The profile samples were analysed in the Institut f\u00fcr Geo\u00f6kologie, Universit\u00e4t Potsdam, from whom full details of the methods are available.\u00a0 Some granulometric analyses were done at the University of Mainz.<\/p>\n
Two general points about laboratory methodology affect the characterisation and correlation of BCI soils:<\/p>\n
Firstly, the granulometric analysis of fine textured kaolinitic and oxidic soils is problematic.\u00a0 The oxidic minerals have strong bonding effects, and much of the clay in such soils is aggregated into pseudo-silt or pseudo-sand particles. These give the soil high macroporosity and free drainage typical of medium or coarse textured soils. However, the aggregates are not dense and relatively inert primary minerals, as in true sands and silts.\u00a0 They have high internal microporosity and the voids are lined with clay surfaces that have low or moderate electrochemical activities.\u00a0 The result is that these soils behave like sands or silts with respect to drainage, aeration and moisture release at low moisture tensions, but like clays with respect to cation exchange and other chemical sorption\/desorption attributes, and also for moisture retention at high tensions.<\/p>\n
The micro-aggregation also affects the determination of clay content.\u00a0 If the soils are treated with a mild dispersant, as normally used in soils with non-oxidic mineralogies, the aggregates remain intact and the soils analyse as high in silt and\/or sand.\u00a0 If dispersed fiercely, the aggregates break up and the soils analyse as clays.\u00a0\u00a0 Reliance on only one method of dispersion gives an incomplete picture of these soils, with mild dispersion missing the clay characteristics and fierce dispersion missing the aggregation and silt- and sand-like features.\u00a0 We have therefore estimated the clay contents of some samples with both sodium pyrophosphate (mild) and sodium dithionite (strong) dispersants and indirectly from moisture retention at tensions above the conventional permanent wilting point (1500kPa) (Soil Survey Staff, 1999).\u00a0 The indirect moisture retention estimates of clay for the same soils are in the range 60 \u2013 90%.\u00a0 The dithionite dispersion gives intermediate estimates, with clay contents in the range 40 – 60%.\u00a0 These are similar to the hand textures in the field. All of the laboratory granulometric methods make heavy demands on resources, and the number of determinations is limited.\u00a0 In contrast, we have several thousands of field textures, many of them by multiple observers.\u00a0 Although only qualitative, hand texturing is in some ways superior to laboratory methods, because it reveal both aspects of micro-aggregated 1:1 clays, with initial impressions of high silt diminishing after continued kneading to give a more clay-rich texture.\u00a0 Such clays are designated as light, in contrast to heavy clays, which have substantial proportions of 2:1 minerals and do not initially feel silty.\u00a0 Our soil characterisations and correlations are based on the field textures<\/p>\n
The second point concerns the determination of cation exchange capacity (CEC).\u00a0 The CEC\u2019s of kaolinitic and oxidic clay minerals are variable and are usually low in their natural proton-rich acid environments.\u00a0\u00a0 The conventional laboratory determination of CEC is done at pH 7. Deprotonation of cation exchange sites occurs at the artificially high pH, and conventional CEC values are considerably higher than the actual working CEC in field conditions.\u00a0 This problem has long been recognised (e.g. Hesse, 1971; Pleysier\u00a0et al.<\/em>\u00a01979). There are other problems with CEC, and they are sometimes considerably lower than the sum of the exchangeable bases and appear to be substantially underestimated.\u00a0 This is not a problem that is unique to BCI and has been noted in somewhat similar soils in Mexico<\/p>\n These problems are partially circumvented by the use of the effective CEC (ECEC).\u00a0 We here use the term in the ST sense, i.e. ECEC = the sum of the exchangeable bases (TEB) + extractable Al (Soil Survey Staff, 1999), rather than the WRB definition of ECEC = TEB + exchangeable acidity (FAO, 1998), although the two are probably similar in BCI soils. TEB is relatively unaffected by pH, and labile Al is extracted at close to field pH, so ECEC is a less variable and more realistic characterisation of the exchange complex in acid soils in field conditions.<\/p>\n ST and WRB differentiate some of their taxa on base saturation (BS = TEB\/CEC).\u00a0 The methodological problems with CEC lead to potential anomalies in BS.\u00a0 In order to better characterise base status, we estimate Effective BS (EBS = TEB\/ECEC). EBS is the complement of Al saturation, and indicates the balance on the exchange complex between nutrient bases and toxic Al.<\/p>\n","protected":false},"excerpt":{"rendered":"