Fuente: Wipo "precision farming" The invention facilitates a determination of the rate at which soil organic nitrogen is converted to inorganic nitrogen or mineral nitrogen. This conversion is known as N-mineralization. The total release of plant available nitrogen in a soil, i.e., the gross N-mineralization rate quantifies the conversion in the soil of organic nitrogen to inorganic nitrogen. The present invention provides a quick and reliable method for measuring the gross N-mineralization rate of a soil sample. The method exploits that a microbial enzyme activity of a functional ornithine acid cycle present in said sample is correlatale with the rate or gross N-mineralization of that same sample. The method of the invention can be used e.g. in a determination of the amount of a nitrogen-containing fertilizer to be applied to a soil such as e.g. a field of agricultural crops.