Where do most of the trace metals found in tissues come from? Since the answer relates to the food ingested, the focus turns to the edible plants, and then, ultimately, to the soil. The origin of these metals can be traced to the rocks which formed that soil by the process of weathering. The minerals are transferred to plants. Many factors must be considered in this transfer process, and these include the concentration of the element, its solubility, and its availability, a factor that depends on the chemical form of the element which is in combination with soil materials. The acidity of the soil is also important and absorption of trace elements from the soil may be pH dependent; molybdenum is not absorbed readily by plants in an acid soil. Plant diseases may result from too little or too much of any one element. Not only are particular metals required but a balance of nutrients may be essential for healthy plants. In the United States, boron deficiency is the most widespread, and molybdenum is lacking in twenty one states; few, if any, regions are deficient in copper.

Trace metal content of plants may easily reflect the mineral distribution of the soil in which the plants are grown. The entire picture of trace elements and biological function is complicated, however, in that some plants may concentrate specific unusual elements in soils; for example, the plant Astragalus cumulates selenium. On the other hand, different species of plants grown in the same region could concentrate identical elements. Shrub leaves, grasses, deciduous trees, and conifers were analyzed and found in certain unmineralized regions to contain chromium, cobalt, nickel, copper, zinc, molybdenum, lead, and vanadium; what is surprising is that the concentration of zinc was high in all plants; copper and nickel values were uniformly moderate in concentration, and chromium was low in each plant. The relative ratios of cumulated minerals were quite similar in different botanical species.

Normal uptake of trace elements from the soil by plants can be modified by adding the nutrients as metal complexes or chelates. On the other hand, the charge on the complexed ion may prevent ready absorption of the mineral; for example, it was found that the EDTA complexes of cobalt, nickel, zinc, and copper were not as easily taken up as were the free ions.