Forest soil amendments are increasingly used in western US forests to dispose of unmerchantable woody residues, reduce wildfire risk, and improve soil properties. Our objective was to determine the effect of fertilizer and organic amendments on tree growth and organic matter decomposition after thinning. Treatments were a control, three single soil amendments (wood chips, fertilizer, and biochar), and one combined soil amendment (biochar + fertilizer), each applied after thinning a ponderosa pine (Pinus ponderosa Dougl. ex. Laws) stand. After 10 years, amendment treatments had no effect on tree diameter increment (p = 0.600), but the biochar + fertilizer and wood chip treatments significantly increased height growth (p = 0.006). To estimate belowground biological changes, we used wood stakes made from aspen (Populus tremuloides Michx.) and loblolly pine (Pinus taeda L.) as an index of microbial activity. Stakes were placed: (1) on top of the litter/amendments, (2) at the interface between the litter/amendments and mineral soil, and (3) vertically inserted into the mineral soil, and stake mass loss was measured over 5 years. Stake mass loss of each species was least on the soil surface and increased with increasing depth. Aspen stakes generally had greater mass loss at all three soil locations in the fertilizer and biochar treatments. In contrast, pine stake mass loss was lower than aspen and less affected by fertilizer. Using thinned tree biomass to create amendments can improve forest productivity by enhancing soil conditions and mitigating wildfire. However, the impact of amendments on tree growth may take decades to be detectable.