This standard BS 5551-4.3.1:1983 Fertilizers. Chemical analysis. Determination of potassium is classified in these ICS categories:
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65.080 Fertilizers
https://www.en-standard.eu/bs-5551-4-3-1-1983-fertilizers-chemical-analysis-determination-of-potassium-preparation-of-the-test-solution-for-determination-of-water-soluble-potassium-content/