Yellowing leaves with dark green veins (also known as interveinal chlorosis) is a common symptom of a nutrient deficiency problem.  However, determining which nutrient is deficient will involve closer inspection of the plant. 

First, you’ll need to determine which part of the plant is exhibiting the symptoms.  If symptoms are present in the older leaves, then we can narrow it down to Magnesium or Potassium.  In broadleaf trees and shrubs, a Magnesium deficiency is characterized as interveinal leaf chlorosis (or in some plant species as an inverted green V at the leaf base), whereas Potassium is typically characterized by leaf tip or leaf margin necrosis, with or without interveinal chlorosis.

If the symptoms are present in the newest or recently mature leaves, then we can narrow it down to Manganese, Iron, and possibly Zinc.  If the leaves are not in rosettes (a circular arrangement that resembles a rose), we can eliminate Zinc as the cause.  A plant with Manganese deficiency often has prominent wide green veins verses the fine green veins exhibited with an Iron deficiency.

A good resource for diagnosing plant nutrient deficiencies can be found at: http://hort.ifas.ufl.edu/database/nutdef/index.shtml

While pinpointing which nutrient is deficient in the soil may help you determine which fertilizer you’ll need, it is important to note that a soil pH test may also be in order if the fertilizer does nothing to correct the problem.  All plants have an optimal soil pH range.  If the soil pH is too high or too low for the species, the plant will be unable to take up certain nutrients even if there is a high concentration of the nutrient in the soil.  For example, plants growing in alkaline soils often exhibit micronutrient deficiency symptoms that can only be corrected by lowering the pH of the soil.  However, for areas with naturally alkaline soils or elevated pH soils near new construction (some construction materials leach alkaline materials), selecting plant species tolerant of high pH soils is recommended.