Phosphorus (P) requirements for major agronomic crops have been currently based on a pre-plant mass balance method.  Fertilizer needs are estimated from crop needs, available soil P and other external nutrient inputs that include animal manure, crop residues, etc...  Thus, this approach uses field-specific information for P recommendations and nutrient use efficiency. With the advent of recent developments in X-ray fluorescence spectroscopy (XRFS), energy-dispersive XRFS methods can be extended beyond elemental characterization of heavy metals in oils, lubricants and additives, consolidated geologic materials, or eroded sediments. In agronomic applications, EDXRFS has been used to determine trace elements in plant materials, metal-contaminated soils, and macronutrients in nutrient-enriched field soils and animal manures. The principles of EDXRFS and its application to the sensing of low atomic number (Z) nutrients will be discussed. Elemental composition of geo-referenced soil samples from a manure-amended field was characterized and mapped to delineate the spatial distribution and zones of management of soil P or any other elements of Z > 11 that were simultaneously determined. Relationships to bioavailable P soil fractions were examined to derive predictors of nutrient availability, thus developing field-scale approaches to managing spatial variability and in-season P changes in soil and corn plants to enhance nutrient-use efficiency on the farm.