Demand-Driven Technologies for Sustainable Maize ... - IITA

144on exchangeable Ca, exchangeable K, and CEC. On the other handthe effect of the two factors and their interaction had no signifi canteffect on exchangeable acidity.Soil pH. There were signifi cant differences among rotation treatmentsand fertilizer application for soil pH (Table 3). Compared to theunfertilized treatments, fertilizer application signifi cantly (P < 0.05)decreased soil pH as was reported by Thomas et al. (1981) and Juoet al. (1995). Mean value of 6.18 was obtained for MM, 5.97 for MF,5.90 for MS2 and 5.77 for MS1.Changes in soil pH are related to crop management and in thisstudy, it was found that soils under legume rotation had lower pH,especially in fertilized soils, than continuous maize system. However,the reduction did not reach levels limiting maize growth. Severalauthors have reported rotation induced changes in the pH of the bulkor the rhizosphere soil of legumes (Chan and Heenan, 1993; Alveyet al. 2001; Xu et al. 2002). These changes are derived through theexudation of organic acids or ligand exchange on the epidermal cellsurfaces of the root and may enhance P availability (Ohwaki and Hirata1992; Ae and Otani 1997). Carbon dioxide from rapidly decomposinglegume residues and root respiration may dissolve in soil water toform weak organic acid to reduce soil pH.Soil Organic Carbon. Rotation and fertilization signifi cantly affectedsoil OC contents of the 0-15 cm layer (Table 4). The mean OC contentwas in the order MS1 ≥ MS2 > MF > MM, with values of 6.5 g kg -1 forMS1 and MS2, 5.6 g kg -1 for MF, and 5.4 g kg -1 for MM. Soil OC waslowest in the unfertilized MM plot perhaps due to poor crop coverboth in the fi rst and the second year. The greater part of the soil wasleft bare and therefore directly exposed to wind and water erosion. TheAlfi sols of the Guinea savanna of Nigeria are poorly structured andhighly prone to crust and compaction, which affects crop emergenceand aggravates soil erosion (Ogunwole et al. 1999; Wuddivira et al.2000). A slope of 0.3% in Samaru led to a 25.2% runoff with soilloss of 3.0 t yr -1 , in sorghum fi elds (Bationo et al. 1996). OC contentsincreased with fertilizer application in all treatments.This is related to the amount of residue left in the soil. About80% of the soybean leaves had fallen before the crop was harvestedfrom the fi eld in 2003. All maize residues were left on the fi eld afterremoving the cobs in 2004. A combination of the legume residuesin 2003 and that from the following fertilized maize in 2004 couldbe responsible for the higher OC observed in MS1 and MS2. Othercomplementary factors include good soil cover, reduction in runoffand losses of fertilizer and topsoil due to soil erosion. There was nosignifi cant difference in OC between MF and MM. Odell et al. (1984)