Direct effects of soil organic matter on productivity mirror those observed with organic amendments

Organic amendments to arable soil build soil organic matter (SOM), which can increase crop yields. However, organic amendments can influence crop yields independently of SOM by providing nutrients directly to plants. The relative importance of native organic matter versus organic amendments is not well quantified. We experimentally manipulated both organic amendments and native SOM concentrations to quantify their relative importance to crop yields. We created OM concentration gradients by (1) diluting an organic-rich A-horizon with a mineral base and (2) amending compost to the same mineral base, generating OM concentrations for both treatments of approximately 2, 4 and 8%. We grew buckwheat and measured plant productivity and a range of soil fertility variables. Higher concentrations of OM, whether native or amended, were associated with higher soil water holding capacity and nutrients, and improved soil structure. Consequently, increases in both native and amended OM were associated with strong positive but saturating impacts on productivity, though amendment effects were greater. Our results suggest that native SOM can support productivity levels comparable to those observed with organic amendments. Although our quantitative findings will likely vary for different soils and amendments, our results lend support to the idea that SOM stocks directly increase productivity.