The impact of Brassica napus seed meal on the microbial complex that incites apple replant disease was evaluated in greenhouse trials. Regardless of glucosinolate content, seed meal amendment at a rate of 0.1% (vol/vol) significantly enhanced growth of apple and suppressed apple root infection by Rhizoctonia spp. and Pratylenchus penetrans. High glucosinolate B. napus cv. Dwarf Essex seed meal amendments did not consistently suppress soil populations of Pythium spp. or apple root infection by this pathogen. Application of a low glucosinolate containing B. napus seed meal at a rate of 1.0% (vol/vol) resulted in a significant increase in recovery of Pythium spp. from apple roots, and a corresponding reduction in apple seedling root biomass. When applied at lower rates, B. napus seed meal amendments enhanced populations of fluorescent Pseudomonas spp., but these bacteria were not recovered from soils amended with seed meal at a rate of 2% (vol/vol). Seed meal amendments resulted in increased soil populations of total bacteria and actinomycetes. B. napus cv. Dwarf Essex seed meal amendments were phytotoxic to apple when applied at a rate of 2% (vol/vol), and phytotoxicity was not diminished when planting was delayed for as long as 12 weeks after application. These findings suggest that B. napus seed meal amendments can be a useful tool in the management of apple replant disease and, in the case of Rhizoctonia spp., that disease control operates through mechanisms other than production of glucosinolate hydrolysis products.

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A series of experiments was established to characterize biochars made from four feedstocks regionally available in the Pacific Northwest (wood pellets, softwood bark, switchgrass (Panicum virgatum L.) straw, and anaerobically digested fiber) to determine their effect on five soils. Soils were amended with 9.8, 19.5, and 39.0 Mg ha−1 of each of the four biochars and evaluated for changes in pH, water holding capacity, N mineralization, and soil C. The C content of biochars derived from the herbaceous feedstocks was 60 to 67 kg kg−1 whereas that of the woody feedstocks was >75 kg kg−1. In amended soils we found that biochars, regardless of origin, significantly raised the pH of all soil types 0.1 to 0.9 units, with the greatest impact on a sand soil. The biochars increased the soil C and water holding capacity at the higher rates of amendment depending on soil and biochar type. Nitrogen mineralization rates decreased in three of the five soils across all feedstocks. There were significant correlations (r ≥ 0.9) between biochar C measured and biochar C added regardless of feedstock or soil type. Our research demonstrates that in temperate soils, biochar feedstock may not be as important a variable as soil type for increasing C content and pH but can influence N mineralization and water holding capacity.