Productivity of grain amaranth can be improved at reduced cost through combined use of low-cost Rock Phosphate (RP) phosphorus sources as well as selection of amaranth varieties with high phosphorus (P) use efficiencies. RP sources are known to exhibit differences with respect to grain yield, disease tolerance and P use efficiency (PUE) in amaranth varieties. However, very little is known about their residual release of P. It is probable that the differential performances exhibited by the RPs might be a function of their P residual properties after cropping. Based on these premises this study was carried out to evaluate the residual effects of P sources under continuous croppings for grain amaranth production. The study was a factorial experiment with four replications. Four P sources: Single Super Phosphate – SSP, Ogun Rock Phosphate – ORP, Sokoto Rock Phosphate – SRP and control; three amaranth varieties: NH84/493, NH84/445 and NH84/452 were factorially combined with two soil types: Kaudiudult and Haplustalf. In order to monitor the residual effects of the P sources the experimental plots were cropped consecutively three times. Results from the study revealed that in the sandy loam Haplustalf, grain amaranth yield was better than that in the clayey Kaudiudult soil under a continuous cropping system. Grain yield and Relative Agronomic Efficiency (RAE) decreased with continuous cropping for SSP but increased with continuous cropping for ORP and SRP till second cropping; thereafter there was a decrease. Averaged over the three continuous consecutive croppings, SRP and ORP were 68.9% and 73.5% as effective as SSP in increasing grain yield respectively. It was concluded therefore that ORP and Haplustalf soil are best options for grain amaranth production under continuous cropping system.