The low productivity of cassava root is mainly attributed to low fertility soils, after successive and long cultivations in the same area. In this sense, the objective of this study was to determine the P2O5 dose that provides the maximum relative efficiency of cassava stem thickness, height and leaf area. Pirarucu (Manihot esculenta Crantz). The study was carried out in the field at the Mangabeira experimental farm of the Federal University of Amazonas - UFAM / IEAA, located at BR 319 km 3. The experimental design was a randomized complete block with four replications, consisting of two factors: two-level liming with and without, and P2O5 doses with five levels (0, 30, 60, 90 and 120 kg ha-1 of P2O5) randomized. At the end of all data collections, the information obtained was subjected to analysis of variance by the F test with 5% significance and regression analysis by orthogonal polynomials for phosphorus dose levels. The evaluated response variables that presented significant values within the liming factors and P doses were plant height and stem thickness. Leaf area showed significant results within liming factor. The components of crop growth, plant height, stem thickness are positively affected by the addition of limestone and increasing doses of P2O5 up to 60 and 120 kg ha-1 respectively. Leaf area was not positively affected by phosphorus dose, but by limestone dose. Root mass showed a productive increase with liming and P rates did not influence significant increase in yield when applied together with limestone.
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