Biochar increases soil pH, but is as ineffective as liming at controlling clubroot.

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The use of pyrolysed carbon, in the form of biochar, as an agricultural soil amendment has recently gained considerable scientific interest. However, its adoption in practice has been slow due to a need to satisfy concerns about the safety of biochar and the need for more evidence of system improvements in temperate soils. Clubroot, caused by Plasmodiophora brassicae, is a disease of economic importance in Brassicaceae production systems. There is only limited plant resistance and disease limitation is currently managed via rotation breaks and liming. Biochar can alter the pH of soils so a series of glasshouse bioassay trials were established to determine if biochar could provide a similar level of clubroot control as traditional liming. Soil pH was raised from an initial pH of 5.4 to pH of 7.2 upon the addition of lime and to between 5.9 to 7.3 by the addition of biochar, without or with lime. However, the incidence of clubroot was either comparable to the lime treatment, which was similar to the unamended negative control, or exacerbated following the addition of biochar. The results support the existing knowledge of clubroot epidemiology, in that solely altering soil pH is not sufficient to manage this disease infection due to a complex interaction of nutrients, pH, moisture, and inoculum load in contaminated soil. The results highlight that further work is needed within temperate soils, as the addition of biochar to Brassicaceae production systems could potentially exacerbate both the incidence and soil load of the pathogen P. brassicae.