From: Jennifer A. Cabbage <fxjac.camelot.acf-lab.alaska.edu>
This is information obtained from Charles Knight (my soils professor).

Element Function Deficiency symptoms in plants
Macronutrients:
N - nitrogen Component of all amino acids, enzymes, proteins, etc. Uniform yellowing appears first on lower leaves. Growth ceases.
P - phosphorous Essential for ADP, ATP, DNA, RNA. Involved in protein synthesis. Purple veins. Some plants: unform ashy color. Stunted growth.
K - potassium Regulates water uptake. Component of cell walls. Tip-burn, brown spots appear first on lower leaves. Weak stems.
Ca - calcium Component of cell walls. Involved in cell division. Uniform yellowing appears first on young leaves. Growing tips (roots and shoots) die. Plants dwarfed.
Mg - magnesium Component of chlorophyll. Enzyme activator. Yellowing of the veins. Yellowed areas die.
S - sulphur Component of all proteins. Important in enzyme reactions and photosynthesis. Uniform yellowing in whole plant. Thin lateral stems (exception: Cruciferae).
Micronutrients (only a few GRAMS per acre needed)
Fe - iron Component or co-factor of many oxidases. Yellowing of the veins. Larger veins stay green.
Mn - manganese Essential for photosynthesis, N-metabolism, N-assimilation. Yellowing of the veins in younger leaves.
Zn - zinc Promotes growth hormones, seed maturation and production Rosette. Small yellowed leaves. Scalloped leaf edges.
Cu - copper Important in photosynthesis, protein and carbohydrate metabolism. Pale yellow-bleached leaves. Leaves and stems not firm. Tip and stem dieback.
B - boron Essential for cell division + development. Synthesis of nucleic acids, plant hormones. Yellowing of younger leaves. Internal browning of cork of stems and fruit. Many flowers abort.
Mo - molybdenum Essential for N-fixation and assimilation. Uniform yellowing of whole plant. Extreme curling of leaves.
Cl - chlorine Influences photosynthesis and root growth. Unknown.
Co - cobalt Essential for N-fixation. Unknown.