Re: leaf burn causes?
- Subject: Re: leaf burn causes?
- From: Steve Marak <firstname.lastname@example.org>
- Date: Sun, 30 Nov 2014 11:33:28 -0600
I'll add a couple of comments to Ted's excellent note on water
Even with relatively good water - low in dissolved salts - there is
a lot of difference in how salts will build up in medium on a
greenhouse bench, where leaching water can run straight through the
pot and carry salts with it, compared with the same pot in a tray or
saucer or other place where water is held at the bottom of the pot
(and the medium inside) until it evaporates.
I keep a very few plants on a windowsill at my office, so they're in
a tray. Even though I use only deionized water, which has
essentially no dissolved salts, salts that dissolve from the medium
itself, plus salts in the fertilizer I use periodically, have
nowhere to go and so build up in the tray. With the next watering
those deposits will redissolve to whatever extent their solubility
allows and can produce surprisingly high TDS values relatively
quickly, and if the pot sits flush in the tray that solution
permeates the bottom of the medium. Since I'm lazy about repotting,
I wash out the tray every few weeks and periodically flush the pots
in a sink.
Also, some media are widely considered to "hold" salts from water
and fertilizer better than others, and so need replacing more often.
I haven't seen any rigorous measurement, so take this with a grain
of (undissolved) salt, but I know orchid growers who believe that
sphagnum moss must be replaced as often as every six months if you
are feeding regularly.
Some water supplies are switching from chlorine to chloramine as a
disinfectant, because it's more stable and thus is effective longer.
But that also means you can't get rid of it just by letting the
water stand for a day or two - it requires special treatment. As
usual - think fluoride - there is a lot of debate over the safety
and desirability of chloramine. I haven't seen any reports of plant
toxicity, but I know there are aquarium enthusiasts who have had big
losses when their water supply switched to chloramine. (Chlorine is
toxic to fish too, just easier to remove.)
Note also that very pure water, which has a theoretical pH of 7.0
dead on, if left exposed to the air will quickly become somewhat
acidic as gaseous acid anhydrides from the air, mostly carbon
dioxide, dissolve into it. What the value will be when equilibrium
is reached depends on what's in your air, but for typical levels of
carbon dioxide the theoretical pH is 5.6-5.7 - and indeed when I
measure rain water here (northwest Arkansas), it's pretty close to
that. That's 100 to 1000 times as acidic as our tap water, which
like Ted's is adjusted up into the 8-9 range. Surprisingly, to me
anyway, most plants don't seem to mind the higher pH of tap water,
but we switched to reverse osmosis water in the greenhouse, and our
unscientific observation is that the plants grow better.
On 11/29/2014 2:50 PM, Theodore Held
Just for information purposes, I exist on city water here
and it is fluoridated. Most municipal waters in the United
States have small amounts of soluble fluoride. Large amounts
can, indeed, be toxic. But the rules for fluoridation keep the
level quite low and it should be safe (see my comment on salt
buildup, later). As I mentioned previously, my experience here
is that the leaf tip burn goes away with sufficient moisture
and my practice to keep them moist uses only plain tap water.
Most municipal waters in the U. S. also contain a small amount
of chlorine as a disinfectant. The chlorine dissipates quickly
upon sitting (overnight). I also have chlorine in my trap
Recall also that all (almost all?) municipal water sources
are also lightly alkaline, meaning a pH in the range of 8 or
9. The reason for that is to minimize water pipe corrosion.
This also applies to my water supply, which originates from
the Great Lakes.
One problem that people sometimes have is keeping their
plantings for extended periods without refreshing the soil
(whatever the composition). If tap water (or well water) is
added from time to time (except for deionized or distilled
water, from which no salts will originate) salts gradually
build and can lead to "burn" phenomena. The kind and extent of
dissolved salts in your water can be seen by evaporating some
of it in a clean glass vessel. Look for the milky white
residuals. These are salts. Water used to keep plants wet will
transpire through the leaves and evaporate from the soil, but
any salts will be left behind. To some extent excess salts can
be purged with leaching and subsequent disposal of the
leachate. People often ask me about the "calculus" of whitish
salt that appears on pot rims over time. This is mostly
calcium sulfate (gypsum). It is diagnostic of an accumulation
of salts in the potting medium, although the calc is not
soluble in new added water. Fluoride used to treat tap
water is a typical salt and is not volatile. It also gets
converted in horticulture to a very insoluble form (as calcium
fluoride), meaning that it will not be flushed with a
leaching. That means fluoride will accumulate as a planting
ages without periodic refreshing of the planting mix. (My
guess is that since calcium fluoride is so insoluble it must
be essentially inert once created.)
Finally, it should go without saying that once tip burn
occurs it will not go away. Brown tips are dead tissue.
Success with any remedy will need to be noted on new leaves as
they appear. If you are successful the new leaves will stay
green while the affected leaves will stay with their brown
tips. If you have a big display of Spaths, it might take a
couple of years or more of better growing conditions to
finally have a planting without brown tips, as old leaves die
off and are replaced by nice green ones.
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