INFLUENCE OF ALDER HEDGES ON NITROGEN NUTRITION OF APPLE TREES

In an experimental orchard of Lombartscalville apples on rootstock M I, a favourable influence of alder hedges dividing the field into plots was observed. This influence was only found in plots with a grass cover and without nitrogen fertilization for several years, where the apple trees showed severe symptoms of nitrogen deficiency. The influence of the hedges was due to an improved nitrogen supply. Trees 5 m or less from the hedge had higher leaf nitrogen contents, a 37 per cent higher yield and made better shoot growth. Apple trees in clean-cultivated plots fertilized annually with 80 to 120 kg N per ha showed no favourable influence of the alder. In the topsoil of the grass plots nodule clusters of nitrogen-fixing organisms were found in abundance on the alder roots whereas in the clean-cultivated plots almost no root nodules occurred; this difference was attributed to a difference in soil nitrate concentration. There was a distinct negative correlation between the leaf nitrogen content of the apple trees and the distance from the alder, indicating that the transfer of nitrogen takes place in the nodule zone and is therefore a function of the degree of overlapping of apple roots by the nodule zone. It was estimated that twelve years after planting, a degree of overlapping of 11 and 28 per cent would equal a fertilizer effect of 26 and 48 kg N per ha respectively. © 1968 Martinus Nijhoff.