Boron and Heavy Metals

I have been always asked from concerned Foresters about what is the acceptable levels of heavy metals in Boron products used to fertilise trees. 

When I first started importing various Borates from around the world in the early 1980's and, every subsequent year after that, I have put restrictions on several items, within the product, one of which is the loading of heavy metals within the product.

What are heavy metals?

Heavy metals are defined as metals with a specific gravity greater than 5.0

When you mine a product out of the ground you get a certain percentage of the product you want, and a lot of little other unwanted, and inert substances that usually come with it.

When we mine borates whether as Ulexite, Colemanite or Hydroboracite our active ingredient of this boron product can range anywhere between 7-14 %.

Assuming a Boron product has got a B203 reading of 34% this means the active ingredient of that boron is 10.5% ( you simply multiply the boric oxide by 0.31- atomic weight) This will give the percentage of boron within the product.

In smaller amounts substances are expressed as parts per million (ppm) e.g.. heavy metals.

There are several types of heavy metals within the earth's crust, these include cadmium, mercury, lead, chrome etc. In the 20 years I have been importing boron, these have never been a problem as far as excess quantity goes. Quantities very rarely exceed 50 parts per million, and therefore are not a problem.

On the other hand there is a heavy metal normally associated with all borate products called arsenic (specific gravity = 5.72). When this word is mentioned it immediately registers with the layman, especially if they have read a few of Agatha Christies novels, or are familiar with copper chrome arsenic (CCA) which is pumped into the million plus fence posts around the country.


I remember an old laborer called Taffy, he lived in Dusky Forest just out of Tapanui, and he had been using arsenic for years. He seemed quite healthy until one day the district nurse took a sample of his hair for analysis, and then all the whistles and bells were set off. 

Taffy was rushed away to hospital, and the following week we were all told that he was going to die, but instead the following week Taffy was back at work cussing the system for not knowing "Sh-T from Clay".

Apparently Taffy had about a thousand times more arsenic in his body than was acceptable to sustain normal growth. Obviously Taffy had produced an immunity over the many years he had been using the product (OSH would have had a heart attack). He would roll a smoke with unwashed hands, as well as eat his lunch.

Taffy never had a days sickness in his life. He argued that he had an immunity to sickness due to the high arsenic levels in his body wiping out all the winter ills that everyone else went down with. He was a tough character who could out work and out drink us all up until his retirement at aged 71 (we think)

So, now getting back to the permitted arsenic loadings in Borates. Since I was responsible for the importing of all the forestry boron into the country at the time, I drew a line in the sand, and said anything over 1,500 ppm would not be acceptable. 

Ever since I have been punished for opening up this can of worms, because now arsenic levels are being used by salesmen as the number one criteria for determining the best boron product for tree fertilization, when in fact it should be one of the least concerns, because there are far more important things to consider than basic arsenic levels, like the makeup of the product (either sodium, Magnesium or calcium),  granule size, hardness of the granule, and active ingredient of the boron.

For the intellectuals out there that want to put some figures on acceptable arsenic levels, lets assume we have a borate product that has an excessively high amount of arsenic for example 4000 ppm. (0.4%).

Assuming we have a product that has 10% boron and we apply this borate product to our trees at a rate of between 60kgs per Ha. The question is, what will happen? In our NZ soils we have naturally occurring arsenic ranging between 25-28 ppm. 

If we apply this borate product on, we will effectively increase our arsenic levels from 25 to 26 ppm. So, all you interested foresters out there, if you get approached by a salesman in a suit, and tie, that talks about the negative responses that you'll get with high arsenic loadings in your borate. Remember, you will have far greater worries with boron toxicity than you will ever have with arsenic problems especially if you are applying a processed product containing sodium borate or applying a product unevenly due to several factors such as, soft granules which shatter on application, or applying the product unevenly, and in dust form.

For the record the Hydroboracite that we now use has a maximum arsenic content of 500ppm, so is well below the arbitrary level of 1500ppm that I set many years ago.