| [College of ACES] | [University of Illinois] | [Illinois CES] |
I may catch some flack from some devout fertilizers here, but let me offer some reasons fertilizing may have killed teh tree.
First, you have to know how water gets into trees. At teh cellular level, roots have a semi-permeable membrane that allows water to pass through but not larger molecules. When there is more salt (minerals) in teh root than the soil, water naturally flows toward the saltier side of teh membrane, into the root. If too much fertilizer builds up in the soil, the water flows the opposite way, because the soil is saltier. The net effect is that the soil pulls water from the tree.
This is one way trees are killed; anyone who fertilizes without first testing a soil sample for a deficiency is taking chances with your trees.
Another possibiity must start with a knowledge of how a tree uses resources. Energy is created in the leaves through photosynthesis, it is stores as starch in live tissue, and it is used for five purposes: Growth, maintenance, replenishment of soil, reproduction, defense. In that order. So, say a tree has 100 units of starch. It puts on a crop of leaves and one inch of twig growth, spending 25 units of starch. It automatically spends another 25 on maintenance. It dumps 10 units into the soil, spends 20 making flowers and nuts, and has 20 left over to defend itself against stress (maybe construction damage, drought, insect problem, whatever).
Now look at the same tree after fertilization. It now spends 35 units on growth, 35 on maintenance, 10 to the soil, 20 on reproduction, and it has nothing left for defense. If you start with a weak tree that only has 70 units, you never even get to reproduction (did you notice how last year's acorn crop compared to surrounding trees? Did it produce as many catkins as its neighbors?). Another factor is that, if the tree is dying, it spends a larger percentage on the soil, to feed nearby trees with its starches before it dies and they are wasted.
Third, blackjack oaks are less tolerant than some of living around development. Has the soil grade changed recently? has increased foot traffic (or parked cars) increased soil compaction? Has grass been planted on top of teh root zone? Has an irrigation systemor utility service recently resulted in severed roots within the outer edge of teh branchline? Has the tree been getting unusually large amounts of water recently, or been subject to a drougth with no supplemental watering? Have large branches been removed to let in more light, provide roof or yard clearance, etc.? All of these factors and more could cause the decline and death of the tree, regardless of fertilization.
There's no way to offer a definite answer without a site inspection, but these are some things to think about when trying to diagnose the problem, and some things the arborists you talk to should be asking you about. If they walk up and verify whatever problem you ask them about without any questions or analysis, look for someone else.
Good luck,
Keith