http://www.legislation.gov.uk/nisr/1991/540/schedule/2/crossheading/determination-of-total-magnesium-atomic-absorption-spectrophotometric-method/paragraph/4.3/madeThe Fertilisers (Sampling and Analysis) Regulations (Northern Ireland) 1991Local GovernmentGovernment Printer for Northern Ireland2016-09-06AGRICULTURESCHEDULE 2METHODS OF ANALYSIS13a.DETERMINATION OF TOTAL MAGNESIUM -ATOMIC ABSORPTION SPECTROPHOTOMETRIC METHODREAGENT4.3Hydrochloric acid, 0.5 N solution.