Question
I live in Wichita, Ks and have a very mature Pin Oak tree.  I have noticed this spring that 1/3 of the S.E. part of the tree is not leafing out.   Leaves are thin and yellow in color.   I have several dead limbs on the bottom of the tree.   I wish to prune them out if that is ok for this time of year.  Currently this tree is approx. 70 foot tall.  It's already pruned up about 10 ft. from years past.  It seems to me that the tree is trying to die or has some kind of disease.   I don't see signs of blight or fungi.  It also stands approx. 30 foot from another pin oak that is the same height and does have full leaves but are also yellow in color.  
Any recommendations?   I hate to lose these trees.
tks,

Answer
Sounds like a soil problem causing iron chlorosis. This will also have a great effect on the growth of the trees.

Yellow leaves and green veins are often the result of a common problem called iron chlorosis. This is the result of either an iron deficiency or iron unavailable for plant uptake.

Iron plays an important role in producing chlorophyll, a green pigment involved in the absorption of light needed for plant growth. A deficiency in iron results in a reduction in chlorophyll producing a yellowing of the leaves. This is an early sign of iron chlorosis. On deciduous trees, the leaf veins remain green while the area between the veins turns yellow-green to yellow. Leaves that emerge in early spring are frequently normal size, while those that emerge later in the growing season are often smaller and more yellow. In severe cases, the leaf color may change from yellow to white and finally brown. In advanced stages, twig dieback may be observed, especially toward the top of the tree and the ends of long branches. After several years, the tree may die if the condition is not treated.

Iron chlorosis is often a problem of pin oaks, though other tree and shrub species can be affected. Chlorosis is typically associated with one of three site conditions--a soil with a pH above 7 (an alkaline soil), a heavy soil containing a high percentage of clay, or a site where there has been extensive soil fill or removal, compaction, or construction injury that has damaged the root system. Older, established oaks are especially subject to the latter type of injury.


I think I would check with the local Agriculture Extension Service and ask about getting a soil sample run. They can give you the information on how to do this. A soil sample will give you what elements are missing and how to correct them and the amount to use. It maybe as simple as adjusting the Ph so the roots can absorb the elements.
Normal fertilizer recommendation are 1 lb of 10-10-10 fertilizer per inch of trunk diameter scattered around the tree and watered in good. But since you are having more lesser element problems I would do the soil test first. Make sure that you tell the soil lab that this is for Pin oaks and they are showing signs of the leaves turning yellow.


Use soil applications to treat individual trees and shrubs, or small areas in a landscape, in the fall or early spring. A mixture of equal parts iron (ferrous) sulfate  and elemental sulfur can produce lasting results and is relatively inexpensive. Select an inorganic iron source with a high concentration of iron and one that is derived from iron or ferrous sulfate. Read labels to determine iron concentrations and forms in different products.

It is not practical or desirable to blanket an entire landscape with the elemental sulfur-ferrous sulfate combination. Instead, treat small areas by making holes 1 to 2 inches in diameter and 12 to 18 inches deep. Space the holes 18 to 24 inches apart around the area within the drip line (outer edge of crown) of affected trees and shrubs. Fill each hole with the iron sulfate-elemental sulfur mixture to within 4 inches of the soil surface. For a small tree make about six holes and use a total of about 2 Pounds of combined elemental sulfur and iron sulfate. Make holes with an auger or soil probe that removes soil to reduce compaction. Avoid damaging large, woody roots when making holes.

Some commonly available sources of iron and sulfur.

Source  Trade name Content (% iron)   
Inorganic iron:      
Ferrous sulfate Iron sulfate  20.5   
Ferrous sulfate Ionate 10.0   
Ferrous sulfate  Bandini 7 iron 7.0   
Ferrous sulfate Ironite  4.5   
Acidified mining residue Iron-Sul  20.0   
     
Iron Chelates:      
FeEDTA Sequestrene 12.0   
FeEDTA  Hampene Iron 9.0   
FeDTPA  Sequestrene 330 10.0   
FeEDDHA  Millers Ferriplus 6.0   
FeEDDHA Sequestrene 138 6.0   
FeHEDTA Che-Gro Iron  5.0   
FeHEDTA Versonol Ag Fe  5.0   
     
Sulfur:  %sulfur   
Elemental sulfur Elemental sulfur (powder form)  90.0   
Elemental sulfur Degra-Sul (prill form) 90.0   
Elemental sulfur Agri-Sul (prill form) 90.0

Check with your local nursery for either Iron sulfate or Iron chelate and sulfur