Fertilize Container Plants
Most container plants have had all the fertilizer that was initially in the soil used up or washed out by frequent watering. Plant roots have a more limited growing area in containers. Container soils are more porous to insure good drainage. These two factors make it necessary to water more frequently. As a result, fertilizer is used more rapidly and it washes out of the soil more quickly.
I apply Osmocote or other coated slow release fertilizer when I plant my containers. I have found that I need to add more fertilizer by the first of August because it runs out. Liquid fertilizers can also be used for containers, but should be applied on a weekly basis.
Yellow lower or older leaves on container plants is a sure sign of nutrient deficiency.