Mineral deficiency. No one wants that - it weakens the plant and makes it vulnerable to other diseases and pests. If you grow stuff in containers or hydrocultures it's particurlary important to be able to diagnose it, since you can't count on Mother Earth to provide what's needed via the soil. And since Mother Earth sometimes can be pretty tight-fisted I think we all can benefit from this homepage
Thanks to Parkettmaken (the Indoor Gardener Hubby) who found the link!