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Water Quality for Orchids
by Aaron J. Hicks
with thanks to Robin (no relation to Batman _or_ Bruce Ward) Garrel,
garrell@chem.ucla.edu
5/21/98
v1.1 (high-strength edition, but still working on it!)
v1.2 (not on drugs, but I dare you to prove otherwise)
v1.3 (who knows when or what I modified for this one)
v1.4 (added Bob Hamilton's stuff on ppm to uS)
v1.5 (added more stuff)
Orchid water quality is a subject that is discussed often in the
orchid realm, typically with large shovels and great force. Hopefully,
this will serve to *snork* muddy the waters, and cause great havoc to be
established.
What is pure water?
Water. Two party hydrogen, one part oxygen. Combine them in the
presence of a high-energy spark, and you get water plus a LOT of energy.
Ever senn pix of the Hindenburg going up? Same deal. Ditto with the
Space Shuttle 'Challenger.' Many rockets use liquid hydrogen and liquid
oxygen as their fuels; very high impulse, and the only "waste" product
is nice, clean water.
Pure water has a high electrical resistivity; although if you were
taking a bath, and someone dropped in, say, a toaster, if there was not a
ground fault interruptor, you would be dead in a fairly short period of time.
Pure water, on the other hand, has no dissolved salts, which are what makes
water conductive to electricity. It also has no chlorine, which many people
regard as a nuisance to orchid culture.
Pure water can be created through distillation. This involves
evaporation, followed by condensation. This can be done many ways (solar;
gas or electric heat; freeze-drying), but is generally done with electric
heating coils. The primary problem with this is that there are a fair
number of organic compounds that may be contaminating the water in very
tiny concentrations that have vapor pressures much lower than that of
water; these will boil off along with the water, and be concentrated in
the condensate. These chemicals (such as benzene, toluene, xylene and
other organic solvents) are readily removed by running the condensate over
an activated charcoal filter, or by distilling the water over an oxidizing
agent, such as potassium permanganate.
Activated charcoal is ma nature's own filtration system. AC is
made from charcoal that has undergone special thermal fracturing to provide
a compound with IMMENSE surface area; these surfaces adsorb (not ABsorb-
ADsorb, which means compounds stick to the surface- and recall that the
AC has huge surface areas) contaminants. Typically, these filters will
remove only organic compounds. There has been a recent development in
AC filter technology with the advent of compressed AC filters. Rather than
trickling water over charcoal nuggets, these new filters use a cartridge
composed of a "hockey puck," if you will, of compressed activated charcoal.
These have relatively low void space, and a high efficiency. Some research
indicates these reconfigured activated charcoal filters may remove lead
and some other contaminants that simple AC filters do not.
What about water filters that use simple AC filters?
These will remove the organic contaminants, chlorine, and some other
"nuisance" chemicals. They do NOT remove calcium and magnesium carbonates
(more on this later!), and probably do not remove lead.
Water hardness- wazzat?
Hardness is determined by the amount of dissolved salts. Typically,
these are calcium and magnesium carbonate (CaCO3 and MgCO3). These are
highest in areas where water is taken from deep subsurface sources, from
areas with large concentrations of limestone and dolomite at the surface,
or both. Typically, water that is very hard does not make "suds" very
readily when you are lathering up in the shower, or washing your clothes.
Special softeners must be used. These replace the calcium and magnesium with
sodium from sodium chloride. There is some evidence that this extra salt is not
good for people (high blood pressure), and there is a lot of discussion as
to whether or not this is bad for orchids or not. Certainly it has _some_
effect, but whether this is significant to the hobby grower or not is best
left to the individual (and their pocketbook) to decide.
Reverse osmosis
Remember high school biology? Remember osmosis?
Osmosis works like this: if you have two salt solutions, separated
by a semi-permeable membrane (such that water can move through, but not
the salts), the water will move to the more concentrated solution, diluting
it until the salt solutions are of equal concentration.
Well, not quite. If one forms a high column of water, therefore
creating a lot of back-pressure, the water will tend to be "pushed"
whichever way the lower pressure is.
