Feature Article: March, 2010
The nutrient solution is the foundation on which hydroponic plant growth is based. Its composition, in terms of essential nutrient ions, oxygen, microbial life and other ‘beneficial’ elements determines the health and growth rate of plants in hydroponics. While most growers are familiar with the essential macro and micro elements required for crop growth, plant matter has been found to contain well over 60 elements and soils are typically composed largely of aluminum, silicon and iron. Plant tissue has been found to contain elements as diverse as lead, gold, mercury, arsenic, uranium, sodium and many others, levels of which often reflect those found in the soil. In hydroponics, it has been found that plants will absorb and accumulate numerous non-essential elements from the nutrient solution, however, being non-essential does not necessarily mean that these extra elements are not beneficial to different plant species.
Most of the potentially beneficial elements only need to be present in the nutrient solution or root zone in such minute quantities that some find their way into the plant’s environment through natural means, in water supplies or growing substrates and even as dust in the air. With many of the beneficial elements required at just a fraction of a part per million, there is such a fine line between a safe amount and toxicity, that the best method of incorporating these into a nutrient solution is with the use of commercially prepared hydroponic supplements, many of which are organically based. On the market these days there are nutrient concentrates that feature a number of the proven beneficial elements such as silica, selenium, nickel and others as well as a good balance of the essential macro and micro nutrients. In the future we can expect to see the range of beneficial elements in commercially prepared nutrients increase as scientists uncover more of the secrets of advanced plant nutrition.
Different plant species often vary in their response to many beneficial nutrients and supplements.
Why incorporate nutrient supplements
There are two main reasons for the use of beneficial elements as nutrient supplements in hydroponics. Firstly, many studies have demonstrated that certain ‘non essential’ elements have beneficial effects on the growth and development of many plant species. In fact it is likely that additional essential micro elements will be identified in the future, those which are currently only required in extremely low levels and thus difficult to identify and quantify. Secondly, many hydroponic growers are interested in the health giving properties of the fruits and vegetables they produce and there is a growing area of research into supplementing some of the human essential elements into plant material. Although plants need a certain diet of their own essential nutrients, humans require many others such as sodium, iodine, cobalt and selenium, as well as silicon, chromium, tin and vanadium in minute quantities. We obtain these nutrients from our food, and while many are present in animal products, we get certain amounts from eating plants. Many of the crops we eat contain the extra elements we need, although they aren’t essential for the plant itself. There has been some concern that hydroponic crops, grown with carefully formulated, salt fertilizer-based nutrients don’t contain the full range of extra elements required for a healthy diet. This is not entirely correct as most water sources contain small amounts of a large number of the same minerals found in soils and these do find their way into hydroponic crops in quantities comparable to well grown field crops. However, the hydroponic nutrient solution can be boosted with small amounts of the same potentially beneficial nutrient elements found in healthy soils and in many cases will then produce more nutritionally complete fruits and vegetables than those grown in many heavily cropped soil systems.
By having plants naturally incorporate human beneficial elements such as selenium into their tissue at increased levels, human nutrition can be improved. Furthermore, not only do some elements such as selenium play a role in health, and in some cases, cancer prevention, but they are incorporated into plant tissue in a much more biologically active form, which is more suited to human intake. Hydroponic garlic grown in selenium-enriched nutrient solution has been shown to have potent anti-cancer properties, which are a consequence of the form of selenium that develops inside plant tissue. The potential for enrichment of human diets with beneficial elements and organic supplements in hydroponic crop production is huge and something that even small growers can take advantage of.
Beneficial micro nutrients
Some of the beneficial micro nutrient supplements have undergone considerable investigation by scientists while others are only just being looked into. Aluminum, cobalt, sodium, selenium and silica all have known roles in certain plant species, which are well documented. Aluminum has been shown to be beneficial to some plants species, particularly those adapted to acidic soils when supplied at low concentration. Tea plants show increased antioxidant properties and increased growth in the presence of aluminum in the root zone. Cobalt concentrations in plants are typically in the range 0.1 to 10 ppm on a dry weight basis and cobalt is often found in low levels in natural water sources (0.04 ppm). At low levels, cobalt can have a number of beneficial effects, particularly in leguminous plants such as peas where application of eight ppm cobalt increases growth, nodule number and weight as well as seedpod yield and quality. Cobalt may also play a role in slowing leaf aging and disease resistance in some species. Since cobalt is also essential for human health, addition of this element to nutrient solutions can enhance the nutritional quality of hydroponic food. While selenium enrichment of food crops for human health has been an area of recent study, selenium may also have a beneficial role in plant growth and development. Plant tissue contains less than one ppm of selenium in most species; however, selenium is chemically similar to sulfur, a plant macro element, and appears to be metabolized via the same mechanisms. Trace amounts of selenium 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 ultraviolet radiation. There is also evidence that boosting selenium levels in hydroponically grown plants can help protect them from biotic stress such as fungal diseases. Titanium is another element whose role in plant development and metabolism has been studied for over 90 years. While plant tissue is generally low in titanium content, being 0.1 to 10 ppm on average, titanium is present in soil in relatively high concentrations. Studies have found that the chlorophyll content of hydroponic tomato plants increased when titanium was added. When maize was provided with titanium in solution, the yield increased by 25 to 30 per cent and the concentration of sugars in the grain also rose.
