Hi Stefano,

Have a look in the pages 15 - 19 of Methods of Phosphorus Analysis for Soils, Sediments, Residuals, and Waters (http://www.sera17.ext.vt.edu/Documents/Methods_of_P_Analysis_2000.pdf), (attached):

Weigh 0.02 g of fertilizers into a 50 mL Erlenmeyer flask

Thank you very much, but i didn't find phosphite!

Quantitative NMR may work well for you.  You'd have to compare it to a standard, but it shouldn't be too hard given that most phosphite fertilizers are water soluble,