So. If we take tap water, and push it across a membrane, we will tend
to push through pure water, and concentrate the salts on the one side of
the membrane (this is simplified; bear with me). Out of one side (the pressure
side) comes slightly concentrated salt solution (tap water that has been
concentrated), and the other side comes clean water. It's not absolute,
though; you'll remove a LOT of the salts, but not all. Plus, the filter
membrane has to be replaced periodically to produce best results.
Contaminants
Tap water is an awful thing to give to your orchids.
At the very least, some people regard this as gospel, and cite
fantastic results after switching over to RO for ALL their plants, or
at least for sensitive plants. Most of these people live in areas with
very hard water, or have water with high levels of chlorine, rust and
other "nuisance" compounds.
Some orchids do not particularly mind these compounds; it's a matter
of tolerance more than anything else. In general, plants with smaller roots
are more susceptible to injury by salts accumulation, chlorine and other
damaging compounds. In particular, terrestrial plants like paphiopedilums
and phragmipediums, with their "hairy" roots, have extremely high surface
area to mass ratios, and are readily damaged by getting "burned" by salts
accumulation. Typically, these plants manifest symptoms with browned leaf
tips and sometimes reduced growth. Many people will choose to use RO or
DI (distilled) water for these plants.
If you have hard water, it is not always necessary to switch to
treated water. Rather, it may be easier and less expensive to flush the pot
each time you water.
Let's say you water a plant with regularity, pouring in water until
a little trickle runs out the bottom. Eventually, as the plant and evaporation
removes all the water, the salts precipitate out and settle in and on the
media. Next time you water, the same thing happens. Eventually, the plant
becomes "poisoned" with the built-up salts; this is compounded by the
addition of fertilizer, which is (after all) mostly salt.
On the other hand, if you add so much water that the pot is flooded,
and plenty of water runs through the pot, it helps to flush out some of the
salt that has accumulated each time you water. Run PLENTY of water through the
pot, and you remove some of the built-up carbonates and sulphates which can
stunt the growth of a plant.
Terminology for water analysis
Contaminants are generally expressed in ppm (parts per million), or
ppb (parts per billion); rarely used is ppt (parts per trillion). A part
per million is 1 milligram in a liter of water (about a quart). A ppb is
1/1000 that, and a ppt is 1/1,000,000 that of a ppm. Both ppm and ppb can
be in units that represent weight for weight (milligrams of compound per
kilogram of solvent) OR weight for volume (milligrams of compound per liter
of solvent). For water, the difference is negligible for our purposes: one
liter weighs almost exactly a kilogram.
pH
pH stands for "potential hydrogen." It is a logarithmic scale of
hydrogen concentration (the difference between pH 4 and 5, for example, means
that there is 10 times as much hydrogen ion between the two). BUT... it's
_inverse_, so there is 10 times as much hydrogen with pH 4 than there is
with pH 5.
pH = - log [H+]
where [H+] is the concentration of hydrogen ions, also known as
protons to us science dudes.
pH goes from 1 to 14 (and, actually, it goes into negative numbers
as well as number over 14). Neutral is 7.0, and pure distilled water is
supposed to be at this level.
There is a complicating issue: air contains carbon dioxide, which
combines with water to form carbonic acid:
CO2 + H2O <--> H2CO3
This lowers the pH of distilled water in air to 5.6, thanks to the
ambient carbon dioxide in air. This is _very_ mildly acid; it's the reason
caves like Mammoth Cave in Kentucky have been formed- but it takes a few
million years to make 330 miles of passageway.
Rainwater is normally at this pH (5.6); so, if we collect rainwater,
and it *seems* mildly acidic, this is natural. Rainwater (neglecting the
famous "Acid Rain" scare) should be from between about 5.2 ot 5.3 and, say,
5.7 or so. Although this is acidic, it is not truly "Acid Rain." There are
reports of rainwater in West Virginia, in heavily industrialized areas,
having a pH comparable to that of soda (Coke is around pH 1.5; depends upon
who you ask and how flat the soda is!).
Fortunately, almost nothing is made in the United States anymore,
so acid rain is pretty rare.
