While science has identified a total of 17 essential elements required for their growth, plants also have the ability to make use of a number of other substances and elements that enhance growth, development and other functions…
As hydroponic growers, we want to make the most of the technology and information we have access to. However, with advances in plant nutritional research occurring all the time, we always have much to learn about crop nutrition and how we can best use hydroponic nutrient formulations to their full advantage.
One area of interest to soilless growers has always been the use of beneficial mineral elements, which are those outside the range of the 17 essential macro- and micronutrients required by plants to grow. Plant matter contains over 60 elements, some as diverse as titanium (Ti), gold (Au), mercury (Hg), arsenic (As), and uranium (U). Since plants evolved in soil, they developed the ability to make use of a number of substances and elements that, while not essential for growth, have been shown to have the ability to enhance growth, development and other functions.
Other beneficial elements
Some of the beneficial elements that have undergone considerable investigation by scientists are aluminum (Al), cobalt (Co), sodium (Na), selenium (Se) and silicon (Si).
Sodium, while often seen as a troublesome and unwanted element that can accumulate in hydroponic systems, does in fact have a role in some plants. It can replace potassium (K) as an osmoregulator and facilitate nitrate uptake. Aluminum is beneficial to some plants species, particularly those adapted to acidic soils. Particularly, tea plants show increased antioxidant properties and increased growth in the presence of Al in the root zone.
Cobalt can have a number of beneficial effects, especially in leguminous plants (like peas) where it increases growth, nodule number and weight, and seed pod yield and quality. Co may also play a role in slowing leaf aging and disease resistance in some species. Trace amounts of Se have been found to stimulate growth in a variety of plant species, including ryegrass, lettuce and potato, and to also provide the plants with more resistance to ultra-violet radiation. There is also evidence that boosting Se levels in hydroponically grown plants can help protect from biotic stress.
Titanium is another element whose role in plant development and metabolism has been studied for over 90 years. When maize is provided with Ti in solution, the yield increases and the concentration of sugars in the grain also rises. Studies have also found that the chlorophyll content of hydroponic tomato plants increased when Ti was added.
Another interesting potential beneficial element is iodine (I). Iodine stimulates the synthesis of cellulose and the lignification of the stem tissue, which helps the mechanical strengthening of the plant. This element also increases the concentration of ascorbic acid and the amount of total free amino acids in crops, and seems to increase salt tolerance by facilitating a lower chlorine (Cl) uptake.
There are increasing reports of other beneficial elements which may play a role in plant growth and development, these include: silver (Ag), cerium (Ce), chromium (Cr), lanthanum (La), rubidium (Rb), tin (Sn), serium (Sr), vanadium (V) and tungsten (W). Lithium (Li) has been reported to affect the transport of sugars from leaves to roots in certain plants and to increase the chlorophyll content of potato- and pepper-plant leaves.
It is likely there are other elements whose quantities in plant tissue are so minute it is hard to quantify their role or presence, but these will lead to some exciting discoveries in plant nutrition in years to come. Also, we only just added one of the extra elements to the pre-established list of essentials—nickel (Ni)—so it is possible that other beneficial elements could follow suit in the future.
Most of the nutrients mentioned above have two things in common: The growth advantage of certain elements is often species dependant and most, but not all, extras are only required in minute quantities and become toxic if the levels are too high. There is, however, one exception: silica.
Silicon is a common element that makes up more than 25 per cent of the Earth’s crust. In fact, the concentration of Si in soil is equal to that of many macronutrients, such as K and calcium (Ca), and well in excess of phosphorus (P) levels. Silicon is mostly available to plants as monosilicic acid—Si(OH)4—which is taken up by the roots from soil water. The element is then deposited as amorphous silica throughout the plant, mainly in cell walls. Once incorporated into the tissues, Si can’t be redistributed thought the plant; so, Si needs to be in constant supply if the entire plant is to contain a useful amount. (Si is many plant tissues at 10 per cent or higher dry weight.)
