CHAPTER ONE

1.1 Introduction

Long-term field experiments are expected to provide important information regarding soil properties as

affected by cropping system and soil management practices. Legumes such as cowpea and soybean add

both organic matter and nitrogen (N) to the soil (Omay et al., 1997; Sainju et al., 2003) and increase soil

fertility. Maize is the second most essential cereal crop after sorghum in sub-Saharan Africa; it is grown

much more intensively than it was estimated since 1985 (FAO, 1999). In Nigeria, maize yield averages

about 1.4 tons per hectare and this is only about 20% of the average in Canada and other parts of the

world where intensive cereal production is carried out (Afolami and Fawole, 1991; FAO, 1999). Studies

have shown that legume-cereal rotation produce relatively higher grain yields than either crop grown

alone (Rao and Mathuva, 2000; Olufemi et al., 2001; Mpairwe et al., 2002; Dapaah et al., 2003).

Rotating maize with grain legumes is often targeted towards utilizing biologically fixed-nitrogen by

legumes for the benefit of the maize (Yusuf et al., 2009). Nitrogen fertilizers are most effectively used

as part of abalanced fertilization plan that aims to maximizeeconomicreturn of acereal–legume rotation

system; Nitrogen fertilizer, apart from increasing the content of nitrate in soil that leads to its leaching

(Porter et al.,1996), results in changes in soil pH and manyother soil properties (Bradyand Weil, 2002).

Long-term field experiments with N fertilization can give valuableinformation about how those changes

occur and indicate the trends of the changes (Dragan et al., 2010).

Nitrogen (N) is most often the yield limiting nutrient with respect to crop production (Factsheet, 2014).

Nitrogen contributes firstlyto grain yield and forage biomass production, and at the same time to protein

(Eche, 2011). Nitrogen is essential for seed formation and maturity. A steady supply is needed during

the early growth stage and this steady supply can be provided by the action of soil microorganisms on

the soil organic matter (SOM) or by application of inorganic fertilizers (Eche, 2011). Nitrogen stress

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was observed to cause reduction in the number of grains per ear (Lemcoff and Loomis, 1994). However,

nitrogen fertilizers can be permanentlylost through ammonia volatilization, denitrification, leaching and

run-off if not well managed. It could also result in changes in soil pH which affects the availability of

other plant nutrients. Nitrogen can be fixed in the soil through symbiotic association of some

microorganism (e.g rhizobia) and leguminous plants (Brady and Weil, 2002). Therefore proper

management of soil in terms of method of tillage practice and cropping system is very important in

determining the amount of N in the soil (Veenstra et al., 2006).