, so I wouldn't call a couple tall drinking cups of water a "flushing".

So the question is:
1) is the salt buildup representative of the salt composition of my irrigation water (mostly calcium and sulfate), or
2) representative of the salt composition of my fertilizer solution (high in NPK), or
3) some combination of both?

Judging from the GH wide shift in plant growth since going low K, but reading into the analysis that I conducted on that squeeze water from the moss, I think the fertilizer is still primarily influencing the salt budget in the substrate. So for my situation I feel that EC values in that 400 or so range would be potentially stressful compared to comparable EC values of nothing but calcium/magnesium sulfate and bicarbonate.

But I don't think you can use any of my numbers as good guides for your potted systems since you use different NPK Ca/Mg ratios and a different form of irrigation water. So what I am advocating is developing your own baseline values for your fresh pots (with known healthy conditions) and some older pots with healthy roots/good growth, and compare to some of your oldest and/or less healthy plants to see if they are "salted up". Then see what it takes to flush them, while watching the meter tick down.

I'm sure after you do this a few hundred times you would develop a second sense for this without needing the meter. But after growing for over 10 years this was quite a revelation for me. And I'm going to need to water with the meter in hand for the next few months to learn a better way to water and feed.

 

[Rick]

Going back to the topic of salt sensitive or salt tolerant plants, I checked the pot EC of my Paph exul (which hasn't been repotted in at least 3 years, and is in a CHC mix).

This plant is found on the same cliffs as niveum so presumably an example of salt tolerant.

I've had this plant since 2001, and its done fairly well (except for some occasional minor bouts of erwinia). I think there are over 50 growths in the pot now. But since going Klite, the leaves are at least 25% longer than they ever were in the previous 9 or so years.

Anyway first flush RO water (last fertilizing a week ago) was in the 600+ range. But the plant looks great has lots of new growths and lots of good roots when I dig down in the substrate. So since the plant and roots look good, I just flushed it down to about 350. I'll give it another week without feeding (just irrigation water) and see what it is next week. But this plant has never been a worry so I'm not going to try to get it down to 200 or so.

 

[JeanLux]

Rick, here around I jusr can find the model Hanna HI98129! Is this still ok, or is that an older one? Jean

 

[Rick]

Rick, here around I jusr can find the model Hanna HI98129! Is this still ok, or is that an older one? Jean

Jean From what I can see on the internet is that the 198129 model does not come with the soil probe (like a thick knitting needle that plugs into the end of the unit). So the only way you can use it would be to collect enough water in a cup to stick the end of the meter into.

For substrates like bark or CHC you pretty much have to do this anyway since I'm finding the soil probe needs total contact (no air) to get a reading. So for the pots with CHC or bark that model would be fine. Just but a shallow dish under your pots and pour some RO water through (maybe 3 or 4 times) and check the value in what you collect.

With the soil probe attached, I could find a container just big enough for the pot, set it in, and fill the pot to the substrate surface with water. Wait a few minutes then push the probe in and read. But I think you should be able to come up with a procedure for the older unit.

 

[JeanLux]

Thanks Rick!!!! and I suppose that the calibration sets offered with that unit are needed too! Jean

 

[keithrs]

Thats the same meter I use.... There is an attachment for soil app. for this meter.... Last I know of anyway.

All interesting info.!!!

 

[Rick]

Thanks Rick!!!! and I suppose that the calibration sets offered with that unit are needed too! Jean

The calibration solution appears to be the same as I use for our standard meter in the lab. A potassium chloride solution with EC of 1413us/cm. It should be cheap enough, but if you want a cheap standard, and you have an accurate gram scale, then put a gram of NaCl in a liter of water and that give you a conductivity of ~2000 us/cm

 

[Rick]

Thats the same meter I use.... There is an attachment for soil app. for this meter.... Last I know of anyway.

All interesting info.!!!

If the port for the probe is in that meter, then just need to look up the probe catalog number (I'm not near the instruction sheet at present).

 

[Stone]

A few important points when reading EC!
At the risk of copyright I'll quote a few lines from Growing Media by Handreck and Black:
The EC of a solution will be mesured to decrease as its temp. falls so its strongly recommended to use a meter with Automatic Temperature Compensation. or at least one with a dial to make adjustments.
If its not available you need to convert the reading to the standard 25C by adding 2% for each 1C that the solution is below that and subtracting 2% for each 1c it is above 25c.

The EC of a medium cannot be measured directly, so you must make an extraction of with water and the EC of the water measured.

Obviously the EC will be different with different amounts of water used so you must use a ''standard'' amount to get usable results.

Use distilled or deionized water to mix with the medium or if you use other water, measure the EC of that and subtract that from the reading.

They give 2 methods which they consider best:

Saturation extract.
After reading that, its too complicated!!

1;1.5 volume extract (Standard method used by Australian labs.)
Thats also too involved!

Here we are!! Pour through technique for mix in pots!
Not as accurate but good enough or us

1. Two hours after normal irrigation, select several pots for testing
2. Suspend each pot over a dish or saucer of a diameter that's a little larger than the base of the pot
3. Pour onto the surface of the pot enough water to give about 50mL of drainage Do this slowly so as to allow the added water to move through the mix to displace some of the water already there.
4. Remove the pot and take a reading.

Pour through extracts give EC values about 1.5 times those using saturated method which is supposed to be quite accurate, so I guess you need to know the methods used when your trying to duplicate something and you can regard your reading as 1.5 times what the roots are really seeing?

My only problem with this method is that with such course particles we use in orchid mixes, does the water remain in the pot long enough to get a accurate idea of the real EC of the mix?
The other methods involve removing the mix.

 

[Rick]

The other methods involve removing the mix.

If you get the meter with the direct soil read probe then you can plug a pot (or insert into a close sized water tight container) fill the pot with RO water to saturate the mix, and start reading direct from the pot.

Take the readings over time until they stabilize, and your good to go:wink:

 

[Rick]

Obviously the EC will be different with different amounts of water used so you must use a ''standard'' amount to get usable results.

I take it you mean the volume and rate used to pour through the pot.

The EC of a solution is the same regardless of the volume. But yes, the transfer and dilution of salts from media into solution is volume/rate dependent.

Also the rate of transfer is quite different from one type of media to the next, and could be very slow.

The EC of the pour through solution of those hydroton balls was only about 250 or so immediately (or within "standing around time") after the pour through. But I left the pot standing in the pour through water overnight, and it went back up to 450 by the next day.

 

[keithrs]

If the port for the probe is in that meter, then just need to look up the probe catalog number (I'm not near the instruction sheet at present).

Come to find out its a pH probe not and EC probe.