To understand the behaviour of mercury in soils and assess the risk of its toxicity, the forms in which the element is found in the environment must be detected. In the occurrence of favourable alkylation conditions (for example in floodplain soils), the content of methyl and ethyl mercury is higher. These Hg forms in comparison with mineral compounds are toxic for the same organisms in concentrations from 10 to 100 times lower. The research concerned eight selected soil profiles representing six different types of soils (Endogleyic Phaeozem, Endogleyic Fluvisol, Eutric Fluvisol, Haplic Luvisols, Brunic Arenosol, Albic Podzol) placed in croplands. The content of mobile (water soluble), available (DTPA-extractable) and bound with organic matter (NaOH-extractable) Hg forms was determined after thermal decomposition using AMA mercury analyser. The mobility as well as availability of mercury in the analysed soils was very low, with average percentage to 0.28 and 2.45 % of the total content of this metal, respectively. It was mainly dependent on texture, the amount of organic matter and soil pH. The percentage of mercury bound with organic matter ranged from 2.34 to 73.70 % of the total content of this metal and was correlated with amount of clay and Fe oxides. Considering these results, the hazard of migration of this element into deeper horizons of the soil profile and ground water is very low. Moreover, crops from the investigated area are not at risk from mercury contamination.

The paper describes rhizospheric (Rs) and non-rhizospheric (nRs) soil to demonstrate the zone of the plant root impact on physical and chemical properties of the soil. The effects of the process accompanying the transformations of organic matter into humic substances in the rhizosphere of “common dandelion” Taraxacum officinale have been determined, and the properties of humic acids (HAs) were described. The importance of iron and clay minerals for the formation of a stable and water-resistant soil structure has been emphasized.