Organic farming practices involve multiple steps in the whole system, from cultivation to harvesting, with the end result being a crop that is cleaner and nutritionally closer to how it may be in a natural environment. We may think of natural environments as those before the industrial revolution, when there was less mass production of crops and virtually no pollution.
Fruit and vegetables are particularly susceptible to trace contaminants that accumulate through the soil, including heavy metals. The most concerning of these are lead, cadmium, mercury, arsenic, because of their toxic effects at low levels. For example, lead and mercury are known to cause problems with learning and cognitive function in children .
As mentioned, heavy metal residues can be found in the air, soil and water. They originate from the byproducts of many industrial processes  including mining, manufacturing plants that process products and raw materials (e.g. metals) and the burning of fossil fuels (e.g. coal).
Fruits and vegetables are vulnerable to absorbing heavy metals mainly from soil and water supply, because they obtain nutrients from the roots, although it depends on the type of fruit or vegetable. Root vegetables, like carrots and potatoes, are probably more at risk, because they grow in submerged in the soil.
A Chinese study published in 2017 analyzed the levels of mercury in a variety of fruit and vegetable crops grown nearby to large coal-fired power plants . They showed a direct relationship between the distance the crops were to the stations and their levels of mercury. This shows that plants can absorb heavy metals, depending on how much they are exposed to.