Crop and are at a different scale (region,

Crop farming in sub-Saharan Africa is characterized
by small family farms that rely heavily on rain-fed production systems,
traditional methods of soil fertility maintenance, and lack of access to credit
and improved inputs such as seeds (WFP, 2012). Majority of the farmers are
smallholder farmers that are highly diverse, operating in complex natural and
economic environments and are largely heterogeneous in terms
of socio-technical conditions, farm typologies, production objectives, and the
biophysical environment (Vanlauwe et al.,
2015). Several literature on soil
fertility in sub-Saharan Africa reported a scenario of continuous nutrient
depletion and severely negative nutrient balances leading to abysmally low crop
yields (Dercon et al. 2010; Sanginga
and Woomer., 2009). Perhaps, the major factor leading to severe decline in per
capita food production in SSA has been the declining soil fertility (Sanchez et al., 1997).

The
poor soil fertility and low nutrients availability in the SSA could be attributed
to inadequate supplies of organic and inorganic fertilizers as organic manures
are available in limited quantities and inorganic fertilizers are often too
expensive, poor cultural practices, lack of soil fertility maintenance plans,
nutrient mining, low nutrient use efficiencies, inappropriate fertilizer
recommendation, differences in crop response to fertilizers Chianu et al., 2012; Smaling et al., 2012; Smale et al., 2011). This has possibly been compounded by climate change
and increased pressure on land due to increasing population. Insufficient
nutrient application happens because inorganic fertilizers are often too
expensive for most of the farmers, whilst organic resources are available in
limited quantities, the consequent which led low productivity (Sheahan and
Barrett, 2014; Sommer et al., 2013;
The Montpellier Panel, 2013).

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There
is wide variation in the growing conditions across the maize belt of Nigeria in
terms of soil, climate and resource endowment of the farming communities. Most
of the soils are spatially heterogeneous as dictated by the underlying parent
materials, geomorphology, position on the landscape and historical management
practices enforced by farmers. Perhaps, much of the soil fertility variation
within farming systems is caused by spatial soil heterogeneity (Tittonell et
al., 2005a). Causes of variability in soil fertility status are both
biophysical and social economic (Tittonell et al., 2005a, b) and are at
a different scale (region, village, farm, and field). And this has led to
steadily stagnated maize yields over the last few decades because of
deteriorating soil fertility (FAO, 2014). Soil properties are an important subset of the environmental
factors limiting the yield. Soil chemical properties include factors such as pH
and availability of macro- and micronutrients. These determine nutrient
deficiencies and toxicity of the soil.

Soil
fertility depletion remains the major biophysical cause of declining crop
productivity on smallholder farms in sub-Saharan Africa (Chianu et al., 2012; Bationo
et al., 2004; Kimani et al., 2004). Most savannas of SSA are
associated with significant soil degradation that has been aggravated by rapid
population growth, cultivation of fragile ecosystems and continuous cropping.
The consequent of this has to been low returns to investments, declining food
security and general high food prices (Okalebo et al., 2006).

Poor soil
fertility and low nutrient availability have been singled out as the most
serious biophysical constraints that result in poor yields in SSA countries
(Tittonell and Giller, 2013; Kibunja et
al., 2012; Giller et al., 2011;
Sanchez, 2010, 1997; Manu et al.,
1991). The poor soil fertility and low nutrients availability are as a result
of inherent differences in parent material, catenal position in the landscape
and household priorities and production strategies regarding preferential
allocation of labour,  manure, compost
and mineral fertilizers to fields near the homesteads (Tittonell et al., 2013; Giller et al., 2011). In addition to the
inherent variability of soils and their fertility, management decisions result
in 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 these
gradients will vary greatly from farm to farm as well as across agro-ecological
zones depending on the many possible combinations of these biophysical and
socio-economic factors (Masvaya et al.,
2010).

Management of poor soil
fertility and low nutrients availability have repeatedly found a nexus between
climate 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 rainfed
with annual precipitation ranging from 850 to 1500mm in the semi-arid region.
Onset and distribution of rainfall are uneven with different rainfall events
occurring during the cropping season. Under this condition, the efficient use
of nutrients is limited by the increasingly uneven rainfall conditions that are
often characterized by high inter-annual variability (Laux et al. 2008). The effectiveness of any soil fertility measure
would, therefore, depends on the rainfall regime as it significantly influences
plant nutrient uptake and the availability of soil nutrients (Zougmore et al. 2010). The availability of water
in addition to soil fertility has been cited as the most critical factor for
sustaining crop productivity in rainfed agriculture (HarvestChoice 2010).
Seasonal variability in rainfall greatly affects soil water availability to
crops, and thus poses crop production risks.