Opinion: The long-term poisoning of our rural soils with the heavy metal cadmium threatens vast tracts of rural land and will soon affect the exportability of crops and animals says scientist Mike Joy

By Mike Joy*

Imagine if someone told you that over the last 70 years we had systematically contaminated large areas of New Zealand’s very best agricultural and horticultural soils with a toxic, carcinogenic heavy metal and that as a result we will be unable to grow food for human consumption on large tracts of this land from now on? 

I’m guessing that you would very likely and reasonably respond with utter disbelief and anger.

It is appalling but sadly true, appalling not just that it has happened but that it continues almost unabated.

The heavy metal is cadmium and the culprit is intensive farming and horticulture and bizarrely it’s not just you who hasn’t heard of this but almost all of the farmers who own the land as well.

I can already hear the predictable and patronising response from industry and government to this news that they have covered up for so long, that pollution is a necessary cost of feeding the world and saving our economy.

But that response is bizarre, obviously once we have poisoned the soil we will feed no one.

The reality is that now our options for feeding ourselves, let alone the rest of the world in the future have been drastically reduced

Of course this contamination wasn’t intentional at least not in the beginning - we just poured on phosphate fertiliser to grow more stuff.

The problem is that cadmium does not readily leave the environment so even incredibly small amounts add up over decades.

This is especially true in intensively farmed areas like the Waikato where now more than 160,000 hectares are contaminated to the point that would have been officially labelled as a contaminated site. It can no longer be classed as contaminated because changes to legislation this year removed agricultural land from any contamination classification, an interesting way to get rid of the problem.

Fortuitously for the dairy industry cadmium doesn’t get passed to milk or we would have been banned from export markets decades ago.

Instead of going to the milk cadmium accumulates in the body like it accumulates in soil.

In mammals it accumulates in the major organs (kidneys and liver).

As a result, the sale of these organs from cattle and sheep over 18 months old are banned for human consumption in New Zealand. 

The problem for humans is that vegetables take up cadmium from the soil so this carcinogen is ingested by us when we consume those plants and then slowly builds up in us. It’s very hard to decide on a safe level of cadmium in soils to keep our intake below danger levels. Currently food standards are used as a guide, the theory is that if soil cadmium is kept below a certain level it should mean that in vulnerable crops like potatoes, onions, root and leafy vegetables and most grains, the standards will be met and we will be safe. 

We know very little about the health effects of long-term cadmium accumulation so the supposedly safe levels of consumption are changing globally.

However, if we use the European standard then the latest results of our 5 yearly total diet survey, horrifyingly shows that New Zealand toddlers, infants and children already eat cadmium at, or near, the limit and the rest of us are not far behind.  The World Health Organisation standard is more lenient and New Zealand uses this limit so the figures look a bit better.

Of course using these more lenient guidelines might backfire if New Zealand standards are not accepted by Europe it could have potentially huge ramifications for our markets there. 

There is no good news for the future either, the national cadmium report produced by government revealed that on-going cadmium accumulation in our agricultural soils has the potential to increase dietary intakes in the New Zealand population. 

So what has the fertiliser industry and government done about this issue?

Apart from producing a national strategy and setting up a working group dominated by industry representatives virtually nothing has changed in the four decades since cadmium accumulation was first identified as a problem. 

Not surprisingly given the industry domination of the working group there is no evidence of net reduction in the rate of contamination.

Every year, about 2 million tonnes of superphosphate fertiliser is applied to pastoral and horticultural soil in New Zealand, so that means we are adding a whopping 30-40 tonnes of cadmium per year. 

One distressing aspect of this debacle is how industry and government have kept the truth about this issue from the public and even worse, from farmers. Undoubtedly because of the potential economic consequences on land values and maintaining growth in production the lid has until now been firmly kept on this sad reality. 

The security of food production in New Zealand for exports, and for feeding the nation, depends on us facing up to this crucial issue.

Public perceptions of the value of New Zealand’s dairy boom are slowly changing with the realisation of the social and environmental costs become more obvious.

Many people are now aware of the multiple impacts on our lakes and rivers, but only a tiny minority know that we are poisoning our soils as well.

The freshwater issues in rivers can be repaired easily, once you stop putting nutrients and sediment in them they will clean up in no time, contaminated soil however is almost impossible to fix.

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Dr. Mike Joy is the Director of Massey University's Centre for Freshwater Ecosystem Management. You can contact him here »

We ran a story on the soil cadmium problem in 2011 and you can find it here »