3 Tree Fertilization Techniques by David Merriman

Landscape and urban trees typically grow in soils that do not contain sufficient elements due to disruption of the nutrient cycle by pavement, buildings and roads. Also, leaves, the driver of the nutrient cycle, are raked up before decomposing and micro-organism's can't break stuff down.

The key to tree fertilization is distributing the right amount of nutrients at the right time. Macro-nutrients are nutrients that trees need the most and is frequently deficient in a tree. Signs are reduced growth, smaller leaves and the yellowing of leaves. Secondary nutrients or nutrients needed in moderate amounts include phosphorus, potassium and sulfur. Micronutrients are nutrients that trees need in small amounts.

Many times it can be hard to tell what nutrient is deficient as the symptoms may overlap. However, it is worth finding as you may be wasting fertilizer on a tree that needs something different. If you see leaf discoloration or other unhealthy signs on the bark, test the tree to get an idea of what might be missing.

Tree fertilization application techniques vary based on the foliage, tree condition, the time of year and your preference for using "greener" techniques.

1. Surface Application

The fertilizer is broadcast over the ground surface using a spreader. The advantage is it's ease, with very little special equipment required. The disadvantage is lots of residual chemicals on your lawn.

2. Sub-surface Application

Sub-surface fertilization techniques are necessary when a tree is surrounded by turf grass or to limit chemical exposure. Turf grass absorbs nutrients more readily than tree root systems so the fertilizer must be applied below turf level. Subsurface fertilization techniques are also necessary where runoff water is common. Two common techniques are drill hole and injection.

The drill hole method involves drilling holes around the tree in concentric circles. Holes should extend to the drip line. This allows you to put fertilizer deep enough that turf grass won't reach it but shallow enough so it doesn't leach (drain away) especially during the rainy months of spring.

Liquid injection uses fertilizer, dissolved or suspended in water. The solution is injected into the soil using a soil injection system. Advantages are better distribution and the benefit of adding water directly into the root zone. A disadvantage is liquid injection can create dark, vigorous patches of grass. I sometimes recommend a lawn aeration and fertilization in conjunction with a liquid injection fertilization to combat that.

3. Foliar Application and Tree Injection

This involves spraying everything with FDA approved chemicals. Which is OK, as long as we remember that DDT was a federally approved chemical at one point! Foliar application is a short term fix to correct minor elemental deficiencies of micro nutrients. Typically spraying the leaves works best in spring, right before a period of active growth.

Implants and injections are for minor nutrient deficiencies. Tree implants and injections provide a systemic application and can be combined with insect control. The main advantage is that it completely eliminates any residue outside of the tree. However, because it involves creating holes in the tree it is limited to a once a year process and trees have to be large enough to handle it. I always look to see if a tree is water stressed before doing this.

Finally, trees may not require additional fertilization at all. Over fertilization can burn out leaves or cause a tree to grow too quickly resulting in frequent pruning or removal. In Denver or other dry climates, homeowners may fertilize a tree that actually just needs a deep root watering.


About the Author

David Merriman owns ArborScape, a tree service in Denver, CO He is an ISA Certified Arborist and writes about lawn care and tree care.tips fertilization,