From: pharvey@quack.sac.ca.us (Paul Harvey)
Subject: Re: question about gypsum, road salt and rhodies
Organization: The Duck Pond public unix: +1 408 249 9630, log in as 'guest'.
Date: 9 Jan 1993 22:00:48 UTC

In article <1993Jan08.165037.32322@watson.ibm.com> 
sundare@watson.ibm.com (C. Sundaresan) writes:
>I have heard that gypsum can neutralize road salt.  Recently my
>rhododendrons ("NOVA ZEMBLA", Canada-hardy and pretty tough) 
>got a pretty bad dose of road salt -- they look OK
>now but I worry that damage will show up this spring.  Can I do something
>preventative, like sprinkle gypsum (from the nursery) over the snow and
>mulch, so at least when everything really starts dissolving, the roots
>will have some help? Will this throw my soil pH off? 

No. Gypsum is also a neutral source of calcium and sulfur. But, I would
not apply any calcium or gypsum to rhodies.

>I do not understand the mechanism (chem) of gypsum's action in the soil.

Clay soils tend to bind up salt and restrict the natural tendency for
the salt to dissolve in water and leach downward. Rain and snow are pure
water and can flush a lot of salt from soils, thus salt accumulation is
rarely a problem in areas with over 20" annual rainfall, but road salt
is a special case. Salt is a poison for plants. Gypsum cancels the
binding effect of some clays and thus aids in flushing salts from the
soil. River and well water typically have dissolved salt and this salt
accumulates in the soil. Salt accumulation is the big problem in
irrigation of arid and semi-arid lands and may have caused the decline
of many ancient civilizations. Road salt, though it does solve the
problem of iced roads, is harsh on roadside vegetation. Heavy snow melt
or spring rains will flush most of the salt away, probably into the
nearby lake or ground water or river where it can cause more problems.

>The comments on the bag sound so overblown.

Typically the case.

>Does it do anything when
>topically applied, or should it be mixed into the soil ... I wouldn't
>do this to my rather shallow-rooted young shrubs.  There are 23 of them
>and it's quite an investment.

It should be scratched into the surface and allowed to leach downward
with water. Water is required to leach salts, gypsum does nothing
alone, it aids water.

>Normally the road (really a lane) they
>are on isn't even plowed much, and might get a light sanding or salting
>once a winter.  The curbing and road grade send the runoff away from
>the plants nicely, but THIS TIME, the salty snow got all plowed up
>over the curb, several times.

Ericaceous plants, (Rhodies, Blueberries, Ericas, etc), are very
sensitive to salt and calcium. I would not use gypsum on these plants,
in fact sulfur may be called for if the soil is not acidic enough. If
the plants suffer every year due to road salt exposure, they may have to
be moved to a safer location. The salty snow can be removed and fresh
snow added to leach the salt already there. If spring melt and rains do
not occur, irrigation would be called for.

>Experience stories, please, and a word from a biochemist would be
>helpful too!

I am none of these, so what I say should be taken with a grain of salt.
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