The objective of this greenhouse study was to evaluate the effects on development parameters in Megathyrsus maximus cv. Mombaça of partially replacing the potassium (K) in fertilizer by sodium (Na)and the impacts of applying calcium (Ca). The experimental design was completely randomized with 4 replicates, in a 3 × 4 + 3 factorial arrangement. Three Ca sources (CaCO3, CaSO4 and CaCl2) were applied at 4 Ca doses (10, 20, 30 and 40 mg Ca/kg of soil) to the grass where 25% of the K fertilizer was replaced by NaCl. The 3 additional treatments were: application of standard fertilizer (N:P:K) with 100% of K recommendation; application of fertilizer with 25% of K replaced by Na but no Ca applied; and grass with no fertilizer of any kind as Control. The variables analyzed were: leaf area, leaf area ratio, net assimilation rate, relative growth rate and photosynthetic rate. Partial replacement of K by Na up to 25% of the K fertilizer recommendation did not cause significant change in the development of Mombaça grass or to any of the measured parameters. Addition of Ca had minimal impact on the analyzed variables regardless of level or source. These measurements helped to explain why dry matter yields of Mombaça grass were not significantly affected by partial replacement of K by Na up to 25% of the K fertilizer recommendation.