Should you fertilize your trees this fall?

Oct. 5, 2018

This paid piece is sponsored by Aspen Arboriculture Solutions.

By Sam Kezar, owner of Aspen Arboriculture Solutions

Wondering if you should fertilize your trees this fall? You’re not alone.

I get asked this question by clients fairly consistently. Let’s go over the basics on what to look for before you decide because fertilizing a tree can actually do more harm than good if done improperly!

What to know about nutrients

The first thing you should realize is that trees do not use, need or metabolize nutrients in the same way as garden plants or agricultural crops. Fertilizer is often coined as “food” for your plants. That is crazy talk. Fertilizer is nutrients: N = nitrogen, P = phosphorus, K = potassium. It would be like telling toddlers they don’t need supper because they had their gummy vitamin instead. Fertilizer is providing only the necessary minerals so the plant can metabolize its food — the sugars produced through photosynthesis.

When you give your tomato plants fertilizer and they grow bigger, that’s because the plants use the additional and easy-to-get nutrients — mainly nitrogen — to photosynthesize more efficiently and to metabolize both the sugar and nutrients together to grow faster. What stays consistent among plants is this: The only thing a plant can do when given nitrogen is use it to metabolize green growth.

Why is that important to trees?

Trees are compartmentalizers — they grow and use their energy in compartments and sections. For example, new growth is pushed at the tips in the spring. Then, growth is put on in the twigs and stems to increase girth to handle the new loads by having grown taller. Then, the roots grow in the fall. Not everything grows all at once.

Plus, since trees live so much longer than most other plants, they store energy to make it through a tough growing year or some other stressor. So if we give a tree a bunch of nitrogen at the wrong time or in the wrong manner  — for instance, quick release of nitrogen versus slow release — it could lead to the tree using more energy than it planned on leaf and twig growth. That might mean it wouldn’t have enough for a major stress event, the necessary roots to keep up with the new growth or extra energy to fight off native pests and diseases.

Conduct a soil test

How are you going to know what to put in the ground if you don’t know what’s there? Anyone who tells you to fertilize your plants without testing the soil first is conducting malpractice! Soil tests are quick, easy and inexpensive. Once you have your results, either the testing lab or a professional like myself can read the test and determine if fertilizing is necessary.

Then consider these factors

Just because you have low nutrient levels doesn’t always mean you should fertilize. There are some advanced considerations with reading the soil test that should be considered too. What is your soil pH? If it is above 7.5, certain micronutrients such as iron and manganese are going to start becoming unavailable to the trees. That would be a situation where fertilizing or adding in those micronutrients will not work.

Also, pay attention to the percentage of organic matter in your soil. Ideal organic matter content is 5 percent to 8 percent in soils around Sioux Falls. If you are below that amount, it could explain the low amount of nutrients you do have and lead to leaching of the fertilizer you use. Finally, testing should be done every year or two to determine if fertilizing is still necessary. Soil chemistry can change over time, and fertilizing may not always be necessary.

When fertilizing, apply only what is necessary and use high-quality, slow-release products. Quick-release or cheap urea-type fertilizers will only harm your trees and can lead to scorching of the leaves if applied at high levels or the wrong time of year. Phosphorus should rarely be needed because it does not leach out of the soil and is barely used by trees at all — not to mention it is a major downstream pollutant of waterways and algae blooms.

Fall is the best time to fertilize trees, but early spring can work with high-quality products with lower nitrogen rates. And all of this will depend on your soil test and the species of trees you have.

Call or text to 605-759-6020, email, or fill out the consultation request form  at

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Should you fertilize your trees this fall?

Wondering if you should fertilize your trees this fall? You’re not alone.

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