NOTE 1 Aqua regia will not totally dissolve most soils and similar materials, and the efficiency of extraction for particular elements differs from element to element. Such efficiency might also differ for the same element in different matrices. Users of this International Standard should carry out a programme of analysis using reference materials to assure themselves that the method given here is appropriate for their needs. Elements extractable in aqua regia cannot, therefore, be decribed as “totals”; conversely they cannot be regarded as the “bio-available” fraction, as the extraction procedure is too vigorous to represent any biological process.