The influence of earthworms (Lumbricus terrestris, Aporrectodea caliginosa, and Eisenia fetida) on the mineralization of nitrogen compounds in soil was studied in experiments with vessels. The activation of earthworms was accompanied by intensified nitrification processes in the soil that improved the nitrogen nutrition of plants, especially on the background of applying plant residues with a high C : N ratio. The earthworm species (L. terrestris and L. caliginosa) with the highest rate of ammonium excretion (calculated per individual) exerted the greatest effect on nitrification. The ammonium of earthworm excreta is suggested to take part in the regulation of nitrification as a direct source and/or promoter of N-mineralization of organic matter in soil.