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Urban toxicity
Despite the steady market in complete fertilisers, trace element deficiencies in urban areas are so rare as to be negligible. In fact, the fertilisers are part of the reason.
We at SESL almost never come across trace element deficiencies in Sydney and other urban centres. The modern lifestyle has a lot to do with this situation. People – both home gardeners and professional landscapers – have been applying fertilisers for so long now that urban soils are no longer the barren grounds that the First Fleet encountered.
One notable exception is sports turf: playing requirements mandate a well-drained sandy soil, which by its nature cannot retain nutrients (see our articles on Moisture retention and CEC in sandy soils and New versus established turf).
Too much zinc
In addition to deliberate application, plenty of trace elements have been added unintentionally. An excellent example is zinc. Zinc is essential for both plant and animal survival, but too much can be harmful. There are two major sources of environmental zinc: galvanised iron and car tyres. Galvanising iron with a thin layer of zinc protects the iron from rusting, initially by forming a physical barrier. Secondarily, in an electrochemical reaction, the zinc corrodes in preference to the iron, thus protecting the iron. This is the basis for the sacrificial anodes used all the way from hot water tanks to shipping wharves. The zinc compounds – oxide, hydroxide and carbonate – enter the environment in runoff, and the zinc slowly dissolves out in acidic soil conditions. Galvanised iron is ubiquitous: roofs, water tanks, water pipes, fencing, nails, wire ...
Car tyres are made of vulcanised rubber. Zinc oxide is added to the rubber as an activator, and is slowly released along our roads as the tyres wear. The zinc is then washed out in runoff to creeks, rivers and harbours, or blown in the wind onto adjacent land.
In Growing Media for Ornamental Plants and Turf, Kevin Handreck and Neil Black refer to an experiment in which rubber from shredded tyres was used in growing medium. As the amount of shredded rubber increased, the growth of petunias decreased, and the amount of zinc in the plants increased to toxic levels. Their advice: “Don’t even think of doing it!”
Other trace elements
In addition to zinc, other trace elements have been built up in urban soils through decades of fertilizer application. Many are also released through urban activities: copper from water pipes; manganese in petrol, paints and batteries; magnesium in engine components and fireworks; selenium in shampoo; and iron in just about everything.
Rural deficiency
In stark contrast to the urban situation, trace element deficiencies are common in agricultural and horticultural production areas. The principal reason is that through regular harvesting, plant nutrients are continually removed from fields and orchards.