Influence of added K+ on ammonium selectivity/mobility by soils with vermiculitic behavior

In soils containing vermiculite, fixed NH4+ (Fix NH:) levels are affected b y exchange reactions with other cations, which may directly impact both NH4 + availability to nitrifiers and mobility through the soil. Some previous s oil suspension studies have shown that in certain soils, K+ is highly effec tive at displacing adsorbed NH4+, whereas in other studies, K+ induced high affinity NH4+ adsorption in soils exhibiting vermiculitic behavior. The pu rpose of this study was to demonstrate the influence of K+ on NH4+ mobility in soils in a heterovalent ternary cation exchange system (K-NH4-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 Pal eudalfs) topsoil (0-6 cm) and an Eden clay loam (fine, mixed, mesic, Typic Hapludalfs) subsoil (15-30 cm). Ammonium breakthrough curves (BTC) were obt ained by leaching packed soil colunms with either 5 mM NH4+ plus 1 mM Ca2solution or 5 mM NH4+, 5 mM K+, plus 1 mM Ca2+ solution. Ammonium ETC with and without added K+ for both soils indicated, as expected, that NH4+ movem ent through the column was impeded by soil retention. For the Maury soil in the presence of K+, the NH4+ ETC appeared approximately 1 pore volume to t he left of the NH: curve in the absence of K+, signifying that K+ completed with NH4+ for soil surface exchange sites, whereas for the Eden soil in th e presence of Ki, the NH4+ ETC appeared approximately 5 pore volumes to the left of the NH4+ curve without K+. This signified an apparently strong com petitive effect by K+, with respect to NH4+ for soil surface exchange sites . However, extraction of soil subsamples from the columns revealed that add ing K+ to the Maury soil reduced ExNH(4) but did not change Fix NH4 levels, whereas for the Eden soil, adding K+ reduced ExNH(4) and greatly increased Fix NH4+. FTIR analysis of the soils revealed NH4+ deformation bands at 14 54 and 1430 cm(-1), suggesting that NH4+ was held in the Maury and Eden soi ls in two chemically distinct binding environments, possibly representing t wo different NH4+ sinks, i.e., exchangeable and fixed. Furthermore, this di stortion of the tetrahedral molecule, evidenced by the shifts of the NH4+ d eformation bands to wavenumbers beyond 1399 cm(-1) for free or uncomplexed NH4+, led us to propose that the vermiculite interlayer was more stereospec ific for the NH4+ ion than for the physically and chemically distinct spher ical K+ ion. That the IR spectra were identical for both soils in the absen ce and presence of added K+ implied agreement with the extraction data that K+ was not able to affect interlayer NH4+ complexation directly. We propos e that K+ fixation collapsed the interlayer around NH4+ ions, thus seeming to induce NH4+ fixation.