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

Lowsoil fertility is one of the main constraints to crop production in the WestAfrican savanna. However, the response of major cereals to fertilizerapplications is often far below the potential yields. Low fertilizerefficiency, the inadequacy of current fertilizer recommendations, and theignorance of nutrients other than N, P, and K may limit crop production.

Thiscall for the need to identify limiting nutrients in maize-based farmingsystems. Most soils in the savannas of West Africa sandy, low in organiccarbon, nitrogen, and nutrient retention capacity (Kwari et al. 2011). Inthe Nigeria savannas, the major plant nutrients (N, P, and K) are added to thesoils in form of NPK or urea fertilizers, whereas little attention has beengiven to other macro and micronutrients leading to imbalanced nutrientmanagement and crop quality.

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Because of the dynamism of soil when externalinputs are applied, it is subjected to change due to physical, chemical andbiological reactions taking place in the soil. For this reason, it is logicalto expect that the plant availability of not only the added element but ofother elements already in the soil, may change (John and Venugopal, 2015). Nitrogen (N) has been fordecades recognized as the major nutrient limiting maize yield in the Sudansavanna of Nigeria (Oikeh et al.

2007; Kamara et al. 2005). Nitrogenfertilizer has been reported to be a necessary input for sustained high maizeyields in an intensive crop production system.

It is the most mobile, volatileand an exhausted nutrients due to its ability to exist in different forms andits easy leachability (Palm et al., 1997). Snapp et al. (1998)observed that maize removes about 40 kg N ha-1 to produce 2 to2.5tons ha-1 of grain yield in the tropics.According to Kwabiah etal. (2003) phosphorus is a limiting nutrient in maize production due to thelow native soil P and high P fixation and that P deficiency is a factorlimiting crop production in tropical and subtropical soils. In addition,Fairhurst et al.

(1999) observed that phosphorus, unlike nitrogen, couldnot be replenished through biological fixation. For many cropping systems inthe tropics, application of P from organic and inorganic sources is essentialto sustain high crop yield. Inother part of SSA, e.g Kenya potassium (K) is an emerging limiting plantnutrient (ICRAF, 1997).

This could be attributed to greater losses than gainsof 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 parentmaterial and application of organic and inorganic fertilizers (Weil and Mughogho,2000).Because of nutrient mining, SMNs limitations remain achallenge 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 thatCa, Mg and Zn are principally deficient as indicated by their strong negativeindices.A number of secondary macronutrients and micronutrients have been found to bedeficient 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 Cuconcentrations at Bauchi state Northeastern Nigeria were 0.26 and 0.36mg/kg of soil, a critical limit far below the levels required forarable crop production.

Weil and Mhughogo (2000) have reported the deficiencyof Sulphur in SSA where annual burning results in losses to the atmosphere asSulphur dioxide. Perhaps, S has been recognized as the 3rd mostlimiting nutrients in maize production apart from N, and P. S constituted partof the two essential amino acids (Lysine and Tryptophan) and it is taken up bymost grain crops in amounts similar to those of P, from 10 to 30 kg/ha (Weiland Mhughogo. 2000).

Sulphur (S) deficiency in the most part of the GuineaSavanna of Nigeria, is becoming prevalent under intensifying agriculturalsystems. This is manifested in the suboptimal or lack of response by crops likemaize to NPK fertilizers leading to a reduction in crop quality and yields(Joshua et al., 2009).

The status ofS in soils of the Guinea Savanna of Nigeria is not well documented. However,Joshua et al., (2009) and Ojeniyi andKayode (1993) have reported significant responses to Sulphur by maize. On soilsdeficient in Sulphur, maize did not respond to N fertilizers unless Sulphur wasadded (McCol. 1984).