High and sustainable crop yields in the tropics have been reported to be only possible with judicious combination of mineral fertilizers and organic amendments. Fertilizing croppings to achieve this has usually been a difficult task to achieve. The growth and yield of maize cultivated with a complementary application of organic and inorganic fertilizers was assessed compared with sole organic and sole inorganic fertilizers between April and July 2003 and 2004 at Ibadan, Nigeria, in the degraded tropical rain forest zone. There was a no-fertilizer treatment as the control. The organic fertilizer was an equal mixture of composted domestic waste and stale cow dung, applied at 10 tonnes ha -1 . Urea and Single super phosphate were applied as the inorganic fertilizer to supply 70 kg N and 13 kg P 2 O 5 ha -1 respectively. The mixture of organic and inorganic fertilizer treatment consisted of half the rates used for sole organic and sole inorganic fertilizer treatments: 5 tonnes organic mixture was applied, with 35 kg N and 6.5 kg P 2 O 5. Maize plant height at 8 weeks after planting was highest with inorganic fertilizer application while the leaf area was highest with organic fertilizer application. Stover yield and cob yields were also highest with inorganic fertilizer. Complementary application of organic and inorganic fertilizers however had similar plant heights; stover yield as well as cob yields with inorganic fertilizer. Nitrogen appeared chelated with organic fertilizer application. Plant earleaf Nitrogen was highest (1.68%) with inorganic fertilizer while the control plots had a Nitrogen content of 1.12% which was higher than 0.84% and 0.98% N from sole organic and a complementary application of organic and inorganic fertilizers, respectively. Plant P content was increased by 136% and 15% with organic and inorganic fertilizers, respectively, but was reduced by 15% with complementary application of organic and inorganic fertilizers. The K content was highest with inorganic fertilizer (1.91%). Complementary application of organic and inorganic fertilizers had a K content of 1.70% while the organic -fertilized leaves had 1.53%. Stover nutrient uptake was highest for N and K with inorganic fertilizer while the P was highest with organic fertilizer application. Cultivating maize with complementary organic and inorganic fertilizers gives a comparable cob yield as inorganic fertilizer and has nutrients higher than from sole organic fertilizer application.

The changes in soil nutrient status following the application of different fertilizer types were studied in field experiments involving maize-melon intercrop relayed into a cassava-soybean intercrop between 1995 and 1997. The soil at the experimental site was a Kanhaplic Haplustalf, which was under continuous cultivation with arable crops for seven years and fallow for four years. The effects of organic and inorganic fertilizers were investigated singly and in combination.The type of fertilizer had no significant effect on the soil pH, although, cropping significantly lowered the pH from 6.0 to 5.7. Cropping also significantly reduced the soil organic matter and total nitrogen (N). The soil organic matter (OM) and total N were most depleted with organic fertilizer application. Complementary application of organic and inorganic fertilizers limited the degree of depletion from 31.0 to 12.1 g kg À1 of OM and 1.8 to 0.6 g kg À1 total N. Soil available P was increased (60%) by inorganic fertilizer while the organic fertilizer increased it by 145% and the combined fertilizer by 186%.Exchangeable calcium (Ca) was depleted by about 12% with organic fertilizer application, 15% by inorganic fertilizer and about 19% with complementary application of organic and inorganic fertilizers. Exchangeable sodium (Na) was reduced from 0.43 to about 0.38 cmol kg À1 while magnesium (Mg) was increased from 0.5 to about 0.6 cmol kg À1 .

Field trials were conducted in the forest vegetation (Ibadan) and the derived savannah (Ilora) zones of south west Nigeria to assess the yield performance of cassava/maize intercrop and soil nutrient changes with sole and some combined ratios of organic-based fertilizer (OBF) and inorganic fertilizer. Sole OBF was assessed at 2.5; 5.0 and 10.0 t ha -1 while 400 kg ha -1 NPK 15-15-15 served as the sole inorganic fertilizer treatment. The combined treatments were: 2.5 t ha -1 OBF + 100 kg ha -1 NPK 15-15-15 and 5 t ha -1 OBF + 100 kg ha -1 NPK 15-15-15. An unfertilized treatment served as the control treatment. Maize grain yield was highest with application of 5 t ha -1 OBF + 100 kg ha -1 NPK. It gave a mean yield of 2.45 t ha -1 at Ibadan and 2.49 t ha -1 at Ilora. Cassava root yield was however highest with 10 t ha -1 OBF. It gave a mean yield of 14.55 t ha -1 at Ibadan and 12.52 t ha -1 at Ilora. Soil N, P, K and Organic C was most increased with 10 t ha -1 OBF and 5 t ha -1 OBF+NPK. Crop yields and soil nutrient status decreased with no fertilizer application. Cassava -maize intercrop gives optimum yields and highest soil N, P, K increase with 10 t ha -1 OBF.

