Applying granular fertilizers requires understanding the product’s formulation and calculating the exact amount needed for a specific area. Using a balanced fertilizer like 13-13-13 requires a methodical approach to ensure plants receive the necessary macronutrients without risking over-application. This process involves translating general nutrient recommendations into a specific weight of product per square foot.
Understanding 13-13-13 Ratios
The three numbers displayed on a fertilizer package, known as the NPK ratio, represent the percentage by weight of the three primary macronutrients: Nitrogen (N), Phosphate (\(\text{P}_2\text{O}_5\)), and Potash (\(\text{K}_2\text{O}\)). For 13-13-13, this means the product contains 13% of each nutrient by mass. This equal proportion makes it a “complete” or “balanced” fertilizer, suitable for general-purpose feeding across a variety of plants and soil types.
Nitrogen is responsible for vigorous foliage growth and green color, while phosphate supports root development and energy transfer within the plant. Potash, the source of potassium, helps regulate water movement and enhances the plant’s overall resilience to stress and disease. The remaining 61% of the product consists of inert filler materials, such as clay or limestone, which help ensure the granules spread evenly.
Determining Required Nitrogen Input
The amount of fertilizer product to apply is primarily governed by the target weight of actual nitrogen needed for the area. Nitrogen is the nutrient that plants consume in the largest quantity, and its application rate is most often used as the standard for determining overall fertilizer use. The requirement varies based on the type of plant, its growth stage, and the desired level of maintenance.
For general turf maintenance, a common target application rate is between 0.5 and 1.0 pound of actual nitrogen per 1,000 square feet. A lower rate, such as 0.5 pounds of nitrogen per 1,000 square feet, is suitable for established lawns needing light feeding, while the 1.0 pound rate is used for more aggressive feeding or turf establishment. The most accurate way to establish a nutrient target for your specific soil is to perform a soil test, which provides precise recommendations for all three macronutrients. Without a soil test, using the nitrogen requirement as the baseline is the standard practice for calculating the necessary fertilizer quantity.
Calculating the Rate Per Square Foot
Translating the target nitrogen weight into a specific weight of 13-13-13 fertilizer involves a straightforward calculation. The standard formula divides the target weight of the nutrient by the percentage of that nutrient in the fertilizer product. Since 13-13-13 contains 13% nitrogen, this percentage is converted to its decimal form, 0.13, for the calculation.
For a maintenance application targeting 0.5 pounds of actual nitrogen per 1,000 square feet, the calculation is: \(0.5\text{ lbs N} \div 0.13 = 3.85\text{ lbs of 13-13-13 product}\) per 1,000 square feet. To convert this to the weight needed for a single square foot, the result is divided by 1,000, yielding \(0.00385\text{ pounds}\) per square foot. Converting this to ounces results in approximately \(0.062\text{ ounces}\) of 13-13-13 product per square foot.
For an establishment or heavy-feeding application targeting 1.0 pound of actual nitrogen per 1,000 square feet, the required product is exactly double. The calculation is: \(1.0\text{ lbs N} \div 0.13 = 7.69\text{ lbs of 13-13-13 product}\) per 1,000 square feet. This translates to \(0.00769\text{ pounds}\) per square foot. When converted to ounces, the required amount is approximately \(0.123\text{ ounces}\) of 13-13-13 fertilizer per square foot.
Safe Application and Timing
Once the precise weight per square foot has been calculated, the next step is the physical application of the granular product. For even distribution, it is recommended to use a calibrated broadcast or drop spreader, rather than attempting to hand-spread the fine granules. Spreader settings vary significantly by model and fertilizer brand, so it is important to calibrate your equipment or use the manufacturer’s suggested setting.
A common technique to achieve uniform coverage and avoid streaking is to split the total calculated amount in half. Apply the first half of the fertilizer by moving the spreader in one direction, such as north to south, and then apply the second half by moving perpendicular, or east to west, over the same area. This cross-hatch pattern minimizes the risk of applying too much product in one spot. To prevent “fertilizer burn,” the treated area should be lightly watered immediately after application to wash the granules off the foliage and into the soil. Avoid applying fertilizer right before heavy rainfall, as this can lead to nutrient runoff and environmental waste.