SOLUTION, EXCHANGEABLE AND CLAY-FIXED AMMONIUM IN SOUTH COAST BRITISH-COLUMBIA SOILS

Solution, exchangeable and clay-fixed ammonium were measured in a vari ety of south coast British Columbia soils in the laboratory using extr action, equilibration and leaching procedures, and with and without am monium amendments in order to evaluate the relative significance of ad sorption and fixation processes on nitrogen behavior. Non-amended soil s contained from 59 to 224 g N g(-1) of fixed ammonium and these amoun ts were correlated with clay (positive) and sand (negative) contents. The amount of native fixed ammonium in the soils was influenced by man agement history. Recovery of ammonium added to eight selected samples by a small volume of solution followed by air drying was the same with 1 and 2 M KCl extractions, but less was extracted by 0.1 M KCl and ev en less by water. This showed that both 1 and 2 M KCl had sufficient K + to displace the NH4+ present (inherent and added) on the exchange si tes without an apparent effect on fixed ammonium. Up to 68% of the amm onium added was not extracted by 2 M KCI and assumed to have been fixe d in the clay lattice. The proportion of the added ammonium that was a dsorbed onto the exchange sites was linear up to 200 mu g N g(-1) appl ication, but the amount fixed by the clays was linear to only about 10 0 mu g N g(-1) application rate. Data from studies where soil columns were leached with NH4NO3 provided only limited information on the rela tive importance of exchange versus fixation processes to the adsorptio n of ammonium. Leaching columns with a limited number of batches of NH 4NO3 followed by water (short cycle) were useful for comparing the abi lity of different soils to adsorb and retain ammonium, but the relativ e importance of exchange versus fixation could not be determined. In t he 18 samples of this study, the amount of NH4+ adsorbed ranged from 8 4 to 25% during shea cycle column leachings. Different patterns of ads orption occurred among the four soil samples that were leached with a large number of batches of ammonium (long cycle), but it could not be determined whether or not these patterns were related to differences i n proportions of fixation relative to exchange. Measurements showed th at ammonium was fixed in the soils during the long cycle leachings but that the amounts of fixed ammonium measured were influenced by air dr ying the sample before analysis. Several studies with and without tolu ene additions showed that microbial activity must be controlled during these measurements. It was concluded that south coastal soils contain ed significant amounts of fixed ammonium and can fix additional ammoni um. The amount of ammonium in the clay fixed pool and the potential to fix added ammonium was different in relation to the soil type, their previous management, and wetting and drying cycles. The biological sig nificance of fixed ammonium and the fixation process must not be disco unted in any nitrogen studies of these soils for proper interpretation of data.