Use of a substituted thiazolidine compound of formula I or a salt, stereoisomer, tautomer, N-oxide, or S-oxide thereof as nitrification inhibitor, wherein R 1, R 2, R 3 and R 4 are independently of each other selected from H, halogen, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, C 2 -C 6 -alkenyl, C 3 -C 6 -cycloalkyl, C 1 -C 4 -alkylsulfonyl, benzyl, and phenyl, wherein said groups are unsubstituted or substituted by one or more, same or different substituents R x; or R 1 and R 2 or R 3 and R 4 together form =O; and wherein R N is H, C 1 -C 6 -alkyl, C 3 -C 6 -cycloalkyl, benzyl, allyl, propargyl, or phenyl, wherein said groups are unsubstituted or substituted by one or more, same or different substituents R y; CHR a C(=O)OR b, CHR a C(=O)O - -Q +, CHR a C(=O)R d, CHR a C(=O)NR b R c, C(=O)R d, C(=O)OR b, C(=O)NR b R c, CHR a OR b, C(=O)(CH 2) 2 C(=O)OR b, or CSNR b R c; R a is H, halogen, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 1 -C 4 -haloalkyl, C 3 -C 6 -cycloalkyl, phenyl, or phenyl-C 1 -C 2 -alkyl; R b is H, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 6 -cycloalkyl, aryl, phenyl, or phenyl-C 1 -C 2 -alkyl; R c is H, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 6 -cycloalkyl, aryl, phenyl, or phenyl-C 1 -C 2 -alkyl; R d is H, C 1 -C 8 -alkyl, C 1 -C 6 -haloalkyl, C 2 -C 6 -alkenyl, C 2 -C 6 -alkynyl, phenyl, phenyl-C 1 -C 2 -alkyl, hetaryl, or hetaryl-C 2 -C 3 -alkenyl, wherein these groups are unsubstituted or substituted by one or more, same or different substituents selected from halogen, OH, OCH 3, and CH 3; R x is halogen, OH, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, C 1 -C 6 -alkylthio, C 1 -C 6 -alkylcarbonyl, or C 1 -C 6 -alkylcarboxy; R y is halogen, OH, C 1 -C 6 -alkyl, C 1- C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, or phenyl; Q + is selected from the group of Na +, K +, Ca 2+, and NH 4 +. R 1, R 2, R 3 and R 4 are independently of each other selected from H, halogen, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, C 2 -C 6 -alkenyl, C 3 -C 6 -cycloalkyl, C 1 -C 4 -alkylsulfonyl, benzyl, and phenyl, wherein said groups are unsubstituted or substituted by one or more, same or different substituents R x; or R 1 and R 2 or R 3 and R 4 together form =O; and wherein R N is H, C 1 -C 6 -alkyl, C 3 -C 6 -cycloalkyl, benzyl, allyl, propargyl, or phenyl, wherein said groups are unsubstituted or substituted by one or more, same or different substituents R y; CHR a C(=O)OR b, CHR a C(=O)O - -Q +, CHR a C(=O)R d, CHR a C(=O)NR b R c, C(=O)R d, C(=O)OR b, C(=O)NR b R c, CHR a OR b, C(=O)(CH 2) 2 C(=O)OR b, or CSNR b R c; R a is H, halogen, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 1 -C 4 -haloalkyl, C 3 -C 6 -cycloalkyl, phenyl, or phenyl-C 1 -C 2 -alkyl; R b is H, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 6 -cycloalkyl, aryl, phenyl, or phenyl-C 1 -C 2 -alkyl; R c is H, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 6 -cycloalkyl, aryl, phenyl, or phenyl-C 1 -C 2 -alkyl; R d is H, C 1 -C 8 -alkyl, C 1 -C 6 -haloalkyl, C 2 -C 6 -alkenyl, C 2 -C 6 -alkynyl, phenyl, phenyl-C 1 -C 2 -alkyl, hetaryl, or hetaryl-C 2 -C 3 -alkenyl, wherein these groups are unsubstituted or substituted by one or more, same or different substituents selected from halogen, OH, OCH 3, and CH 3; R x is halogen, OH, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, C 1 -C 6 -alkylthio, C 1 -C 6 -alkylcarbonyl, or C 1 -C 6 -alkylcarboxy; R y is halogen, OH, C 1 -C 6 -alkyl, C 1- C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, or phenyl; Q + is selected from the group of Na +, K +, Ca 2+, and NH 4 +. R N is H, C 1 -C 6 -alkyl, C 3 -C 6 -cycloalkyl, benzyl, allyl, propargyl, or phenyl, wherein said groups are unsubstituted or substituted by one or more, same or different substituents R y; CHR a C(=O)OR b, CHR a C(=O)O - -Q +, CHR a C(=O)R d, CHR a C(=O)NR b R c, C(=O)R d, C(=O)OR b, C(=O)NR b R c, CHR a OR b, C(=O)(CH 2) 2 C(=O)OR b, or CSNR b R c; R a is H, halogen, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 1 -C 4 -haloalkyl, C 3 -C 6 -cycloalkyl, phenyl, or phenyl-C 1 -C 2 -alkyl; R b is H, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 6 -cycloalkyl, aryl, phenyl, or phenyl-C 1 -C 2 -alkyl; R c is H, C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 3 -C 6 -cycloalkyl, aryl, phenyl, or phenyl-C 1 -C 2 -alkyl; R d is H, C 1 -C 8 -alkyl, C 1 -C 6 -haloalkyl, C 2 -C 6 -alkenyl, C 2 -C 6 -alkynyl, phenyl, phenyl-C 1 -C 2 -alkyl, hetaryl, or hetaryl-C 2 -C 3 -alkenyl, wherein these groups are unsubstituted or substituted by one or more, same or different substituents selected from halogen, OH, OCH 3, and CH 3; R x is halogen, OH, C 1 -C 6 -alkyl, C 1 -C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, C 1 -C 6 -alkylthio, C 1 -C 6 -alkylcarbonyl, or C 1 -C 6 -alkylcarboxy; R y is halogen, OH, C 1 -C 6 -alkyl, C 1- C 6 -haloalkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -haloalkoxy, or phenyl; Q + is selected from the group of Na +, K +, Ca 2+, and NH 4 +.