Combining grass seed and fertilizer applications into a single step seems logical for saving time and ensuring emerging seedlings have immediate access to necessary nutrients for growth. However, successfully pairing these materials requires careful attention to the fertilizer’s chemical composition. Applying the wrong type of fertilizer at the time of seeding can negate all efforts, transforming the intended nutrient boost into a source of irreparable damage for the delicate germinating seeds.
The Danger of Seed Burn
Applying a standard lawn fertilizer alongside grass seed introduces a significant risk known as “seed burn” or salt injury. Most common fertilizers intended for mature lawns contain high concentrations of nitrogen and are formulated with highly soluble mineral salts. When these salts dissolve in the soil’s moisture, they temporarily create a high salt concentration around the seed and the young root structure.
This elevated salt concentration interferes with the natural process of osmosis, which is how the seed and roots typically draw water from the surrounding soil. Instead of the plant absorbing water, the osmotic pressure gradient reverses, causing water to be drawn out of the seed and fragile root cells toward the higher salt concentration in the soil. This process effectively desiccates the germinating seed or seedling, leading to dehydration and failure to establish. The delicate period during germination is when the seed is most susceptible to this salt-induced dehydration.
Identifying and Using Starter Fertilizers
To prevent the damaging effects of salt injury, a specialized product known as a starter fertilizer must be used instead of a standard lawn food. Starter fertilizers are chemically formulated to support the specific needs of new grass and are designed to have a lower salt index. This formulation ensures they do not create the high osmotic pressure that causes seed burn, allowing the seedlings to absorb water freely.
The key difference lies in the NPK ratio, which represents the percentage of Nitrogen (N), Phosphorus (P), and Potassium (K) in the blend. Starter fertilizers emphasize phosphorus, the middle number in the ratio, which is crucial for promoting robust and rapid root development. Typical starter fertilizer ratios might appear as 10-20-10 or 18-24-12, indicating a higher proportion of phosphorus compared to standard maintenance fertilizers.
These formulas also often incorporate a portion of nitrogen in a slow-release form, which provides a steady, measured supply of this nutrient over time. This slow-release mechanism further mitigates the risk of a sudden, high concentration of soluble salts that could harm the new growth. Reading the product label to ensure the fertilizer is explicitly marketed as a “starter fertilizer for new lawns” is the most reliable way to select the correct, low-salt product for seeding.
Application Methods: Mixing and Layering
Once the correct starter fertilizer is selected, the application method becomes the next practical consideration. While it is technically possible to physically mix the starter fertilizer and the grass seed together in a spreader, this is generally not the preferred method. Mixing can lead to uneven distribution, resulting in patches where the seed density is too low or where fertilizer granules are clustered, potentially still causing localized burn.
A more reliable method is a separate application, often referred to as layering. This technique involves spreading the starter fertilizer first, and then immediately following with a separate pass to spread the grass seed, or vice versa. Applying the materials in two passes with a calibrated spreader ensures each component is distributed uniformly across the entire area.
After both the starter fertilizer and the seed have been distributed, lightly working them into the top half-inch of soil helps ensure good seed-to-soil contact. This step maximizes the seed’s ability to draw moisture and the roots’ ability to access the phosphorus-rich fertilizer as soon as germination begins. The layering method is favored by professionals because it offers greater control over coverage, leading to a consistently dense and healthy turf establishment.