If the veins stand out as a little darker than the surounding area then it is iron deficiency.  Zinc deficiency usually goes along with it.  Nitrogen deficiency causes the leaves to look evenly lighter, no darker veins.  If you add iron now, you will notice little if any improvement as the tree takes up most iron as the leaves are first being formed.  The later in the summer, the less good it will do to add iron.  There are iron foliar sprays you can apply that will work this time of year.  They are not long lasting, but will help for now.  Next spring at bud swell, apply chelated iron.  Chelated is formulated to work best in our alkaline soils.