Crop and are at a different scale (region,

Crop farming in sub-Saharan Africa is characterizedby small family farms that rely heavily on rain-fed production systems,traditional methods of soil fertility maintenance, and lack of access to creditand improved inputs such as seeds (WFP, 2012). Majority of the farmers aresmallholder farmers that are highly diverse, operating in complex natural andeconomic environments and are largely heterogeneous in termsof socio-technical conditions, farm typologies, production objectives, and thebiophysical environment (Vanlauwe et al.,2015). Several literature on soilfertility in sub-Saharan Africa reported a scenario of continuous nutrientdepletion and severely negative nutrient balances leading to abysmally low cropyields (Dercon et al. 2010; Sangingaand Woomer., 2009). Perhaps, the major factor leading to severe decline in percapita food production in SSA has been the declining soil fertility (Sanchez et al.

, 1997). Thepoor soil fertility and low nutrients availability in the SSA could be attributedto inadequate supplies of organic and inorganic fertilizers as organic manuresare available in limited quantities and inorganic fertilizers are often tooexpensive, poor cultural practices, lack of soil fertility maintenance plans,nutrient mining, low nutrient use efficiencies, inappropriate fertilizerrecommendation, differences in crop response to fertilizers Chianu et al., 2012; Smaling et al.

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, 2012; Smale et al., 2011). This has possibly been compounded by climate changeand increased pressure on land due to increasing population. Insufficientnutrient application happens because inorganic fertilizers are often tooexpensive for most of the farmers, whilst organic resources are available inlimited quantities, the consequent which led low productivity (Sheahan andBarrett, 2014; Sommer et al., 2013;The Montpellier Panel, 2013). Thereis wide variation in the growing conditions across the maize belt of Nigeria interms of soil, climate and resource endowment of the farming communities. Mostof the soils are spatially heterogeneous as dictated by the underlying parentmaterials, geomorphology, position on the landscape and historical managementpractices enforced by farmers. Perhaps, much of the soil fertility variationwithin farming systems is caused by spatial soil heterogeneity (Tittonell etal.

, 2005a). Causes of variability in soil fertility status are bothbiophysical and social economic (Tittonell et al., 2005a, b) and are ata different scale (region, village, farm, and field). And this has led tosteadily stagnated maize yields over the last few decades because ofdeteriorating soil fertility (FAO, 2014). Soil properties are an important subset of the environmentalfactors limiting the yield.

Soil chemical properties include factors such as pHand availability of macro- and micronutrients. These determine nutrientdeficiencies and toxicity of the soil.Soilfertility depletion remains the major biophysical cause of declining cropproductivity on smallholder farms in sub-Saharan Africa (Chianu et al.

, 2012; Bationoet al., 2004; Kimani et al., 2004). Most savannas of SSA areassociated with significant soil degradation that has been aggravated by rapidpopulation growth, cultivation of fragile ecosystems and continuous cropping.The consequent of this has to been low returns to investments, declining foodsecurity and general high food prices (Okalebo et al.

, 2006).Poor soilfertility and low nutrient availability have been singled out as the mostserious biophysical constraints that result in poor yields in SSA countries(Tittonell and Giller, 2013; Kibunja etal., 2012; Giller et al., 2011;Sanchez, 2010, 1997; Manu et al.,1991).

The poor soil fertility and low nutrients availability are as a resultof inherent differences in parent material, catenal position in the landscapeand household priorities and production strategies regarding preferentialallocation of labour,  manure, compostand mineral fertilizers to fields near the homesteads (Tittonell et al., 2013; Giller et al., 2011).

In addition to theinherent variability of soils and their fertility, management decisions resultin the development of strong spatial fertility gradients (Tittonell et al., 2013, 2007a,b; Giller et al., 2011, 2006; Masvaya et al.

, 2010; Zingore et al., 2007a). The magnitude of thesegradients will vary greatly from farm to farm as well as across agro-ecologicalzones depending on the many possible combinations of these biophysical andsocio-economic factors (Masvaya et al.,2010). Management of poor soilfertility and low nutrients availability have repeatedly found a nexus betweenclimate variability, farmer’s management decisions and on-farm investments(Cooper et al., 2008). Specifically,maize production in the guinea savanna (GS) of Nigeria is primarily rainfedwith annual precipitation ranging from 850 to 1500mm in the semi-arid region.

Onset and distribution of rainfall are uneven with different rainfall eventsoccurring during the cropping season. Under this condition, the efficient useof nutrients is limited by the increasingly uneven rainfall conditions that areoften characterized by high inter-annual variability (Laux et al. 2008). The effectiveness of any soil fertility measurewould, therefore, depends on the rainfall regime as it significantly influencesplant nutrient uptake and the availability of soil nutrients (Zougmore et al.

2010). The availability of waterin addition to soil fertility has been cited as the most critical factor forsustaining crop productivity in rainfed agriculture (HarvestChoice 2010).Seasonal variability in rainfall greatly affects soil water availability tocrops, and thus poses crop production risks.


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