Organic amendments and cover crop effects on soil organic matter and disease suppression in potatoes

Early potatoes come first to the market in summer and hence are a very profitable crop. In Norway, they are produced in small areas repeatedly without crop rotation. Potato production is often conducted on sandy soils, with intensive soil tillage reducing the content of soil organic matter (SOM) in the topsoil (0-20 cm). With less SOM, soilborne diseases may thrive, also fungal disease may also survive on decaying plant residues. Cover crops (CC) and application of organic amendments to the soil may increase SOM, but concurrently provide sites of survival for Rhizoctonia (black scurf), a very serious potato disease. We have examined the effects of winter rye as cover crop, sown after potato harvest, and addition of different organic materials on soil characteristics, potato yields and soilborne potato diseases in a field experiment in Norway during 2019-2021. Organic materials applied were biochar mixed with liquid digestate; solid digestate; and horse manure with sawdust bedding material. One single addition of these materials, applied as 15 Mg total C per hectare in April 2019 increased SOM assessed in 2021, especially with horse manure, while the presence of CC did not significantly affect SOM in 2021. The CC significantly increased potato yield in 2021 and reduced the severity of potato diseases by 10% in the post-harvest potatoes in 2020 and 2021, while the number of marketable potatoes after storage in 2021 was increased by 37% with CC. Application of organic materials did not significantly affect potato yield or quality, but the proportion of potatoes with diseases tended to be higher after application of biochar + liquid digestate. These results indicate that in the short-term, the use of a CC over winter after early potato harvest may enhance productivity by increasing the number of marketable potatoes, while addition of organic amendments lead to an increase of SOM levels without affecting the potato yield and productivity in the short term.