Cold Root Zone, Poor Fertility
Phosphorus deficiency is most often manifested as purpling of the leaves, particularly the leaf veins. In severe cases the whole plant may take on a purple hue. Tomato roots growing in cold soil, either in the greenhouse or the field, take up phosphorus poorly. Deficient plants lose vigor and yield poorly.
While phosphorus deficiency due to improper fertility may be a problem in hydroponic, pot culture, and some sandy soil conditions, it is most likely a result of cool root zone temperatures. Plastic mulch will alleviate the problem in early field plantings, and a balanced liquid feed fertilizer will alleviate the problem in greenhouse and pot culture.