On-farm assessment of soil quality in California's central valley

The high-value, large-scale crop production systems in the San Joaquin Valley (SJV) of California typically entail intensive tillage and large fertilizer and water inputs but few C additions to the soil. Such practices often contribute to a decline in soil quality. Our objective for this participatory study was to examine the effects of supplemental C management practices (SCMPs) on various soil quality indicators. To increase farmer participation, we conducted the study on farms using a variety of SCMPs, including cover crops, compost and manure amendments, and several different crop rotations common to the region. The SCMPs significantly changed a number of soil properties, including soil organic matter (SOM); total Kjeldahl N; microbial biomass C and N; exchangeable K; Olsen P; and extractable Fe, Mn, and Zn. A comparison including previously established, adjacent organic, conventional, and transitional fields in addition to the treatment fields at one farm revealed significant differences in 16 of 18 soil quality indicators. A soil quality index computed for this farm scored the established organic system significantly higher than the conventional system. Our results suggest that significant changes in several soil quality indicators occur with a variety of SCMPs. This is especially noteworthy considering the intensive tillage, irrigation, and hot, semiarid environment of the SJV, California, where increases in SOM and related soil properties are generally not expected in a 3-yr study.