TECHNICAL
DEALING WITH SOIL POLLUTANTS opinion
By Robin Boom : CPAg, Member of the Institute of Professional Soil Scientists
Soil pollution can be defined as the build up in soils of toxic compounds, chemicals, salts, radioactive materials or disease causing agents which have an adverse effect on plant growth, animal and human health. Pollutant materials are those which are not naturally there, but which have been deposited or have accumulated through human activity. Some soils contain naturally high amounts of certain elements which can be toxic to plants and animals, but these are not regarded as pollutants per se.
If the land was to be used for housing only, and no food plants or crops would be grown on the land, then the zinc level found would be in the acceptable range Recently I was asked to take four soil samples on a 14-acre block of land next to an urban area for a client who is in the process of subdividing it for residential development. He needed to ascertain the heavy metal status to ensure that these were not excessive for land which future residents might use for growing their own produce. The elements tested for were arsenic, chromium, lead, nickel, mercury, copper, zinc and cadmium. Three of these samples were taken from grazed pasture paddocks which had relatively low heavy metal levels, but one sample we deliberately took from around an old shed where for decades, wooden fencing materials, concrete posts and old corrugated roofing iron sheets had been stored and run-off from the shed roof had fallen. On this sample the arsenic, lead and zinc levels were above BioGro limits acceptable for organic farming and growing standards. The high arsenic in this sample would be from the treated fence posts and wooden gates which lay on the ground over the years, and the high lead and zinc from the roofing iron.
42 NZGROWER : JULY 2021
It may be necessary for my client to scrape away the topsoil around the old shed and dump this if the levels are deemed too high by the local council. In New Zealand the only set of standards for heavy metals appears to be the BioGro limits, where the bar is set a lot higher than most other countries. In Britain for instance, for this sample the level for arsenic and lead would be acceptable for residential land where food plants are grown, and only the zinc level would be excessive. If the land was to be used for housing only, and no food plants or crops would be grown on the land, then the zinc level found would be in the acceptable range. For heavy metals, having a good soil pH (acidity/alkalinity), high organic matter levels and good overall fertility, the availability of these for plant uptake is lowered. In the case of dairy farming land in the upper North Island where zinc has been given to cows at high rates over the summer and autumn to prevent facial eczema, zinc levels are commonly several times the natural background levels and on effluent areas up to ten times the natural levels. In the case of copper which has been extensively used on orchards as a fungicidal spray for many decades, and in recent years on kiwifruit orchards to prevent Psa, copper levels can be up to thirty times the natural background levels. While doing some work in Britain just over 20 years ago I sampled former hop growing land which had copper levels higher than anything I had ever seen before or since. Continued spraying of fungicides is not good for the soil biology and even the plants themselves, as high zinc and copper levels can impede the uptake of other elements, such as phosphorus.
In the case of dairy farming land in the upper North Island...zinc levels are commonly several times the natural background levels and on effluent areas up to ten times the natural levels
