Effect of Topography on Phosphorus Forms and Distribution in Soils Formed

in Mica Schist in Ife Area
ADEGBENRO, R.O., OJETADE, J.O., AND AMUSAN A.A.
Department of Soil Science and Land Resources Management,
Obafemi Awolowo University, Ile-Ife, Nigeria.
E-mail: kemiadegbenro@yahoo.com
Abstract
This study was conducted to examine the effect of topography on
phosphorus forms and distribution in soils formed in mica schist in
southwestern Nigeria. The toposequence was delineated into
different physiographic units and soil profiles were established,
described and sampled at each of the units. Soil samples collected
were subjected to routine analyses along with total-P, organic and
available P forms. The active forms of P (Al-P, Fe-P, Ca-P and Occl-P)
were determined by fractionation. Result showed that the available
P was less that the critical level (10ppm P) set for soils of the humid
tropics, and it decreased down the slope. The organic P ranged from
50 – 232.50 ppm (19 to 29 % of the total P) in the surface soil
horizons and decreased with soil depth. It was highest at the upper
slope and lowest at the valley bottom for the A-horizon. The relative
abundance of other inorganic P forms was in the order of Fe – P > Al-
P > Ca –P> Occl-P, with Fe-P varying from 137.2 to 224.2 ppm while
Al-P ranged from 91.6 to149.5 ppm. Total P ranged from 763.1 to
418.7 ppm and was highest at the upper slope and lowest at the
middle slope. The distribution of total P within the soil profiles was
rather uniform for all the soils. The various forms of P decreased
down the slope for the A-horizon while P forms in the B and C
horizons were irregularly distributed. The soils of Egbeda, Olorunda
and Oba series were classified as Typic Paleustult, Jago as Aquic
Ustipsamment (USDA Soil Taxonomy) and as Lixisol and Fluvisol
(FAO/UNESCO) respectively. In conclusion, P forms and within the
profiles distribution varied with different topographic units. Due to
this variability, P fertilizer recommendation would need to be made
with due consideration to the physiographic units.