The phytotoxicity of metals is established as a function of soil content; however, their potential negative effect is dependent on the type of soil and fundamentally on the presence of colloids in the soil, mainly of the organic type. For this reason, an experiment on the influence of Zn on the germination of Lepidium sativum L. in two acid soils with different organic matter contents was carried out. The results indicate that the potential phytotoxic effects of Zn at high levels are less pronounced in soils with high organic matter content compared to a soil with lower organic matter content. Therefore, it is of special interest to maintain adequate organic matter contents in agricultural soils in order to make them more sustainable and less vulnerable to potential contaminants. The experimental results obtained by students were highly satisfactory, showing experimental values similar to those previously found by the teachers. According to postlab surveys, the students were highly satisfied and in favor of the laboratory exercise developed from both sides, the didactic point of view and the importance of the exercise for the understanding of soil science concepts.