On‐Farm Assessment of Soil Quality in California's Central Valley

TLDR

Intensive tillage, high fertilizer and water inputs, and limited carbon additions in California’s San Joaquin Valley have led to a decline in soil quality. The study aimed to evaluate how supplemental carbon management practices affect soil quality indicators. Farmers participated in a field‑based experiment applying cover crops, compost, manure, and diverse crop rotations to assess their impact. Supplemental carbon practices markedly altered soil organic matter, nutrients, microbial biomass, and trace metals, with 16 of 18 indicators differing from adjacent conventional, organic, and transitional fields and the organic system scoring highest on a soil quality index.

Abstract

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.