Q&A

DEAR NEW FARM:

Could you please explain the harmful effects, if any, that potassium chloride fertilizer has on soil.

Thank you,
Marvin Miller
Ohio

DEAR MARVIN:

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.

NF

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