The fertilizer of the invention is a fertilizer containing a sparingly water-soluble phosphatic fertilizer and a urea/aliphatic aldehyde condensation product. The process for controlling inoganizing speed of urea/aliphatic aldehyde condensation product of the invention comprises adding to the urea/aliphatic aldehyde condensation product a sparingly water-soluble phosphatic fertilizer having special elution characteristics. The fertilizer of the invention and the process for controlling inoganizing speed of urea/aliphatic aldehyde condensation product of the invention make it possible to control the inorganizing speed of the condensation product without changing the particle size or form, that is, to control development of the efficacy of the condensation product used as a fertilizer. The fertilizers of the invention or the urea/aliphatic aldehyde condensation products whose inorganizing speeds are controlled according to the process for controlling the inorganizing speed of the invention can provide nitrogen components to plant in accordance with the periods of culture of desired crops and a pattern of absorbing nutrition. This is effective in saving of the number of additional fertilization or nitrogen components left in soil after the period of culture.