Bromine and its effects on plants
It was recently announced that bromine (Br) is an essential element in animals and plays a critical role in tissue development. In one experiment, fruit flies died when bromine was removed from their diet, but regained health and survived when it was added. However, there has been little recent research on bromine in plants. One report from the 1950s suggests that although it seems like bromine can be a partial substitute for chlorine in plants, the reason for it is not entirely clear. The data showed that plants grown under chlorine deficiency actually contained significantly more chlorine when bromine was present than they did in the absence of bromine. That is an interesting and kind of strange result, but it does not suggest plants require bromine. More research may indicate otherwise, but given that the biological function of bromine is in the animal-specific structure of tissues, a bromine requirement in plants does not seem likely.
Chromium and its effects on plants
Chromium (Cr) resides in the transition metal section of the periodic table of elements and is a close neighbor to manganese and molybdenum, which most definitely are required by plants. For that reason, and since chromium is essential in animals, it is worth taking a look at it from a botanical perspective. Complicating matters is the different chemical natures of the two main ionic forms of chromium: the hexavalent Cr(VI), which is quite hazardous, and the trivalent Cr(III), which is more benign.
Plants grown in soils high in chromium do not do well, but some experiments have shown a small amount of it to be a plant growth stimulant. In one early experiment, chromium at a concentration of 0.1 ppm in a nutrient solution appeared to slightly benefit lettuce, but only sometimes.
That seems pretty sketchy. Reports suggest that any beneficial response to chromium depends highly on the species and dose given, with only relatively low doses being effective. Because of its toxicity, the hexavalent form should not be considered for use in hydroponic nutrient solutions. Testing how your plants respond to Cr(III) might make for an interesting research project, but given the uncertainties surrounding its effect, general use in nutrient solutions is not recommended.
(Learn more about The Effects of Cobalt, Copper, and Chromium in the Garden)
Strontium and its effects on plants
Strontium (Sr) has a bit of a bad reputation mostly because of the existence of strontium 90, a radioactive isotope created in nuclear fission reactions. However, most strontium is not radioactive and poses no threat to humans or plants. Plants will take up strontium if it is present in the root zone and is able to take up strontium and calcium almost equally.
Research reveals that strontium has slight influences on plant health at low concentrations and may partially substitute for calcium, having a positive effect on chlorophyll. It is detrimental at higher concentrations. There is little evidence to suggest strontium plays a beneficial function in plants as long as sufficient calcium is available.
Selenium and its effects on plants
Selenium (Se) is another element required by animals but not for higher plants (although some photosynthetic micro-organisms do require it). Selenium is chemically similar to sulfur and plants take it up and transport it using mechanisms that are intended to work on sulfur.
Selenium can even take the place of sulfur in proteins, but it disrupts the activity of any protein it finds its way into. It has been suggested that some hyperaccumulators of selenium require it, since they grow better in soil with a high concentration of selenium, but this has not been proven and a selenium hyperaccumulator would not be desirable as a food plant.
If there is a known dietary selenium deficiency in a geographical area, it might make sense to include it at a low concentration in a nutrient solution for uptake by a food crop. Otherwise, there seems to be no need to consider selenium further, since it is more often considered a plant toxin rather than a plant nutrient.
Sodium and its effects on plants
Plants move sodium (Na) ions in and out of their cells to regulate osmotic uptake of water and maintain turgor pressure. They do this by means of transport proteins in the cell membrane. As is the case with many other elements that are possibly beneficial, plant responses to sodium depend on the species grown and the doses applied.
In general, plants tolerate a considerable amount of sodium in their environment and a certain amount can often be put to use. Whether we can consider that work worthy enough for an element to be called a “nutrient” is questionable, but it is completely fine if your feed water or nutrient solution reagents contain some sodium (sodium borate, sodium phosphate, sodium nitrate, etc.).
(Learn more about The Role of Sodium and Chloride in Plant Culture)
Titanium and its effects on plants
Titanium (Ti), the transition metal to the left of vanadium on the periodic table, has a notable effect on plants over a wide range of concentrations. Titanium is not readily soluble so not much ends up in aquatic systems, but it is present in most rocks and soils. Our knowledge of how it might act as a substitute for other elements or otherwise have a physiological effect on plant cells is rather sparse.
What is known is that in that species-specific manner once again, low concentrations (1 to 2 ppm) result in notable changes in the concentrations of other elements in plant tissues and may counteract the effects of nitrogen deficiency. The only way to know for sure if a small dose of titanium will improve a particular crop is to try it under specific conditions.
Putting Together the Pieces
It is unlikely we are on the verge of discovering additional elements required by the vast majority of plants. Only in a few cases does it make sense to be concerned about the presence, or lack thereof, of the more obscure elements that have been mentioned in this series.
Even if some plants do require an as-yet-unknown essential element, their requirement is so minuscule and they are most likely getting all they need from trace contaminants in the solution reagents and process systems currently in use.
However, we may still be surprised one day, as we were recently by the announcement of bromine as an essential element for animals. Nature has a way of keeping some of her secrets hidden from us for a long, long time.