Lawn fertilizer supplements the nutritional needs of turfgrass, providing elements often missing or depleted in the soil. While grass can survive on existing soil nutrients, a dense, vibrant lawn requires a steady supply of specific elements to support its fast growth cycle. Fertilizer composition is designed to deliver these elements, manage their release rate, and provide a physical medium for even application. The contents are divided into nutritional components and non-nutritional carriers and additives that support grass health and appearance.
Understanding the Primary Macronutrients (NPK)
The most prominent components of lawn fertilizer are the three primary macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K). These elements are required in the largest quantities for robust plant development. They are always represented by the three numbers, known as the NPK ratio, displayed on the fertilizer bag. The first number represents the percentage of Nitrogen, the second is Phosphate (P₂O₅), and the third is Potash (K₂O), signifying the concentration of each nutrient available by weight.
Nitrogen (N)
Nitrogen is responsible for the rapid growth of the leaf and stem, driving the deep green color in grass blades. As a component of chlorophyll, nitrogen plays a direct part in the plant’s ability to convert sunlight into energy through photosynthesis. Applying too much nitrogen can lead to excessive, weak growth that leaves the turf vulnerable to disease.
Phosphorus (P)
Phosphorus focuses on below-ground growth, promoting strong, deep root development. This nutrient is particularly important during the establishment of new sod or seedlings. It helps the plant efficiently transfer energy for cell division and growth, and adequate levels are linked to improved overall plant health.
Potassium (K)
Potassium helps regulate internal processes like water uptake and enzyme activation. It strengthens the grass’s ability to tolerate various environmental stresses, including drought, extreme temperature fluctuations, and disease. By promoting stronger cell walls, potassium ensures the turf remains resilient throughout the growing season.
Secondary and Trace Nutritional Elements
Beyond the NPK trio, lawn fertilizers may contain secondary macronutrients and micronutrients, which are essential for healthy grass but are needed in far smaller amounts. The three secondary macronutrients are Sulfur (S), Calcium (Ca), and Magnesium (Mg). Calcium and Magnesium are often present in adequate amounts in many soils. Sulfur is sometimes included in fertilizer formulations because it is a constituent of plant proteins and helps improve the turf’s uptake of the primary nutrients.
Micronutrients, or trace elements, are required in minute quantities yet are necessary for specific physiological functions within the grass plant. Iron (Fe) is one of the most common micronutrients added to fertilizer. It plays a direct role in chlorophyll production, giving the grass a deep, vibrant green color. A deficiency in Iron often results in chlorosis, which is a yellowing of the grass blades.
Other trace elements include Manganese (Mn), Zinc (Zn), Copper (Cu), and Boron (B), which contribute to overall metabolic health and enzyme activation. These elements are typically found in sufficient quantities in the soil. However, deficiencies can occur, especially in sandy or highly alkaline soils. Soil testing is the only way to accurately determine if a secondary or trace element supplement is necessary.
Non-Nutrient Carriers and Chemical Additives
A significant portion of granular fertilizer consists of non-nutritional components designed to ensure the product is effective and safe to apply. These materials, known as carriers or fillers, serve to dilute the concentrated nutrients. Without fillers, the small amount of pure nutrient material would be nearly impossible to spread evenly across a large lawn area.
Common inert materials used as fillers are selected for their low cost and ability to improve the fertilizer’s physical properties. These substances prevent the granules from clumping and ensure the product flows smoothly through a spreader for uniform distribution. Some fillers, like granulated limestone, can also provide the secondary nutrients Calcium and Magnesium while helping to manage soil pH.
The common inert materials include:
- Sand
- Clay
- Limestone
- Diatomaceous earth
Modern fertilizers often incorporate specific delivery mechanisms, such as slow-release technology. This is achieved by coating the nutrient granules with a semi-permeable polymer or resin. This coating acts as a barrier that controls the rate at which water can enter and dissolve the nutrients. The controlled release minimizes the risk of fertilizer burn, where highly concentrated nutrients damage the grass, and provides a steady supply of nutrients over an extended period.
Lawn fertilizer products are frequently combined with other active chemical additives to create multi-purpose products. The most common combination is a “weed-and-feed” product, which incorporates herbicides to control broadleaf weeds or pre-emergent herbicides to prevent weed seeds from sprouting. Some formulations contain insecticides to protect the turf from common pests like grubs or surface-feeding insects.