Fertilizer use should not be a fourth principle to define conservation agriculture: Response to the opinion paper of Vanlauwe et al. (2014) ‘A fourth principle is required to define conservation agriculture in sub-Saharan Africa: The appropriate use of fertilizer to enhance crop productivity’ - Sommer, Rolf; Thierfelder, Christian L.; Tittonell, Pablo A.; Hove, L.; Mureithi, J; Mkomwa, S (Elsevier, 2014)