Fall Urea Application

Apple growers in Vermont, traditionally, apply their nitrogen fertilizer, ground and/or foliar in the spring despite experimental controversy.  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.  This was attributed to ovule longevity.  This should be of interest in areas where the conditions for pollination, due to climatic conditions are marginal.  Any method that increases ovule longevity would lead to a better chance for fruit set.

In a recent study conducted in Oregon appearing in the October issue of HortScience Journal (Distribution of urea-derived nitrogen supplied to apple leaves.  H. Khemira, T.L. Righetti, and A. Azarenko) , three experiments were conducted which I thought would be of interest to you.  The objective of the first study was to determine the mobility of urea-derived N from young leaves.  The results indicate, as previous studies have, that at the low concentrations at which urea is applied, the foliar applied-urea remains in the leaf where it was applied.  However, since this study was done using young leaves that were less than 30% of their normal size, it is possible that different age leaves might exhibit different mobility.  The second experiment was to study the mobility of urea-derived N from fall foliar applications where both spur and branches were sprayed.  The results of this experiment also indicate that there is little mobility of the fall-applied urea ten weeks after the urea was applied.  However, in the treated buds, 16% of the N came from the applied urea.  In the last study, the relationship between time of urea application in the fall and the movement of urea-N from leaves to buds was determined.  The urea applications consisted of dipping spurs of Red Delicious in 5% urea solution on 10 Oct,  1 and 18 of November (leaf fall started on 1 Nov. and the trees had lost 50% of their leaves by 18 Nov).  Results indicate that the 10 Oct date was the most effective of the three applications in supplying N to the buds.  The results of these studies indicate that a fall urea foilar application should provide N to the developing flower buds, but this application should be done soon after harvest for urea to be better absorbed. 
Since these studies were conducted in Oregon, what would be the applications to Vermont? Most of the time it is difficult to extrapolate the results of experiments conducted in a totally different climate and only few trees were tested.  However, in the NE Pest Management Guide, it is recommended to spray 5% urea in the fall to decrease the scab inoculum.  Now, there is a evidence of a additional benefit to this fall urea application.  This should be an added incentive to keep your sprayer out, but as their results indicate, this application should be done soon after harvest, before the abscission layer forms, in order to achieve maximum benefits.