Abstract:
With the aid of Global Positioning System (GPS) and Satellite Aerial Photo, a comparative characterization and mapping of soil gully erosion features on two geological formations were carried out in Nsukka area of eastern Nigeria. The two geological formations were Ajali and Mamu formations. The study involved the use of base map created using a Geographic Information System (GIS) (GPS Track Marker) and Satellite Photo downloaded from the internet using the same GIS (GPS Track Marker). This aided the field work for erosion site study and data collection. A total of seventy (70) erosion sites with an average length of about 1606.5 meters, average width of about 64.2 meters and average depth of 8.6 meters were visited in Ajali formation. On the other hand, only nine with an average length of about 484.2 meters, average width of about 6.5 meters and an average depth of about 3.7 meters were visited in Mamu. In Ajali formation, forty three new erosion sites were identified to add to the twenty seven old sites while in Manu formation only five new sites were identified to add to the four old sites. Three profile pits were dug in each formation to represent the soils. They are sites of Ada (Mpt1), Agu-Orba (Mpt2) and Agu-Ekwegbe (Mpt3) on Mamu Formation while Iheaka (Apt1), Ede-Oballa (Apt2) and Aku (Apt3) were sites on Ajali Formation. The soils from the pits were sampled and analyzed for some physical and chemical properties. The properties were, colour, texture, soil reaction, organic matter, exchangeable bases, total nitrogen and available phosphorus, exchangeable acidity, cat ion exchange capacity, and aggregate stability. There was a significant difference in the value of gully length and width while the depth was statistically the same. There was a positive significant correlation between length and width (r = 0.409), depth and width (r = 0.862), but non significant correlation between length and depth (r = 0.188) in Mamu Formation, while a positive and significant correlation was found between length and depth (r = 0.635), length and width (r = 0.578), depth and width (r = 0.689) in Ajali formation. The results of the soil percentage state of aggregation (PSA) and percentage aggregate stability (PAS) was low at both soils. There was no significant difference between their mean weight diameters (MWD). Their low MWD values (1.1mm) in the soil of Mamu formation and 1.2mm in the soil of Ajali formation were indication that the soil were highly susceptible to erosion. Both soils of the studied area have low silt content (8% in the soil of Ajali and 5% in the soil of Mamu), but moderate to high fine sand values (18% in Ajali and 49% in Mamu). These could be one of the factors promoting the soil erodibility. At micro level, the water/calgon dispassion ratio (DR) was very high. There was no significant difference in the chemical properties of the soils of the two underlying geological formations using t-test analysis. Gully erosion affected both Mamu and Ajali formations and led to loses to all the soil nutrients. Low soil pH due to heavy rainfall and the acidic nature of the underlying geology (false bedded sand stones and coal measures) and possible acidic precipitation affected the soil structure and promoted erodibility. The organic matter content of the soils was generally low (5.8g/kg in Ajali Formation and 4.3g/kg in Mamu Formation). The total nitrogen values were low (average of 0.1g/kg in the soil of Ajali and 0.07g/kg on the soil of Mamu formation) .The effective cat ion exchange capacity (ECEC) (cmol kg-1) values were also very low. The available phosphorus (mg/kg) was very low (6.6m/kg in Mamu formation and 6.7m/kg in Ajali formation) compared to the critical value 8-15m/kg. All these signify low soil fertility status partly due to severe land wash by soil erosion. This is getting worse due to anthropogenic effects on the soil cover (deforestation) and soil disruption due to sand and stone excavation. To this, effort is urgently needed to rescue the inhabitants of the agricultural areas such as Agu-Ukehe, Agu Ekwegbe, Agu-Orba, Imilike-Agu, Ezimo and Obollo-Eke where threatening gullies were identified. Farmers should be encouraged to practice conservation tillage and use more organic manure as against inorganic fertilizer. These should be reforestation especially at the eastern aspect of the Ajali formation where the soil structure is becoming poorer year after year. Government should set-up a task-force to control sand and stone excavation which was identified as part of the major initiator of gullies in the studied area.
