Chromium (Cr) occurs naturally in soils from weathered rocks, but mostly as the chromium(III), or Cr3+, ion. This ion clings tightly to the negatively-charged sites of clays and organic matter, or forms insoluble compounds unavailable to plants.
The chromium(VI), or Cr6+, ion widely used in industry, however, is regarded as an environmental pollutant as it is highly soluble in water and soil and extremely toxic to plants and animals.
(For those wondering about the ‘chromate’ in chromated copper arsenate (CCA) treated timber, this is chromium trioxide, CrO3. The chromium ion here is Cr6+ yet does not contaminate soils significantly as it is ‘fixed’ to the wood. More on this at the bottom of this article.)
Both Cr3+ and Cr6+ ions have very low uptake rates by plants, and neither moves readily through a plant — the concentration of chromium in roots (already low) can be up to a hundred times greater than in the shoots.
The uptake of Cr3+ is passive while that of Cr6+ is active — chromate (CrO42-, with a Cr6+ ion) is structurally similar to sulfate (SO42-) and phosphate (PO43-) and can be uptaken by the sulfate and phosphate transporters in root cells. Uptake of CrO42- can thus be reduced by the presence of SO42-.
High levels of chromium in soils can affect the availability to plants of essential elements such as iron (Fe), magnesium (Mg), phosphorus (P) and calcium (Ca) by forming insoluble compounds with them. If in a plant at high levels, chromium becomes toxic by interfering with growth and development, photosynthesis, and enzymes and their associated metabolic activities.
Selenium (Se), like iodine, is an element not needed by plants but which is essential for animals and us. It is often given as a supplement to livestock on Australia’s naturally selenium-deficient soils. (Selenium deficiency in animals causes hair and feather loss, weakened muscles, and a weakened immune system.)
Selenium sits below sulfur on the Periodic Table and resembles it in its chemical properties. The highly soluble selenate ion (SeO42-) competes with sulfate ions (SO42-) for the same uptake sites, but this ion is mostly found in very dry selenium-rich alkaline soils. Uptake of selenium from such soils can be reduced by the application of SO42-. But generally, selenium exists in very low concentrations in most soils, of no more than 0.2 ppm (parts per million), and its availability to plants is very low in acid and neutral soils.
Selenium may have a mildy-stimulatory effect on plants at low levels, and some species can tolerate very high levels of selenium. In others though, high levels lead to stunted growth and chlorosis. Selenium can substitute for sulfur in amino acids, creating selenomethionine instead of methionine, and selenocysteine instead of cysteine, and some selenium compounds can interfere with sulfur metabolism also through replacement.
While some plants can tolerate high levels of selenium, animals grazing on them can suffer from selenium toxicity if the amount of selenium in their diets is over 5 ppm. This is known as ‘alkali disease’ as it occurs on those selenium-rich alkaline soils mentioned above, but this is a misnomer. The disease is better known by the more accurate name selenosis, or selenium poisoning. One route is by ingesting those plant-produced seleno-amino acids, selenomethionine and selenocysteine, which then are incorporated into the animal’s proteins in place of methionine and cysteine. (This can affect some biochemical functions.) Another route is by ingesting plant matter that has simply accumulated high levels of selenium that remain in the tissue. Selenium toxicity is marked by weakness, lameness and an emaciated condition.
Lead (Pb) is a well-known toxin and pollutant, and prior to the introduction of lead-free petrol, accounted for around 80% of total lead in the atmosphere. The natural level of lead in atmosphere is less than 0.1μg/m3, less than 0.1 ppb (parts per billion), and in soil averages about 15 ppm (parts per million).
Lead is toxic as it mimics calcium in several metabolic processes and disrupts many enzyme pathways.
In soil, lead forms compounds that are mostly unavailable to plants, and applications of calcium and phosphorus decease this further.
Plants uptake lead in the soil solution, via calcium transporters in the root cells. The lead tends to concentrate in the roots, but can affect the rest of the plant by impeding the uptake of other nutrients such as magnesium and iron.
Lead in plants interferes with root growth, growth, transpiration and chlorophyll production.
Cadmium (Cd) sits below zinc on the Periodic Table and is very similar chemically. Cadmium can mimic zinc in both its uptake by plants and in metabolic functions within the plant. However, zinc is an essential micronutrient, while cadmium is toxic to both plants and animals.
The main source of cadmium pollution is from zinc smelters.
Cadmium’s toxicity is due to its higher affinity for thiol (-SH) groups in enzymes and other proteins, which thus disrupts enzyme processes. Cadmium in plants can also disrupt iron metabolism and cause chlorosis.
Cadmium is uptaken by a plant via zinc transporters in root cells, but unlike lead which mostly stays in the roots, cadmium is readily transported to upper plant tissues, and chlorosis, leaf rolls and stunting are the main signs of a cadmium toxicity. Its availability to plants is dependent on soil pH and the presence of other cations (positively-charged ions) — the presence of zinc and calcium can especially suppress its uptake. And as with zinc, cadmium becomes less available with increasing soil pH.
Some plants can tolerate high amounts of cadmium, but as with selenium, if these plants are then ingested toxicity can arise in those that consume them. Cadmium in animals accumulates in the kidneys, spleen, liver, and bones, causing damage and inflammation. One particularly nasty example is Itai-itai disease.
Arsenic occurs naturally in soils — its the 53rd most abundant element and is found in over 200 minerals in the Earth’s crust — but is not used by plants and is toxic to both plants and animals. It does have important industrial uses, and is regarded as an environmental pollutant from these. Some soils may also have high levels of arsenic from applications of arsenic-containing pesticides.
Arsenic toxicity in plants is more likely in sandy soils than in clays. Symptoms of toxicity are wilted leaves, stunted growth, and death. Roots are more affected than shoots.
Arsenic is the A in chromated copper arsenate (CCA) treated timber mentioned in the chromium section at the beginning of the article. The arsenic is in the form of arsenic pentoxide, As2O5. (The copper is copper(II) oxide, CuO.) The arsenic compound controls termites, the copper compound controls fungi, and chromium’s role is to fix these two into the wood.
I included this section on arsenic at the bottom rather than immediately below the chromium section at top (where it would more logically flow). The reason for this was simply to segue into a fascinating CSIRO publication on The facts about CCA-treated timber! This is a very measured and highly informative seven-page article that is very quick and easy to read. The original link was here, now missing, perhaps through a site restructure? But now archived here.