How To Fertilize Shade Trees

Introduction

Healthy trees usually do not need fertilizer; however, its use can improve appearance, growth and vigor. Fertilizer is used to help trees resist or recover from:

  • stress caused by adverse weather
  • chemical injury
  • physical damage to roots, stems or branches
  • attacks by pest organisms

In some cases, fertilizer is necessary to correct nutrient imbalances or deficiencies.

Organic fertilizers and mulches improve both the physical and chemical suitability of soils for trees. If the soil around a tree appears to be in good condition, yet the tree is growing poorly, or its foliage is sparse and off-color, then fertilizer may help.

Nutrient availability to tree roots depends on soil quality. Soil aeration and drainage is more important than nutrient deficiencies. The quality of a soil is determined by:

  • Physical properties,
  • moisture characteristics,
  • temperature,
  • organic content,
  • micro-organisms
  • pH (acidity or alkalinity)

Getting Started

Soil tests and foliage symptoms are often used to diagnose problems resulting from a nutrient deficiency or imbalance. Nitrogen is the most commonly deficient soil nutrient, and trees usually respond to a nitrogen application. Adequate amounts of the other essential nutrients are available to trees in most soils. It is usually most economical to apply only nitrogen unless another nutrient is known to be deficient.

High nitrogen fertilizers are commonly available through farm and garden supply stores. They are usually about 30 percent nitrogen by weight. Nitrate formulations are immediately available to plants, but are readily leached from the soil; ammonium and urea formulations are not immediately available to plants, but last longer in the soil. Slow release fertilizers provide nitrogen over a much longer period. These varieties are ideal for sandy soils that receive heavy summer rains or irrigation.

In addition to nitrogen, most fertilizer formulations include phosphorous and potassium. These formulations use numbers, such as 10-10-10 or 10-6-4, to indicate the percentages by weight of these ingredients. For example, a 40-pound bag of 10-6-4 fertilizer has 4 pounds of nitrogen, 2.4 pounds of phosphorous and 1.6 pounds of potassium. The remaining 32 pounds of material is an inactive carrier. A fertilizer composed primarily of nitrogen, such as 27-3-3 or 26-4-6, is a good one for trees.

Nitrogen is also released through the decomposition of organic matter. Packaged organic fertilizers, such as manure, cost more per unit of nutrient, and do not release nutrients uniformly. Organic fertilizers have the advantage of improving soil condition as well as fertility, and are derived from renewable resources. Appropriate application of organic fertilizers must be based on experience.

Nutrient deficiencies such as potassium, phosphorous, iron, maganese, zinc, boron, magnesium, copper and calcium occasionally affect shade trees. These deficienies usually occur in extremely acid or alkaline soils or where the topsoil has been removed by grading.

Applying Fertilizer

  • Using the tree's size, determine how much fertilizer the tree needs. Use about one pound of high-nitrogen fertilizer (e.g., ammonium nitrate; 27-3-3), or three pounds of 10 percent nitrogen fertilizer (e.g., 10-6-4) for every inch of trunk diameter at chest height.

For example, a tree that is 10 inches in diameter at chest height needs about 10 pounds of high nitrogen or 30 pounds of 10 percent nitrogen fertilizer. This amount of fertilizer should be spread evenly around the tree over an area of 1000 square feet, which is equivalent to a circle about 36 feet in diameter. If less space is available, do not apply the full amount all at once; water it in gradually over a period of several days.

  • Apply nitrogen on the soil surface and water it in lightly. Loosen the soil first if it is severely compacted. Arborists sometimes apply nitrogen as a foliage spray to obtain immediate results.
  • Phosphorous, potassium and calcium should be added to the soil if they are found to be deficient. These deficient nutrients can be sprayed on foliage to alleviate symptoms quickly and to determine a plant’s response. Some nutrients can also be injected directly into the stem. In most cases, soil application is least expensive and the best for long-term results.
  • Apply nitrogen in late winter or early spring. Late season release of nitrogen can make plants more susceptible to cold injury. Additional applications may be needed on sandy soils after heavy rains or irrigation.
  • For trees in lawns, apply nitrogen before rapid growth begins and when grass is dry. Water it in. A second irrigation the following day will minimize injury to grass from high concentrations of fertilizer, and will help move nitrogen to the tree roots.
  • Nutrients that are sprayed on foliage can be applied as soon as leaves are expanded. Phosphorous, potassium and calcium can be applied at any time.
  • Avoid soil compaction and try to maintain moisture at moderate levels. Mulching usually helps.
  • Follow the directions on the fertilizer package. Too much fertilizer or improper application can injure plants and pollute water.
  • Do not fertilize trees every year.

Inorganic fertilizers are manufactured primarily from nonrenewable resources. Don’t waste them.