Heavy metals when present in amounts equal to the geochemical background do not interfere with the soil metabolism, which is associated with the growth and development of soil microorganisms as well as the processes of synthesis and re-synthesis, governed by intra- and extracellular enzymes. In the said concentrations, heavy metals do not cause undesirable changes in the development of plants. On the contrary, such elements as copper and zinc are essential constituents of physiological processes in all living organisms, including microorganisms and plants. Some soils suffer from zinc and copper deficits, which is why they are enriched with fertilizers containing copper or zinc to satisfy the nutritional requirements of crops. Cadmium is different in that its essential role in the proper functioning of living organisms has not been proven yet. In Poland, soils contaminated with heavy metals, including cadmium, copper and zinc, occur only locally. The purpose of this study has been to discuss the characteristics of these elements in terms of the chemical properties and the role in the natural environment, the effect they produce on plants when present in excessive concentrations in soil and the response of soil microbes and enzymes to such contaminants. Crops cultivated on soil with an elevated content of heavy metals typically present inhibited growth, reduced transpiration, chlorosis of leaves, limited germination of seeds and deformations of the root system. The effect induced by heavy metals is more pronounced in the early development of plants. Mobility and plant availability of heavy metals depend on a series of factors, for example the soil pH, content of organic matter, grainsize composition of soil, content of iron and manganese oxides, soil sorption capacity and the type of metal. Higher bioavailability of heavy metals is observed in soils with a low content of humic acids. As the soil pH increases (within 6.5-7.5), metals, especially zinc and - to a lesser degree - copper become less toxic to plants. The mechanism building tolerance of plants to heavy metals is closely connected with processes which reduce the uptake and transport of metals and with detoxification on cellular membranes and inside cells. An increase in the concentration of metals induces the synthesis of phytochelates, whose main function is to sustain the homeostasis of metals in the cell. These proteins also transport metal ions to vacuoles, where they can be bound by oxalates. Excessive amounts of cadmium, copper and zinc disrupt the homeostasis of soil by interfering with the control mechanisms on the level of genes, thus inhibiting the activity of microbial enzymatic proteins. They cause damage to metabolic pathways, often resulting in the apoptosis of cells. Consequently, the counts and species diversity of soil microorganisms suffer. Such process as nitrification and ammonification are inhibited, alongside the activity of soil enzymes. The adverse influence of cadmium, copper and zinc on microorganisms and enzymes can be alleviated by application of organic and natural fertilizers. For soil phytoremediation, microorganisms resistant to these metals but enhancing their availability can be used.
