Fertilizing Trees

Question from Michelle:
A year or two ago I switched from conventional to organic gardening. Occasionally I find some old synthetic fertilizer in my garden shed that I have not yet used up. That is why yesterday I fed all my citrus trees with a commercial citrus food that was synthetic fertilizer instead of using organics.

Then last night I got into bed and realized—oh dear I never watered the citrus trees! I was doing so many tasks that I simply forgot. I have turned the sprinklers on and am watering them now, but my question is will the roots get burned?

Answer from Pat:
Don’t worry about your citrus trees.

The purpose of watering well after spreading fertilizer is simply to water the fertilizer into the ground.  The reason is that a certain amount of nitrogen escapes into the air unless one waters it in immediately after spreading it or unless there is a good rain. (This is why we often fertilize ahead of rain.) When one uses synthetic fertilizer more nitrogen is lost into the air than when one uses organics, which are slower acting. Leaving dry synthetic fertilizer lying on top of the ground thus somewhat reduces the amount of nitrogen you gave your trees and for this reason is not a good practice, but it will not make a huge difference. Luckily it will not burn your trees. Dry fertilizer lying on top of the ground does not harm plants, it only harms the fertilizer.

On the other hand, fertilizing when the ground is completely dry and then watering inadequately afterwards can result in burned roots because strong nitrogen may hit dry roots, thus burning them. This problem is most often seen with lawns. Fertilizing a lawn when the ground is bone dry is a good way to burn it, even when using chicken manure. The safest time to fertilize a lawn is when the soil is moist but the blades of grass are dry, but even if the ground is bone dry you can perhaps avoid burning a lawn by watering deeply and well after spreading the fertilizer.

These are the reasons why I always say “Water deeply after fertilizing” or “Water the fertilizer in well.” The main point here is that in this specific case you did no harm. Also, it is wise of you to gradually use up your old fertilizers when you find them. Always follow package directions regarding amounts to use.