Assessment of some heavy metals and physiochemical parameters of soil samples from Biu, Biu Local Government Area, Borno State, Nigeria

Biu is central to the known Biu province which harbors mostly the Pabirs and Buras who considered farming not only as a source of livelihood but as a pride. Cereal and other agricultural products are produced for both commercial and consumption purposes, thus, a lot agrochemicals and other soil amendments are employed to improve harvest. Also, the soil of the area is mostly found either amongst or surrounded by mountains or rocky environment, thus the need to assess level of some heavy metals and physicochemical parameters of the soil samples is critical. Atomic Absorption Spectrophotometer (AA-6800 SCHIMADZO) was used to determine the concentration of the heavy metals in the soil. Determinations of pH, electrical conductivity, cation exchange capacity (CEC) organic carbon, organic matter, calcium, sodium and potassium were carried out using standard methods. The results revealed the concentrations of Mn ranging between 6.45 and 16.98 mg/kg; 5.34 and 14.324 mg/kg Fe; 0.98 and 1.67 mg/kg Ni; 0.19 and 0.916 mg/kg Co; 0.43 and 0.838 mg/kg Cu; 0.326 and 1.269 mg/kg Zn; 0.012 and 0.435 mg/kg Pb; 0.043 and 0.813 mg/kg Cd and 0.02 and 0.80 mg/kg As. The levels of heavy metals in the sampled areas are in the following order Mn > Fe > Ni > Zn > Co > Cu > Cd > As > Pb. The predominant high concentrations of manganese and iron in all the depths for pre- and post harvests soil samples observed from the results could be associated to nature of soil of the area which is mostly found amongst or surrounded by mountains or scattered rocks. It is also clearly observed that the concentration of the heavy metals is higher in the post-harvest compared to pre-harvest, this could be attributed to the use of agrochemicals applied to boost harvest and to control pest. The levels of some physicochemical parameters in soil samples from different agricultural locations during pre-harvest and post-harvest revealed pH values ranging between 5.74 and 6.32; 28.0 and 151.0 (μScm-1) electrical conductivity; 0.92 and 1.58 % organic carbon; 1.59 and 4.17 % organic matter; 6.21 and 90.9 c.mol/kg CEC; 1.40 and 37.0 mg/kg Ca; 0.12 and 0.26 mg/kg Na and 0.23 and 2.94 mg/kg K. The decrease in heavy metals concentration with increase in depth is in agreement with increase in pH with increase in depth. Low pH increases the solubility and mobility of heavy metals since the mobility and availability of heavy metals are considerably greater in acidic soils than in near neutral or alkaline soils. In this study organic carbon and organic matter are observed to decrease with depth both in the pre-harvest and post-harvest, high organic matter content helps in high retention capacity of many pollutants. The cation exchange capacity (CEC) is found to decrease in depth with rise in pH values. Levels of Na, Ca and K detected in the soil are within the safe limit set by FAO, however, the heavy metals were found to be above the limit.