Influence of compost and digestates on plant growth and health: potentials and limits

Summary

Composts can influence soil fertility and plant health. These influences can be positive or negative, depending of the quality of the composts. In order to estimate the potential of Swiss composts to influence soil fertility and plant health, one hundred composts representative of the different composting systems and qualities available on the market were analyzed.
The organic substance and the nutrient content of the composts varied greatly between the composts; the materials of origin were the major factor influencing these values. The respiration rate and enzyme activities also varied greatly, particularly in the youngest composts. These differences decreased when the composts become more mature. Maturity, the degradation stage of the organic matter, depended not only on the age of the compost, but also on the management of the process. The Nmineralization potential of compost added to soil showed that a high proportion of young composts immobilized the nitrogen in the soil. Two compost parameters allow to predict the risk of nitrogen immobilization in soil: the NO3- and the humic acids contents. The phytotoxicity of the composts varied very much even in mature composts, showing that the storage of the compost plays a decisive role. While the majority of composts protected cucumber plants against Pythium ultimum, only a few composts suppressed Rhizoctonia solani in basil. With respect to disease suppression, the management of the maturation process seems to play a major role.
In field experiments, some biologically immature composts immobilized nitrogen in soil and reduced growth of maize. With additional fertilization, however, it was possible to compensate this effect. Digestates and composts increased the pH-value and the biological activity of soil. These effects were observable also one maize season after compost application. In conclusion, big differences were observed in the quality of composts and digestates, and in their impact on soil fertility and plant health. The management of the composting process seems to influence the quality of the composts to a higher extent than the materials of origin or the composting system. More attention should be paid to biological quality of composts, in order to produce composts with more beneficial effects on crops.