Guidance for timing of sampling is seldom included in soil sampling protocols for agronomic phosphorus (P) fertilizer recommendations or P runoff risk assessment. Our objectives were to determine the influence of soil temperature and moisture content on (i) Mehlich-3 and Morgan soil test P (STP) levels and (ii) the accuracy of Mehlich-3 to Morgan STP conversion models used for nutrient management planning in New York. An incubation study with three silt loam, excessive P soils, three temperatures (5°C, 10°C, and 20°C), and three moisture contents (50%, 70%, and 90% of field capacity) showed that an increase in soil temperature from 10°C to 20°C decreased Morgan STP and showed inconsistent and small changes for Mehlich-3 STP. Both tests showed higher STP levels with soil moisture increase. The increase from 10°C to 20°C resulted in a decrease in the ratio of measured to predicted Morgan STP, reflecting a reduction in measured Morgan STP and an increase in predicted Morgan STP, mostly driven by an increase in Mehlich-3 calcium (Ca). The increase in moisture impacted the ratio of measured to predicted Morgan STP for one soil only (a pasture with native Ca content). Repeated sampling of 20 corn (Zea mays L.) fields in May, June, July, October, November, and the following April showed a large seasonal variability in STP results, with the most accurate predictions of Morgan STP from Mehlich-3 data upon sampling in October after corn harvest. We conclude that for the most accurate prediction of STP status, samples should be taken directly after corn harvest, especially when conversion equations that include pH and Mehlich-3 P, Ca, and aluminum are used.