The role of Si in plants is multi-functional. It contributes to strength and thickness of cell walls, helping to keep plants upright and to position the leaves for good light interception. It also plays a role in resistance to attacks by fungi and insects and provides greater tolerance to plant stress due to high salinity, heavy metal toxicity, drought, UV radiation from sunlight, extremes of temperature, ect. Several plants, including rice, sugar cane and tomatoes, actually require Si as an essential element, while a large number of other species have shown this element to offer growth benefits like increased absorption and translocation of several macro- and micronutrients.
Beneficial elements and human health
Higher animals like humans eat to obtain certain elements they require to survive, many of which are beneficial to plants as well—for example, Na, I, Co, Se, Si, Cr, Sn, V and F. In the past there was concern raised that crops grown in soilless systems, such as hydroponics, could be deficient in some of the minerals people need for good health. More recent research, however, has shown that most hydroponically grown produce contains similar levels of elements to plants grown in soil and, what’s more, using beneficial-element-enhanced nutrient solutions can turn hydroponics into a tool for producing nutritionally superior food.
Indeed, human nutrition and health can be improved by having crops naturally incorporate human-benefiting (and plant-benefiting) elements into their tissue at increased levels, especially since these elements are incorporated into plant tissue in a more biologically active form that is more suited to human intake. For example, hydroponic garlic grown in a Se-enriched nutrient solution has potent anti-cancer properties that are direct results from the form of Se that developed inside that plant tissue.
The potential for enrichment of human diets with beneficial element and organic supplements in hydroponic crop production is huge and something that even small growers can take advantage of.
How to add beneficial elements to hydroponic nutrient solutions
Many water sources, particularly well water, already contain a range of the additional elements found in plant tissue; however, the levels and types of elements will vary depending on the location and soil type. Some beneficial elements also find their way into hydroponic nutrient solutions from dust, growing medium or tiny amounts of soil contamination, but growers can intentionally boost these levels by supplementing their nutrient solution. In the early days of hydroponics some growers added small volumes of soil extract or sea water to their nutrient solution as a source of diverse micro-elements, but this often led to issues with salt accumulation. Now, there are far more advanced ways of adding in the beneficial elements we want and excluding those we don’t.
While it’s not practical for a hydroponic grower to obtain and weigh out the extremely tiny quantities needed of the pure salts of these elements, large scale manufacturers of nutrient formulations and hydroponic additives can do this with a high degree of accuracy and ensure growers are using the correct levels of these elements. As a simpler approach, there are other organic- and mineral-based hydroponic additives and nutrient products—such as seaweed concentrates, concentrated vermicast extract, organic nutrients, and mineral clay supplements—that contain the minute amounts of many beneficial elements.
So, planning a full menu of elements for your plants includes giving thought to both the essential macro- and micronutrients and a whole host of other potential beneficial mineral elements. Since we have the knowledge and technology to create beneficial element supplements and solutions for hydroponics, this is something well worth taking advantage off.
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References and sources of information
The role of titanium in biomass production and its influence on essential contents in field grown crops. P Tlustos et al. Plant and Soil Environ., 50, 2005 (1): 19-25.
Physiological functions of beneficial elements. E AH Pilon-Smits et al. Current Opinion in Plant Biology 2009, 12:267-274.
Effect of seaweed concentrate on hydroponically grown spring barley. C M Steveni et al. Journal of Applied Phycology 4: 173-180, 1992.
Effect of silica on the growth of solution culture cucumber plant. Miyake Y and Takahashi E. . Soil Sci. Plant Nutri. 29(1) 71 – 83, 1983.
Effect of silica on the growth and fruit production of strawberry plants in solution culture. Miyake Y and Takahashi E. 32(2) 321-326, 1986.
The 17 nutrients essential for plant growth:
Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), chlorine (Cl), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), nickel (Ni), oxygen (O), carbon (C), hydrogen (H)