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Potassium
Your plants source of good nutrition
Interestingly enough, plants take more potassium out of the soil than any other element except for nitrogen and calcium. Why, you ask? Because only 1% of the potassium that naturally occurs in soil is in a form plants can use. The usable stuff is called exchangeable potassium. (Not to be confused with the conservationist's exchangeable cardigan.) When shopping for plant foods and fertilizers, make sure that the potassium you're buying will actually go into your plants.
Another good thing to know is that plants get their potassium by moving their roots through the soil. You should mix fertilizers and plant foods deep into pots and containers, where the roots are. You can also use fertilizer sticks to reach the root areas of plants and flowers that are already well-established in containers.