Home About Us Our Services Publications Testing Submissions Contact Us Links

The Loam Ranger – Heavy metal content of fertilisers

Our client Martin Yates, of Ecofertiliser, asked:

I frequently get enquiries from our customers about the heavy metal content of fertilisers we sell. As you would know, we are required to put warnings on the bags if the levels of mercury, lead and cadmium are above certain levels. The concern of the customers is that these heavy metals will cause problems and that they shouldn’t use the fertilisers because of them.

The NSW Fertilisers Act 1985 No 5, through the Fertilisers Regulation 1997, requires all fertilisers sold in NSW to be labelled with the contents of cadmium (Cd), lead (Pb) and mercury (Hg). The maximum allowable concentrations are 10 mg Cd/kg of fertiliser (or 350 mg/kg of P if it is a phosphate fertiliser), 100 mg Pb/kg and 5 mg Hg/kg.

The fertiliser labels must also show warning statements if the concentrations are greater than 1 mg Cd/kg, 20 mg Pb/kg and 0.2 mg Hg/kg. The warnings refer to possible build-up of these heavy metals in the soil with continued application.

Background

Phosphate fertilisers derive from either mineral deposits or ancient accumulations of guano (seabird droppings). In either case, Pb, Zn and particularly Cd are associated with the deposits. Being at the top of the food chain, seabirds accumulate these metals naturally. It is very difficult and expensive to refine these deposits to produce Cd-free phosphate fertilisers.

Back before the current fertiliser legislation was drafted, phosphate fertilisers with very high Cd levels were common, and several instances of Cd contamination, particularly in potatoes, occurred. Fertiliser Acts now regulate this and force manufacturers to produce cleaner fertilisers.

It is necessary to warn consumers when heavy metal levels exceed the warning statement levels, but users should be aware that it would take many years of continuous application to exceed safe levels in soils.

Build-up of heavy metals in soil

If you repeatedly apply any heavy metal to a soil, the level will build up. This occurs because they are fixed, or “stuck” firmly to soils. Crops would normally remove only small amounts, and rain will not normally wash the metals down the soil profile (they do not leach, but remain in the soil for long periods). So there is no common mechanism to remove them once they’re present.

Detrimental effects on soil, crops and consumers are mitigated by the following circumstances:

• If the soil pH is in the range suitable for normal crop growth (5.5 to 7.0), the plant will usually take up only very small amounts, because the heavy metals will not dissolve in the soil water. (If the pH is too low, then liming will be necessary.)
• Clay in soils will bind and fix the metals, thus reducing plant uptake – the heavy metals will bind to the clay particles and become unavailable to the plant roots. The higher the clay content, the greater the capacity of the soil to build up heavy metals without harm.
• The longer the metal is in the soil, the less available it becomes, because it becomes bound into the crystal lattices of the soil minerals and therefore unavailable to plants.
• Organic matter will bind and fix the metals, also reducing availability.

These mechanisms add up to a fairly comprehensive protection mechanism for the food chain. Consumers should not be put off by a warning label on fertilisers: it signifies that the fertiliser should be used only in reasonable amounts, as needed, to correct soil deficiencies. Heavy metals are mostly a problem with phosphorus and trace element fertilisers. These fertilisers should not be used unless a need is established (see Soil Testing for Phosphorus).

The short-term answer to avoiding heavy metal build-up in soil is to use fertilisers that comply with the relevant state fertiliser legislation, which exists to protect soils, and to use them only when a need is established.

Removal of heavy metals from soils

It is not economically feasible to remove heavy metals from soils. It is, of course, far better to avoid adding them in the first place, but past fertiliser practices might have left you with a heavy metal legacy. If it becomes essential, various technologies have been developed to remove heavy metals from soils.

• The first, and most effective on a small scale, is to remove the soil and replace it. This brings obvious drawbacks of disposing of the soil and sourcing appropriate new soil. The cost can be very high.
• Soil washing has been developed as a way to remove the heavy metals from soil in a water-based solvent that contains iron chloride. Again this is time-consuming, labour-intensive and costly and is applicable only to small, usually urban sites.
• One apparently promising technology, already tried in several countries, is to plant crops that preferentially take up the heavy metals from the soil, called “hyperaccumulators”. The crops are more often than not common weeds that have shown an ability to accumulate heavy metals. Once the crop is harvested it is removed from the field and burned to concentrate the metals in the ash. Many crop cycles might be necessary to reduce the concentrations to acceptable levels. It is not likely to be economic on an agricultural scale.
• Fix them using lime, organic matter, or even clay additions, and time.
• Grow crops that do not take up the metals. For example, fruiting and seed structures such as tomatoes, grains and citrus fruits are largely protected from metal accumulation. In contrast, foliage crops, for example silver beet and spinach, are known accumulators. It is possible to grow perfectly safe food even in heavily contaminated soils if the right selections are made.

If you have concerns ...

SESL can test soils and plant tissues for heavy metals if you have any concerns. We can also verify the contents stated on the fertiliser labels if you want. The most common situations where we have found accumulations of heavy metals are:

• heavily urbanised inner city sites, often but not always next to main roads
• old cattle and sheep dips where arsenic was used
• old gardens, where excessive fertiliser, often manure-based, was applied over many years.

Download as a PDF