(Mg,Fe)O ferropericlase-magnesiow & uuml;stite has been proposed to host the majority of Earth's sodium, but the mechanism and capacity for incorporating the alkali cation remain unclear. In this work, experiments in the laser-heated diamond anvil cell and first-principles calculations determine the solubility of sodium and favorability of sodium incorporation in iron-rich magnesiow & uuml;stite relative to (Mg,Fe)SiO3 bridgmanite. Reaction of Mg/(Mg + Fe) (Mg#) 55 and 28 olivine with NaCl at 33-128 GPa and 1600-3000 K produces iron-rich magnesiow & uuml;stite containing several percent sodium, while iron-rich bridgmanite contains little to no detectable sodium. In sodium-saturated magnesiow & uuml;stite, sodium number [Na/(Na + Mg + Fe)] is 2-5 atomic percent at pressures below 60 GPa and drastically increases to 10-20 atomic percent at deep lower mantle pressures. For these two compositions, there is no significant dependence of the results on Mg#. Our calculations not only show consistent results with experiments but further indicate that such an increase in solubility and partitioning of Na into magnesiow & uuml;stite is driven by the spin transition in iron. These results provide fundamental constraints on the crystal chemistry of sodium at lower-mantle conditions. If the sodium capacity of (Mg,Fe)O is not strongly dependent on Mg#, (Mg,Fe)O in the lower mantle may have the capacity to store the entire sodium budget of the Earth.|Sodium is among the most abundant elements on the Earth, but where it can be stored in the Earth's largest layer, the lower mantle, has not been understood. Whether sodium dissolves into the most common minerals in the mantle affects the interpretation of the Earth's composition and structure. This study uses experiments and computer simulations of reactions of sodium with the two most common minerals in the lower mantle to determine how much sodium can be dissolved in the mantle. Both methods show that enough sodium can dissolve into magnesium-iron oxide, called ferropericlase or magnesiow & uuml;stite, to store all of the Earth's sodium budget. The amount of sodium that dissolves into this mineral increases with depth in the Earth because of a change in the arrangement of electrons around iron, which takes part in the chemical reaction with sodium.|Experiments show that sodium strongly partitions to (Mg,Fe)O magnesiow & uuml;stite and is not incorporated in bridgmanite Sodium is soluble in iron-rich (Mg,Fe)O in the deep lower mantle at multiple percent level First-principle calculations show that spin transition in (Mg,Fe)O drives a pressure-driven increase in sodium solubility