Should I Fertilize My Trees?
My clients in Sioux Falls ask me this question fairly regularly. And initially is can be easy to answer, but can be very complicated if we dive into the why. Lets go over the basics on what you should look for before you decide to fertilize your trees. Because fertilizing a tree can actually do more harm than good if done improperly!
The first thing you should realize is that trees do not use, need, or metabolize nutrients they way garden plants or agricultural crops. Fertilizer is often time coined as “food” for your plants. That is crazy talk. Fertilizer is nutrients (N=nitrogen, P=phosphorus, K=Potassium). It would be like telling your toddler they don’t need supper because they had their gummy vitamin in stead. Fertilizer only provides the necessary minerals to the plant so it can metabolize its food (i.e. sugars produced through photosynthesis). When you give your tomato plants fertilizer and they grow bigger, thats because the plants use the additional and easy to get nutrients (mainly nitrogen) to photosynthesize more efficiently and metabolize both the sugar and nutrients together in order to grow faster. That last portion is where fertilizing trees can be harmful. What stays consistent among plants is this; A plant can only metabolize green growth when fertilized.
Why is that important to trees?
Trees are compartmentalizers, they grow and utilize 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 have energy storage in order 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 (e.g. quick release Vs slow release nitrogen) it could lead to the tree using more energy than it planned on leave and twig growth and not 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 whats there or not 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.
Other Factors to Consider
And just because you have low nutrient levels doesn’t always mean you should fertilize either. 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 micro-nutrients are going to start to become unavailable to the trees like iron (Fe) and manganese (Mn). That would be a situation where fertilizing or adding in those micro-nutrients will not work. Also, pay attention to your percent organic matter in your soil. Ideal organic mater content is around 5-8% in our 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 put in.
Lastly, testing should be done on a regular basis (once a 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, only apply 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 since it does not leach out of the soil and is barely used by trees at all. Not to mention it is a major down stream 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.