"Typical" values of ions from groundwater:
(in ppm)
Florida Pennsylvania
Ca +2 34 83
Mg +2 5.6 17
Na + 3.2 8.5
K + .5 6.3
HCO3 - 124 279
SO4 -2 2.4 27
Cl - 4.5 17
NO3 - .1 38
SiO2 12 ----
pH 8.0 7.36
(from Langmuir, 1971; Back and Henshaw, 1970)
Higher calcium ion (Ca +2) levels indicate hard water; the same is
true with magnesium (Mg +2). Paphiopedilums and phragmipediums actually LIKE
these minerals, and do well with relatively high concentrations. These ions
will be higher in areas with large amounts of karst topography- areas where
limestone and dolomite prevail. All plants need these ions, and unless these
figures are _extremely_ high, will not harm them in and of themselves. If
they are allowed to accumulate, like anything else, they _can_ hurt plants.
Levels depend upon how sensitive the plant is, of course.
If Na + (sodium) and Cl - (chloride) ions are very high, and you
live near the coast, you may have drawn down your well far enough to
start taking in seawater. Other ions will also be high, but these two are
considered key. The only solution (pardon the pun) is to back off on pumping
to prevent a seawater "lens" from infiltrating your home supply.
F- (fluoride ion) is known to the public as the sodium fluoride
that is added to commercial water supplied and toothpastes to help prevent
dental carries. It is a very strong oxidizer, and is chemically related
to chloride, bromide and iodide. In high concentrations, it is toxic. As
a cute little side-note, the discovery that areas with naturally occurring
high concentrations of fluoride coincided with areas having relatively low
numbers of dental cavities was the impetus for addition of fluoride
to commercial water supplies- arguably the first medical use of geochemistry.
NO3 - (nitrate) naturally occurs in many areas, but only in
small concentrations. In higher concentrations, it can cause harm to
humans (blue baby syndrome). It is most commonly found in agricultural
areas where water supplies have been contaminated by fertilizer runoff.
Does not harm plants, unless _extremely_ high.
SO4 -2 (sulphate) is also naturally occuring; typically high in
groundwater. Can damage plants in high concentrations. Calcium sulphate
(CaSO4 * 2H2O, as a hydrate) is also known as gypsum, and is one compound that
people use to help supply their Paphs and Phrags with more calcium.
pH adjustment
To adjust pH DOWN: add vinegar (acetic acid), or nitric acid.
Nitric acid has the beneficial effect of adding nitrate to your water
supply, which your plants can use. Phosphoric acid will also work, and has
the added benefit of adding phosphorous (see section at the end on this).
To adjust pH UP: add sodium hydroxide, also known as lye, or as
"caustic soda." Very nasty. It is a primary constituent of Drain-o and other
household plumbing relief compounds, but not the _only_ one: most contain
aluminum and special compounds to keep the pipes from being directly attacked
by the caustic soda. If left in contact with your skin (particularly wet skin),
this stuff will turn your tissues to soap. Very nasty. Some people advocate
the use of potassium hydroxide, which has the pleasant property of adding
potassium to your water, which your plants can use (see same section at the
end of this document). Potassium hydroxide is just as nasty as sodium
hydroxide, and carries the same use warnings.
Sodium bicarbonate (plain old Arm and Hammer baking soda) will
also serve to raise solution pH, and isn't as nasty as the hydroxides are.
GFI's
Ground Fault Interruptors (GFIs) are a MUST-have if you are going
to put electricity in your growing area/greenhouse.
A GFI works by detecting if any electricity goes from the circuit
to your electrical "ground," which would indicate current is flowing
somewhere it shouldn't. If this is the case, the GFI automatically kicks
off the current in a fraction of a second- enough to get a quick shock,
but (hopefully) not enough to kill you. Remember that, if it goes in
and out the wrong places (i.e., through the heart), it only takes a
few milliamps of current to kill. The old radio operator's saying, "It's
the volts that jolts, but the mills that kills!" holds true.
