Plant Nutrition 101

#4   Potassium (K)


by Riley Holly


The third number on the fertilizer bag is potassium.  It is usually supplied as K2O (potash), but sometimes as KCl (0-0-60), K2SO4 (0-0-50), and KNO3 (13-0-44). It is taken up by the plants thru the root hairs as K+ ions.

Potassium is usually found in plant tissue at between 0.5 & 0.8%.  In the soil about only 1-2% is available, as it attaches to negative ions.  Sandy soils tend to leach out the potassium.  It is also mobile in the plant and is available at a soil pH greater than 4.5. It should be applied during the growth phase to be sure it doesn’t leach out during irrigation.


Potassium has many functions in the plant:

*increases the size and quality of fruits and vegetables

*regulates turgor pressure which increases protection of the plant from diseases and insect damage

*control stomate opening

*winter hardiness

*photosynthesis in low ambient light

*carbohydrate translocation

*protein synthesis

*promotes root growth


Symptoms of potassium deficiency are usually slow to show on the plants, especially in low temperature soils and dry soils

*one of the first is a slowing of growth

*weak stems and stalks (loss of turgor)

*chlorosis, and necrosis later, which develops first along tip and margins

*small or shriveled fruit

*interveinal chlorosis of older leaves

*mottling, curling, spotting beginning on lower leaves

*shriveled seeds

*crinkled edges on margins of older leaves


Corn shows deficiency on older (lower) leaves as chlorotic or burned edges, the midrib stays green.


An excess of potassium shows up as light green foliage, and calcium (Ca) & magnesium (Mg) deficiencies.  An excess also reduces the uptake of magnesium, manganese (Mn), zinc (Zn), nitrogen, & iron (Fe).

Potassium slows down the absorption of Ca & Mg, and vice versa.  A high N to K ratio will cause a potassium deficiency. Potassium is usually adequate in soils, except for sandy soils and low pH soils


Organic sources of potassium are many, most are slow acting.  Some of those with a higher percentage are:

*greensand (5-7%)

*granite meal (3-5%)

*kelp / seaweed (12%)

*sul-po-mag (22%) {sulfur-potassium-magnesium}

*wood ashes (6-20%) fast acting


A high concentration of positive ions affects the amount of potassium sorbed and increases the availability and mobility of potassium.

The desirable amount of potassium ions in solution is between 100 and 400 ppm.


Carrots, beets, celery, tomatoes, citrus, melons, and potatoes use lots of potassium and it is removed from the soil with each harvest.