Tree Fertilization...

here is conflicting information on the benefits of fertilizing shade trees, and on the type of fertilizer that should be used. The only way to know for sure what is needed in your soil is to take a soil sample to a lab for testing. Texas A&M University has a soil lab that will perform this service. Your county extension office can provide more information.

The most current research on fertilizing shade trees indicates that nitrogen is most often the limiting factor. If you choose to fertilize without a soil test, look for a slow-release fertilizer that is high in nitrogen. The fertilizer can be broadcast on the ground surface in late spring or early summer and watered-in. If you have adequately mulched the tree, you probably do not need to add fertilizer..

Be aware that mature post oaks do not seem to tolerate intensively maintained lawn situations. They are sensitive to construction, soil compaction, drainage changes, excessive water, turf competition, etc. These stress factors and others cause the tree to decline and set it up for disease. Do not provide extra water and nutrients to mature post oaks. The more you leave them alone, the better they like it! (Texas Forest Service).

Some general rules from research suggest that:.

1. Injection of fertilizer into the stem of the tree is no more effective than other methods and in some cases is detrimental to tree health because of wounding.
2. Deep root treatments are much less effective than surface or shallow soil injection treatments because over 90% of the roots capable of absorbing nutrients are located in the top 18 inches of soil. Deeper roots are structural support.
3. Most trees do not need fertilizer, especially native species. Fertilizer applications may cause some trees to be more prone to insect and disease problems. Mulch can eliminate the need for fertilizer and provide many other benefits.
4. More expensive treatments and application of “complete” fertilizers often do not help the tree any more than an inexpensive application of only the needed elements.
5. Fertilizer is not medicine, it may cause additional stress to trees that already have been impacted.
6. Applications of mychorrizae, aeration, mulch, compost, and organic matter may increase nutrient uptake and improve root health. These applications may be preferred to applying fertilizer or can be used to reduce the amount of fertilizer needed.
7. Fertilizer causes accelerated growth in most cases. Make sure your tree does not outgrow its space. Tree roots can expand 2-3 times the width of the canopy.