The role of depths and soil pH in the content and occurrence of the forms of magnesium in soils of the four parent materials in Edo and Delta States (Coastal plain sand, Shale, Basement complex and Alluvium) was investigated. Soils samples were collected from profiles that developed on these parent materials, with dimensions 1.2 x 1.2 x 1.5 m and demarcated according to depths ranging from 0 to 150 cm. Reagents were applied under regulated conditions and durations ranging from 10 min to 1 h. The results showed that different forms of magnesium were extracted from varying depths in each profile. In the alluvium the non available Mg content in the study ranged from 7.79 to 24.34 c mol kg-1 and was highest in the top soils of the profile examined,. The exchange Mg was between 0.16 to 0.24 c mol (+) kg-1 decreasing also across depths, in the coastal plain sand, the non available Mg ranged from 17.56 to 19.97 c mol kg-1 with greater amount realized from the bottom depths. Exchange Mg pool ranged from 0.12 to 0.52 c mol kg-1with higher records on surface depths of 0-3, 3-15 and 15-45 cm. In the Basement complex the non available Mg ranged from 12.26 to 18.97 c mol kg-1 with greater value recorded from 45 to 62 cm depths. While exchangeable Mg ranged from 0.22 to 0.46 c mol kg-1 and 3.22 to 6.25 c mol kg-1 respectively with both forms distributed oppositely across the depth. The structural Mg ranged from 6.28 to 8.84 c molkg increasing down the depths. The water sol Mg in the four parent materials were distributed in similar pattern in three out of the four parent materials, it ranged from 0.16 to 0.52 c mol kg-1 and 0.09 to 0.19 c mol kg-1 in coastal plain sand and alluvium. In Shale it ranged from 0.21 to 0.58 c mol kg-1, while in the basement complex 0.25 to 0.40 c mol kg-1 with values decreasing downs the depths across the four parent materials. The soil ph across the four parent materials ranged from 4.41 to 5.63 decreased from surface to the bottom.
Key words: Soil pH, horizon, depths, profile, available magnesium.
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