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Experiments were conducted in the growing seasons of 2005 and 2006 at Ibadan, Nigeria, in the degraded tropical rain forest zone to assess the growth and yield of maize with Nitrogen-enriched organic fertilizer made from municipal waste and cow dung (2.5t ha -1 Pacesetter fertilizer + 100kg ha -1 urea)and also with Nitrogen-fortified poultry manure. Their performance was compared with those of inorganic NPK fertilizer and no fertilizer control. Maize growth was significantly (P=0.05) affected by an enrichment of the organic manures. They had plants comparable in height with inorganic fertilizer application. At harvest, plants treated with fortified poultry manure were about 259cm tall while those treated with fortified Pacesetter fertilizer and the plants treated with inorganic fertilizer were about 253cm tall. Average plant leaf areas were similar with the fortified fertilizers and with inorganic fertilization. Length of days taken to achieve 50% tasselling was also reduced with fertilization. Inorganic fertilizer application gave plants that achieved 50% tasselling in 50days while fortified poultry manured -plants took 52days and the fortified Pacesetter fertilizertreated plants took 53days. Fertilization of maize gave significantly (P=0.05) higher seed yields. Fortified poultry manure gave an average yield of 3.97t ha -1 while fortified Pacesetter fertilizer had an average of 3.78t ha -1 .Inorganic fertilizer gave a yield of 3.70t ha -1 while a significantly lower yield of 2.48t ha -1 was given by the unfertilized plants. Maize growth and yield from the enriched organic manures were comparable with inorganic fertilizer, indicating the potentials of the use of fortified organic manures as alternatives to inorganic fertilizers. Poultry manure required lesser N-fortification to give comparable seed yields as cow dung. Although both organic manures increased the soil N and P, poultry manure gave higher values while the soil K, Ca and Mg contents were more increased with the cow dung than poultry manure. Poultry manure, fortified with 100kg Urea can be applied at 2.5t ha -1 to cultivate maize. It gives a comparable yield as inorganic fertilizer and increases the soil N and P.

Field trials were conducted at the Institute of Agricultural Research and Training Ibadan in 2000 and 2001 cropping seasons to evaluate the efficiency of some phosphorus (P) sources, rates, and frequency of application on grain yields of maize and cowpea grown sequentially. The experiment was laid out in a split-split plot arrangement using randomized complete block design, with three replications. The main plot was frequency of fertilizer application (seasonal and annual). Source of P [single superphosphate (SSP: 18% P 2 O 5 )]; Ogun rock phosphate (OPR: 20.21% P 2 O 5 ), and crystallizer super (CS: 31.4% P 2 O 5 ) constituted the subplots. The P rates (0, 20, 40, and 60 kg P ha −1 ) were the sub-subplots. An early maize cropping followed by a late-season cowpea was the sequential cropping, while an early maize followed by a late-season maize was the continuous cropping. The croppings were repeated the second year. Frequency of P application and source of P were significantly different while rates of application were not. Maize and cowpea grain yields from the control treatment were significantly less than yields from P-treated plots. Maize grain yields were slightly greater in plots alternated with cowpea, compared to continuous maize. Relative agronomic efficiency (RAE) values of OPR and CS were 67% and 75%, respectively, in annual application and 140% and 134% in seasonal applications, respectively, in 2000. In 2001, the trend was almost similar to 2000 except that there was a decline in RAE values of cowpea. Each Naira invested in the annual application returned about N =3.12 while about N =2.13 was returned with the seasonal application (N =125.00 is the equivalent of USD$1.00). The OPR and CS can serve as alternatives to water-soluble sources such as SSP.

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