With the need to develop sustainable agricultural systems, biochar, a recalcitrant solid material that is generated from the pyrolysis of organic material under controlled conditions, has currently driven much interest to soil scientists. Biochar additions to agricultural soil have been shown to result in numerous net potential benefits; however, most studies have been conducted in greenhouse or laboratory trials with few being conducted in the field and particularly in association with organic farming systems. Herein, we address this gap by conducting on-farm studies on the efficacy of locally produced biochar as a soil amendment in small-scale organic agriculture on six farms in San Juan County, WA. Biochar produced from local timber harvest residues was applied at a rate of 20 Mg ha-1 in factorial combination with a poultry litter based fertilizer to replicated, randomized 4m2 plots on all six organic farms that were growing Kabocha squash (Cucurbita maxima) in the summer of 2016. A series of soil physicochemical and microbial properties (particularly C, N and P related) were examined during the growing season; squash samples were evaluated for productivity and nutrient uptake. Multivariate analytical methods were subsequently conducted to assess the patterns in soil characteristics and crop productivities following biochar addition.
Results showed that biochar additions significantly enhanced surface soil water holding capacity (14%), total C (44-47%), available NH4+-N (81-96%), active organic N (47-54%), active inorganic P (21-29%), and active organic P (52-56%) throughout the mid-growing season; total dry biomass of squash plants in biochar-treated plots was enhanced by 31% after 5 months period. Increased availability of soil nutrients were reflected in nutrient concentration of harvested squash. Principle component analysis (PCA) identified soil total C content, microbial biomass C, enzyme activities, and several P-cycling related variables as those variable most responsive to biochar, five months after application. Redundancy analysis (RDA) further indicated that soil biochemical variables, particularly soil enzyme activities and available P concentrations, were associated with higher crop productivity in biochar-treated plots. Organic farming systems strive to create closed nutrient cycles that have lower immediately available nutrients compared to conventional farming, our study demonstrated the potential benefit of locally produced wood biochar in short-term soil and plant nutrient management, particularly P management of sandy agricultural soils on the San Juan Islands.