**2. Soil factors influencing availability of heavy metals to plants**

Physico-chemical properties of soil influencing contents of phytotoxic forms of heavy metals include the type of soil, its grain size distribution, reaction, organic matter content, sorption properties and redox potential [43, 67, 68, 73].

Mother rock is a natural source of heavy metals in soils. The amounts of elements coming from mother rock constitute the geochemical background posing no threat to soil fertility [67]. Other sources of heavy metals include geochemical processes and anthropogenic factors. In soil heavy metals are subjected to processes affecting their concentrations and chemical forms [4]. In individual soil horizons their content depends on anthropogenic and climatic factors [67, 114].

Soil reaction is a major factor influencing the form and availability of heavy metals to plants [67]. Soil acidity may lead to their increased concentrations in soil [3, 13, 30, 48, 93, 112] and their excessive uptake by plants [42, 93]. According to Tyler and Olsson [118], concentrations of Cu and Pb increase also at pH 7.5 – 7.8 as a result of formation of stable complexes with ligands, which solubility is connected with solubility of organic substance. Liming results in a reduced content of available forms of heavy metals in soil [39]. Soil reaction is a factor determining the force with which heavy metals are bound by organic substance and mineral compounds [2, 60].

Organic substance found in soil contributes to the limitation of the amounts of heavy metals available to plants [38, 47], since it binds very strongly Cr3+, Fe3+, Pb2+, Ni2+ and Co2+ ions and to a lesser extent also Mn2+ and Zn2+ [68]. Organic matter binds heavy metals into water insoluble forms or forms sparingly soluble in water [79], thus reducing the share of plantavailable forms of heavy metals [19], and in this way it limits their toxicity to plants [48, 87].

Heavy metals differ in the force, with which they are bound by the sorption complex and they are connected mainly with the silt fraction [67]. In soil they undergo exchange and biological sorption. They may also be precipitated in the form of insoluble compounds [83]. Availability of heavy metals in soil is influenced by the cation sorption capacity. Introduction of compounds enhancing sorption capacity to soil causes a reduction of amounts of available metal forms in soil [106].

Availability of heavy metals is also dependent on the redox reactions taking place in soil [83]. Oxide forms of heavy metals become readily available to plants at a low redox potential [33].

The application of mineral fertilizers changes amounts of phytoavailable forms of heavy metals in soil. The effect of used fertilizers on physico-chemical and biological properties of soil causes a reduction or an increase of amounts of phytotoxic forms in the soil environment [105]. When applying mineral fertilizers we introduce heavy metals to soil, which contents in fertilizers are connected both with the raw material from which they were produced and the technological process of fertilizer production. The greatest contamination with heavy metals, particularly cadmium, is found in phosphorus fertilizers [52] and calcium fertilizers, mainly being byproducts of various branches of industry [27, 69]. Heavy metal contents in phosphorus fertilizers depend on the fertilizer type [52] and solubility [57]. The application of phosphorus fertilizers leads to the transition of soluble phosphate forms into sparingly soluble zinc, copper, cadmium and lead phosphates [30], leading to the reduction of heavy metal contents in plants.
