Fertilizing a tree requires precise nutrient delivery to supplement the soil for healthy growth without causing harm. Tree fertilizers are typically formulated around the NPK ratio, which represents the percentage of Nitrogen (N), Phosphorus (P), and Potassium (K) by weight. Nitrogen is the element most often required, promoting vigorous shoot and leaf development. Phosphorus and potassium support root health and cellular function. Determining the correct amount is a balance, as under-fertilization provides no benefit, but over-fertilization can severely damage the tree’s root system and foliage.
Calculating Fertilizer Requirements Based on Tree Size
Calculating the exact amount of actual nitrogen (N) required is the foundation of tree fertilization. Arborists use two primary methods, focusing on the root zone where nutrient absorption occurs.
The first method uses the canopy spread. A common recommendation is to apply 1 to 4 pounds of actual nitrogen per 1,000 square feet beneath the canopy. A moderate rate for established landscape trees is 1.5 pounds of actual nitrogen per 1,000 square feet annually.
To use this, calculate the square footage of the area extending from the trunk to the drip line (the circle defined by the outermost branches). For example, a tree requiring 1.9 pounds of actual nitrogen would need 1.5 pounds per 1,000 square feet over its canopy area.
The second method, often used for smaller or younger trees, is based on the trunk diameter measured at breast height (DBH), 4.5 feet above the soil line. A standard recommendation uses a rate of 0.1 to 0.5 pounds of actual nitrogen for every one inch of trunk diameter. A healthy, growing tree might receive 0.25 pounds of actual N per inch of DBH.
Once the required amount of actual nitrogen is determined, convert that figure into the total weight of the commercial fertilizer product. This conversion depends on the fertilizer’s NPK ratio, specifically the percentage of nitrogen listed first. If a tree requires 1.9 pounds of actual nitrogen and the product is a 10-10-10 blend (10% nitrogen), divide the required amount by the decimal form of the percentage (1.9 lbs / 0.10). The result is 19 pounds of the 10-10-10 fertilizer product needed.
Application Methods and Dosage Distribution
Once the precise dose is calculated, the delivery method must ensure nutrients are evenly distributed across the root zone. Fertilizer should be applied starting a few feet from the trunk and extending outward.
The simplest approach is a surface or broadcast application, where granular fertilizer is spread uniformly over the soil. This technique works well for nitrogen, which is highly water-soluble and moves readily into the soil with irrigation or rainfall.
A more effective strategy, especially for non-mobile nutrients like phosphorus and potassium, is the drill-hole method. This involves creating a grid pattern of holes, 8 to 12 inches deep and spaced 2 to 3 feet apart, extending to the drip line and slightly beyond. The total calculated fertilizer amount is divided equally among all the drilled holes to ensure uniform coverage.
Fertilizer spikes offer a convenient alternative for homeowners, driven into the soil following a similar grid pattern to the drill-hole method. However, the fertilizer concentration is highest immediately surrounding the spike, which can cause localized root damage if the total dosage is not accurately distributed. The calculated total amount of fertilizer must be spread or divided evenly over the entire root zone to prevent hot spots.
Annual Timing and Frequency of Feeding
The optimal time to apply fertilizer is when the tree’s roots are actively growing, which is outside the canopy’s peak growing season. The two most beneficial periods are late fall, after the leaves have dropped and the tree is dormant, or early spring, just before buds break. Applying fertilizer during these cooler periods allows roots to absorb and store nutrients as reserves for the next flush of growth.
Avoid fertilizing in late summer or early fall. This can stimulate a late-season flush of tender growth that does not have time to harden off before cold weather, making the tree susceptible to winter injury.
The frequency of feeding depends on the nitrogen source. Quick-release nitrogen forms may require annual application, especially for younger trees. Controlled-release or slow-release formulations extend the feeding period, often requiring application only once every two to three years.
Recognizing and Addressing Over-Fertilization
Applying too much fertilizer, or failing to water it in adequately, can cause “fertilizer burn,” resulting from a high concentration of soluble salts. These excess salts disrupt the tree’s ability to absorb water through osmosis, drying out the roots and foliage. Early signs of this osmotic stress include the browning or “scorching” of leaf margins and tips.
Other indicators of excessive nutrient load are wilting leaves, a white or yellow crust of salt residue on the soil surface, or excessive, weak succulent growth. This rapid, soft growth is often structurally weak and attracts pests.
To mitigate damage, leach the soil by flushing the area with water. This involves slowly and deeply saturating the root zone for an extended period, dissolving excess salts and carrying them below the root zone.