22 Continental J. Agronomy 5 (2): 22 - 31, 2011 ISSN: 2141 - 4114 © Wilolud Journals, 2011 http://www.wiloludjournal.com Printed in Nigeria PARTICLE SIZE AND FREE IRON OXIDES DISTRIBUTION ALONG TWO TOPOSEQUENCES IN SOUTH WESTERN NIGERIA Enya, O.O, Omueti, J.A.I and Akinbola, G. E. Agronomy Department, University of Ibadan, Ibadan. Nigeria. ABSTRACT Particle size and free iron oxides content distribution along Sedimentary and Basement complex toposequences in South Western Nigeria was investigated. Representative soil samples totaling nine mini-pits (45 cm x 45 cm x 70 cm) were taken based on topographic position. Conventional methods were used in the determination of pH, particle size distribution while the free iron oxides were determined using Dithionite-Citrate-Bicarbonate (DCB) and Acid Ammonium Oxalate methods. Results indicated that, soil texture was loamy sand surface horizon and increasing proportion of clay with depth, except the valley bottom profile in the basement complex toposequence which was clayey on the surface horizon, decreasing with depth. Silt content showed that those of basement complex have an average of 105.4 g/kg while those of sedimentary formation were 32 g/kg. pH values of the soils ranged 4.9 to 6.0 indicating strongly acid to medium acid. Dithionite extractable Fe oxide was 51.5 g/kg and 16.8 g/kg at Basement Complex toposequence and Sedimentary toposequence respectively. Oxalate extractable Fe were low (9.1 g/kg) at Basement Complex toposequence and (6.0 g/kg) at the Sedimentary toposequence. Basement Complex toposequence have low Fe-ratio and could be highly weathered as compared to the Sedimentary formation. This may be due to the coarse nature of the Sedimentary formation. There is high negative significant correlation between Fe d /clay and clay content in Sedimentary formation (r=-0.913; p<0.01) and high positive significant correlation when Fe d /clay was correlated with silt (r=0.887; p<0.01) in Basement Complex toposequence, indicating co-migration of Fe and clay and Fe and silt respectively. KEYWORDS: particle size, free iron oxides, sedimentary and basement complex toposequences, dithionite extractable, oxalate extractable. INTRODUCTION The majority of the total irons found in soils exist as oxides. These oxides are found as iron concretions as well as coatings on the soil minerals or bind the different soil particles (Olsen, 1948) Particle size and free iron oxides distribution has been extensively studied at both small and large scales (Juo et al, 1973; Udo, 1980; Ogunsola and Omueti, 1990; Mosugu et al., 1999; Agbenin, 2003; Igwe, 2005; Obi and Akinbola, 2009) . This work is to further contribute to the existing works that has been done on the sedimentary and basement complex toposequences in South Western Nigeria compared with other regions. Forms of free iron oxides are important parameters for the proper understanding of the soil. Their amount and distribution in the soils are known to influence some soil properties such as anions adsorption, surface charges, specific surface area, swelling and aggregate formation, nutrient transformation and pollutants retention in soils (Deshpande et al., 1968; Greenland et al., 1968). The various forms of free iron oxides have been extracted by different extractants (Mehra and Jackson, 1960; Mckeague and Day, 1966; Holmgren et al., 1967; Blumme and Schwertman, 1969; Blakemore et al., 1987; Walker, 1983). Dithionite extractable Fe and Al have been widely considered to give a reasonable estimate of the total pedogenic free iron oxides in soils. The pattern of distribution of free iron oxides is closely related to that of clay where clay content is more than 40% in the B horizon and closely related to silt for soils less than 40% clay (Juo et al., 1973; Udo, 1980; Ogunsola and Omueti, 1990; Mosugu et al, 1999; Ibia,2001). Relative contribution of oxalate extractable to total iron oxides (DCB extracts) is low (0.01 -0.36%) in soils overlying limestone areas in Southern Nigeria suggesting that Fe oxides were essentially crystalline (Ogunsola and Omueti, 1990). Also, the dithionite iron extractable (Fe d ) content increases with increase of depth and the constant clay/dithionite Fe-ratio indicates the co-migration of clay and Fe oxides from the A horizon into the B horizon (Juo et al., 1973; Ogunsola and