The field experiment was carried out in a block design with
four replicates. The following fertilization treatments were used:
(1) biofertilizer (NPKB) 4.0 t ha−1; (2) NPKB 6.0 t ha−1; (3) NPKB
8.0 t ha−1; (4) mineral fertilizers (NPKF) at the recommended rate
(RR); (5) NPKF (two times RR); (6) PK rocks (2000 kg ha−1); and (7) a
control treatment with earthworm compound (20 t ha−1). All of the
treatments were inoculated with G. etunicatum mycorrhizal fungi.
Fertilizers treatments were estimated following the recommended
rate for irrigated cowpea in Pernambuco state (IPA, 2008),
applying the mixed soluble fertilizer (NPKF) with the following:
N fertilizer (ammonium sulfate – 400 kg ha−1) + P fertilizer (simple
super phosphate – 500 kg ha−1) + K fertilizer (potassium sulfate
– 100 kg ha−1). The biofertilizer NPKB was calculated based on the
total N analyzed in the NPKB (20 g kg−1) that corresponded to application
of 4000 kg ha−1 (10 times ammonium sulfate applied).