Effects of sample pretreatment and sequential fractionation by centrifuge drainage on concentrations of minerals in a calcareous soil solution

Soil solution is the direct source of mineral nutrients to vascular plants. Soil solutions of sieved samples at water-holding capacity of a seminatural calcareous soil, never cultivated or treated by fertilizers, were collected by sequential centrifugation at 24-18,900 X g relative centrifugal force (RCF), followed by ultrafiltration (0.2 mu m), and analysed for Al, Ca, Fe, K, Mg, Mn, Mo, Na, P, S, Si and pH. Soil solution held by weak forces (low RCF applied) had higher concentrations of base cations, Fe, and Si than soil solutions obtained at higher RCF. Opposite relationships were measured for P-TOT, molybdate-reactive P and Mo, whereas concentrations of Mn and S did not vary much according to RCF level. A comparison between centrifugation of soil cores at undisturbed stratification and mixed samples uncovered no great differences in soil solution pH or concentrations of major constituents, though concentrations of Ca, Si and S were 10-20% lower in solution from undisturbed cores. Possible mechanisms accounting for the observations are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.