Investigations were carried out on the effect of rainfall pattern on some soil chemical properties during 2011 in the Southern Guinea savanna ecosystem in Nigeria. The study was carried out in Oro Forest Reserve in Kwara State of Nigeria. Twenty plots were randomly selected for soil sampling at for different seasons namely: January (dry season), May (beginning of rains), September (peak of rains) and November (end of rains). Different soil depths were sampled: 0 to 5, 6 to 10, 11 to 15 and 16 to 20 cm at five randomly selected locations. The chemical properties that were mostly influenced by rainfall pattern are soil organic matter, total nitrogen, soil pH, available phosphorus, exchangeable cations (Ca, Mg and K), and cation exchange capacity (CEC). The two major seasons that show profound influence on soil properties are dry season (January) and peak of rainy season (September). Soil pH and available phosphorus were higher in dry season (January) and at the beginning of rainy season (May) and remain low at the peak of the rainy season (September). In contrast, soil organic matter and total nitrogen were low in dry season (January) due to burning of the vegetation. However, nitrogen content increased at the peak of the rainy season (September), due to nitrogen fixation. The increase in the total exchangeable bases (TEB) could be attributed to their importance in the tissue synthesis. There was decline in most soil nutrients during active growth of the woodland savanna trees. Therefore, the limitation of N, P, Ca, Mg, Na and K is most likely to occur in September (peak of rainy season).
Key words: Soil properties, seasonal changes, Southern Guinea savanna, sampling depths.
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