Potassium

Potassium (K) is one of the three primary plant nutrients, but what does it actually do for plants and how do we know if our plants have enough (or too much)?

There’s a lot of potassium on Earth. It is the fourth most plentiful mineral, making up 2.5% of the Earth’s crust and upper mantle (lithosphere), but most of that potassium is unavailable to plants. Plants can only use potassium that is in solution (like the sugar in kool-aid). As plant roots absorb mineral rich water from the ground, some of that potassium is pulled in and put to work. If you were to dry out a plant completely, between 2 and 10% of the remaining weight would be potassium.

Potassium deficiency in tomato leaf (Goldlocki) CC BY-SA 3.0

How plants use potassium

Potassium, also known as potash, is concentrated in leaves and growing tips. Found in guano and wood ashes, potassium is a highly mobile element within the plant and it serves several functions:

• creates proteins and carbohydrates
• supports root growth
• regulates moisture levels, by opening and closing the stomata, increasing drought tolerance
• determines fruit shape, color, and sugar content (brix)
• activates enzymes related to photosynthesis and respiration, boosting photosynthesis in low lights situations, such as shade gardens
• builds cellulose, the stuff that holds plants upright

Symptoms of potassium deficiency

Plant roots can only absorb potassium when the balance of other nutrients is within certain ranges. Our Bay Area clay soil tends to have an overabundance of potassium, but plants can rarely get to it because of low iron levels in the soil. Too much nitrogen, calcium, or sodium, high soil alkalinity, and temperatures over 80°F can also interfere with potassium absorption. Compacted soil does not seem to interfere, other than by restricting root growth, but heavily compacted soil should still be aerated for better air flow. Potassium deficiencies result in reduced nitrogen absorption and a build up of sugars that can give leaves a burnt appearance. These common signs of potassium deficiency generally move from older/lower growth to higher/newer growth:

• wilting
• brown spotting
• brown areas between veins
• brown or yellow leaf veins
• yellow or white spots on new leaflets
• more vulnerable to pests and diseases
• more susceptible to heat and frost damage
• chlorosis (yellowing) on leaf edges
• dieback

Symptoms of potassium toxicity

Potassium is one nutrient that plants can absorb at levels higher than they can use, in an action called ‘luxury consumption’. If you see a white crust developing on leaf margins (edges), it is the sugar and potassium residue from guttation. When toxic levels are reached, older leaves will start turning brown at the bottom, between and alongside of the veins, working upwards through the plant. This is the same symptom that would indicate a magnesium deficiency, so a soil test from a local, reputable lab is really important before you start trying to adjust your garden’s chemistry.

​Before you toss another bag of fertilizer at your plants, make sure they really need it. The only way to know for sure what your plants are working with is to invest in a soil test from a local, reputable lab. It will save you a lot of money in terms of replacement plants, reduced harvest, unnecessary soil amendments, and chemical treatments. If you are growing in San Jose, California, too much potassium could easily be a problem worth investigating.