Two rates of nitrogen application were compared over an eight-year period in a plantation of raspberries (Rubus idaeus), cv. ‘Malling Jewel’, in which a number of management systems for the control of weeds and sucker growth were also evaluated. Despite considerable differences in cane and fruit production between management systems involving cultivation and non-cultivation, the response of the crop to additional nitrogen was independent, no interaction between cultivation and nitrogen being found. Over-all, the higher rate of nitrogen significantly increased yields per hectare in the first two cropping years, had no effect during the next three years, and significantly depressed yield in the final two years. These changes in response as the plantation matured resulted from changes in the relative importance of cane numbers and yield per cane as components of total yield.
In six out of seven years, added nitrogen significantly increased the numbers of fruiting canes tied in, but had little effect on average cane length. The numbers of new (first-year) canes growing between the stools, in the alleys and within the stool itself were also increased. This resulted in greater competition between fruiting canes and new canes within the stool and also increased the need for rigorous control of new canes-all of them true suckers arising from roots - growing outside the stool area.
Extra nitrogen produced a significant increase in the incidence of cane death in several seasons. It further affected yield per cane by depressing the mean number of berries per metre of live cane, although this was offset in some years by larger berry size. The rate of nitrogen application did not, however, affect the number of laterals per metre of live cane. The evidence suggests, therefore, that apart from actual cane death, the major cause of adverse response to increased application of nitrogen in the later years was a reduction in mean fruit production per lateral.