Fall Fertilizing

I know you’re not supposed to fertilize fruit trees now, but I’ve noticed that landscapers seem to give the opposite direction, encouraging some fertilizer as the trees go dormant in order to harden them for the winter. Why the disconnect?

I mostly ask because I have a lot of fish tank water, and would like to dump it somewhere useful.

Fish wastes in water contains high amounts of ammonia which turns to nitrites by a relationship with nitrifying bacteria and eventually bacteria turn nitrites to nitrates in nature. Nitrate, ammonia, nitrites are readily used by trees and other plants so I don’t use it in the fall because the trees need to go to sleep and not grow. I would recommend you store the water because it’s valuable fertilizer in the spring. The fertilizer we apply on hay or grain crops such as wheat in the fall is phosphorus § and potassium (K). This link will help to understand why we apply P,K on those crops in the fall http://msue.anr.msu.edu/news/fertilizer_practices_in_the_fall. In the early spring we apply only nitrogen (N) because that’s when (N) does the most good. The idea is applying fertilizer when the plant will use it and the fertilizer does not have a chance to be washed away by rains (leaching) Leaching (agriculture) - Wikipedia. Back to trees and fertilizer, fertilizer should be available when the trees need to grow which is like I mentioned is in the spring. Read this link to better understand fertilizers and application of those http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/tree-fertilization-guide/. Trees don’t need nitrogen at all in many cases which you can judge by growth of trees or by soil tests. Boron as one example should be applied on fruit trees as a foliar spray in the fall when the leaves are still on the tree if the soil is deficient in boron. The buds will use that boron in the spring. Not everyone agrees with my philosophy of applying nitrogen in the spring when trees are growing and you should read this link http://www.ext.colostate.edu/ptlk/1216.html. When you do apply nitrogen this guide will help you know how much to apply http://extension.colostate.edu/docs/pubs/garden/07612.pdf. One thing you can use that fish water on is fall green crops if you have those growing such as turnips, mustard etc. because they continue to grow until the lack of sunlight slows them down.

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In Suffolk county NY it is illegal to to apply fertilizer after Nov 1. The idea is that the nitrogen is not used after that date and just pollutes the ground water.

No reason to apply nitrogen once the tree is dormant, N is very mobile in the soil profile and most will be unacessible to the plant come the next growing season. As Clark said there are some benefits of applying other nutrients to trees before leaf drop.