Potassium (K+) is important ion in sugar/starch formation and the synthesis of proteins. It is a catalyst for enzymatic reactions and nuetralizes organic acids. K+ also acts as a ionic balance functioning to open stomates (how the plant "breathes"). It also effects how "juicy" a fruit can be by functioning to move water into the cell of the developing tomato fruit.


Potassium deficiency is easily identified by marginal necrosis (death of tissue along the leaf edges) of the older leaves. This may be preceded by small areas of yellowing (chlorosis) near the margins. As the deficiency progresses younger leaves will become necrotic.

Mild deficiencies can occur during fruit maturation. This is expected as it is a normal part of the growth cycle as most of the K is translocated to developing fruit. Fruit that develop during K+ stress conditions can be puffy, soft, low in acidity, irregular shaped and susceptible to ripening diseases. Later harvests from mature plants may be deficient in potassium. It is important to make sure there is enough K+ as the fruit develop.

Lack of potassium can result in misshapened fruit, ripening problems, precocious seed germination (seed germinate inside the fruit), soft, mushy or mealy fruit flesh texture, low acidity and puffiness.

Example of precocious seed germination
and poor flesh texture
photo courtesy of Bren @ bggarden.com

K+ is easily leached from most soils.

Conditions which favor K+ deficiencies

    • light, sandy soils

    • leaching rains

    • acid soils

    • organic soils

    • inadequate fertilization

Excess K+

Excessive amounts of K+ can compete for the uptake of other ions such as calcium. Lack of sufficent calcium supply may lead to blossom end rot.

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