PEDOGENIC CHANGES AND PHOSPHORUS AVAILABILITY IN SOME SOILS OF NORTHERN GHANA

ABSTRACT

Pedogenic changes and P saturations in five ferruginous soils (Lixisols or Alfisols) and their associated nodules on two different landscape positions in northern Ghana were investigated. Tingoli, Tolon and Kumayili series are well drained upland soils which occur on one landscape (toposequence 1). Kpelesawgu series which is imperfectly drained and Changnalili series which is poorly drained occur on a relatively lower landscape (toposequence 2). Chemical, physical and mineralogical properties of the soils and their associated nodules were determined including factors which tend to affect the P saturation levels.

The amounts of dithioni te-ci trate-bicarbonate (DCB) extractable Fe and A1 are higher than the oxalate Fe and A1 indicating that the Fe and A1 oxides are very crystalline. Both DCB and oxalate forms of Mn are very low. The DCB-Fe and-A1 generally increase with depth in all the soils. On toposequence 1, the content of free Fe and A1 follows the order Tingoli series > Tolon series > Kumayili series. In the low lying soils on toposequence 2, the order is Kpelesawgu series > Changnalili series. The oxalate Fe and A1 are, however, higher in the low lying soils than in the well drained soils due to the low oxidising conditions in the soil environments of toposequence 2. Crystallinity is higher in the well drained soils than in the imperfectly and poorly drained soils. Crystallinity which reflects the stage of maturity is found to be in the order of nodules (> 2mm) > nodules (< 2mm) > clay > total fine earth > silt. The sequence of maturity in the well drained soils also follows the trend Tingoli series > Tolon series > Kumayili series. In the soils of the low lying areas, Kpelesawgu series is more matured than the Changnalili series. The sand and silt fractions in all the soils generally lack weatherable minerals indicating advanced stage of weathering in the soils. The clay fraction of the well drained soils is dominated by kaolinite and quartz with traces of goethite and haematite, while the low lying soils have illite in addition to the kaolinite and quartz.

The major minerals in the nodules are goethite, haematite and quartz. The CEC is generally low in all the soils (< 10 cmol (+)/kg) due to the nature and activity of clay minerals. This is also supported by the very low specific surface area of the clay fraction. In each respective soil, total phosphorus (TP) concentration follows the sequencenodule (< 2mm) * clay > nodule (> 2mm) > total fine earth > silt. The phosphorussaturation levels in the well drained soils on toposequence 1 correspond to the stage of profile maturity. In the Tingoli series where the highest TP levels are found, only DCB-A1 and oxalate forms of Fe and A1 appear to exert any influence on phosphorus saturation. The TP saturation in the Tolon and Kumayili series is greatly affected by the clay content, oxalate-Fe and -A1 and DCB-Fe. In the low lying area soils, total phosphorus content is higher in the Changnalili series than in the Kpelesawgu series probably due to translocation. In the Kpelesawgu series TP saturation is influenced by the silt content, DCB -Fe and -A1 and oxalate-A1 whereas in the Changnalili series, DCB and the oxalate forms of A1 and Fe enhance total phosphorus saturation. The Tingoli and Tolon series can be classified as Plinthic Lixisols (FAO-UMESCO) or Plinthustalfs (USDA Soil Taxonomy) while the Kumayili series is placed under Ferric Lixisols or Haplustalfs. Both the Kpelesawgu and Changnalili series are classified as Plinthic Lixisols or Plinthaqualfs. It also appears that poorly crystalline forms of Fe and A1 oxides and oxyhydroxides control the P saturation and distribution in the soils. However, as nodules are formed the P saturation tends to be controlled by both crystalline and poorly crystalline forms of the iron and aluminium oxide minerals. This would imply that the nature and properties of the clay fraction are very important in controlling the P saturation levels in the soils studied. It is suggested that for sustainable agricultural production the factors which affect phosphorus saturation in soils should be addressed.