Stoma in plant leaf

Without this exchange, nearly all life would cease to exist. Plants use their chloroplast cells to make chlorophyll, which is essential for photosynthesis, and most of that activity takes place in the plant leaves. Thus a potassium deficiency shows up in the leaves first, notably in the older leaves. They can turn a very blackish-green, or begin to yellow (chlorosis) which is caused by insufficient chlorophyll. If the potassium deficiency is really bad, the leaves will become lace-like, then die and fall off. Plant disease-resistance is enhanced by adequate potassium levels, and low Brix is a problem of too much potassium. Another problem with excess potassium is that it can replace the necessary calcium in cell walls.

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Chloroplast cells
Leaf structure

Plants accumulate the potassium they need in their leaves, bringing it up from their roots. As the roots absorb potassium from the soil, potassium from farther away migrates to the root area, and there is a lot of potassium in most soils. However, only a very small fraction of all the potassium in soil is available for uptake by plants. The available potassium in solution is in balance with “exchangeable potassium” (colloid and chelated forms) and it’s the exchangeable potassium that plants can use. The solution potassium (“available potassium”) is like a reservoir just waiting to replenish the exchangeable potassium.

Where do we get potassium?
The natural potassium in never-fertilized soils comes originally from mineral deposits like feldspar and mica (found in igneous, metamorphic and sedimentary rocks) that have weathered and released potassium. Most potassium cannot be used by plants as is. When the minerals weather, some potassium ions bind to clay particles in the soil. Eventually the clay weathers, releasing potassium, which can then form a simple bond to negatively-charged soil particles. This allows potassium to be released into soil solution, but it also allows potassium to leach away, especially in sandy, silty and water-logged soils.

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Feldspar crystals in granite
Feldspar crystals

Potassium in soils is found in three forms: unavailable, slowly available (“fixed”) or readily available (“exchangeable”). Depending on soil type, approximately 90-98% of total soil K is found in the form of “unavailable”.[1] Potassium added via fertilizers to soils can quickly increase solution potassium in clay soils, known as potassium “fixing”, resulting in a slow release of potassium to plants. How much potassium can be “fixed” is a complex equation depending on parent soil type, CEC (cation exchange capability), existing potassium in the soil, pH, and organic matter. A soil test will help with this determination.


Potassium soil additives
Potassium chloride is a common source of K, and most of what we use is mined in Canada or the western United States. Potassium-magnesium sulfate is good when you need to add more magnesium. Manure is also a good K source but should be tested to know how much potassium you are actually adding. There are other potassium compounds used in fertilizers (and usually more expensive) like potassium oxides, potassium carbonates, and
potassium nitrates. Potash is a form of potassium carbonate; the term is often used for potassium. Potassium sulfate provides both potassium and sulphur.[2] Of course if you need potassium, you could always till in thousands of pounds of bananas, a natural source high in potassium.

Potassium is an alkali capable of raising soil pH, so it’s wise to know how much your soil needs, and to apply it carefully.


Photo credits
Tomato leaf stoma, Public Domain
Green Leaf, Public Domain
Chloroplasts, licensed under Creative Commons Attribution ShareAlike 2.0 Germany
Potash, Public Domain
Potassium chloride, Public Domain
Mica, GNU Free Documentation License
Feldspar crystals in granite, Public Domain
Feldspar, Public Domain