Fall Fertilization Sets Trees Up for Spring Success

Fall is an excellent time to fertilize trees and shrubs. With cooler temperatures and consistent rainfall, root development is typically vigorous during this season. As roots grow, they absorb nutrients that are either transported to the leaves or stored for use in the spring. When new leaves and twigs emerge, they can draw from these reserves to produce the food trees need to thrive.

The goal of fertilization is to supply the nutrients plants need without overapplying or introducing unneeded elements. A soil nutrient analysis is recommended to determine exactly what’s necessary, improving both tree health and environmental outcomes.

If a soil test hasn’t been performed in the past three years, a sample should be collected and sent to a laboratory for analysis. The resulting report outlines soil conditions and provides a detailed prescription for the essential nutrients required to maintain tree and shrub health.  

Urban Soil Often Lacks Key Nutrients

Trees and shrubs require 17 different nutrient elements. A deficiency in even one of these can weaken a tree and increase its susceptibility to pests and disease.  

In urban landscapes, nutrient deficiencies occur for a number of reasons. New construction often involves removing topsoil across the building area, leaving behind compacted low-nutrient subsoil for new plants. In established landscapes, removing fallen leaves each year reduces the amount of nutrients returned to the soil—unlike in forests, where leaf litter naturally decomposes and replenishes soil nutrients.  

Turf fertilizers are not always beneficial to trees. Turf grasses require a higher soil pH than most tree species, so annual turf applications may negatively impact surrounding trees. Fertilizer should only be applied when a known deficiency exists. Laboratory soil and foliar nutrient analyses are more effective than visual assessments of leaf color and twig growth at detecting deficiencies and identifying underlying causes such as improper soil pH. Laboratory tests can also reveal secondary nutrient deficiencies or excesses that may be missed through visual observation alone.

Soil pH Affects Nutrient Absorption

Soil pH, a measure of soil acidity or alkalinity, plays an important role in plant health. Most trees and shrubs prefer slightly acidic soil for optimal nutrient uptake. If the pH is too high or too low for the species, adjustments may be necessary. 

Understanding Fertilizer Labels

Most fertilizers are a blend of nitrogen (N), phosphorus (P) and potassium (K), available in both liquid and dry forms. Complete fertilizers contain all three macronutrients. Their labels include three numbers, such as 10-10-10, indicating the percentage by dry weight of each nutrient: total N, available phosphoric acid (P₂O₅) and soluble potash (K₂O). The order always remains N-P-K.

For example, a 10-6-4 fertilizer contains 10 percent N, 6 percent P and 4 percent K. A 50-pound bag would contain 5 pounds of N,
3 pounds of P and 2 pounds of K.

Fertilization methods vary by need. Fertilizers can have fast-release, slow-release or a combination. Understanding the specific nutrient deficiency and desired outcome helps determine the best formulation and application method.

Choosing the Right Application Method

There are four primary methods for applying fertilizer, each with specific goals and outcomes. Broadcast application involves spreading fertilizer directly onto the soil surface, while subsurface application places it below the surface. Trunk injections are typically used to address micronutrient deficiencies around the circumference of the tree. And foliar application involves spraying fertilizer onto the surface of the leaves.

Certified Help Is Available

Given the complexities of soil health and fertilization strategies, expert guidance can make all the difference. For best results, property owners are encouraged to consult a certified arborist from an accredited tree care company. These professionals have the training and equipment needed to ensure appropriate nutrient management and support long-term tree health.