Landscape and urban trees typically grow in soil that does not contain sufficient nutritional elements due to disruption of the nutrient cycle by,
- lack of leaf regeneration
The key to tree fertilization is distributing the right amount of nutrients at the right time. Macro-nutrients are needed the most and most frequently deficient in a tree. The deficient tree will show signs of reduced growth, smaller leaves, and the yellowing of leaves.
Secondary nutrients or nutrients needed in moderate amounts include phosphorus, potassium, and sulfur. Micro-nutrients are nutrients that trees need in small amounts.
Many times it can be hard to tell which nutrient the target tree it is deficient in as symptoms may overlap.
However, it is worth finding out as you may be wasting fertilizer on a tree that needs a different treatment. If you see leaf discoloration or other unhealthy signs on the bark, test the tree with a soil kit
to get an idea of what it 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
Granular fertilizer is broadcast over the ground surface using a spreader. The advantage of this fertilizing method is its ease, which requires very little special equipment. The disadvantage is the large number of residual chemicals on your lawn.
2. Sub-surface Application
Sub-surface fertilization techniques are necessary when a tree is surrounded by turf grass, has limited chemical exposure, or the homeowner wants residual chemicals on the lawn. Turfgrass absorbs nutrients more readily than tree root systems so the fertilizer must be applied below turf level or it will never reach the tree and shrub roots. Sub-surface fertilization techniques are also necessary where runoff water is common. Two common techniques of sub-surface fertilization are the drill-hole method 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 turfgrass won’t reach it but shallow enough so it doesn’t leach (drain away) especially during the rainy months of spring.
The 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. We sometimes recommend lawn aeration and fertilization in conjunction with liquid injection fertilization to combat that.
3. Foliar Application and Tree Injection
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. We will use a growth stimulator such as Envirotree.
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 the trees must be large enough in terms of circumference, to handle the holes created.
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 needs deep root watering. We always look to see if a tree is water-stressed before making our proper recommendation.