These are commonly included in most any areas around water in
modern construction; they can be found in bathrooms, around pools, and
so forth. Typically, they have a "TEST" and "RESET" button. As with
fuses and breakers, if you trip the circuit, find the specific cause
for the interruption and correct it before resetting it. Test them at
regular intervals, as regularly as the manufacturer suggests. They are
TRUE lifesaving devices, and should be a part of everyone's greenhouse if
they have any current flowing through them.
Tabular data from Barnstead Catalog:
Conductivity Resistivity Dissolved Grains per
uS/cm, 25' C at 25' C Solids, ppm US gallon
=============================================================== ================
0.055 18.3 megohm/cm 0 0
0.063 16 0.00363 0.0002
0.071 14 0.00776 0.00045
0.083 12 0.0133 0.00078
0.1 10 0.0210 0.0012
0.5 2 0.206 0.012
1.0 1 0.44 0.026
10.0 0.1 4.6 0.27
100 0.01 47 2.7
200 5000 ohm/cm 91 5.3
1000 1000 ohm/cm 495 29
10000 100 ohm/cm 5400 315
20000 50 ohm/cm 10800 631
Date: Sat, 18 Apr 1998 14:56:20 -0700 (PDT)
From: "Robert M. Hamilton" <bob@argon.eecs.berkeley.edu>
Subject: [18353] uS to ppm
>Sun, 05 Apr 1998 17:40:57 -0400
>From: Onorio Farina <farina@sprint.ca>
>Subject: [18258] Fw: Water chemistry
>Can someone tell me how to convert ppm to uS. I can find references in
>articles pertaining to EC but I have not been able to find a table to
>convert that number to microSiemens.
Terse Answer:
Simple conversion methods for fairly accurate values in the ranges used in
horticulture are:
uS X .64 = ppm
1.57 X ppm = uS
Some history:
The Siemen (S) is the current International System of Units (SI) term
for conductivity. Conductivity is the ability of a material to conduct
electricity. The Siemen is the reciprocal of the resistivity unit, Ohm.
Previously, the Siemen was known as the "mho" (Ohm backwards) because
conductivity is the reciprocal value of resistivity -- cute! (Note:
International Scientific Units are named to honor scientists. When
abreviated, these units are capitilzed while scaling factors remain lower
case; hence, uSiemens for millionths of a Siemen. Another common example:
kW for one thousand Watts.)
In horticulture, conductivity is used to measure the purity of water and
to measure feed strengths when fertilizer is added to water. Units which
make sense for horticulture measurements are millionths of Siemens or uS.
To confuse matters the horticulture industry often refers to conductivity
in a bastard unit called the EC (short for "electrical conductivity" or
"earth conductivity" depending on which side of the Atlantic you are on).
An EC of 1 is equivalent to 1000 uS or 1 mSiemen.
Pure water has exceptionally low conductivity. As chemical compounds
(salts) are added to water, conductivity increases (up to a point).
Excellent tap water has a conductivity of about 50-100 uS. A meter which
measures conductivity does not directly measure parts per million (ppm)
however an approximate equivalent can be determined. Altough you find
"ppm" meters on the market these are really conductivity meters with a
scale reading in aproximate ppm.
The reason you cannot directly measure ppm's with a conductivity meter
is because different combinations of salts in water have different
conductivities. For instance 10 ppm of sodium chloride in pure water
has a different conductivity than 10 ppm of calcium carbonate. This is
not much concern for plant growers because we can use an average value
which works fairly well.
I grow cool growing Andean orchids and their hybrids. My tap water has
a conductivity of ~80 uS (good water). I add enough fertilizer for a
conductivity of 700 uS. I feed my plants every watering and they grow
very well at this feed strength and I do not have any leaftip dieback
which I can attribute to feed strength.
I find a conductivity meter a good tool for growing.
Bob Hamilton
------------------------------
[Ed. note: I have never grown conductivity meters]
Date: Wed, 29 May 1996 08:53:22 -0400 (EDT)
From: brneise@andrews.edu (Harvey Brenneise)
Subject: [8827] Raising pH, fertilizers, etc.
Alan,
DynaGro markets a product called pH Up. Also their ProTeKt product is a
powerful alkalizer.
