A goal that is common to both soil fertility and environmental quality is to regulate the dissolution of nutrient elements from soil solids into solution. Nutrient solubility and plant availability is largely controlled by the solid-phase chemical species present. Macro-scale chemical analyses estimate nutrient availability to plants while spectroscopic analyses identify chemical species. This presentation will summarize research that utilizes synchrotron x-ray absorption spectroscopy (XAS) to characterize the chemical speciation of nutrients, specifically iron and phosphorus, in soil materials. Several applications of XAS will be discussed, including: identifying multiple chemical species of a single nutrient element in a soil; characterizing changes in chemical forms of Fe in organic matter; and demonstrating how interactions between multiple elements (Fe, P, and Al) affect speciation.  These examples illustrate the utility of XAS to characterize both mineral and non-crystalline forms of certain nutrients, and to show how Fe speciation affects both the molecular and macroscopic properties of phosphate binding.  Being able to predict and control the solubility of plant nutrient elements based on solid-phase chemical speciation across a wide range of soil conditions is a goal that would ultimately aid in optimizing plant nutrient availability with minimal environmental impacts.