Your reference has some clues (there are citations about preparing samples of soil). Start with these and work through references that will be found in those initial citations. Literature research always gives clues about how to proceed and achieve reproducibility in any analysis.
Another reference (found in many science libraries) would be the A.O.A.C Methods which are matrix specific analyses for foodstuffs, crops, soil, and other agricultural i.e. They can be somewhat difficult to navigate but official methods can be found for plant and soil analysis.
Official Methods of Analysis of AOAC INTERNATIONAL,18th Edition
Wiki AOAC International...