For calcium for paphs (but not for phrags), I use dolomitic limestone
(60% calcium carbonate) in the mix. This is also sold as horticultural
or agricultural lime (and may be powdered or granulated). I've also
bought some in liquid form to see if it works (never satisfied, you
know). You can also use calcium nitrate (I've seen recommendations from
1 Tablespoon to 1/2 teaspoon/gal.--I go with the latter). Peters also
markets a Cal Mag fertilizer in their Excel professional line. A number
of paph growers regularly top dress with lime.
Someone asked about when to use high phosphorus fertilizers. I try
never to use the same fertilizer twice in a row. I figure the plant
will use what it can, but by doing this I make sure it has a wide
variety of nutrients to "choose from." I probably use a high phosophorus
fertilizer at least half the time (perhaps less in spring when there is
heavy growth). Perhaps as important is to make sure not to use too much
nitrogen when growths are mature as this may encourage growth at the
wrong time (at least in some genera). My suspicion is that many of us
use more fertilizer than the plants can use anyway.
Harvey Brenneise
(brneise@andrews.edu)
Date: Wed, 29 May 1996 12:08:00 -0700 (PDT)
From: "ISLAM, YASSIR" <Y.ISLAM@CGNET.COM>
Subject: [8833] lowering pH, liming paphs, blossom booster
Aaron Hicks wrote:
> Quick question:
> What do people suggest for _raising_ the pH of their water for
>horticultural use? Ditto with _lowering_ the pH. Is anybody out there
>using chemicals for either of these on a regular basis?
> Further (I lied... MORE than a 'quick question'): what compounds
>do people use/apply to give their Paphs and Phrags more calcium? What
>works?
* Aaron, since my pH is too high (8.1) , I have to lower it. I use ordinary
distilled white vinegar at 2 tsp. per gallon of water which brings the pH
down between 5-6. I use vinegar because 1) it is cheap, and 2) it is safe;
will not burn the skin of your hands. The only problem is that it is
volatile, so you can't store your pH adjusted solution but have to make it
fresh each time. I measure the PH using a simple test kit with a colour
chart which is sufficient. I can usually judge colour to about .5 of a pH
'point'. I was once told that since vinegar (acetic acid) is an organic
compound, it encourages certain bacteria and micro-organisms that can be
harmful to orchids to flourish; this does not seem to be a problem for me.
Note than I use only about 1 tsp in a fertilizer solution, since most
fertilizers will bring the pH down a point or two themselves.
As for liming paphs ( I don't lime phrags as they seem to grow in rather
acidic environments), I tend to only lime my brachys and others that grow on
a limestone substrate. I use powdered dolomite which has the benefit of
containing Magnesium in addition to Calcium. I lime plants about twice a
year. I use about a 1/4 tsp per 3" pot and about 1/2 tsp per 5" pot. The
lime is sprinkled lightly on the surface of the mix and watered in. I also
incorporate dolomite in the mix when repotting 'lime loving' paphs. Some
other orchids, such as Oncidium papilio, also benefit from applications of
lime. Others mix up a saturated solution of lime (easy to do since the
stuff doesn't dissolve very easily) and water with their plants occasionaly
with this solution. Paphs appear to respond well to this treatment, even
though most city tap water is high in Ca to begin with.
Aaron, once a month I apply calcium nitrate at a concentration of 0.4 grams
per gallon of distilled water to my paphs. My calcium nitrate is quite old
(wet) so the actual concentration is probably slightly less (some of its
weight is water). This mixture approximates (I hope I did the calculations
right) the concentration recommended to me (100 ppm) by Jerry Fisher of
Orchids Limited. I've been doing this since last December and haven't
killed anything yet, in fact my P. bellatulum bloomed very nicely for me
this spring (very anecdotal, I know). I hope this helped.
