There are elements known to be absolutely essential for all plants to grow, develop and reproduce. But are there elements that, although not strictly required, are beneficial for plant health and productivity?
Plants are good at absorbing metals, and just because an element is commonly found in plant tissues doesn’t mean it has an important role to play in that plant’s metabolism or life cycle.
But in some cases, the answer to the above question is yes. Beneficial usually means anything that increases growth, enhances the production of desired plant constituents, or improves drought or disease resistance.
In Hydroponics: A practical Guide for the Soilless Grower, Dr. J. Benton Jones, Jr. discusses four beneficial elements: cobalt, silicon, nickel and vanadium.
Dr. Lynette Morgan, another well-known hydroponics author, has developed a list of an additional nine beneficial trace elements: sodium, strontium, aluminum, iodine, silver, rubidium, lithium, selenium and titanium.
Historically, general purpose nutrient solutions have not included most of these elements. But recently, providers of commercial solutions have included some of them “just to be sure.”
If you make your own solutions, whether to include them or not is up to you, but hopefully this article and the ones that follow will give you some guidance. This article will take a look at the first four elements mentioned—cobalt, silicon, nickel and vanadium—to see what they can do (or not do) for your plants.
Cobalt and its effects on plants
Cobalt (Co) has diverse effects on plants, including stimulation of growth and expansion of stems and leaves, as well as an effect on the growth of buds. The influence of cobalt salts, at low concentration, is generally positive. This seems to result from cobalt’s effect on the regulation of the plant hormone ethylene.
Legumes certainly don’t do well without cobalt, since cobalt is a required element for rhizobium species, the nitrogen-fixing bacteria that exist symbiotically in the roots of leguminous plants. This, however, is an indirect effect.
Should cobalt be included in nutrient solutions? In general, it is not needed. It doesn’t take much cobalt to cause toxic effects, and there is usually a trace amount as an impurity in solution reagents. That should provide enough for any plants that will do better with a bit of it.
Nickel and its effects on plants
Nickel (Ni) is now considered an essential nutrient element for all plants (making it No. 17 on the list of essentials). Although the benefits of nickel have been acknowledged for many years, especially for legumes and grains, it was officially recognized as essential in 2004 by the American Association of Plant Food Control. Plants have a very low nickel requirement, which explains why it took so long to figure out its importance.
Plants deficient in nickel have difficulty breaking down the waste product urea and build up an excess of it in their tissues. In addition to its role in urea catabolism, nickel is found in a number of other proteins needed for nitrogen metabolism. Symptoms of nickel deficiency include leaf tip necrosis.
Nickel is not listed as an ingredient in many commercial nutrient formulations and it does not seem to make much difference in their performance. This indicates there is sufficient nickel present as a contaminant to meet plant requirements. If you want to be sure, a small amount (about 0.04 to 0.06 ppm) of nickel in a nutrient solution in the form of nickel chloride (NiCl2) will do the trick.
Silicon and its effects on plants
Given how common silicon (Si) is on Earth—pick up virtually any rock—it would be surprising indeed if plants have not developed some kind of relationship with silicon during their evolutionary history. Evidence is good that they have, and plants often contain a significant amount of silicon in their tissues. Is it doing anything important?
Silicon is not very soluble, so the way to make it available to plants is to provide it as an anion in the form of silicate (SiO4-4 or SiO3-2, depending on the positively charged counter ion involved). Field trials have shown that the addition of calcium silicate to soil makes pumpkin and wheat plants more resistant to powdery mildew and, in some cases, increases yields.
Tomatoes and cucumbers are also more resistant to fungal diseases when silicate is provided. Corn can become less susceptible to damage from the European corn borer when grown in soil amended with calcium silicate. In the case of rice, silicon is considered essential, since the plants cannot maintain upright growth without it.
Hydroponic solutions can include soluble silicate as either the potassium (K) or sodium (Na) salt, or possibly as silicic acid to achieve a silicon concentration of around 100 ppm.
Vanadium and its effects on plants
Vanadium (V) is one of those elements your plants probably won’t miss if they don’t have it. Unless, that is, they are short on molybdenum, in which case vanadium can provide partial substitution. Molybdenum (Mo) is essential in enzymes required for nitrogen metabolism. This substitution is somewhat surprising since vanadium and molybdenum do not occupy the same period (column) of the periodic table of elements, meaning they do not have the same outer shell electron configuration.
Vanadium is common in soils but rarely at a concentration to cause plant toxicity. It is probably present in tiny amounts as a contaminant in hydroponic solution reagents. If vanadium does play a vital role in plant physiology, the requirement for it is in the nanogram range (or less), and below the ability of current instrumentation to detect it. The bottom line is, as long as your nutrient solution contains adequate molybdenum, there is no need to provide vanadium.
Stay tuned for the next installment in this three-part series, which will cover another round of elements that may be of use in a hydroponic nutrient solution.