DEAR NEW FARM:
Could you please explain the harmful effects, if any, that
potassium chloride fertilizer has on soil.
We posed your question to Dave Wilson, a research agronomist
here at The Rodale Institute. Dave said:
Potassium chloride (KCl) is a
salt and contains chlorine as the chloride ion as is found
in table salt. This is not the same form of chlorine used
to kill microbes, in bleach or in chlorinated drinking water.
(I am not sure if the effect of chloride from KCl on soil
microbes has been measured.)
The main harmful effect of KCl
use is the increase and buildup of chloride in the soil,
which reduces the yield of salt-sensitive crops.
Tobacco is extremely sensitive to excess amounts of chloride.
Crop plants such as potato, sweet potato, peaches, citrus
and some legumes are sensitive to high quantities of chloride.
High rates of Potassium chloride should be avoided on these
The potential harmful effects
of using potassium chloride fertilizer is from the salt
concentration of the material. The lower-grade potassium
materials will have a greater content of soluble salts per
pound of potassium supplied. With some crops and with some
methods of fertilizer placement, this higher salt content
may be inhibit plant growth. Fertilizers can cause injury
to plants when applied in excess or when too much is placed
too close to the roots of the crop. This injury is referred
to as "burning.” Low levels of burn, even without
visible damage, can impede the roots’ ability to uptake
nutrients. Salts become a problem when enough concentrate
in the soil solution near the root zone to negatively affect
plant growth. Excess salts in the root zone hinder and reduce
the roots’ ability to absorb water from the surrounding
soil. This lowers the amount of water available to the plant,
even if there may be adequate amounts of soil moisture for
that particular crop. The presence of excess salts in the
soil solution causes the plants to exert more energy extracting
water from the soil. This cause plant stress and "burning."
The extent to which fertilizers
can burn plants is evaluated by the "salt index"
of the material. The salt index rating compares the fertilizer
with an equal weight of sodium nitrate. The higher the salt
index, the higher the fertilizer "burn" potential.
Potassium chloride has a salt index rating of 116, one of
the highest salt indexes of all commercial fertilizers.
The salt index of table salt (sodium chloride) is 153. Other
Potassium fertilizers are available that have a lower salt
index; the lower the salt index, the less likely it will
cause burning. Potassium nitrate has a salt index of 74;
potassium sulfate has a salt index of 46.
us with comments, suggestions and questions.