St. Augustine grass is a warm-season turfgrass valued for its dense, thick growth and vibrant, blue-green color, making it a popular choice across the Southern United States and Gulf Coast regions. This turf variety forms a thick carpet through surface runners called stolons. Proper fertilization is necessary to keep the lawn dense and healthy, which helps suppress weeds and defend against environmental stress. Meeting the distinct nutritional needs of St. Augustine grass is paramount to successful lawn care.
Essential Nutritional Requirements
The foundation of a healthy St. Augustine lawn lies in the proper balance of the three primary macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K). Nitrogen is the most needed nutrient, fueling the grass’s vigorous growth and promoting the deep green color homeowners desire. A high-nitrogen formula is generally recommended for St. Augustine grass during its active growing season to support its rapid development.
The ideal N-P-K ratio for general maintenance is typically high in nitrogen, low in phosphorus, and moderate in potassium, often expressed as a 4-1-2 or 3:1:2 ratio (e.g., 16-4-8). Phosphorus is crucial for developing strong root systems, but established lawns require minimal amounts. Excessive phosphorus can interfere with the grass’s ability to absorb other micronutrients, especially Iron.
Potassium strengthens the grass plant against environmental and biological threats. It enhances the plant’s ability to withstand stresses like drought, heat, and cold, and improves resistance to turf diseases. Ensuring a sufficient level of potassium, particularly in later applications, supports the overall health and resilience of the turf system.
Micronutrients are important, and Iron is the most frequently required supplement for this type of grass. Iron is a component of chlorophyll, the pigment responsible for the green color of the grass blades. A deficiency in Iron can cause the grass to turn yellow while the veins remain green, a condition known as chlorosis. Iron supplements are often applied separately to achieve a deeper green color without promoting the excessive growth flush caused by nitrogen.
Selecting the Right Fertilizer Type
The method of nutrient delivery is important, making the choice between fertilizer types significant. Slow-Release Nitrogen (SRN) formulas are favored for St. Augustine grass because they provide a steady, continuous stream of nutrition over an extended period. This controlled release prevents the rapid growth surges, or “flushes,” that quick-release fertilizers cause, which lead to excessive mowing and a weakened turf stand.
Slow-release mechanisms vary, with some products utilizing a polymer coating that slowly dissolves based on soil temperature. Others, like organic options, rely on microbial activity to break down the material. This gradual process greatly reduces the risk of fertilizer burn on the leaves and minimizes the potential for nutrient runoff into the environment.
Granular fertilizers are the most common delivery method, offering the best longevity and typically being the more cost-effective choice for large areas. However, liquid fertilizers provide a distinct advantage when a quick fix or supplemental feeding is needed. Liquid products are absorbed immediately through the leaf tissue, allowing for a rapid green-up, and are the preferred delivery system for micronutrients like chelated iron.
Organic fertilizers (e.g., composted manure or Milorganite) release nutrients based on soil microbial action, which is slower and builds soil health. Synthetic fertilizers offer a more immediate and precise nutrient delivery based on the exact N-P-K ratio listed on the bag. A hybrid approach is often suggested, using granular slow-release synthetics for bulk feeding and liquid applications for quick iron supplements.
Seasonal Timing and Application Schedule
Fertilizing St. Augustine grass must align with its active growing cycle, as the grass is dormant in the winter months and cannot absorb nutrients effectively. The active feeding season runs from early spring through early fall, with the first application triggered when the soil temperature consistently reaches 65 degrees Fahrenheit. This initial feeding, typically in mid-to-late spring, should be a lighter application, often combined with a pre-emergent weed control product.
The second and heaviest feeding should occur in late spring or early summer, coinciding with the grass’s peak growth period. This application should contain the highest concentration of nitrogen to support vigorous growth and color during the warmest part of the year. A mid-summer feeding may be necessary, particularly on sandy soils that leach nutrients quickly.
The final application of the year happens in the early fall, usually six to eight weeks before the first expected frost. This “winterizing” application should focus on a higher proportion of potassium relative to nitrogen to help the grass prepare for cold stress. Potassium strengthens the cell walls and increases the grass’s cold tolerance, supporting root health throughout the dormant season. Nitrogen should not be applied too late in the fall, as new growth stimulated by the nutrient will be vulnerable to frost damage.
Application Techniques and Troubleshooting Iron Deficiency
Before beginning any fertilization program, conducting a soil test is recommended to determine existing nutrient levels and the soil’s pH. St. Augustine grass thrives best in a slightly acidic to neutral pH range of 6.5 to 7.5, and the soil test results indicate if soil amendments are necessary. Knowing the soil pH is helpful, as alkaline (high pH) soils are a primary cause of unavailable iron, leading to yellowing.
Applying granular fertilizer evenly is necessary to avoid a striped appearance or localized burning, which requires properly calibrating the spreader. Calibrating the spreader to the manufacturer’s recommended setting ensures the correct amount of material is distributed across the lawn. After application, the lawn must be watered immediately with about a quarter to a half-inch of irrigation to dissolve the granules and move the nutrients into the soil.
Failure to water in granular fertilizer allows the concentrated salts to sit on the leaf blade, resulting in chemical burn spots. For liquid fertilizers, however, it is necessary to allow the product to dry on the leaf surface for a few hours before watering. If the lawn begins to show signs of interveinal chlorosis, which is yellowing despite adequate nitrogen, the issue is likely iron deficiency. A separate foliar spray application of liquid chelated iron provides a rapid-acting solution to restore the deep green color without encouraging unwanted excessive growth.