Top 10 Heavy Metals in Soil and Fertilizer

heav·y met·al

noun

plural noun: heavy metals

a type of highly amplified harsh-sounding rock music with a strong beat, characteristically using violent or fantastic imagery.

a metal of relatively high density, or of high relative atomic weight.

As much as I would love to discuss my favorite metal bands this is a plant nutrition blog so we'll be focusing on the second type.  This is a very broad term and includes many elements.  Many people have the false assumption that they are all toxic, but this isn't the case.  Gold and platinum are both heavy metals but because they're extremely inert (don't readily react with other materials) they are among the safest of all the elements.  We will be focusing on the ones that are the biggest concern to plant and ultimately human health.  There are several that are very toxic but are extremely rare, we'll be looking at the ones that are more common.

Ultimately the purpose of this post is to educate the public rather than to frighten people.  Remember the dose makes the poison so a small amount of exposure to a toxic substance doesn't mean that it will cause ill effects. In addition bio availability is a big factor with heavy metals.  If they are in a soluble (salt) form they are far more dangerous than the metallic form.

10. Nickel

While not extremely toxic, it is of concern because of its wide usage in industry and the contamination of soils from processing.  Allergic reactions to the metal are not uncommon. 

  

9. Cobalt 

An essential nutrient for animals in the form of Vitamin B₁₂ and is used by leguminous plants in the nitrogen fixation process.  Toxicities are rare but in high dosages can cause heart problems in humans and iron deficiencies in plants, as the two elements have similar properties.

8. Molybdenum 

It is an essential element for all complex life (fungi, plants, and animals) but in very minute concentrations.  Large doses can compete with copper uptake, but overall it's not particularly dangerous in comparison to others on this list.  Since its being used more some in metal alloys there is increased concerns over contamination of soil and water.

7. Zinc

It is not seen as a dangerous heavy metal since partially because it's an essential nutrient for both plants and animals and is used very extensively in modern society.  Contamination can be a concern because of the mining and processing of the metal but soils deficient in zinc are far more common.  High levels of zinc can compete with the absorption of copper and iron and cause deficiencies with those nutrients. 

6. Copper

One of humans' most widely used materials from the ancient to present times and being an essential element for plants and animals.  While metallic copper is safe under normal conditions, soluble sources such as copper sulfate can potentially be quite dangerous.  In high doses it can damage the brain, liver, kidneys and stomach.  It is especially dangerous to aquatic environments which are very sensitive to copper toxicities. 

 

5. Selenium 

Also an essential trace element in mammals and is a nonessential beneficial element in some species of plant.  It is a fairly rare element having properties similar to sulfur and they can occur together in certain rocks.  High doses can cause damage to the heart and liver and even death.

4. Cadmium 

Commonly used in the past for industrial purposes it has been mostly phased out due to safety concerns but is still an environmental contaminant from usage in the past.  Can be a significant contaminant in some rock phosphate deposits used in fertilizer and so is of particular concern in agriculture.  Cadmium is a Group 1 carcinogen, which is the highest designation.  Causes damage to bones, liver and respiratory tract as well as severe damage to the kidneys.  

 

3. Mercury

Usage was common in the past but is less common in modern times, contaminated fish being the largest cause of exposure presently. Mercury is also associated with damage to the nervous system and is fatal in large doses.  Like most elements on this list the form of the contaminant is a big factor in its toxicity.  The salt forms such as mercury chloride are the worst as they're them easiest to absorb.

 

2. Lead

Has been used extensively from ancient times to present, being most commonly used in batteries and bullets.  The list of toxic effects is quite extensive, the most notable being the effects on the nervous system, children being the most susceptible.  Large doses may be fatal. 

*Did You Know*

Lead was phased out from gasoline in the 1980's.  This may have helped to lower the crime rate since less exposure to lead meant fewer learning disabilities in children.  This translated to fewer violent crimes as these children grew into adulthood. 

1. Arsenic 

Relatively common in the environment and quite dangerous due the the fact that it's chemically very close to phosphorus.  Organisms will absorb it readily because of its similarity.  Used extensively in the past as an insecticide and as a wood treatment because of its toxicity.  The most common source of exposure is contaminated ground water, a problem effecting 200 million people worldwide. Arsenic is a Group 1 carcinogen and large dosages are fatal.  This has been placed in the number 1 position not necessarily because it's more toxic than lead or mercury but because it's a lot more common in the environment.

 

For customers be aware of where your food is coming from.  Countries which have lax environmental standards are more likely to have higher levels of contaminants.  Buying organic food isn't a guarantee that it's going be free of heavy metals, copper sulfate is very toxic and is permitted for use in organic farming and rock phosphate can be high in heavy metals.

For growers, be aware of any type of mining or industry in the area which might pose a risk. If there's a concern about contaminated soil have it tested by an accredited lab.  Be aware of the fertilizers being used.

In conclusion, with public awareness and better environmental regulations, heavy metal poisoning is less of a threat than in the past when they were used indiscriminately.  That being said, having an understanding of contaminants will lessen the likelihood of exposure even further.