Abstract: The present invention relates to a microbial preparation enriched for and comprising a consortium of nitrifying micro-organisms comprising at least ammonium oxidizing micro-organisms chosen from bacteria of the group of Nitrosomonadaceae, comprising the genus Nitrosomonas, the genus Nitrosospira and the genus Nitrosovibrio, and/or from archaea of the group of Thaumarchaeota, of which bacteria and archaea at least two different species are present and at least nitrite oxidizing bacteria selected from the genera Nitrobacter and Nitrospira of which at least two different species are present. It further relates to a method for preparing such a microbiological preparation comprising the steps of a. Aerating an amount of compost in water; b. Extracting a sample of microorganisms from said aerated compost sludge; c. Culturing said microorganisms under aeration for several days and adding an ammonium compound at temp 10-35° C., preferably between 15 and 30° C., more preferably between 20 and 30° C.; d.