Foliar nutrient concentrations vary during the day. Field research was conducted to quantify and better understand this variation in corn (Zea mays L.) and soybean (Glycine max L. Merr.) with foliar sampling during the vegetative and reproductive stages. Time of day effects occurred inconsistently across nutrients. Nitrogen (N), manganese (Mn), iron (Fe), and zinc (Zn) foliar concentrations were generally high early in the day. Phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) foliar concentrations varied inconsistently with time of day, while concentrations of boron (B) in both crops and copper (Cu) in corn were not affected. Limiting foliar sampling to after 10:00 AM reduced the variation for soybean but not for corn. Interpretation by Diagnosis and Recommendation Integrated System (DRIS) did not reduce the time of day effect. The variation caused by time of day, along with other causes, affects confidence in interpretation of foliar results suggesting use of the information with either additional foliar sampling or soil testing in making nutrient management decisions.