Abstract: Our objective was to determine temporal effects on medium pH caused by decomposition of three organic amendments incorporated with topsoil. Pine (Pinus taeda L.) bark, pine (Pinus taeda L.) straw, and red oak (Quercus falcata Michx. var. falcata) were ground to uniform particle size, incorporated with a silt loam topsoil at two rates (1:29 and 1:10 amendment:soil, w:w basis, referred to as 1X and 3X, respectively), placed into greenhouse pots, and sampled during 12 months to determine medium pH in comparison to an unamended topsoil (control). Compared to the control, pine straw, pine bark, and red oak 3X increased soil medium pH. All media except pine straw increased pH during the study. At any given sampling date, pine straw 3X had lower pH than the control, while red oak either did not differ from, or had higher pH than the control. By the end of the sampling period, pine bark and pine straw media had lower pH than the control. While statistically significant, change in medium pH caused by any of these substances would be trivial for most horticultural crops, and easily corrected by use of other liming or acidifying amendments.