Preheating soil in an effort to inactivate any enzymes or other thermolabile catalysts in the soil had only a slight effect upon the ability of the soil to remove amino acids from percolating solutions. Though some evidence was obtained in support of the existence of small but thermostable, small but thermolabile, and either a weak microbial activity or a catalyst inactivated by toluene as responsible for the removal of nitrogen from percolating solutions of amino acids, still the most important factor in removing amino acids from solutions was adsorption by the solid phase of the soil. Adsorption isotherms of a number of the more soluble amino acids with preheated Yolo fine sandy loam were obtained by the usual laboratory methods of analyzing a series of equilibrium extracts from the soil. The dicarboxylic mono-amino acids, aspartic and glutamic, were most weakly adsorbed, the monocarboxylic mono-amino acids, glycine, alanine, leucine, and norleucine, were somewhat more strongly adsorbed, and the monocarboxylic amino acids with one or more nitrogen-containing radicals in addition to the alpha-amino radical, such as lysine, histidine, and arginine, were most strongly adsorbed--even more strongly so than the NH4 ion of NH4Cl. Evidence was obtained which indicates that lengthening the hydrocarbon chain in the amino acids or transferring the NH2 radical farther out on the chain away from the carboxyl group increases adsorption; and that replacing a hydrogen on the terminal carbon atom of the chain with a phenyl group decreases adsorption, while an OH substitution increases the adsorption some and an SH substitution increases it somewhat more.

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