ADSORPTION OF THIAMIN (VITAMIN-B-1) ON SOILS AND CLAYS

Recent research suggests that thiamin applied to soils or coated onto seeds may stimulate plant growth. The behavior of thiamin in soils has not been investigated. Therefore, studies were carried out to determi ne how thiamin hydrochloride inyl)methyl]-5-(2-hydroxyethyl)-4-methylt hiazolium chloride hydrochloride) is adsorbed by 17 soils and three cl ays. The dominate mechanism in binding of thiamin is thought to be cat ion exchange with clay minerals and organic matter. In soils with low to medium organic matter content, thiamin adsorption occurred primaril y on clay minerals and depended particularly on the amount and composi tion of the clay. Adsorption is species dependent (pK(a1) [negative lo garithm of the first dissociation constant] = 4.85) and takes place pr incipally in the acidic pH range, probably with position 1' of the pyr imidine ring. Adsorption equilibrium is attained in <30 min. Adsorptio n in all soils, and in kaolinite and illite clays, could be described by a one-surface Langmuir isotherm at initial concentrations ranging f rom 16.3 to 995 mu mol L(-1). Adsorption by smectite in the same range was log-linearly related to the equilibrium concentration. Extending the initial concentration range from 16.3 to 9890 mu mol L(-1) showed that a two-surface Langmuir equation more adequately described adsorpt ion in hydroxy-interlayered vermiculitic and chloritic-illitic soils, whereas a one-surface Langmuir equation was found to be adequate in mi xed layer-smectitic soils. Only in the case of smectite clays is thiam in allowed to lie in a monolayer configuration parallel to the basal p lane.