A recent study from the University of California Davis (UC Davis) confirmed what many of us have known for our professional careers: compost makes the difference. Researchers tested soils from long-term field plots that had been established 19 years prior. They measured total soil organic carbon and nitrogen as well as bulk density (the weight of the soil per unit volume) at depths to 200 cm. By multiplying the soil bulk density and the percent carbon, you can calculate how much organic matter is stored in the soil. As increased carbon usually makes the soil lighter (decreasing bulk density), higher soil carbon concentrations do not always mean increased carbon storage.
To calculate the total carbon stored in a soil, you multiply the bulk density times the carbon concentration. The decrease in bulk density has to be accompanied by sufficiently high increases in carbon or increases in carbon across soil depths. In the UC Davis study, the plots had been used to grow corn and tomatoes in Northern California. Treatments included fertilizer, winter cover crops and winter cover crops plus (+) chicken manure composts. While both the cover crop and cover crop + compost treatments increased soil carbon concentrations and carbon storage in the surface soils (0-30 cm), only the cover crop + compost treatment increased carbon through the entire profile. The cover crop alone treatment had decreased soil carbon storage in the 60 to 200 cm depth in comparison to the control. This does not necessarily make sense as all additions and most root growth are concentrated in the top 30 cm, but that is what the researchers found.
In comparison, the cover crop + compost treatment had higher stored carbon at depth, compared with both the cover crop and the fertilizer treatments. This treatment, the one with the compost, was the clear winner — the only one that showed increased carbon through the measured depth. No study is perfect and as we all know, only hindsight is 20:20. It would have been really helpful to have a treatment with compost alone to see if the compost would be sufficient to increase soil carbon storage. Even without that treatment, this study shows that cover crops and compost are the most effective combination for increasing soil carbon storage for the climate, soil and crop rotation tested.