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Secondary macro and micronutrients play an important active
role in the plant metabolism process, from cell wall development to
respiration, photosynthesis, chlorophyll formation, enzyme activity and
nitrogen fixation (Das, 2000). Secondary and micronutrients (SMNs),
including Ca, Mg, S, Zn, Cu, Mn, Fe, B, and Mo, often limit crop growth,
especially in soils are continuously cropped without returning these nutrients.
Sulfur (S) is often the third
limiting nutrient in soils after N and phosphorus (P) (Randhawa and Arora,
2000), yet it is seldom included in the fertilizers commonly available. Szulc et
al. (2008) indicated that Ca and Mg plays a vital role in plant nutrition as
they constitute plants protoplasm. Micronutrient deficiencies occur not only
due to low contents of these elements in the soil but also nutrient mining by
growing plants (Brady and Weil, 2002). Studies across SSA indicate that many
soils become deficient in Zn, Fe, and S even though macronutrient status is
corrected (Lipinski, 2005). Sustained removal of micronutrients means they need
to be replaced after some time (Giller et al., 2006).

Most of the commonly
applied fertilizer in SSA contains mainly N, P, and/or K, which do not
replenish SMNs. Nutrient depletion can be further aggravated by soil
acidification, which interferes with the availability of specific nutrients.
The considerable extent of SMN deficiencies in SSA is gradually becoming
apparent. In the savannas of some SSA countries, there has been an attempt to
map out nutrients and the extent of their deficiencies. For example, the
Ethiopian Soil Information Service is currently involved in mapping the entire
country for all nutrients and has found extensive areas of S, Zn, and B
deficiency (www.africasoils.net/EthioSIS).
Similarly, soil nutrient maps of Rwanda and Burundi show that the majority of
the arable land is affected by multiple nutrient deficiencies, including P, Ca,
Mg, S, Zn, and B, as well as low soil pH (www.ifdc.org/Nations/Rwanda/;www.ifdc.org/Nations/Burundi/).
Significant maize responses to S (e.g. Wendt and Rijpma, 1997; Weil and
Mughogho, 2000), Mg (e.g. Abunyewa and Mercer-Quarshie, 2004), Zn (e.g.
Abunyewa and Mercer-Quarshie, 2004; Zingore et
al., 2008), Cu (e.g. Lisumu et al.,
2006), and B (Wendt and Rijpma, 1997) have been demonstrated across several
sub-Saharan Africa countries. It has been reported that application of
secondary and micronutrients can have significant effects on crop yields in
sub-Saharan Africa but has received less attention than the macronutrients N,
P, and K. the major reason for this has been that there is no need to address
other nutrient limitations while the region is still struggling to adopt
macronutrient fertilizers. The consequence of this is that where SMN
deficiencies exist, they can limit crop response to NPK fertilizers. Similarly,
most researches attempted to define the effect of single secondary macro or micronutrient
and this has not shown much effects on yields. There is growing evidence in the
literature that multiple rather than individual SMN deficiencies are the norm
in much of sub-Saharan Africa. In an omission trial from Burundi, attainable
yields with the balanced nutrient application were >5 Mt ha-1 but
eliminating either Cu or B limited the response of all other nutrients to 3.7
Mt ha ha-1, demonstrating the importance of including all
potentially deficient nutrients in an omission trial. However, trials that
examine the response to multiple nutrients are few and far between. Application
of SMNs on soils exhibiting secondary nutrient limitations is an effective way
to enhance fertilizer nutrient agronomic efficiency (AE), provided that all
limiting nutrients are addressed (Vanlauwe et al., 2015). 

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