St. Augustine Grass Fertilization Guide

By: Ken Wilson

Fertilizing St. Augustine grass correctly is crucial for maintaining its health and beauty. Without adequate nutrition, the turf might become sparse and yellowed. So, how does one feed this popular, hardy grass species to keep it thick and verdant?

Fertilize St. Augustine grass with nitrogen during the summer growing season. Depending on soil test results, one might need to apply fertilizers containing potassium and phosphorus. If the turf is yellowing, applications of nitrogen, iron, or manganese usually make the grass greener.

Turfgrass maintenance is not complicated provided one adheres to a few fundamental rules and guidelines. Here is a guide for anyone wishing to understand the essentials of St. Augustine fertilization.

Guide to Fertilizing St. Augustine Grass

While St. Augustine grass (Stenotaphrum secundatum) grows aggressively, fertilizer applications help this spreading turfgrass achieve and maintain an even and luscious appearance.

The general rule is to feed S. secundatum nitrogen (N) fertilizers during the summer growing season when additional nutrition is needed to sustain the development of new vegetative growth. It might also be necessary to add small amounts of phosphorus (Ph) and potassium (K) if soil tests indicate these nutrients are deficient.

Adding nitrogen fertilizer in winter is not necessary or advisable since the grass is dormant. There may, however, be a need to apply micro-nutrients like iron or manganese if the turf is yellowing. In colder climates, it may be beneficial to add some potassium to increase the grass’ resistance to freezing winter temperatures.

Use the soil type as a guide when deciding how much fertilizer to apply to the lawn and the frequency of these applications. The following principle serves as a rough guideline:

  • Feed higher amounts of fertilizer more frequently in sandy soils.
  • Add lower amounts of fertilizer less with less frequency in heavier clay soils.

Here are the details about feeding St. Augustine grass during the growing and dormant seasons:

Fertilizing in the Growing Season

The task of feeding this wide-bladed turfgrass is primarily undertaken during the northern hemisphere’s summer growing season, roughly between May and August.

Nitrogen is the primary nutrient required to maintain the lawn’s health, vigor, and visual appeal during summer. When working without a soil test, it is advisable to apply a balanced fertilizer with low quantities of phosphorus and potassium (such as an NPK ratio of 3-1-2). For those fortunate enough to have soil test results, you can tell how much potassium and phosphorus you need in addition to nitrogen.

Feeding Frequency

It is ideal to fertilize St. Augustine grass two to three times in the growing season. If the soil is sandy, feed the turf three times (in early, mid, and late summer). Heavier clay soils hold higher concentrations of nutrients, so applying fertilizer twice during the growing season is sufficient.

Fertilizer Amounts

The recommended fertilizer application rates for feeding St. Augustine grass depend on the soil type and the lawn size. For sandy soils, the turf should receive three pounds of nitrogen fertilizer p/1000 square feet each growing season. Add one to two pounds of nitrogen during the growing season for clay soils.

In mid and late summer, apply potassium and phosphorus to the St. Augustine grass if necessary (refer to soil test results).

Potassium is often needed in sandy soils a month to six weeks before the end of the growing season, as a way to enhance the hardiness of the lawn over the winter months. Common potassium fertilizers and application rates are:

  • Potassium sulfate (0-0-50) – 2 pounds p/1000 square feet
  • Muriate of potash (0-0-60) – 1.6 pounds p/1000 square feet

Calculating Nitrogen Fertilizer Application Rates 

Here is the method to calculate the application rate for a particular fertilizer product and the amount required for a specific area.

To figure out how much fertilizer is equivalent to one pound of nitrogen, divide 100 by the amount of nitrogen in the product. You can check this amount by referring to the first of the three nutrient numbers on the packaging, which represent N-P-K (such as 10-7-8).

For example:

  • 100 / 10N = 10
  • This means 10 pounds of fertilizer = 1 pound of pure nitrogen

Next, work out the amount of nitrogen you need for the size of the grassed area. Divide the square footage of the turf by 1000, and then multiply by the amount of fertilizer calculated in the previous step.

So, for example:

  • 700 square feet / 1000 = 0.7
  • 0.7 x 10 pounds = 7 pounds
  • This means you should apply 7 pounds of the fertilizer product equates to 1 pound of N for the entire area.

Fertilizing to Address Yellowing (Chlorosis) of the Lawn

Nutrient deficiencies can make St. Augustine grass suffer from yellowing (chlorosis). Inadequate nitrogen, iron, and manganese are probably the most common deficiencies that cause yellowing. Chlorosis usually occurs at the beginning of the growing season or towards the end of summer.

In early summer, iron and manganese might be inadequate because:

  • The soil temperatures are still low.
  • The soil pH is too high (excessively alkaline).
  • Phosphorus levels are elevated (making iron and manganese unavailable).

Later in the growing season, iron and manganese deficiencies may be the result of:

  • pH levels in the soil are too high.
  • An overabundance of phosphorus.

Yellowing is also commonly observed at the end of the growing season when the lawn has depleted the available nitrogen in the soil. Nutrient problems are complex and call for long-term, site-specific solutions, so you can establish a balanced soil nutrient profile more conducive to growing thriving St. Augustine grass.

Fortunately, direct applications of nitrogen, iron, or manganese (depending on which nutrient is deficient) provide a quick way to make the lawn greener, at least temporarily. Add these nutrients in the following forms and application rates:

  • Nitrogen (ammonium sulfate) – 2 to 4 pounds p/1000 square feet
  • Soluble iron (ferrous sulfate) – 2 ounces in 3 to 5 gallons of water p/1000 square feet
  • Manganese (manganese sulfate) – 2 pounds p/1000 square feet

Final Thoughts

St. Augustine grass requires routine fertilization to ensure the turf stays evenly dense and retains its attractive deep green color.

Feeding the turf revolves around the application of nitrogen-based fertilizers during the growing season, supplemented with limited quantities of phosphorus and potassium. Iron and manganese are also added if the grass starts yellowing.

About the author 

Ken Wilson

Long time career in the home services industry from remodeling to patio construction. Currently residing to in SWFL and active contributor to multiple home & garden publications.