INFLUENCE OF ADDED K+ ON AMMONIUM SELECTIVITY/MOBILITY BY SOILS WITH VERMICULITIC BEHAVIOR

Abstract

In soils containing vermiculite, fixed NH 4 + (Fix NH 4 + ) levels are affected by exchange reactions with other cations, which may directly impact both NH 4 + availability to nitrifiers and mobility through the soil. Some previous soil suspension studies have shown that in certain soils, K + is highly effective at displacing adsorbed NH 4 + , whereas in other studies, K + induced high affinity NH 4 + adsorption in soils exhibiting vermiculitic behavior. The purpose of this study was to demonstrate the influence of K + on NH 4 + mobility in soils in a heterovalent ternary cation exchange system (K-NH 4 -Ca). For this study, we selected two Kentucky soils of mixed mineralogy with varying quantities of vermiculite: a Maury silt loam (fine, mixed, mesic Typic Paleudalfs) topsoil (0-6 cm) and an Eden clay loam (fine, mixed, mesic, Typic Hapludalfs) subsoil (15-30 cm). Ammonium breakthrough curves (BTC) were obtained by leaching packed soil columns with either 5 mM NH 4 + plus 1 mM Ca 2+ solution or 5 mM NH 4 + , 5 mM K + , plus 1 mM Ca 2+ solution. Ammonium BTC with and without added K + for both soils indicated, as expected, that NH 4 + movement through the column was impeded by soil retention. For the Maury soil in the presence of K + , the NH 4 + BTC appeared approximately 1 pore volume to the left of the NH 4 + curve in the absence of K + , signifying that K + competed with NH 4 + for soil surface exchange sites, whereas for the Eden soil in the presence of K + , the NH 4 + BTC appeared approximately 5 pore volumes to the left of the NH 4 + curve without K + . This signified an apparently strong competitive effect by K + , with respect to NH 4 + , for soil surface exchange sites. However, extraction of soil subsamples from the columns revealed that adding K + to the Maury soil reduced ExNH 4 but did not change Fix NH 4 levels, whereas for the Eden soil, adding K + reduced ExNH 4 and greatly increased Fix NH 4 + . FTIR analysis of the soils revealed NH 4 + deformation bands at 1454 and 1430 cm -1 , suggesting that NH 4 + was held in the Maury and Eden soils in two chemically distinct binding environments, possibly representing two different NH 4 + sinks, i.e., exchangeable and fixed. Furthermore, this distortion of the tetrahedral molecule, evidenced by the shifts of the NH 4 + deformation bands to wavenumbers beyond 1399 cm -1 for free or uncomplexed NH 4 + , led us to propose that the vermiculite interlayer was more stereospecific for the NH 4 + ion than for the physically and chemically distinct spherical K + ion. That the IR spectra were identical for both soils in the absence and presence of added K + implied agreement with the extraction data that K + was not able to affect interlayer NH 4 + complexation directly. We propose that K + fixation collapsed the interlayer around NH 4 + ions, thus seeming to induce NH 4 + fixation.