St. Augustine grass holds up well to the heat and humidity of the southern Atlantic coast, the Gulf Coast, Florida and coastal California. It has a coarse texture and an undergrowth that has given it the common name "carpetgrass." Its nutrient requirements are not extreme, but it needs a steady flow throughout the growing season, along with 1 inch of water per week and careful mowing.
Texas A&M University recommends applying fertilizer to St. Augustine lawns monthly from early spring until late fall.
TAMU suggests one pound of nitrogen per 1,000 square feet in sandy soils. Denser soils require less, about one-half pound per 1,000 square feet.
Established St. Augustine has little need for added phosphorus. What it needs can generally be found native in the soil. Newly planted lawns benefit from a light application of phosphorus.
St. Augustine needs approximately half as much potassium as nitrogen. Therefore in sandy soils, it needs one-half pound per 1,000 square feet or one-quarter pound per 1,000 square feet in denser soils.
Shopping for Fertilizer
Given the above requirements, the proper ratio for fertilizer to be used on St. Augustine lawns is 2-0-1. The label doesn't have to be 2-0-1, but the ratio should be that. For example, 12-0-6 is the same thing. Continuing that example, using a 12-0-6 fertilizer on sandy soil would require 8.34 pounds per 1,000 square feet (one pound divided by 12 percent). If you cannot find a product with twice as much nitrogen as potassium and no phosphorus, look for something close. The presence of some phosphorus isn't going to do any damage, but neither will it add any benefit. The important issue is that the nitrogen and potassium ratio remain 2:1.
St. Augustine is sensitive to iron deficiency. If you notice a yellowing in your St. Augustine lawn, apply iron chelate or iron sulfate.