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 crops.
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.