maybe this'll help...

I got this from the anarchists cookbook....

SYMPTOM PROBABLY PROBLEM/CURE

Larger leaves turning yellow - Nitrogen dificiency - add nitrate of soda or organic fertilizer.
smaller leaves still green.
----------------
Older leaves will curl at edges, Phosphorsus dificiency - add commercial
turn dark, possibaly with a purple phosphate.
cast.
--------------
Mature leaves develop a yellowish Magnesium dificiency -
cast to least veinal areas. add commercial fertilizer
with a magnesium content.
---------------
Mature leaves turn yellow and then Potassium dificiency -
become spotted with edge areas add muriate of potash.
turning dark grey.
-------------
Cracked stems, no healthy support Boron dificiency - add
tissue. any plant food containing
boron.
------------
Small wrinkled leaves with Zinc dificiency - add
yelloish vein systems. commercial plant food
-------------
Young leaves become deformed, Molybedum dificiency -
possibaly yellowing. use any plant food with a
bit of molydbenum in it.