You can apply starter fertilizer to established grass, but it is rarely the most beneficial or appropriate choice for routine maintenance. Fertilizer products are formulated with specific nutrient ratios to address different needs. The unique composition of starter varieties is primarily designed for the demanding process of germination and initial root establishment. Understanding the specific function of these nutrients helps explain why their use is generally reserved for new turf.
The Purpose of Starter Fertilizer
Starter fertilizer is chemically distinct from typical lawn maintenance products because it has a significantly higher proportion of phosphorus (P), the middle number in the N-P-K ratio. This nutrient ratio is intentionally skewed to support new growth beneath the soil surface. Phosphorus is involved in energy transfer, cell division, and the development of nucleic acids. Its primary function in grass is to stimulate robust and deep root systems in young seedlings. Since new grass plants have limited root structures, they are poor at scavenging for nutrients already present in the soil. The concentrated, readily available phosphorus in a starter blend gives these tender new roots the immediate boost they need to anchor and develop into a dense turf. For example, while a maintenance fertilizer might have a 20-4-12 ratio, a starter product is often balanced or high in P, such as 10-10-10 or 18-24-12. This composition is not optimized for the established, nutrient-scavenging root system of mature grass.
Specific Scenarios for Using High-P on Established Lawns
Despite the general advice to avoid high-phosphorus fertilizers on established turf, there are two specific scenarios where this application is justified.
Correcting Soil Deficiency
The first and most reliable reason is when a professional soil test confirms a severe deficiency of phosphorus in the soil. Established grass, particularly if clippings have been consistently removed over time, can sometimes deplete the available phosphorus to levels that impede healthy growth. In this case, the soil test will provide a specific recommendation for the amount of phosphorus needed to correct the imbalance, making a high-P product a temporary, targeted treatment rather than a routine application.
Major Lawn Renovation
The second scenario involves major renovation or repair of an existing lawn, such as heavy aeration, deep dethatching, or significant overseeding. When overseeding, the goal is to introduce new grass plants, which require the same high-P environment as a newly seeded lawn to germinate and establish their roots. Applying a starter fertilizer during this process ensures the new seedlings have the necessary nutrient concentration right near the soil surface. This temporary use stimulates the new root growth without over-applying the nutrient.
Choosing the Right Maintenance Fertilizer
For a mature, established lawn that is already dense and healthy, the nutritional focus shifts away from root development toward leaf growth and overall plant resilience. Standard maintenance fertilizers are therefore formulated to be high in Nitrogen (N) and Potassium (K) while containing little to no Phosphorus (P).
Nitrogen is the nutrient responsible for the vibrant green color and rapid shoot growth that homeowners typically desire. Potassium, the third number in the N-P-K ratio, enhances the grass plant’s internal strength and tolerance to environmental stressors. It helps the turf withstand periods of drought, extreme heat, or cold, and improves its resistance to various diseases. A common maintenance ratio might be 27-0-10, indicating a focus on Nitrogen and Potassium, with zero Phosphorus. Selecting a maintenance fertilizer that includes a blend of both quick-release and slow-release Nitrogen components will provide immediate color while ensuring sustained feeding over several weeks.
Application Guidelines and Environmental Safety
When applying any granular fertilizer to an established lawn, proper technique is necessary to ensure the product is effective and does not cause damage. Always calibrate your spreader to the recommended setting for the product you are using to avoid applying too much nitrogen, which can lead to chemical burn, often called “fertilizer burn.” A general guideline is to never apply more than one pound of actual nitrogen per 1,000 square feet in a single application.
Immediately after spreading, the fertilizer must be watered into the turf with about a quarter to a half-inch of irrigation to dissolve the granules and move the nutrients into the root zone. This step is necessary to prevent the concentrated salts from drawing moisture out of the grass blades.
Furthermore, strict environmental regulations in many states and municipalities now prohibit the use of phosphorus fertilizers on established lawns unless a soil test proves a deficiency. This is due to the risk of phosphorus runoff into storm drains and waterways, where it can contribute to water quality issues. Granules that land on impervious surfaces like driveways or sidewalks should be swept back onto the lawn immediately to prevent this runoff.