Chutibut, Wannarut, Jariya Wongtree, Tongchan Pimpet, Chadaporn Khongnam, and Omara Hanjavanich. 2009. “Proficiency Testing in Chemical  Fertilizer Analysis ”. Thai Agricultural Research Journal 27 (1):43. https://doi.org/10.14456/thaidoa-agres.2009.3.