“Like humans, plants need a certain diet of their own essential nutrients.”
Another interesting potential beneficial element is iodine. Iodine has been found to stimulate the synthesis of cellulose and the lignification of the stem tissue that helps the mechanical strengthening of the plant. Iodine has been found to increase the concentration of ascorbic acid in plants and the amount of total free-amino-acids in crops grown in solution culture. Iodine also seems to increase salt tolerance in plants by facilitating a lower chlorine 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). 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.
Organic nutrient supplements
While individual beneficial elements such as selenium, chromium and nickel can be added to nutrient products in fairly precise quantities via the use of fertilizer salts, they can also be supplemented with organic additives and mineral products. Organic concentrates derived from natural materials such as seaweed, fish waste, compost and vermicast, mineral clay and earth extracts contain a wide spectrum of elements including levels of macro nutrients, micro nutrients and beneficial trace minerals. Organic supplements may not be as precise as using fertilizer salts of beneficial elements; however, good quality products are likely to contain a wide range of beneficial elements and potentially other growth promoting compounds such as humic/fulvic acids amongst others. Seaweed, for example, contains a wide range of minerals, some of which are known to be beneficial for plant growth and disease resistance, however, many decades ago scientists also found that naturally occurring cytokinins in seaweed could provide a growth stimulator effect. The level of naturally occurring cyctokinins and the growth effects are of course largely dependant on the species and source of seaweed and obtaining a stable extract for use in hydroponics. However, extracts of certain species of seaweed have been shown to improve root and shoot growth and improve stress resistance in some crops by increasing the natural production of antioxidants in the plant. Compost and vermicast-based extracts may also have the added advantage of boosting beneficial microbe levels in the root zone as well as providing a wide range of beneficial elements, although the composition of different supplements can be quite variable. Natural mineral, clay or earth extract products have been available for boosting the menu of elements for hydroponic plants for many years; these are a good way of safely incorporating rarer elements into nutrient solutions although they should be considered a slow release form of supplement.
Silica – the missing macro element
Silica is much more than a trace element in many plant species. In fact silica is considered to be a beneficial macro element for many crops with a wide range of benefits for hydroponic crop production. Several plants need silica for growth including rice, sugar cane and tomatoes. Silica is transported from the roots and travels up to the shoot in the xylem vessels and is deposited mainly as hydrated silica dioxide or as polysilicic acids. Once silica has been incorporated into plant issue in this form, it can’t be redistributed throughout the plant, so it needs to be in constant supply if the entire plant is to contain a useful amount.
The use of soluble forms of silica as an additive in hydroponics is not new; many cucumber and rose growers are aware of the benefits of adding silica to the nutrient solution, whether it is in an organic or non organic form. Silica in cucumbers, for example, reduces the incidence and severity of powdery mildew and other fungi. Silica contributes to the strength and thickness of cell walls, helping to keep leaves in a good position for good light interception and to resist attacks by fungi and insects. Silica also assists with the absorption and translocation of several macro and micro nutrients and plays a role in allowing plants to survive and thrive in adverse growing conditions such as high salinity or excess elements in the solution or soil. Silica in solution should, however, not be considered a micro element; levels as high as 140 ppm have been shown to have the most significant results since silica is naturally found in many plant tissues at up to 10 per cent or higher (dry weight).
In the past silica has been a difficult element to supply at high rates in hydroponic nutrient solutions. Chemical forms such as potassium metasilicate not only have a very high pH, but tend to form a glassy like substance, which blocks drippers and emitters when the enriched nutrient solution comes into contact with the air. These days more forms of silica are available on the hydroponic market and nutrient products and supplements containing silica as a macro element are readily available and easy to use.
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