FIELD: agriculture. SUBSTANCE: described are methods of preparing N-hydroxyalkylaminomethyl phosphonic acid I: 
wherein n is integer of 2 to 6, N- phosphonometaminocarboxylic acid II: 
and N- phosphonomethyl glycine III: 
or salts thereof. Compounds of formula I are semiproducts for preparing compounds II and III which are highly effective for controlling weeds. Compounds I are prepared by reacting alkanolamine with formaldehyde and trialkylphosphite at 50-150 C and subsequently hydrolyzing the resulting reaction mixture. Compounds II are prepared by catalytic oxidation by contacting said compounds with alkali metal hydroxide in the presence of Raney catalyst comprising element selected from chromium, titanium, nyobium, tantalum, zirconium or the like. Compounds II are prepared by reacting ethanolamine, formaldehyde and trialkylphosphite at 50-150 C followed by hydrolysis and catalytic oxidation of the resulting reaction mixture. EFFECT: more efficient preparation methods. 38 cl, 1 ex, 4 tbl
