Fertilizer is essential for healthy grass growth, but whether it kills grass seed depends entirely on the type used and how it is applied. Using the wrong product or applying too much standard fertilizer can prevent germination or kill tender new seedlings. However, specialized fertilizer is beneficial for establishing a strong, healthy lawn from seed. The key is understanding the difference between maintenance fertilizers designed for mature turf and starter fertilizers formulated for new grass.
The Critical Difference: Salt Content and Nitrogen
The primary mechanism by which fertilizer can harm grass seed is through a process known as “fertilizer burn,” which is osmotic stress caused by the product’s salt content. Commercial granular fertilizers contain mineral salts, such as ammonium sulfate and potassium chloride, that provide nutrients to plants. When these salts are highly concentrated in the soil directly surrounding the grass seed, they prevent the seed or emerging root from absorbing water.
This high salt concentration draws moisture away from the delicate seed or newly germinated root through osmosis, causing the seed to dry out and fail to germinate. This effect is magnified when using general-purpose lawn fertilizer, which has a high percentage of nitrogen (N) and contributes to a higher “salt index.” Established grass can tolerate higher salt levels, but tender new seedlings are vulnerable to this chemical desiccation. Applying fertilizer on dry soil also amplifies this danger, as the salts immediately bind to the limited moisture, creating a localized drought around the seed.
Essential Nutrients for Seedling Establishment
The solution to avoiding fertilizer burn while still nourishing new growth is to use a starter fertilizer. Unlike standard lawn foods that focus on high nitrogen for green leaf growth, starter fertilizers prioritize different nutrients. These specialized products are designed with a lower salt index to protect vulnerable seeds and emerging roots.
The primary nutrient for new turfgrass is phosphorus (P), the middle number in the N-P-K ratio found on fertilizer bags. Phosphorus plays a direct role in energy transfer and is vital for developing a strong, extensive root system. This well-developed root system allows the seedling to anchor itself and absorb water and nutrients efficiently. Starter fertilizers typically have a higher phosphorus content, often seen in ratios like 12-24-8 or 18-24-12, contrasted with maintenance fertilizers. Potassium (K), the third nutrient, is also included to help new grass develop resistance to environmental stress and disease.
Timing Fertilizer Application Around Seeding
The proper timing of fertilizer application is nearly as important as using the correct product to ensure the success of the new lawn. Starter fertilizer should be applied either just before or immediately after the grass seed is sown.
Applying the fertilizer simultaneously with or right after seeding ensures that essential nutrients, especially phosphorus, are available in the soil profile as soon as the seeds germinate and roots begin to emerge. This application method allows the nutrients to be worked into the top layer of soil, maximizing their availability to the young roots. It is helpful to water the area lightly after application to begin dissolving the fertilizer granules and move the nutrients into the soil.
The first follow-up feeding should occur four to six weeks after the initial application, or after the new grass has been mowed twice. The grass is then established enough to handle a transition to a higher-nitrogen fertilizer to promote leaf density and color.