Potassium (K)

Marijuana Horticulture

by Jorge Cervantes

Potassium is used at all stages of growth. Soils with a high levels of potassium increase a plant’s resistance to bacteria and mold.

Potassium helps combine sugars, starches, and carbohydrates, which is essential to their production and movement. It also is essential to growth by cell division. It increases the chlorophyll in the foliage and helps regulate the stomata openings so plants make better use of the light and air. Potassium is essential in the accumulation and translocation of carbohydrates. It is necessary to make the proteins that augment the oil content and improve the flavor in cannabis plants. It also encourages strong root growth and is associated with disease resistance and water intake. The potash form of potassium oxide is (K2O).

Potassium-starved plants initially appear healthy. Deficient plants are susceptible to disease. Symptoms include the following: older leaves (first tips and margins, followed by whole leaves) develop spots, turn dark yellow, and die. Stems often become weak and sometimes brittle. Potassium is usually present in the soil, but it is locked in by high salinity. First, leach the toxic salt out of the soil and then apply a complete N-P-K fertilizer. Potassium deficiency causes the internal temperature of the foliage to climb and the protein cells to burn or degrade. Evaporation is normally highest on leaf edges, and that’s where the burning takes place.

Treat deficiency of potassium by fertilizing with a complete N-P-K fertilizer. Occasionally, a grower will add potassium directly to the nutrient solution. Organic growers add potassium in the form of soluble potash ()wood ashes) mixed with water. Be careful when using wood ash, the pH is normally above 10. Use a pH lowering mix to bring the pH to around 6.5 before application. Foliar feeding to cure potassium deficiency is not recommended.

Toxicity occurs occasionally and is difficult to diagnose because it is mixed with the deficiency symptoms of other nutrients. Too much potassium impairs and slows the absorption of magnesium, manganese, and sometimes zinc and iron. Look for signs of toxic potassium buildup when symptoms of magnesium, manganese, zinc, and iron deficiencies appear.

Treat toxicity by flushing the growing medium of affected plants with a very mild and complete fertilizer. Severe problems require that more water be flushed through the growing medium. Flush with a minimum of three times the volume of water for the volume of the growing medium.