Chuck Meyer
csmeyer@wolf.co.net
Austin Community College
1600 8th Ave. NW
Austin, MN 55912
Date: Wed, 29 May 1996 21:32:23 -0700 (MST)
From: markdim@AZStarNet.com (Mark A. Dimmitt)
Subject: [8840] Lowering pH
Aaron Hicks asks how to lower pH of water. (Also how to raise it, but
I've never encountered acid water out here in the West.) I use citric
acid because it's safe to handle, nontoxic, and fairly cheap. Sulfuric
acid is cheapest but is hazardous to handle and is very corrosive to
your distribution system too. Phosphoric acid is often recommended
because it's also a nutrient, but I have never found a source. Citric
acid does not harm my Dosmatic injector (you must have the green
acid-tolerant cylinder). It can be purchased in 50 pound bags from
chemical or scientific supply warehouses and costs less than $2.00 a
pound. A bag takes care of my 30 x 72 foot greenhouse and a couple of
hundred outdoor potted plants for about 8 months.
Tucson's city water is pH ca. 8 and has about 250 ppm TDS, mostly
calcium. By lowering the pH to 6.0 leaf spotting is nearly eliminated
and salt-sensitive plants such as soft-leafed oncidiums can be grown
well.
Mark Dimmitt
Tucson, Arizona USA
From 76771.3603@CompuServe.COM Thu May 30 16:02:38 1996
To: Aaron Hicks <ahicks@mailhost.nmt.edu>
Subject: Calcium
Aaron,
I use bone meal in the potting mix when repotting. Additionaly, I top dress
with powdered dolomite lime when repotting and periodically afterwards. Our
water tends to be acid and I feel that the fertilizer has an acid effect. The
lime appears to slow down the breakdown of the mix so that I can get two years
from each repotting and I have had excellent results.
Dennis Dayan
Deal, NJ
From jweiss@airmail.net Wed May 29 21:23:38 1996
To: ahicks@mailhost.nmt.edu
Subject: Water quality
X-Mailer: MR/2 Internet Cruiser Edition for OS/2 v1.00
Hello Aaron,
First, may I say, I have enjoyed your posts to OLD very much. I post there
infrequently, probably as much from being a natural lurker as from lack of
expertise.
However, as it happens, I have summat knowledge of your questions.
First, the safest method of raising the ph of water a lot is to add the
appropriate amount of potassium or calcium hydroxide. I should say the
safest way to avoid disturbing the other chemicals you might want to add
at the same time, such as nitrates or phosphates. Both materials are
extremely corrosive in concentration. You could add calcium bicarbonate,
if you can find an inexpensive source. Let me qualify this: what is
*really* the best method depends upon what the acidifying salt(s) are. A
good place to start is an anylisys of your water followed by a talk with
the county ag. agent.
Our water in Dallas is, of course, rather alkaline (ph 8 to ph 9!) and
with orchids and I solve that problem with phosphoric acid, killing two
birds.
Regarding Paphs: I've been growing plants of various sorts, both
professionally and non for longer than I shall admit, and I must say that
insofar as nutrition, though there are a few remarkable exceptions,
growing a rose, say or a geranium and growing an orchid is largely the
same, though concentration may differ: there is much more 'buffering'
going on in a peat based potting soil, for example, than in a plant
growing in volcanic rock. If you wish to shift the ph of the potting
medium, I would suggest adding dolomite pellets to the potting mix until
the correct ph is reached: a long-acting source of both calcium and
magnesium. If you don't want to shift the ph, then calcium sulphate
(gypsum).
*Though I don't grow Paphs*, I have found that a very light top dressing
of calcium sulphate on the orchids that I have potted in various mediums
(dendrobian, cattlea, encyclia, maxillaria, oncidium, laelia) *seems* to
have a beneficial effect. (Haven't figured out how to top dress a plaque!
:]) As my collection grows, and I can divide the various plants, I plan to
do a trial on the effect.
-----------------------------------------------------------
jweiss@airmail.net
-----------------------------------------------------------
Date: Thu, 30 May 1996 21:09:05 -0400 (EDT)
From: aquaedu@shore.intercom.net
Subject: [8850] Lowering pH (phosphoric acid source)
Hi All;
Saw the post looking for phosphoric acid to lower pH. I
have same in both diluted (8.5%) and tech grade (85%).
Suggest using the dilute, lots easier to handle, and LOTS
easier to adjust pH with, the strong blend can swing pH FAST
after you pass buffering point.
