PERFORMANCE OF YOUNG APPLE TREES PLANTED IN REPLANT SOIL

Young apple trees of cv 'Delia' on rootstock M 26 were planted a year after pulling out an apple orchard maintained for 22 years. Trees for the new orchard were planted either in old rows (IR), or between rows (BR) of the former plantation. We have investigated the probable case of replant problem depending on timing of nitrogen supply by various (either NH₄ or NO₃) forms of N and groundcover management systems. IR-trees that received treatments of N-forms had longer shoots than BR-trees. BR-trees had more flowers than IR-trees. There were similar relationships concerning growth and flowering of trees grown in plots of groundcover treatments. Total bacterial numbers and population of aerobic proteolytic bacteria were higher in soil strips of IR- than BR-trees. Numbers of nitrifying bacteria were consequently similar. The possible role of mycorrhizal fungi in eliminating replant problem has been suggested. Because of the inverse relationship between shoot growth and flowering potential in young apple trees, no real evidence has been found for existence of replant problem in the fertigated (or irrigated) orchard in a sandy soil with low pH and humus content.