RESPONSE OF COWPEA (Vigna unguiculata L. Walp) TO CHANGES IN HYDRAULIC PROPERTIES OF A SPENT CRANK-CASE OIL CONTAMINATED ULTISOL UNDER BIO-STIMULATION

A study was carried out (2007 - 2009) at the Teaching and Research Farm of the University of Nigeria, Nsukka, Nigeria. Main plot treatments were 0% (control), 1%, 2%, and 3% spent crankcase oil (Sp) applied in a single dose at 0, 10,000, 20,000 and 30,000 mg kg-1 soil, respectively. The sub-plot treatments were control (no amendment = NA), palm oil mill effluent (PE), oil palm bunch refuse (PR) and cassava peels (CS) applied at 12 Mg/ha each, per year. The treatments were arranged in a split-plot format in Randomized Complete Block Design (RCBD) with three replications. Samples were collected from 0 – 20 cm soil depth at 3, 6, 12, 18, 24, 30 and 36 months and analyzed for macro-porosity, micro-porosity and saturated hydraulic conductivity (Ks).Cowpea was planted in three seasons to evaluate the phyto-toxicity of spent crank-case oil and soil quality following amendment. Results indicated a trend of decreasing macro-porosity values with increasing spent oil contamination with values in 3 % oil contaminated plots ranging from 7.5 – 14.0 % whereas in 0 % treated soils values ranged from 22.0 – 25.5 %. Micro-porosity increased with increase in rate of oil contamination, increasing from 31.0 % in untreated plots to 44.0 % in plots treated with 3 % spent crank-case oil in 36 months. Amendment of contaminated soils significantly increased macro-porosity whereas micro-porosity was reduced. The Ks of plots changed from rapid (> 25 cm h-1) to moderately rapid 'permeability' (< 6 cm h-1) following oil contamination. Phyto-toxicity test indicated that increasing the contamination of spent oil beyond 1 % led to a significant decline in the percent emergence of cowpea seeds. Emergence was reduced from 99 % in control soil to 78 and 71 % in 2 and 3 % spent crank-case oil treated soils respectively following oil contamination.