According to De Wit (1992), crop growth potential at a givenlocation is determined by genotype and climate, whereas actual crop yieldsresult from the interactions of local growth-limiting and growth-reducingfactors. Therefore, thevariability in crop growth and yields observed within individual farms reflectthe effects of interactions and spatial distribution of these factors, many ofwhich are directly influenced by management decisions (Tittonellet al., 2013). In SSA, smallholder farms consisted of multiple fieldsmanaged differently in terms of allocation of crops, labour and fertilizerresources; making within-farm soil fertility gradients caused by managementstrategies a common feature ((Tittonell et al., 2013; Giller et al.,2011; Zingore et al., 2007a;).
Within the smallholder farms, soil fertilitystatus of different plots vary considerably due to both inherent factors anddifferent resource management strategies 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). Specifically, the variability observed in crop growth andyield observed in the smallholder farms of SSA have been attributed to severalfactors; both natural and biophysical. For example, soil properties (e.
g.Tamene et al., 2015; Casanova et al., 1999), agronomic practices (e.
g.Zingore et al., 2007a), farmers’resource allocation decisions (e.g. Tittonell et al.
, 2013; Zingore et al.,2007a; Nkonya et al., 2005), orcombinations of these (e.
g. Beza et al.,2017). Perhaps, both the long-term and- current soil management decisionsinfluence the prevailing soil quality, spatiotemporal patterns of resourceallocation, and the timing and effectiveness of agronomic practices (e.g.planting dates, weeding, fertilizer application time etc).
In the Guinea savanna of Nigeria, agronomic managementdecisions play an important role in determining resource use efficiency andconsequently crop productivity. Management factors such as the amount oflabour, size of farms, timing of farm operations such as land preparation,sowing, weeding, and fertilizer application and harvesting have been recognizedto causes spatial variability in yield obtained in smallholder farms.Similarly, in many cases, both organic and mineral fertilizer resources arepreferentially allocated to the part of the farm used for growing the main foodsecurity crop, often close to the homestead, whilst plots further away areneglected. In addition, the availability of organic and mineral nutrient resources,their management under spatially variable soil fertility conditions hasconsequences on the soil resource base, cropping patterns and crop yields onsmallholder farms. Such management decisions culminate in the creation ofgradients of decreasing soil fertility with distance from the homestead. Thisgradient has been defined as soil fertility gradients. These factors togetherwith other limiting factors such as N (Adnan et al.
, 2017) and by growth-reducing factors such as Striga infestation (Kamara et al., 2009), the recent fall armywormsoutbreak have led to wide gap between maize yield potential, attainable yieldand farmer actual yield (FAO, 2016).In the SSA, attainable yields of improved OPVmaize varieties and hybrids stood at 6-8t/ha and 8-10t/ha (Figure 2), but thishas rarely been achieved. Some of the reasons for this wide yield gaps includemanagement factors such as residue management, N application rate (Adnan et al., 2017b) and weed management (Ekeleme et al.
2014). Other factors are environmental factors such as waterstress, sloped surfaces, the sand content of the soil, soil fertility(exchangeable Mg, available P, and pH) and soil texture (Tanaka et al. 2013; Fermont et al.
2009; Becker et al. 2003).
