Phosphorus (P) availability is surprisingly variable in cool desert ecosystems of the Colorado Plateau. We analyzed eolian dust and surface soils for phosphorus fractions using seasonal (dust collection) and monthly (surface soil) samples. Analysis of eolian dust shows that plant available (labile) P is a large fraction of total P input, accounting for more than 50% of the total P input during the winter at one site. In contrast, there is generally less total P and also less labile P during summer months. For soils, using the same analysis (Hedley fractionation), we found that labile P fluctuates seasonally as well. In early summer (May through July), soil labile P increases abruptly, more than doubling in all four soils that we tested, and increasing 10x at one site. For soils at most sites there is a second, less pronounced increase in labile P in the fall (October and November). At landscape scales, eolian dust P and soil P provide peaks in labile P that occur at different times of the year. On a local scale the effect of eolian P inputs may be insignificant for some soils that are rich in plant available P. The enormous seasonal variability of labile P in soils, along with eolian source fractions, are important ecosystem processes that may control production in some communities.