While inorganic and organic fertilisers and pesticides may have contributed to arsenic found in soils, in most soils used for agriculture and rice production, it is not the main source of arsenic. Organic and inorganic farming inputs also contain other metals such as chromium, copper, lead, cadmium etc, These metals occur naturally within the environment and in the raw materials which are used for farm inputs.
It is agreed that in some cases where there has been a concentrated use of arsenic based compounds (e.g. dips, timber treatment facilities, industrial/smelting plants, mining etc), which has resulted in high localised concentrations of arsenic in soils under and immediately adjacent to these activities. These soils may, depending on the As concentrations and their chemical structures, may be classed as contaminated under State legislation. Often, As based activities are also nominated in the same legislation as being ones which require further investigation/assessment, especially in the case of a change in land use.
It is also worth noting that under normal farming practices, the contribution of As through organic and conventional farming practices is not overly significant compared to that which naturally occurs in soils.
Generally, most of the arsenic present in soils has occurred through the weather of the underlying rocks or parent material into soil. This weathering process releases arsenic which occurs naturally in the rock minerals.
When rice is often grown in flooded conditions, arsenic in the soil solution (and which may have higher availability) increases due to the anaerobic (reducing) conditions which are present in the flooded soil. This is typically why arsenic levels of rice can be considerably higher than other crops grown in similar soils, but not under flooded conditions. Rice has also been reported to be more efficient in the uptake of arsenic than other cereal gains as well. Both the flooded soil conditions and increased arsenic uptake leads to potentially higher arsenic levels in rice grains.
