The best time to fertilize St. Augustine grass in the south is in the spring, a few weeks after the last frost. Fertilizer should be applied every four to eight weeks until the end of August.
A high phosphorous fertilizer can be applied after planting to encourage quick spreading. St. Augustine grass is prone to iron deficiencies but mature grass does not need much phosphorous because sufficient amounts are usually found in the soil. Established lawns should be fed with a high nitrogen fertilizer. The proper dosage is one pound of nitrogen for each 1,000 square feet of grass on sandy soil and half that for dense soil. Overfeeding produces very thick grass which encourages disease and insects.