Section 180.1210 - Phosphorous acid; exemption from the requirement of a tolerance(a) An exemption from the requirement of a tolerance is established for residues of phosphorous acid and its ammonium, sodium and potassium salts in or on all food commodities when used as an agricultural fungicide and in or on potatoes when applied as a post-harvest treatment at 35,600 ppm or less phosphorous acid.(b) An exemption from the requirement of a tolerance is established for residues of calcium salts of phosphorous acid, including its metabolites and degradates, in or on all food commodities when used as a fungicide or as a systemic acquired resistance (SAR) inducer. 83 FR 3605 , Jan. 26, 2018