The objective of this study was to determine the effect of phosphorus applied through fertigation on growth and root yield of cassava. This was achieved through a greenhouse pot-experiment consisting of 1, 4, 7, 10, 20 and 30 mg P L−1. Increasing P from 1 to 30 mg P L−1 realized a 57.1 and 150.0% increase in leaf blade P in 2014 and 2015, respectively. Similarly, chlorophyll content and shoot growth increased as P concentration in solution increased. However, leaf stomatal conductance and net photosynthesis reached a maximum in 7 and 20 mg P L−1 in 2014 and 2015, respectively. This trend of stomatal conductance and net photosynthesis was consistent with that of dry root yield and storage roots numbers. Regressing dry root yield against storage root numbers showed that R2 = 0.80. Phosphorus encourages formation of storage roots and the duration of cassava’s growth affects the amount of P required for maximum root yield.