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Changes in the bioavailability of nitrogen (N) and phosphorus (P) due to biochar have been frequently reported, raising concerns about the management and effectivity of biochar as soil amendment. We investigated the availability of N and P to maize plants cultivated in soil amended with 20 t ha−1 and 30 t ha−1 of biochar from sewage sludge (SSB20, SSB30) and coconut husk (CHB20, CHB30), in a greenhouse experiment. Plant biomass and concentration of N and P in plant tissue and in the rhizosphere and bulk soil were evaluated. All biochar treatments reduced the concentration of NH4+-N by 22–88% in the soil. CHB20, CHB30 and SSB20 increased NO3–N by 17–40% in the bulk soil and reduced it by 21–38% in the rhizosphere, whereas SSB30 increased NO3− -N by 17–25% in both compartments, but decreased plant N and P uptake efficiency. Biochar increased shoot biomass and P availability by 12% and 12–50%, respectively, except the CHB30, which reduced P availability and N and P uptake efficiency. Biochar improved plant growth during the first 60 days of cultivation in the Ultisol, being suitable as soil amendment at lower rate of application. [ABSTRACT FROM AUTHOR]