I’ll open this post by briefly referring back to nickel (Ni) in last week’s post: the element that walks a tight line between being beneficial and being toxic.
Nickel (Ni) sits right next to cobalt (Co) on the Periodic Table, and closely resembles cobalt’s chemical and physiological properties. Yet until recently, nickel had been regarded as a toxin, while the effects of cobalt deficiencies in animals, and its importance to nitrogen-fixing bacteria, were already known. Cobalt simply showed as more ‘needed’ and less toxic than nickel, even though cobalt too could be toxic at certain levels.
And that’s the point I’m (hopefully) making here: that how any element — beneficial or otherwise — affects a plant is always a matter of degree. Nickel was considered a toxin, until it was discovered that at very small concentrations it actually was beneficial. Cobalt was known to be beneficial, but not at too high a concentration. Even the ‘big three’ nutrients [nitrogen (N), phosphorus (P) and potassium (K)], so crucial for plant growth, can cause adverse effects at high levels.
(Even water has a LD50!)
Other nutrients already covered that are beneficial at low levels but toxic at high ones are iron (Fe), manganese (Mn), zinc (Zn) copper (Cu), and boron (B).
The following are elements with no, or very little, stimulatory effects on plants at low concentrations, but which produce toxicity at higher ones. Usually the toxicities arise because the element either replaces another in a biochemical process, or prevents the uptake of an element that is needed in a biochemical process.
Referring to the Periodic Table might help to see these relationships more clearly.
Iodine (I) and Bromine (Br)
Iodine and bromine both form minus one ions — iodide (I-) and bromide (Br-) respectively — and sit below chlorine (Cl) in the same group of the Periodic Table. Neither have been shown to be essential to plants, but both have been reported to have stimulating effects at low levels.
Just 0.1 ppm (parts per million) of iodide can have a stimulating effect while 0.5 ppm can induce toxic effects. Iodide concentrations are typically 0 – 0.5 ppm in plants, but those showing toxic effects could have over 8 ppm. Bromide is less toxic than iodide and typically can be found in soils anywhere from 0 to 260 ppm. Some plants, like tomato and carrot, are particularly insensitive to bromide, and in experiments could accumulate over 2,000 ppm without showing any signs of toxicity.
Toxic levels of either in soils are actually quite unusual, with very low concentrations more the norm. (Though soils fumigated with methyl bromide can have elevated levels of bromide that can induce toxicity.) Natural levels of iodine can be so low in fact, that it is added to our table salt, as well as given as a supplement to livestock, as it is an essential element needed by the thyroid to make the hormone thyroxine.
Iodine toxicity shows in older leaves first, and is marked by severely restricted growth, and leaves that curl back and with necrotic tips and edges.
Bromide toxicity resembles exposure to excess salt, with chlorotic leaves with necrotic tips and edges. There may also be poor seed germination.
High levels of chloride (Cl-) can reduce iodide toxicity by competing for uptake, while leaching reduces bromide toxicity.
Flourine sits above chlorine in the same group of the Periodic Table. The 1 ppm concentration of its ion, fluoride (Fl-), in tap water is not generally enough to induce toxicity, though some carnivorous plants and other ornamentals have a reputation for sensitivity to this. The life of cut flowers in a vase is also reduced in fluoridated water.
Fluoride ions in irrigation water will readily ‘fix’ in soil and become unavailable to plants quite quickly. Similarly, one way to remove fluoride from tap water is to shake that with a small amount of soil.
In general, total flouride content in soil is unrelated to flouride availability. Plants will take up the soluble form of flouride (the ion itself), but how much of this ion is available depends on pH and the amount of calcium (Ca) and phosphorus (P) present — high levels of any of these will ‘fix’ flouride into unavailable compounds such as calcium flouride (CaF2). Even at low soil pH chloride is preferentially uptaken over fluoride.
Toxicity usually only occurs in industrially-polluted areas where hydrogen fluoride (HF) gas is produced, and even then this is usually only when the mandatory scrubbers in brickworks, aluminium smelters, or superphosphate works fail. Symptoms present as a leaf toxicity, either as an interveinal chlorosis or as a mild necrosis on leaf edges.
Aluminium is the third-most abundant element in the Earth’s crust after oxygen and silicon (Si). Around 8% of the crust by weight is aluminium oxide (Al2O3). Aluminium, along with silicon, are major components of clay minerals.
Aluminium is not soluble enough to be toxic to plants in neutral to high soil pH. Solubility increases as pH drops and the soil becomes more acidic. It is not the acidity of soil that harms plants so much as the increasing levels of aluminium which become available in these soils. An aluminium toxicity first presents as reduced root growth, with thickened tips and lateral roots.
Phosphate uptake and movement through a plant can become affected, and the plant may take on the appearance of a phosphate deficiency, with stunted growth, purpling stems, and dark green leaves. In the plant itself, aluminium will form stable (hard to break down) aluminium-phosphate complexes that interfere with phosphate metabolism.
Aluminium may also bind to DNA, and prevent the helix from unravelling — separation of the DNA strands is necessary for DNA replication and cell division, as well as protein synthesis.
Aluminium toxicity is often accompanied by high iron (Fe) and manganese (Mn) levels and low calcium (Ca) and magnesium (Mg) levels in plant tissue — this is due to the low soil pH that makes aluminium available also increasing concentrations of the former, while decreasing concentrations of the latter due to leaching. Application of lime (calcium carbonate, or CaCO3) overcomes aluminium toxicity in acidic soils.
Some plants can tolerate high levels of aluminium — the blue pigment of hydrangea flowers contains aluminium and this is why blue flowers indicate an acid soil of pH 4.5 – 5.5, pink hydrangea flowers indicate an alkaline one (pH above 7), and the in-between colour of pale purple indicates a pH also in between (5.5 – 7)!
A very interesting article.
Was aware that aluminium was plentiful in the world’s crust but didn’t realise it’s to the extent mentioned.
You learn something every day; in this instance as to the effect of aluminium on the colour of flowers. I always thought iron was the main contributor to the blue colour but now see that the iron is carried via aluminium. Will now start experimenting with our hydrangeas and will from now on only use tank water for our cut flowers!