Low kg N ha-1 to produce 2 to 2.5tons

Low
soil fertility is one of the main constraints to crop production in the West
African savanna. However, the response of major cereals to fertilizer
applications is often far below the potential yields. Low fertilizer
efficiency, the inadequacy of current fertilizer recommendations, and the
ignorance of nutrients other than N, P, and K may limit crop production. This
call for the need to identify limiting nutrients in maize-based farming
systems. Most soils in the savannas of West Africa sandy, low in organic
carbon, nitrogen, and nutrient retention capacity (Kwari et al. 2011).

In
the Nigeria savannas, the major plant nutrients (N, P, and K) are added to the
soils in form of NPK or urea fertilizers, whereas little attention has been
given to other macro and micronutrients leading to imbalanced nutrient
management and crop quality. Because of the dynamism of soil when external
inputs are applied, it is subjected to change due to physical, chemical and
biological reactions taking place in the soil. For this reason, it is logical
to expect that the plant availability of not only the added element but of
other elements already in the soil, may change (John and Venugopal, 2015).

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Nitrogen (N) has been for
decades recognized as the major nutrient limiting maize yield in the Sudan
savanna of Nigeria (Oikeh et al.
2007; Kamara et al. 2005). Nitrogen
fertilizer has been reported to be a necessary input for sustained high maize
yields in an intensive crop production system. It is the most mobile, volatile
and an exhausted nutrients due to its ability to exist in different forms and
its easy leachability (Palm et al., 1997). Snapp et al. (1998)
observed that maize removes about 40 kg N ha-1 to produce 2 to
2.5tons ha-1 of grain yield in the tropics.

According to Kwabiah et
al. (2003) phosphorus is a limiting nutrient in maize production due to the
low native soil P and high P fixation and that P deficiency is a factor
limiting crop production in tropical and subtropical soils. In addition,
Fairhurst et al. (1999) observed that phosphorus, unlike nitrogen, could
not be replenished through biological fixation. For many cropping systems in
the tropics, application of P from organic and inorganic sources is essential
to sustain high crop yield.

In
other part of SSA, e.g Kenya potassium (K) is an emerging limiting plant
nutrient (ICRAF, 1997). This could be attributed to greater losses than gains
of soil nutrients leading to a negative balance in intensively cropped soils
(Smaling et al., 1997). Potassium losses through leaching, soil erosion,
runoff and crop uptake are higher than addition through weathering of parent
material and application of organic and inorganic fertilizers (Weil and Mughogho,
2000).

Because of nutrient mining, SMNs limitations remain a
challenge in SSA as they are used without replenishment (Alley and Vanlauwe,
2009). In the long-term experiments in the savannas of Nigeria, Togo and Benin,
Nziguheba et al (2008) reported that
Ca, Mg and Zn are principally deficient as indicated by their strong negative
indices.
A number of secondary macronutrients and micronutrients have been found to be
deficient in the savanna soils of SSA (Rusinamhodzi et al., 2013; Jeng, 2011; Tittonell et al., 2007a). In Nigeria for example, Ephraim (2012) reported that Zn and Cu
concentrations at Bauchi state Northeastern Nigeria were 0.26 and 0.36mg/kg of soil, a critical limit far below the levels required for
arable crop production. Weil and Mhughogo (2000) have reported the deficiency
of Sulphur in SSA where annual burning results in losses to the atmosphere as
Sulphur dioxide. Perhaps, S has been recognized as the 3rd most
limiting nutrients in maize production apart from N, and P. S constituted part
of the two essential amino acids (Lysine and Tryptophan) and it is taken up by
most grain crops in amounts similar to those of P, from 10 to 30 kg/ha (Weil
and Mhughogo. 2000). Sulphur (S) deficiency in the most part of the Guinea
Savanna of Nigeria, is becoming prevalent under intensifying agricultural
systems. This is manifested in the suboptimal or lack of response by crops like
maize to NPK fertilizers leading to a reduction in crop quality and yields
(Joshua et al., 2009). The status of
S in soils of the Guinea Savanna of Nigeria is not well documented. However,
Joshua et al., (2009) and Ojeniyi and
Kayode (1993) have reported significant responses to Sulphur by maize. On soils
deficient in Sulphur, maize did not respond to N fertilizers unless Sulphur was
added (McCol. 1984).