Distribution of Phosphorus Fractions down the Soil Depth in the Savanna Zone of Nigeria

The distribution of phosphorus (P) fractions down the soil depth varies depending on the soil type as being affected by the surrounding soil management practices. A 10 km by 10 km site was chosen in the savanna zone of Nigeria for the study. Modal soil profile pits were dug to identify the different soil types while soil sampling was done at 0-20 cm, 20-40 cm, 40-80 cm and 80-120 cm depth. Resin P was the largest extractable P fraction while residual P was the least in the soil series characterized, down the soil depth. Fractionation results showed that there was variation down the soil depth. The conc. HCl-Pi was not observed at the 0-20 cm depth, NaHCO3-Pi and Po was small while NaOH-Pi and Po was observed at this depth. Initial decrease at the 20-40 cm depth was observed for most of the P fractions while more of the P fractions were observed at the 40-80 cm depth. The values observed at the 80-120 cm depth was however small compared to other soil depth. Resin P was the sink of P and would serve as a reserve of P in the soil series. However, organic and inorganic P extractable with NaOH i.e. NaOH Pi and Po which were high at the 0-20 cm depth would be mainly responsible for the availability of P in the soil solution for plant uptake.