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