ABSTRACT 

Characterization, classification and evaluation of soils in Osisioma Ngwa, South Eastern Nigeria was carried out. A preliminary reconnaissance survey delineated the study area into three (3) mapping units based on toposequence namely, floodplains, gentle slopes and crests. Flexible grid method was adopted to identify different physiographic units of the area. Nine profile pits: three at floodplains, four at gentle slopes and two at crest were dug; described and examined to represent the study area. Soil samples were collected from genetic horizons. Soil map was produced in a GIS environment, using morpho-physical and soil chemical properties. The results indicated a moist top soil colour of greyish brown (7.5YR 5/2) to brownish grey (5YR 5/1) epipedons over greyish brown (5YR 6/2) endopedons that had dark reddish brown (5YR 5/6) to yellow orange (7.5 YR 7/8) redoximorphic features (mottlling) for floodplain soils. The gentle slope mapping units had dark reddish brown (5YR 3/2) to brown (10YR 4/6) epipedons over yellowish brown (10YR 5/6) endopedons. Crest mapping units had dark reddish brown (5YR 3/3) to dull reddish brown (2.5YR 4/4) epipedons over reddish brown (2.5YR 4/6) to bright brown (2.5YR 5/8) endopedon. Physically, floodplain units had an almost equal proportion of sand (140-550gkg-1), silt (140-570gkg-1) and clay (190-460gkg-1). Sand particles were more dominant in the surface soils of gentle slopes and crest units (580-880gkg-1). Chemically, the soils of floodplain unit were moderately acid (pH 5.3 – 6.4). The gentle slope and crest physiographic units had strongly acid reactions (pH 4.7 – 5.5). At floodplains basic cations were medium (1.4 – 2.6 cmolkg-1) for Ca, low (0.4 – 2.63 cmolkg-1) at gentle slope and crest, high for Mg, moderate for K and low for Na in all mapping unit. Cation Exchange Capacity in the study area was low to moderate (5.1 – 9.8 cmolkg-1) at floodplain and crest (5.4 – 10.7 cmolkg-1), but moderate to high (5.5 – 16.2 cmolkg-1) at the gentle slopes. Total Nitrogen was high (0.5-5.4 gkg-1) at the floodplain, but low at the gentle slope (0.1-0.9gkg-1) and crest (0.1- 0.8gkg-1).  Available phosphorus was low to medium. Floodplain soils were characterized by adequate micro-nutrients (Cu, Fe, Mn, Zn). At the gentle slopes and crest micro-nutrients were marginal to adequate. The fertility capability classification of soils in the study area indicated that floodplain soils had loamy top soils having less than 35% clay sub soils and evidence of gleization. Gentle slopes and crest had Sandy (S) top soils with fertility problems. Land Capability classification placed the floodplains under capability class III and capability subclass IIIw with limitations in poor drainage (wetness). Gentle slopes were placed capability class IV and capability sub-class IVf, while crest as capability class IV and capability sub class IVft due to limitations of fertility and slope. The soils are fit for crop cultivation under good management. Major pedogenic processes in: floodplains include leaching, gleization.  Gentle slope and crest units include erosion, illuviation, lessivage, braunification, and leaching. The floodplain soils were classified as Fluvaquentic Eutrudept (USDA) and Fluvisol (FAO-UNESCO). Gentle slopes and crests as Arenic Kanhapludults  (USDA) or Acrisol (FAO-UNESCO).