Deficiencies of nutrients in plants have various visual symptoms that are usually similar regardless of the species. The most common deficiency symptom is reduced growth, which is difficult to detect and diagnose at a glance. Other visual symptoms usually involve changes in coloration following a specific pattern, such as from the leaf tip down the midrib towards the base of the leaf or from the leaf margin toward the midrib, or between the veins of the leaf. Such symptoms may appear in new leaves or old leaves, indicating the phloem-mobility of the deficient nutrient and the ability of the plant to translocate existing stocks of the deficient nutrient. In many cases, internodal distances will shorten as well.
Many nutrient deficiency symptoms are ambiguous unless they are well-developed, and a visual diagnosis can be regarded as an educated guess until tissue samples are gathered and chemical analyses are used to compare elemental composition with healthy leaf tissue. In fact, many types of environmental and management damage can masquerade as visual nutrient deficiency symptoms.
Lest we become too centered on what plants require and their deficiencies, it may be remembered that plants are autotrophs, gathering solar energy for fixation of atmospheric carbon into energy-rich compounds consumed by heterotrophs, which gather the energy-rich plants for consumption and completion of their (and our) life cycles. In addition to energy, plants also harvest the mineral nutritients required by animal life. For the most part, the essential elements required by higher animals are similar to those of plants. However, animals require sodium (only beneficial in plants), selenium (beneficial to only a small group of Se-hyperaccumulating plants, iodine (essential only to certain marine alga), silicon (beneficial to a number of plant species), and cobalt (essential to N2-fixing Rhizobium symbiotically associated with leguminous plants). Yet other trace elements essential to animal and human nutrition, but apparently neither essential nor beneficial for plants, are chromium, lithium, fluorine, and vanadium (although some propose that chromium and vanadium might be essential or beneficial to plants as well but are required at levels too low to demonstrate the effect of their absence.) Were it not for the fact that plants accumulate not only the elements essential to plants but also incidentally accumulate the elements required by animals, life on earth would be quite different!
(see: L. R. McDowell 1992. Minerals in animal and human nutrition. Academic Press, San Diego.)
This page was last modified by Phillip Barak, Univ. of Wisconsin, on 5 Aug 99. All rights reserved.