Sprinkling fertilizer on top of the soil, known as top dressing or broadcasting, is a common and effective practice. This method involves applying granular or powdered fertilizer directly onto the surface without tilling or mixing it into the soil. When done correctly, surface application delivers necessary nutrients to lawns, garden beds, and container plants. Success depends on using the correct fertilizer type and following up with proper irrigation.
How Surface Application Works
For nutrients to become available to plant roots, solid fertilizer granules must first dissolve and move down into the soil profile. This process depends entirely on moisture; the fertilizer must be watered in or rely on rainfall to be activated. As water penetrates the soil, it dissolves the salts and compounds within the granules, creating a nutrient-rich solution.
Nutrient availability is governed by its mobility within the soil structure. Highly mobile nutrients, particularly nitrogen (in the form of nitrate), move easily downward with water flow, a process called leaching. This mobility allows nitrogen to quickly reach the root zone even from a surface application.
Conversely, nutrients like phosphorus and potassium have low mobility because they bind tightly to soil particles. For these nutrients to be taken up, roots must grow into the immediate area where the dissolved nutrients are concentrated. Surface application is most efficient for delivering mobile nutrients but still works for others, provided sufficient moisture is applied.
Matching Fertilizer Type to Sprinkling
Choosing the right fertilizer formulation is the most important factor for successful surface application. Different types of granules release nutrients at varying rates, affecting application frequency and the risk of plant damage. Matching the release rate to the application method ensures maximum nutrient uptake and safety.
Slow-Release Granular Fertilizers
Slow-release and controlled-release granular products are the ideal choice for sprinkling directly onto the surface. These fertilizers often use a polymer coating or organic material to regulate the nutrient dissolution rate. The slow, steady release minimizes the risk of over-fertilization and burn, providing consistent nourishment over weeks or months. Organic granular products, which rely on microbial activity to break down, are also gentle on plants when applied as a top dressing.
Quick-Release Granular Fertilizers
Quick-release granular fertilizers, also called water-soluble, can be sprinkled on the soil surface but require immediate attention. These products are highly concentrated salts that dissolve rapidly upon contact with moisture. If left sitting on the soil or turf, they create a high concentration of dissolved salts that can quickly dehydrate and damage plant tissue. To prevent this, quick-release granules must be watered in immediately after application to dilute the salts and move them away from plant stems and foliage.
Powdered/Water-Soluble Fertilizers
While water-soluble powders can technically be sprinkled, this is generally the least effective application method. These fertilizers are formulated to dissolve instantly in water for liquid application, ensuring uniform coverage and immediate root absorption. Sprinkling the dry powder can lead to uneven distribution and a high risk of localized burning if the powder clumps or is not completely dissolved by irrigation. It is safer and more efficient to mix these products with water before applying.
Preventing Fertilizer Burn and Runoff
The primary risks associated with surface application are fertilizer burn and environmental runoff, both manageable with careful technique. Fertilizer burn occurs when the high concentration of soluble salts draws moisture out of plant cells. This results in the characteristic browning or scorching of leaves, stems, or grass blades.
To prevent this damage, immediately water the fertilized area after application, especially when using quick-release products. Watering dilutes the salts and flushes them down into the soil where they can be absorbed by the roots, rather than sitting on the plant tissue. Aim for about a quarter to a half inch of water to move the nutrients past the immediate surface.
To mitigate nutrient runoff, avoid applying fertilizer just before a predicted heavy rainfall or storm. Applying fertilizer to saturated soil or steep slopes also increases the chance that the product will wash away, wasting material and polluting nearby waterways. Applying the granules during a dry period and then lightly watering them in provides the best control over nutrient placement and reduces off-site transport.