Studies on the Basaltic Soils of Northern Ireland

Abstract

Results are given of the exchangeable calcium, magnesium, sodium, and potassium contents of the sand, silt, and clay fractions separated by purely physicaI methods from sixteen horizons, mainly subsoils from basaltic soil profiles in Northern Ireland. Relatively high exchangeable-calcium values are recorded, ranging from 6.9 to 20.9 m.e. per cent. for sands, from g to 28 m.e. per cent. for silts', and from 8.3 to 35 m.e. per cent. for clay separates.Exchangeable-magnesium values are also high, particularly for separates from subsoils with imperfect drainage, and values as high as 17.2, 27.3, and 32-2 m.e. per cent. are recorded for sand, silt, and clay separates, respectively. The exchangeable-magnesium contents usually increase with depth in the subsoil and in many cases the exchangeable-magnesium values for silts are as high as or higher than values for corresponding clay separates. Comparisons of the values for sands, 'silts', and for corresponding clays suggest that the high values exhibited by the silts (and often by the sands) cannot be attributed, to any considerable extent, to incomplete dispersion, e.g. to the presence of normally dispersible clay aggregates in the silts or sand separates. It is suggested that these high values and the associated high cation-exchange capacities of these silts (and coarser fractions) may be due to the presence of partially weathered rock minerals which may be broken down, e.g. by acid treatments and dispersion procedures prior to detailed examinations. " IN neutral and slightly acid soils calcium is generally found to be the main exchangeable metal cation, but there are many references in the literature to such soils in which the exchangeable-magnesium contents approach or even surpass the quantities of exchangeable calcium. Mitchell (1g37), Hallsworth et al. (1952), Glentworth (1954), and Brown (1954) have recorded values of the order of 20-25 m.e. exchangeable magnesium per IOO g. soil for some subsoil horizons of basic igneous soils, and Kelly ('948) has recorded a value of 40 m.e. per cent. for a heavy clay from California. Such high values are usually associated with imperfect drainage conditions and often, but not always, with high clay contents. In a recent paper on basaltic soils in Northern Ireland McConaghy and McAleese (1957) gave results showing a reciable increases in exchangeable magnesium with increasing pro Kp e depth, although the percentage clay contents showed considerable decreases I Formerly of Ministry of Agriculture, Northern Ireland, and Queen's University of Belfast.