Feed king protea in the spring and fall with a nitrogen-rich or organic fertilizer. Apply the fertilizer at a ratio of 60 lbs. per acre. Avoid fertilizers with high levels of phosphorus.
Plant the king protea away from other plants that require frequent fertilization. The fertilizer may seep into the soil and kill its roots. Do not use fertilizer on new or freshly planted proteas.
Inject soluble ammonium sulfate in a drip irrigation system twice a year in April and September beginning in the second year. Mix 25 lbs. of ammonium sulfate and 5.25 lbs. nitrogen per acre. Apply half of the solution each time. Irrigate the soil to a depth of 6 inches.