Bokashi promotes general arable soil disease suppressiveness in short term but not in long term

Soil-borne diseases can cause significant crop losses and should be tackled sustainably in current and future agroecosystems. Increasing the capacity of soils to suppress the effects of soil-borne diseases (soil suppressiveness) is an important tool in sustainable crop production. Soil suppressiveness can be improved by adding organic amendments to the soil for multiple years, but the effects vary between the type of amendment (composted, fermented, or fresh material) and timing of application. The objective was therefore to test suppression capacity of sandy arable soil from a multi-year field-experiment where fresh plant cuttings, compost originated from same cuttings, or Bokashi originated from same cuttings were applied across 10 fields. Additionally, the effect of short-term application of the same amendment treatments on soil suppressiveness was tested using control arable sandy soil from 2 field sites. Disease suppressiveness was measured with a bioassay using the Lepidium savitum (cress) – Pythium ultimum model system. Multiple fields showed a strong impact on cress growth independent of the amendment treatment. Additional chemical and biological measurements show evidence of an effect from inherent pathogen load. Focussing on sites with low inherent pathogen load we found no significant impact of long-term amendments on soil suppressiveness. However, short-term application of Bokashi did significantly promote soil suppressiveness. This effect can likely be attributed to the increased metabolic activity of the soil’s inherent microorganisms in response to Bokashi, containing more easily-decomposable compounds as compared to the other soil amendments but
the longevity of this effect requires further field tests.