NOVELTY - At least part of phosphorous content of slurry is extracted, and phosphorous and optionally nitrogen are recovered from slurry by transferring slurry to crystallization container (3), adding magnesium source and optionally ammonium source or anhydrous ammonia such that phosphorous is precipitated as ammonium magnesium phosphate (NH4MgPO4), precipitating NH4MgPO4 on inner surface of vessel and/or as particles in crystallization container, extracting phosphorous reduced slurry fraction, and collecting struvite precipitate by extracting precipitated NH4MgPO4.    USE - Extraction of at least part of phosphorous content of slurry, such as manure, wastewater, and/or sludge, and recovery of phosphorous and optionally nitrogen from slurry.    ADVANTAGE - Overdosing or under-treating with phosphorus and/or other nutrients, such as nitrogen, sulfur, and optionally, potassium (K), and formation of struvite in e.g. biogas and/or wastewater treatment plants are prevented, and method and system can use existing equipment suitable for other tasks, which helps reduce the investment of a farmer in necessary equipment.    DETAILED DESCRIPTION - At least part of phosphorous content of slurry is extracted, and phosphorous and optionally nitrogen are recovered from slurry by transferring slurry to crystallization container, adding magnesium source, such as magnesium salt, e.g. magnesium chloride (MgCl2), magnesium sulfate (MgSO4), and/or magnesium oxide (MgO), and optionally ammonium source, such as ammonium chloride (NH4Cl) and/or ammonium nitrate (NH4NO3) or anhydrous ammonia, in surplus in relation to stoichiometric amount necessary for precipitating phosphorous as NH4MgPO4 to crystallization vessel and/or its inlet, precipitating NH4MgPO4 on inner surface of vessel and/or as particles in crystallization container, extracting phosphorous reduced slurry fraction, and collecting struvite precipitate by extracting precipitated NH4MgPO4 from crystallization container by re-dissolving precipitate in acidic aqueous solution or suspending precipitated struvite in aqueous phase. INDEPENDENT CLAIMS are included for:    (1) system for extracting at least part of phosphorous content of slurry and recovering phosphorous and optionally nitrogen content from slurry comprising crystallization container which has unit (4) for introducing magnesium salt and/or MgO or its aqueous solution, optionally, unit (5) for introducing ammonium salt or its aqueous solution, and optionally, unit for introducing pH regulating substance into crystallization container and/or its inlet in which crystallization container is a mobile slurry tanker; and    (2) use of mobile slurry tanker as crystallization container in the system or the method for extracting at least a part of phosphorous content of slurry and recovering phosphorous and optionally nitrogen content from slurry.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a stationary system for precipitating struvite from a slurry.    Slurry storage container (1)    Slurry pipe/tube connection (2)    Crystallization container (3)    Unit for introducing magnesium salt and/or MgO or its aqueous solution (4)    Unit for introducing ammonium salt or its aqueous solution (5)