POTASSIUM SUPPLYING CAPACITY OF SOILS FORMED ON DIFFERENT GEOLOGICAL DEPOSITS IN THE NIGER DELTA REGION OF NIGERIA

The potassium supplying capacity of soils formed on three geological d eposits along an east-west transect in the Niger Delta region of Niger ia was investigated by determining the various forms of soil K and K u ptake by repeated maize croppings in pots. Soils (Inceptisols and Enti sols) formed on recent alluvial materials of the Meander Belt deposits (MBD) had mica and feldspars resulting in very high levels of total K (4.4 and 5.1%) and nonexchangeable K (NEK) extracted by concentrated H2SO4 and boiling HNO3 (H2SO4-K: 378 and 477 mu g g(-1); HNO3-K: 158 a nd 174 mu g g(-1)). Soils formed on the other two geological deposits (Sombreiro-Warri deposits, SWD, and Coastal Plain Sands, CPS) had low levels of total K (CPS, 0.18-0.37% and SWD, 0.28-1.76%) and NEK (H2SO4 -K and HNO3-K for CPS were 40-87 and 18-49 mu g g(-1) respectively and for SWD they were 40-158 and 14-80 mu g g(-1) respectively) indicativ e of the highly weathered nature of these soils (Ultisols) with traces of K-containing minerals. The trend in soil K levels in the three dep osits are opposite of the previously reported soil P trend. Potassium uptake by maize induced release of NEK to the plant-available K pool. Potassium supply to plants from the NEK pool for 3 successive maize cr ops in MBD, SWD and CPS soils were 303-435, 32-57 and 21-38 mu g K g(- 1) soil, respectively. NEK uptake, as a percentage of total K uptake, decreased with successive croppings in CPS soils and reached zero at t he third cropping, while in MBD soils the percentage remained constant up to the third and last crop. In this respect, some SWD soils behave d similar to MBD soils and others to CPS soils.