I copied this from: Marijuana Plant Care
By Robert Bergman
Marijuana Nutrient Deficiency – Potassium
Potassium deficiencies in marijuana are somewhat common.
When using natural fertilizers like guano, you have to take into
account the fact that potassium is going to be the least abundant
of the three macronutrients (nitrogen and phosphorus being
the other two). A potassium deficiency might actually make the
marijuana plants appear taller and more vigorous at first glance,
but the bottom leaves might be dying. The leaves may also be turning a tan or brown color
and developing necrotic spots in some locations.
Chlorotic spots will start to show up as the deficiency persists. Slow growth and smaller
growth are all possibilities with a potassium deficiency. Potassium is important in the
transport of water and the development of buds later on. Even so, small deficiencies are
really only cosmetic. Fixing larger deficiencies takes a water-soluble fertilizer that is high in
potassium
