Studies of cation exchange in some soils of Central Trinidad
AbstractDetails of 12 profiles are reported, including descriptions, mechanical (particle size) analyses, and determinations of C, N, sticky point moisture percentage, pH, total exchangeable bases and a, Mg, K, Na, and Mn extracted by ammonium acetate, exchangeable H and exchangeable Al extracted by unbuffered BaCJ2, and the cation exchange capacity (CEC) (by summation) at pH 8·1, 7·0 and at the actual pH in contact with the soil. In most samples, with pH below 5·5, the latter CEC value could be termed 'permanent-charge CEC', whereas the values at pH 7·0 and 8·1 include some or all of the 'variable-charge CEC'. Amongst the 'younger mature soils', a typical profile of Ecclesville Series, four profiles of Freeport Series and one Washington-Cunupia intergrade profile show similar distributions of the specific cations, and a limited range of variation in clay mineralogy is indicated by actual-pH CEC value per 100 g of clay between 25 and 50 mequiv. (Mostly about 35 to 40). Exchangeable Al becomes high in the lower layers, with values between 45 and 80 per cent of actual-pH CEC in part of the B horizon; it is suggested that this must have adverse effect on the roots of many species and that investigations should be made into the limitations to the penetration of roots of various crops. The problem is raised of lime applied to the topsoil being entirely retained there and failing to affect the subsoil. One profile each of Las Loma Series and Talparo Series ('older mature soils') and Piarco Series ('senile soil') give results indicative of some further leaching and weathering, and a Couva Series profile ('immature soil') gives higher CEC and much higher exchangeable Ca values. Two profiles formed from alluvium give intermediate results, with negligible exchangeable Al; one of these, from site and profile features, can only be recognized as Freeport Series, whilst the other is distinguishable as L'Ebranche Series. It is suggested that more accurate information on the location of the better soils of terraces and undulating plains, particularly L'Ebranche Series, in some parts of Trinidad, would be valuable.