7. Two Pan Soils of the Lower Volta Basin: Proposals for improving their Classification according to Soil Taxonomy and the World Reference Base

Abstract

Kpejeglo series and Agawtaw series are two closely related pan soils in the Lower Volta Basin of Ghana. They have previously been classified variously according to Soil Taonomy and the FAO/World Reference Base (WRB) systems without positively identifying the subsurface hardpan horizons. The hardpans have generally been described as compact clay loam or claypan horizons. This study was designed to 1) identify the hardpan horizons and 2) classify the soils according to the latest Soil Taonomy and the WRB classification systems. Both soils had argillic and natric horizons. Bulk density of the pan horizons was higher than that of the horizons above them. Air-dried peds of the pan did not slake in water and 1M HCl or 40 NaOH. Up to 50 reduction in the size of the peds was only observed after alternating acid and alkali soaking for more than 5 hours. Peds from the lower limits of the hardpan of Kpejeglo Pedon 2, however, slaked in 1M HCl. The slaking characteristics show that the hardpan horizon could be a duripan, which possibly occurred with a petrocalcic horizon. Both soils were classified under Soil Taonomy as Typic Duraqualfs. Natric horizon did not reect in the classification because it is not presently provided at the subgroup level of Duraqualfs. We propose that due to the agronomic importance of natric horizon, Natric Duraqualfs would be more appropriate classification of the soils. The soils were also classified as Gleyic Solonetz (Duric) under the WRB system. Although Gleyic Solonetz places more emphasis on natric horizon than on petroduric horizon, the latter could be more limiting to plant growth. It would therefore be important if petroduric horizon reects in the classification of the soils. Presently, under Solonetz, duric is only recognized as a supplementary qualifier so we propose that duric be elevated to a principal qualifier and should key out before gleyic so that the soils could be appropriately classified as Duric Solonetz.