To Fertilize or Not to Fertilize?

“Should I fertilize my trees?”

Through my years as an arborist, I have gotten this question a lot. The answer varies depending on which arborist you ask. I thought I’d offer some pros and cons for fertilizing trees to help with the decision.

Why Should I Fertilize My Tree?

Trees in yards and urban areas often have some of the following issues limiting healthy root growth: Competition with turf, limited root zones due to paved areas and buildings, or compacted soils. Since there are limits to the root system, fertilizers will help give the tree with limited root systems more nutrients. 

Why Shouldn’t I Fertilize My Trees?

I don’t know what my tree needs

Ideally, before fertilization, there should be a soil test or foliar test (preferably both) to know what nutrients the tree needs more of, and whether or not they are in the soil. If a tree company doesn’t do any testing before prescribing fertilizer, be wary.

There will be pollution

Some methods of tree fertilization are lower-impact than others, But there will be some runoff that will fertilize algae in the James River and Chesapeake Bay.

There can be a potential harm to the tree

            If too much quick-release fertilizer is used, it is basically adding salt to the soil which can dry out roots.

Flushes of new growth can promote some insect pests

It may harm beneficial soil organisms

Should I do anything instead of fertilization?

Most tree fertilization adds nitrogen, phosphorous, and potassium to the soil. What urban soil needs is more pore space for air, and to let water drain through. Adding more nitrogen when roots really need more air may green your plant up for a while, but it may be masking problems instead of solving them.

To improve the soil quickly, have a soil invigoration (radial trenching, vertical mulching) done instead. Arborists have the ability to use compressed air to blow out compacted soil in areas around the tree with minimal negative impact to the root system. Soil can be put back with a compost mix to improve air and water penetration into the soil. This is a messy process, and you may need to give up some turf around the tree. But it works.

To slowly, and inexpensively improve soil, one of the easiest things to do is give up a little bit of grass and add a few inches of wood chips. Just remember to avoid piling up chips and mulch around the trunk and root flare.