In the first part of this series, we began to take a look at elements that, although not strictly required, may have a positive role to play in plant growth and development. Last time, cobalt, silicon, nickel and vanadium were discussed. Now we will take a look at aluminum, iodine, lithium, rubidium and silver.
Aluminum and its effects on plants
Aluminum (Al) is a light metal of great economic importance. It is the most common metal in Earth’s crust. Plants vary considerably in their responses to aluminum—some have a low tolerance while others are able to accumulate significant amounts. For typical crop plants, too much aluminum is not a good thing and leads to poor root growth.
A few plants, for example rice and some flowering shrubs in the hydrangea genus, seem to benefit from a low concentration of aluminum, but these species are not often seen in indoor gardens. Aluminum-tolerant species show increased overall growth and increased uptake of nitrogen, phosphorus and potassium when it is added. Plants that require a low pH in the root zone are more likely to benefit from aluminum.
Unless you have a specific species-dependent need, it is best to keep aluminum out of your hydroponic nutrient formulations, although because it is such a common element, it is impossible to reduce its concentration to zero. Even if your plants do benefit from the presence of aluminum, they probably get enough as it is and there is no reason to add it.
Iodine and its effects on plants
Iodine (I) exists naturally as a gray metallic solid, although you are probably more familiar with the dissolved iodide form where it appears as a violet-colored liquid. Iodine is an important nutrient element for humans because of its role in hormone synthesis in the thyroid gland. Because of this nutritional need, some work is being done to fortify plants with iodine to improve their nutritional value.
Historically, the research on iodine and plants has been contradictory, with some researchers reporting it as beneficial, some calling it harmless and others claiming it is toxic. The ionic form and concentration available to plants undoubtedly has something to do with how a plant responds to iodine, and it is clear that high concentrations are detrimental. Further clouding the issue, most early experiments indicating a beneficial role were not subsequently able to be replicated.
Careful work during the 1950s indicated low concentrations (5 to 10 ppm) of iodine in the form of potassium iodide did have a reproducible positive effect on growth of tomatoes. However, it is far from clear as to whether or not the results of these soil and sand experiments are applicable to plants under hydroponic conditions.
The matter is not settled, and iodine may yet prove to be required in a very small amount, possibly to be incorporated into an iodine-containing protein. Including a small amount of it might be a good idea, especially in plants meant for human consumption. Careful experiments under specific conditions still need to be carried out to determine the usefulness of iodine.
Lithium and its effects on plants
Lithium (Li) is not all that common in soil, but a small amount is present pretty much everywhere. Plants take up lithium more in acidic soils than in basic soils. As is so often the case with elements that are not proven to be required in plants, a plant’s response to lithium is based on both the concentration being added and the plant species.
For example, 50 ppm of lithium in the nutrient solution reduces growth and causes leaf necrosis in maize. On the other hand, a 5 ppm concentration increases biomass production in both maize and sunflower plants. This general result seems to hold for a variety of other species as well.
In addition, lithium salts reportedly confer resistance to some fungal and bacterial diseases. So, if you want to add a bit of lithium to your solution, go ahead, but keep the concentration low (5 ppm or less).
Rubidium and its effects on plants
Rubidium (Rb) is a soft, light metal. The effect of rubidium on plants seems to follow the trend we are starting to see, in that high concentrations are toxic, but low concentrations increase growth in some cases.
Rubidium works especially well in situations of low potassium availability, which is unlikely to occur in a properly operating hydroponic system. Rubidium salts, depending on the source, can be as much as 20 times more expensive than more common reagents such as potassium chloride, so it is questionable as to if it is worth using it.
Experiments involving a combination of sodium and rubidium suggest that one or the other might be essential for plant growth. As is the case with much micronutrient research, there was a lot of activity between 1940 and 1970, but it is not a hot topic these days, so it may be a while before the matter is settled.
Silver and its effects on plants
Silver (Ag) is a precious metal with antimicrobial properties. Silver salts are toxic to animal life, especially in aquatic environments, and colloidal silver causes a permanent skin discoloration in humans called argyria when consumed over time in small doses. I found no reliable information on silver being metabolically beneficial for plants.
Application of colloidal silver to seeds may protect them from diseases during germination, but there is no valid reason for people to consume silver so we don’t need to be adding it to food plants. And we do not need to be adding silver to the environment. Surely you can find better uses for silver than giving it to your plants. Leave it out of the garden.
Stay tuned for the final installment of this series, which covers bromine, chromium, strontium, selenium, sodium and titanium.