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September, 1998

Can fall-applied nitrogen applications be beneficial?

By Rob Crassweller

Penn State University

Recently on the internet apple crop discussion group there has been considerable discussion about the application of nitrogen in late summer to early fall.

The original work was done by Tim Smith with Washington State University Cooperative Extension service in cooperation with Mike Mrachek and Tim Meyer. The original premise of the work was an attempt to increase fruit size and reduce alternate bearing on Red Delicious trees that were at sub-optimal levels of nitrogen. Based upon their work under Washington state growing conditions they reported the following results:

• Greater return bloom in light cropping seasons,

• Higher yields in light cropping years,

• Larger and higher percentage of N in flowers the following spring,

• Larger and higher percentage of N in early season leaves,

• No significant winter cold damage following -20 F in January 1996.

All this sounds too good to be true; and it may well be. First consider that to begin with they are located in a more arid climate that has significantly less rainfall than we have. They have the capability of controlling the moisture needed to sustain growth. In the east we do not have the ability to stop late season growth flushes after rainfall. Second the trees they were working with were marginally deficient in nitrogen to begin with. The highest nitrogen level at the end of the study was 1.92%.

Can their work be applied to Pennsylvania conditions? Maybe with some modifications. First, I do not believe that we can apply nitrogen at the same time; they were making applications beginning in August. I think to apply nitrogen at that time of year may result in late season growth particularly when we have a dry August followed by a rainier September. Second, most of our trees are already above the nitrogen deficient levels. In all the years I have been looking at leaf analysis results I can probably count on one hand the number of times I have seen deficient levels of nitrogen on apples. The results reported in Washington are comparisons between deficient and low levels. In the east we can not be certain that if we already have a luxury supply of nitrogen in the trees we would see a significant response.

What can we consider here in the mid-Atlantic then? Possibly a late fall application of nitrogen. By late fall I mean mid-October and later. It has been known for many years that early spring activities within the tree draw upon nitrogen reserves. Therefore, it is in a tree’s best interest to have a large reserve to draw upon the following spring. Since roots are believed to be physiologically active as long as soil temperatures are optimum, they can be absorbing the nitrogen and transferring it to reserves. Unfortunately, there is little orchard nutritional work being done in the mid-Atlantic area to firmly determine if late fall applications would help.