If you are trying to RAISE your pH, I suggest using
Potassium Hydroxide. Supplies K, and DOESN'T salt up the
growing media as sodium hydroxide does. Once again, have it
in both concentrated (flake) form and 10-1 dilute form.
Once again, unless you are using it for MAJOR size pH shift,
use the dilute, lots easier to handle and use.
These are the chemicals that most major hydroponic growers
use for pH adjustment, they WORK!
Anyone else wanting to join the rockwool trial, email me.
Scott Jones
Hydro/Aquatic Technologies
specializing in hydroponics, aquaponics, and aquaculture
aquaedu@shore.intercom.net
http://www.intercom.net/biz/aquaedu/hatech/
Date: Tue, 05 May 1998 23:35:13 +0100
From: Iain Wright <iain.wright@which.net>
Subject: [18528] Re: OLD - Volume [1225] - Subject: [18353] Conductivity
The only thing a conductivity meter measures is conductivity.
Conductivity is a complex subject. Any conversion of conductivity into
concentrations of dissolved solids is based on a set of assumptions. If
any of these assumptions are wrong, the concentrations are wrong.
I agree with most of what Bob Hamilton said on conductivity but think he
was wrong to offer numbers for the relationship between conductivity and
ppm.
There are three reasons for this. The first he gives himself.
> The reason you cannot directly measure ppm's with a conductivity meter
> is because different combinations of salts in water have different
> conductivities. For instance 10 ppm of sodium chloride in pure water
> has a different conductivity than 10 ppm of calcium carbonate.
If you restrict yourself to a single source of dissolved solids - your
favourite fertiliser - then you can indeed use conductivity to help you
do the same thing repeatedly - just as Bob reports.
> I grow cool growing Andean orchids and their hybrids. My tap water has
> a conductivity of ~80 uS (good water). I add enough fertilizer for a
> conductivity of 700 uS.
So long as he doesn't change his fertiliser ( ditto the manufacturer -
horror, horror, horror) this is a good way to proceed. As he doesn't
tell us what the fertiliser actually is, I couldn't attempt the same
process with my standard fertiliser and reasonably expect to establish
the same concentration.
Changing between fertilisers is particularly difficult when one of them
uses urea as a nitrogen source. Urea is an organic molecule, does not
ionise in water and consequently does not conduct.
Secondly, conductivity is temperature dependent and some meters, mine
included, do not compensate. I just did a kitchen sink experiment to
make a point. I took a panful of our rather poor tapwater, heated it
gently on the stove whilst stirring vigorously and recorded these
conductivities as the temperature rose.
C(F) Conductivity(uS)
8(46) 270
16(61) 340
21(70) 370
25(77) 380
34(93) 450
The conductivity at 25C looks low but otherwise there is a near linear
relationship - quite a steep one with an ~80% increase for a 26C change
of temperature. So, if I tried to use my conductivity meter to set my
fertiliser concentration in a cool greenhouse on a cool day (10C - 50F),
I might add twice as much fertiliser as I would have added on a warm day
in a warm greenhouse (30C - 86F). I assume that the water is
equilibrated to greenhouse temperature. (I think pH adjustment will
also have an impact on conductivity but can't think of a simple
experiment.)
The third reason is that the numbers are not right - well they certainly
don't work for me. The water in my greenhouse is presently at 15C and
reads 330 uS. I made up a gallon of fertiliser and that raises the
conductivity to 570 uS. So using the supplied numbers, I had 210 ppm
before adding the fertiliser and 365 after ie I added 155. The trouble
is I mix my own fertiliser and know that the correct difference should
have been 316. A 100% error is not what I call fairly accurate.
Where did you get these numbers, Bob? Could they actually be the
figures for calcium bicarbonate? Reporting the concentration of this
salt is a standardised way of reporting water hardness and I understand
that the 'ppm' meters you refer to are calibrated in this way.
Though I agree with most of what Bob said, I think the terse answer to
the question "What is the relationship between conductivity and ppm?"
should have been "Complex!".
Best wishes
Iain
Just back from Corsica, where the orchids were wonderful - though the
Ophrys were also complex.
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