Risks and benefits of soil amendment with composts in relation to plant pathogens

The supply of composts has increased in many countries due to the enforced diversion from landfill of organic biodegradable wastes. Disposal is often the primary financial and environmental incentive for composting this organic waste, with benefits from low value end-products such as organic soil amendments being a secondary or negligible consideration. The use of composts can also pose risks from populations of plant and animal pathogens which may have survived the composting process. However, the suppressive effect of amending soils with composts on crop and turf grass diseases caused by soil-borne plant pathogens can be a significant benefit. The increasing environmental unacceptability and unavailability of effective chemical fungicide treatments has increased this potential value of composts. Information on the effects of compost amendment of soil or sand on diseases caused by soil-borne pathogens was reviewed. Out of 79 container experiments, where soil was generally amended with >= 20 %v/v compost, 59 showed a disease suppressive effect and only 6 showed a disease promoting effect of the compost amendment. Following amendment of soil in the field with compost, generally at >= 15 t/ha, disease suppression occurred in 45 out of 59 experiments, and disease promotion in only one. Although abiotic factors such as increases in soil pH or release of volatiles following compost amendment explained control of some diseases, loss of suppressiveness following compost sterilisation frequently demonstrated a biological control mechanism such as microbial antagonism or induced plant resistance. Prediction of the disease suppressiveness of composts or compost amended soils from their microbial activity by measurement of the hydrolysis of fluorescein diacaetate, dehydrogenase activity, or basal respiration has been variable. Changes in microbial community structure detected using T-RFLP and DGGE analyses have been more reliable indicators of the disease suppressiveness of compost amended soils. Inoculation of composts with biological control agents is also a promising technique for improving the efficacy and reliability of disease suppression following soil amendment.