Last week we explored why nitrogen is an essential macronutrient in plants, by looking at the molecules it appears in, and the biochemical processes relying on those molecules. From that we could better understand where and why nitrogen deficiencies and toxicities can arise.

This week we shall look at elemental potassium, or K [from the Latin word kalium (’potash’, from where the word ‘potassium’ itself comes), from the Arabic al-qaliy (’the ashes, burnt ashes’, and from where we also get alkali)]. We’ll examine how it becomes available to a plant, and where it appears in a plant. From that we’ll learn what the symptoms of a deficiency are.

Availability to Plants

Potassium makes up about 2.3% of the Earth’s crust, mostly as the components of clay and other rock minerals. Potassium ions (K⁺) are positively-charged (cations) and thus adhere strongly to the negatively-charged clay and humus colloids in soil. These ions are very soluble, thus soils rich in clay — which can hold onto these ions — tend to be rich in potassium, while very sandy soils — which can’t — are very poor in potassium.

Potassium in soils is not always available to plants however. Some of it is locked away irretrievably in rock minerals, but does become available gradually over time as the minerals break down and slowly release potassium into the soil water. Some potassium is trapped between the sheets that make up clay particles but is slowly released as these expand and separate when wetted. ‘Poverty in the midst of abundance’ can apply in these situations.

The most readily-available potassium is that on the surfaces of colloids and the potassium ions already dissolved in the soil water. As ions in the soil water are uptaken by plant roots, more ions on colloid surfaces are released to take their place. Excess ions in the soil water can reattach to colloids, and an equilibrium becomes established between the concentrations in the soil water and on colloid surfaces. This is called exchangeable potassium.

The more moist a soil, the more potassium can enter the soil water and be uptaken by the roots, but at the same time very wet conditions increase leachability.

Meristematic Growth

Meristematic cells in plants are akin to the stem cells in animals, in that they too are undifferentiated (unspecialised) cells that undergo cellular division and form (differentiate) into specialised tissue such as leaves, flowers and roots. Potassium is essential for this task of producing sturdy stems, well-developed flowers and strong roots. It aids in loosening cell wall material (essential for cell expansion to occur) and regulates phytohormones (’plant hormones’) such as indole acetic acid (IAA) and cytokinins (more nitrogen-containing compounds) which then initiate that cell division.

Strengthened Cell Walls and Disease Resistance

Potassium strengthens cell walls by thickening them. Nitrogen produces a growth spurt, and potassium strengthens that growth spurt.

Strong cell walls are more resistant to pests and diseases in that it is much harder for insects and fungi to penetrate those walls.

Water Movement and Cell Turgor

Potassium enables the uptake of water by cells and tissues. This increases cell turgor, or rigidity, which is essential for cell expansion in young tissue and a strong plant overall. Turgor is also important for the correct opening and closing of the pores in leaves called stomata (plural form of stoma). These are the entry points for carbon dioxide and the exit points for oxygen during photosynthesis. The stomata also allow excess water to leave a plant via transpiration and minimise water loss by closing when conditions are hot and dry. A plant with weak stomata cannot control gaseous exchange efficiently and is more likely to wilt.

Photosynthesis, Sugar Movement and Fruit Development

Potassium has indirect and direct importance in photosynthesis. We’ve seen how it regulates the closing and opening of stomata; it also has a role in the diffusion of carbon dioxide from the atmosphere into the chloroplasts. Potassium also facilitates the movement of photosynthates (products of photosynthesis, or sugars) around the plant via the phloem (specialised tissue for the transportation of sugars).

Potassium not only assists in the translocation (transport process) of newly-made sugars, but also helps mobilise stored proteins in leaves and stems. This ensures the formation of well-developed flowers and fully-formed, sweet, juicy fruits with a longer shelf-life.

Enzyme Activation and Cation-Anion Balance

Potassium as a nutrient doesn’t form molecules as others do, and exists solely in ionic form, ie as K⁺ ions. The main function of K⁺ in biochemistry generally is to act as a catalyst in the activation of enzymes, and in plants potassium catalyses over 60 enzymes to do with plant growth. Potassium also catalyses enzymes involved in regulating the rate of photosynthesis and the production of ATP.

Potassium, being an ion, regulates the cation-anion balance in plant tissue and the rhizosphere (the region surrounding the roots where microbiological and chemical processes are influenced by the roots).

Potassium Deficiency Symptoms

Depending on species, elemental potassium makes up 1.5–6% of dried plant tissue by weight, and deficiencies will appear below this concentration. As with nitrogen, potassium is highly mobile within a plant, and deficiencies show first in the older leaves as potassium is redirected to newer, growing leaves.

In most plants a deficiency first begins with a reduced growth rate, followed later by chlorosis (the insufficient production of chlorophylls resulting in yellowing leaves) and necrosis (localised death of cells) along the leaf margins (edges) and tips. These are similar to nitrogen deficiency symptoms, but occur very early for potassium rather than very late as for nitrogen.

Root, flower, and fruit development are typically reduced in potassium-deficient plants. Turgor decreases and plants become less drought-tolerant and wilt easily under water stress — symptoms can actually be confused for drought.

Plants deficient in potassium are also more susceptible to frost damage, insect and fungal attack, and saline conditions.

When the nitrate:ammonium ratio in soil is low, excessive amounts of ammonium interfere with the uptake of potassium to cause a deficiency.

Potassium ‘Toxicity’ Symptoms

There is no such thing as potassium toxicity, but excessive amounts of potassium can interfere with the uptake of magnesium (Mg) and calcium (Ca), causing deficiencies in those. The reverse also applies, in that excessive amounts of either can cause deficiencies in the others. These are all cations (positively-charged ions) and compete for uptake by a plant.

We shall cover magnesium and calcium in due course, but next will be the third ‘big’ macronutrient: phosphorus (P).