Used coffee grounds are a popular organic material often considered for sustainable lawn care as a soil amendment or natural fertilizer. However, the effectiveness of any additive depends on the specific needs and sensitivities of the plant. Understanding the composition of coffee grounds and the unique biology of St. Augustine grass is necessary to determine if this practice is helpful or potentially harmful.
Nutritional Profile of Coffee Grounds
Used coffee grounds are rich in organic matter, which improves soil structure and water retention. They contain macro- and micronutrients that plants utilize as the grounds decompose. Analysis shows the grounds contain approximately 2% nitrogen, 0.06% phosphorus, and 0.6% potassium by volume, along with trace elements like calcium, magnesium, and iron.
The nitrogen is locked within organic molecules, resulting in a slow-release effect as soil microorganisms break the material down. This slow nutrient delivery is an advantage over quick-release synthetic fertilizers, which can cause nutrient burn. The grounds also attract earthworms, which further aerate the soil and contribute beneficial castings.
A common misconception is that used coffee grounds are highly acidic. While the coffee bean is acidic, the brewing process washes out most of the acid, leaving used grounds with a pH typically close to neutral, ranging from 5.4 to 6.8. This near-neutral pH is important, as drastic shifts in soil chemistry can interrupt the nutrient cycle and impede nutrient absorption.
St. Augustine Grass Specific Needs and Sensitivities
St. Augustine grass is a warm-season turf that grows best when soil conditions are within a specific range. Agronomists recommend an optimum soil pH between 6.5 and 7.0 for most turfgrasses, though St. Augustine tolerates slightly more alkaline conditions. The grass is particularly susceptible to iron chlorosis, a condition that causes the leaves to turn yellow while the veins remain green.
Iron chlorosis commonly occurs when the soil pH is too high (alkaline), making the naturally present iron unavailable for plant uptake. Although coffee grounds are near-neutral, improper application that temporarily lowers the soil pH too much can inhibit the absorption of other nutrients. St. Augustine grass is also known for developing a thick layer of organic matter called thatch.
Thatch is a dense layer of dead and living stems, roots, and debris that accumulates above the soil surface. While a thin layer is beneficial, excessive buildup acts as a barrier, preventing air, water, fertilizer, and pest control from reaching the soil and roots. Applying organic material like coffee grounds in heavy layers or clumps can exacerbate this issue, creating a dense, water-repelling crust that hinders healthy growth.
Safe Application and Usage Guidelines
For homeowners who wish to use coffee grounds on their St. Augustine lawn, the method of application is paramount. It is necessary to use only used grounds and ensure they are completely dry before spreading. Wet, clumpy grounds will not distribute evenly and can form a mat on top of the thatch layer, potentially creating dead spots on the turf.
To ensure an ultra-thin and even layer, the grounds can be mixed with an equal volume of sand or topsoil before broadcasting. A light application is roughly one-half cup of grounds per square yard, which should be barely noticeable on the surface of the grass. The grounds must be scattered widely to prevent clumping, which could block water from reaching the roots.
Following application, lightly watering the lawn helps the finely dispersed grounds settle into the turf canopy. Due to the slow-release nature of the nitrogen and the risk of contributing to thatch, applications should be infrequent, perhaps only once or twice per year during the active growing season. Homeowners must monitor the grass closely for any signs of yellowing or stunted growth, which could indicate a nutrient issue or pH imbalance.
If the grass shows signs of stress, the application should stop immediately. A safer alternative is to mix the used coffee grounds into a compost pile with brown materials like dry leaves. Once the grounds are fully broken down into mature compost, they can be applied as a nutrient-rich top-dressing, which significantly reduces the risk of pH shock or thatch buildup.