| Popis: |
Ferruginous soils formed from Volta shale deposits in northern Ghana are highly weathered, have high iron and aluminium oxide contents, and a mineralogy dominated by quartz and low activity clays with low effective cec. Despite their advanced weathering state, base saturation values of top soils are generally above 80%, and deficiencies of basic cations are uncommon. We postulated that the annual deposition of dust from the Sahara, carried by the Harmattan weather system may be responsible for the high base saturation. Therefore, we collected dust material for two entire dry seasons at Nyankpala (surrounded by highly weathered Volta shale deposits), and during several events of Harmattan and local dust storms at Bolgatanga (surrounded by a variety of parent materials). Long-distance Harmattan dust had a clearly finer particle size distribution than dust collected during local storms, but angularity and evidence of weathering on dust particles did not distinguish dust from different sources. In Bolgatanga, soils contained unweathered minerals derived from local parent materials, and both local storm and Harmattan dust contained large amounts of bases. Significant amounts of micas and feldspars, and K contents up to 3% in the dust collected at Nyankpala showed Harmattan dust to be the source of K and other bases found on Volta shale soils. Short-distance dust in Nyankpala was mostly quartz and vegetation ashes. Dust deposition during one Harmattan season at Nyankpala amounted to about 15 g m−2, and carried a total of 140, 400, 300 and 60 mg m−2 of Mg, Ca, K and Na, respectively. These amounts could account for the high base saturation of the low cec in Volta shale soils. |
