Native Fixed Ammonium and Fixation of Added Ammonium in Relation to Clay Mineralogy in Some Ontario Soils

Six soil profiles belonging to the Brookston, Lockport, Guelph, Chinguacousy and Haldimand series were sampled in southern Ontario. Illite contents of the clay fractions ranged from 25 to 56% and vermiculite contents ranged from 13 to 53%. The native fixed ammonium content varied from 57 to 367 μg/g and accounted for between 3 and 44% of the total nitrogen, being proportionally less in Ap horizons. Results of simple linear regression analyses demonstrated significant correlations of native fixed ammonium with percentages of clay, illite, illite plus vermiculite and K 2 O. The amount of ammonium fixed increased with increasing concentration of added ammonium up to the highest rate of application (2000μg NH 4 + /g soil). Regression analysis indicated that vermiculite was more important in the fixation of added NH 4 + than illite. Studies on Ap horizons comparing the amount of NH 4 + fixed with the amount of K − fixed for equivalent amounts of added cation showed that K + was fixed in greater quantities than NH 4 + .