Fertilization

The best time to fertilize your trees is in mid to late fall or early spring. Start by conducting a soil test, which is available through your local tree or landscape service, or your County Agricultural Extension Office. A soil test will indicate whether the soil pH is within the proper range for your trees and identify any nutrient deficiencies.

The primary macronutrients—nitrogen, phosphorus, and potassium—are used in the largest quantities and are the most likely to be deficient, particularly nitrogen. Be cautious when using fertilizers containing phosphorus near waterways, as it can contribute to unwanted algal growth.

The secondary macronutrients—calcium, sulfur, and magnesium—are also required in relatively large amounts. Calcium is often lacking in soils and can be increased by adding gypsum or lime. However, be cautious with lime, as it raises soil pH, and most trees prefer an acidic environment. Dolomitic lime provides both magnesium and calcium, while gypsum adds sulfur along with calcium and can help improve soil structure.

If your trees are surrounded by turf, surface-applied fertilizer may be absorbed by the grass before reaching the tree roots. Most professional tree services address this by injecting liquid fertilizer several inches below the surface using a soil probe under pressure.

If your trees are mulched, you can pull back the mulch and gently work in some composted or dehydrated manure, or another organic compost, using a rake before replacing the mulch. Granular organic or slow-release chemical fertilizers can also be applied.

In addition to macronutrients, trees require various micronutrients in smaller quantities, including iron, manganese, zinc, copper, boron, chlorine, and molybdenum. These elements are typically present in sufficient quantities in the soil and rarely need supplementation.

However, soil pH affects micronutrient availability, and trees may exhibit deficiency symptoms even when the soil test indicates adequate levels. This is common in acid-loving plants, such as those in the Ericaceae family, which includes rhododendrons, azaleas, pieris, mountain laurel, cranberries, and blueberries. In neutral or alkaline soils (pH > 7), iron becomes less accessible, often resulting in yellowing between the leaf veins while the veins themselves remain green—a condition known as interveinal chlorosis.

Alkaline pH can similarly limit manganese availability, leading to similar symptoms, particularly in maples, dogwoods, and birch.

A soil test will reveal whether the pH needs adjustment or if any nutrients are deficient. Soil can be acidified with elemental sulfur. Iron sulfate and aluminum sulfate can also be used, but care should be taken with the latter, as excessive aluminum can be toxic to trees. Lime will raise a low soil pH, making it more alkaline, and should be avoided around acid-loving trees and shrubs. Always follow the labeled application rates on these products.

Finally, remember that any soil amendments should target specific deficiencies. Adding lime or fertilizer without a clear need is often a waste of money and can potentially harm your trees.