The pot experiments on spinach have been carried out in the greenhouse of the Agricultural University of Szczecin. The purpose of the experiment was to examine how the dolomite nutrition influences selected factors determining biological value of the plant. The examined spinach was fertilized in two ways: with sufficient quantity of nitrogen (N1) and excessive amount of nitrogen (N2). The obtained results show that plant affected by magnesium deficit in undersoil accumulate excessive amount of nitrogen nitrate in their leaves. Those plants are also affected by low activity of nitrate reductase. Results of the experiments indicate great importance of magnesium during the nitrate conversion of spinach.