For ground application, this practice apparently originated on the dependability of Spring rains to move the fertilizer into the roots zone, to be available to the trees when growth commences in the Spring. However, scientific evidence indicates that the initial growth during the spring comes from materials that have been previously assimilated in the fall, and are redistributed during growth.
Controversy also exists over the effectiveness of Spring foliar application.
Some research indicates that urea application in the spring is as effective, or more, than ground application in improving fruit set, size and yield whereas, other research indicates that foliar urea stays primarily on the sprayed leaves, and does not affect fruiting or the N status of the whole trees. This lack of mobilization may be due to the small concentrations at which urea is applied to avoid phytotoxicity. Although N nitrogen uptake by apple trees appears to be continuos process through out the season, the application of N in the fall is not a common practice. This is mostly due to the evidence of delayed senescence associated with winter damage. However, previous research indicates that trees that were given fall fertilizer application had a higher fruit set number.